Biomedical subjects

C H Irvine

Publications and source records attributed to C H Irvine.

At least 19 recordsLinked to original sources

PubMed2000

The effect of the alpha-2-adrenergic agonist, clonidine, on secretion patterns and rates of adrenocorticotropic hormone and its secretagogues in the horse.

Alpha-2-adrenoceptor activation may lower adrenocorticotropic hormone (ACTH) by reducing secretagogue input and/or increasing the release of an inhibitory factor (CIF). To investigate this, we gave clonidine, an alpha-2-agonist, to seven horses, and collected pituitary venous blood every minute for 20 min before treatment and 40 min after treatment. Six horses were given saline vehicle. Mean secretion rates of corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP) and ACTH were calculated before and during four 5-min then two 10-min periods after clonidine or saline. Reduction in ACTH secretion without corresponding changes in CRH and/or AVP would imply the presence of CIF. Secretion rates of ACTH (P = 0.008) and AVP (P = 0.0005) fell after clonidine and remained lower than baseline values for 20 min and 10 min, respectively. The CRH secretion rate decreased slightly but not significantly after clonidine. In controls, hormone secretion rates did not alter during the experiment. Multiple linear regression showed that CRH and AVP secretion accounted for 69% (treated) or 45% (controls) of the variation in ACTH secretion (P < 0.0001 for each). CRH alone contributed 80% (treated) or 76% (controls) of the fit to this model, which is consistent with the concept that CRH 'sets the gain' of the response of corticotrophs to fluctuations in AVP. Accordingly, minute-to-minute changes in pituitary concentrations of AVP and ACTH were synchronous when all data were considered (% concordant changes: controls, 68%, P < 0.0001; treated, 76%, P < 0.0001) and the percentage of concordant movement was unaffected by clonidine (before 72%; after 73%; P = 0.80). In treated horses but not controls, the ratio between the secretion rates of ACTH and AVP fell (P = 0.009), while the ACTH : CRH ratio tended to fall after clonidine, implying reduced responsiveness to stimulation. Moreover, one horse showed a drop in ACTH and a rise in CRH and AVP secretion after clonidine. We conclude that in horses alpha-2-adrenoceptor activation lowers ACTH secretion primarily by reducing the secretion of AVP and possibly CRH. While there was some evidence that a CIF may participate in the clonidine-induced suppression of ACTH, the subtlety of the discordance between ACTH and its secretagogues in most horses and the rarity of complete dissociation indicate that it does not play a major role.

Adrenergic alpha-2 Receptor Agonists↗

PubMed2000

Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles.

The aim was to define precisely the FSH secretion pattern in mares during the two ovulatory cycles before, and for 24 days after, the last ovulation of the season and to compare this with the profiles of other reproductive hormones and follicular growth to identify changes which may lead to the termination of follicular cycles. Jugular blood was collected every 6 h from ten light horse mares for 6 weeks in autumn. Samples were assayed for FSH, LH, prolactin, inhibin, oestrone conjugates and progesterone. Luteolysis occurred earlier and periovulatory oestrone, but not inhibin, concentrations were significantly lower in the last than in the second to last cycles. In ovulatory and anovulatory cycles, daily mean FSH concentrations were low at the expected time of ovulation and high between days 9 and 11 (day 0 = ovulation), which were usually after luteolysis. However, the periovulatory FSH nadir was prolonged in the last compared with the second to last cycles, and the difference between peak and trough values was not significant in anovulatory cycles. Between day 5 and day 8, the FSH interpulse interval was approximately 2 days, and did not vary in successive cycles. The LH profile also showed progressive changes as mares entered acyclicity; the surge terminated sooner in the last than in the second to last cycles, and failed to occur when expected in acyclicity. Sporadic prolactin pulses occurred at luteolysis in a similar proportion of ovulatory and anovulatory cycles. These results indicate that inadequate gonadotrophin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, and eventually acyclicity. Moreover, the time relationships amongst changes in pituitary and ovarian hormones and follicular growth become increasingly disrupted during the autumn transition, which may contribute to the cessation of cyclicity.

Analysis of Variance↗

PubMed1998

Daily intake and urinary excretion of genistein and daidzein by infants fed soy- or dairy-based infant formulas.

Our aims were to measure isoflavone intake from soy- and dairy-based infant formulas and breast milk and to assess the ability of infants to digest and absorb soy isoflavones by measuring daily urinary excretion rates. We recruited 29 infants: 4 received soy-based formula and 25 received dairy-based formula. We collected pooled urine samples from 3-5 disposable diapers worn during a 24-h period and developed and validated methods for extracting isoflavones from the diapers. Infants were studied every 1 or 2 wk, starting at 2-6 wk of age and continuing until 16 wk. Only soy-based formulas contained isoflavones in concentrations detectable by HPLC (limits: 0.05 mg/L for liquids and 0.1 mg/kg for solids). Soy-based formulas provided a mean (+/-SEM) daily dose of isoflavones (genistein plus daidzein) of 3.2 +/- 0.2 mg/kg body wt, which remained fairly constant (CV: 12%) regardless of age < or = 16 wk. Isoflavones were measurable in all samples from soy-fed infants, but not in urine from dairy-fed infants. Daily isoflavone excretion rates varied little among infants [range of mean individual values (mg x kg(-1) d(-1)): daidzein, 0.37 +/- 0.03 to 0.58 +/- 0.06; genistein, 0.15 +/- 0.03 to 0.32 +/- 0.04] and did not change with age < or = 16 wk. The mean percentage of the daily intake recovered in the urine of soy-fed infants was 38 +/- 4% for daidzein and 13 +/- 3% for genistein, and remained constant with age. These values are similar to those for adults and indicate that young infants are able to digest, absorb, and excrete genistein and daidzein from soy-based formulas as efficiently as do adults consuming soy products.

Adult↗

PubMed1998

Gonadotrophin profiles and dioestrous pulsatile release patterns in mares as determined by collection of jugular blood at 4 h intervals throughout an oestrous cycle.

In mares, dioestrous FSH profiles based on once-a-day sampling are variable; however, the pulsatility of plasma FSH, which has been suggested by limited windows of intensive sampling, may contribute to this variability. Jugular blood from six mares was sampled at 4 h intervals throughout an ovulatory cycle to determine cyclic FSH and LH patterns more accurately and to measure gonadotrophin pulse frequency during dioestrus. Synchronous pulses of FSH and LH occurred regularly in all mares between day 4 and day 12 (ovulation = day 0) with a mean (+/- SEM) frequency of 1.9 +/- 0.1 (FSH) or 1.6 +/- 0.1 (LH) pulses day-1. LH pulse amplitude declined (P < 0.0001) between day 4 and day 10, but FSH pulse amplitude remained large and stable, dipping slightly but not significantly on day 6. Daily mean FSH concentrations exceeded (P < 0.0001) early oestrous values between day 4 and day 5, and between day 7 and day 10. However, significantly different patterns were obtained when once-a-day sampling was simulated by selecting samples collected at 08:00 h or noon. LH was higher during the periovulatory surge than during dioestrus (P < 0.0001) and profiles were similar whether daily means or selected samples were used. It is concluded that: (1) the marked pulsatility of plasma FSH during dioestrus makes once-a-day sampling misleading for determining FSH profiles; (2) the dioestrous pattern of large, slow FSH pulses was consistent among mares, unlike that of the daily mean FSH profiles; and (3) no discrete FSH 'surges' were observed during dioestrus, although FSH pulse amplitude tended to undergo alternate increases and decreases. A period of higher amplitude FSH pulses preceded ovulation by 10.2 +/- 0.7 days, which corresponds to the approximate time the ovulatory follicle emerges. Therefore, it is possible that the signal for follicular recruitment in mares is intermittent excursions of plasma FSH above a threshold value.

Animals↗

PubMed1998

The effect of social stress on adrenal axis activity in horses: the importance of monitoring corticosteroid-binding globulin capacity.

Plasma cortisol is largely bound to corticosteroid-binding globulin (CBG), which regulates its bioavailability by restricting exit from capillaries. Levels of CBG may be altered by several factors including stress and this can influence the amount of cortisol reaching cells. This study investigated the effect of social instability on plasma concentrations of CBG, total and free (not protein bound) cortisol in horses. Horses new to our research herd ('newcomers') were confined in a small yard with four dominant resident horses for 3-4 h daily for 3-4 (n = 5) or 9-14 (n = 3) days. Jugular blood was collected in the mornings from newcomers before the period of stress began ('pre-stress'), and then before each day's stress. Residents were bled before stress on the first and thirteenth day. Residents always behaved aggressively towards newcomers. By the end of the stress period, all newcomers were subordinate to residents. In newcomers (n = 8) after 3-4 days of social stress, CBG binding capacity had fallen (P = 0.0025), while free cortisol concentrations had risen (P = 0.0016) from pre-stress values. In contrast, total cortisol did not change. In residents, CBG had decreased slightly but significantly (P = 0.0162) after 12 days of stress. Residents and newcomers did not differ in pre-stress CBG binding capacity, total or free cortisol concentrations. However, by the second week of stress, CBG binding capacity was lower (P = 0.015) and free cortisol higher (P = 0.030) in newcomers (n = 3) than in residents. Total cortisol did not differ between the groups. In conclusion social stress clearly affected the adrenal axis of subordinate newcomer horses, lowering the binding capacity of CBG and raising free cortisol concentrations. However, no effect of stress could be detected when only total cortisol was measured. Therefore, to assess adrenal axis status accurately in horses, it is essential to monitor the binding capacity of CBG and free cortisol concentrations in addition to total cortisol levels.

Adrenal Cortex↗

PubMed1998

Phytoestrogens in soy-based infant foods: concentrations, daily intake, and possible biological effects.

Exposure to estrogenic compounds may pose a developmental hazard to infants. Soy products, which contain the phytoestrogens, genistein and daidzein, are becoming increasingly popular as infant foods. To begin to evaluate the potential of the phytoestrogens in these products to affect infants, we measured total genistein and daidzein contents of commercially available soy-based infant formulas, infant cereals, dinners, and rusks. We also assayed phytoestrogens in dairy-based formulas and in breast milk from omnivorous or vegetarian mothers. In most cases, the glucoside forms of the phytoestrogens were hydrolyzed before separation by HPLC. Mean (+/-SEM) total genistein and daidzein contents in four soy infant formulas were 87+/-3 and 49+/-2 microg/g, respectively. The phytoestrogen content of cereals varied with brand, with genistein ranging from 3-287 microg/g and daidzein from 2-276 microg/g. By contrast, no phytoestrogens were detected in dairy-based infant formulas or in human breast milk, irrespective of the mother's diet (detection limit = 0.05 microg/ml). When fed according to the manufacturer's instruction, soy formulas provide the infant with a daily dose rate of total isoflavones (i.e., genistein + daidzein) of approximately 3 mg/kg body weight, which is maintained at a fairly constant level between 0-4 months of age. Supplementing the diet of 4-month-old infants with a single daily serving of cereal can increase their isoflavone intake by over 25%, depending on the brand chosen. This rate of isoflavone intake is much greater than that shown in adult humans to alter reproductive hormones. Since the available evidence suggests that infants can digest and absorb dietary phytoestrogens in active forms and since neonates are generally more susceptible than adults to perturbations of the sex steroid milieu, we suggest that it would be highly desirable to study the effects of soy isoflavones on steroid-dependent developmental processes in human babies.

Child Development↗

PubMed1997

Patterns of secretion of GnRH, LH and FSH during the postovulatory period in mares: mechanisms prolonging the LH surge.

To study the mechanisms responsible for the unusually slow decline of the ovulatory LH surge in mares, secretion patterns of GnRH, LH and FSH were monitored in pituitary venous blood collected every 2 or 5 min for 10.5-18.0 h from five mares on the third (n = 4) or fifth day after ovulation (first sampling period). To determine the effectiveness of progesterone negative feedback, mares were then given a luteolytic dose of a prostaglandin analogue (PGF2 alpha) and pituitary venous sampling (every 2 or 5 min for 16 h) recommenced 20-22 h later (second sampling period). During the declining arm of the LH surge, large peaks (detected by the Cluster algorithm) of concurrent LH and FSH secretion occurred infrequently, with four peaks being detected in a combined sampling period of 75 h. Outside the peaks, LH or FSH secretion continued (as assessed by a pituitary to jugular-venous concentration ratio > or = 1.25) during 46% +/- 13 or 40% +/- 10, respectively, of the sampling period. GnRH immunoactivity was detected during each spontaneous gonadotrophin peak, but at other times was generally at assay sensitivity. After PGF2 alpha, plasma progesterone fell (ng ml-1, mean +/- SEM; first sampling period: 8.6 +/- 0.8; second; 2.0 +/- 0.3; P = 0.001) and the frequency of LH (P < 0.05) and FSH (P < 0.02) peaks rose, with 28 peaks detected for each hormone in a total of 80 h sampling. Peaks in LH were smaller during the second period, with decreases observed in maximum (P = 0.027) and mean (P = 0.025) secretion rates. Maximum GnRH secretion rate during peaks also declined (P = 0.010); however, the decrement (-30 +/- 6%) was less than that in maximum LH secretion rate (-82 +/- 5%; P = 0.040), suggesting that other factors contribute to the reduced LH peak amplitude. In summary, gonadotrophin peak frequency during the downswing of the surge in mares is slow, as in the midluteal phase, and the slow rate of decline in peripheral gonadotrophin concentrations is due, at least in part, to continued secretion between pulses. Moreover, progesterone negative feedback is highly effective in early dioestrus, in that lessening it without complete removal markedly accelerates gonadotrophin pulse frequency.

Animals↗

PubMed1997

Effect of insulin-induced hypoglycaemia on secretion patterns and rates of corticotrophin-releasing hormone, arginine vasopressin and adrenocorticotrophin in horses.

To study the effect of hypoglycaemia on secretion rates of corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP) and ACTH in a non-ruminant species, a non-surgical method was used to collect pituitary venous (PitVen) blood every 0.5 or 1 min from seven horses before and after insulin administration (0.4 U/kg i.v.). To assess the effect of PitVen cannulation on results, peripheral hormones were also measured before and after insulin in five horses without PitVen cannulae. Insulin administration lowered plasma glucose in all horses (P < 0.0001; paired t-test). Cortisol concentrations, which were similar in horses with and without PitVen cannulae before insulin, rose significantly after insulin administration in both groups. Most horses showed discomfort as glucose fell. When data from horses with and without PitVen cannulae were pooled, the peak fractional change in cortisol (Spearman's rank correlation coefficient (rs) = -0.94, P < 0.001) and the severity of hypoglycaemic symptoms (rs = -0.61, P < 0.02) were inversely ranked with the glucose nadir. In horses with PitVen cannulae, insulin administration increased secretion rates of ACTH (P < 0.0001), AVP (P < 0.0001) and CRH (P < 0.02). Increments in ACTH (rs = -0.96, P < 0.005) and CRH (rs = -0.81, P < 0.05), but not in AVP, measured during the second half-hour after insulin (i.e. the peak response), were inversely ranked with the glucose nadir. Moreover, ACTH increments were positively ranked with those in CRH (rs = 0.81, P < 0.05), but not in AVP. Nevertheless, in individual horses, minute-to-minute AVP and ACTH concentrations in PitVen blood were always correlated, whereas minute-to-minute CRH and ACTH concentrations were correlated only when glucose dropped below 3.4 mmol/l. In less hypoglycaemic horses, ACTH secretion rose despite little or no change in CRH. We suggest that in horses AVP is the primary acute signal for ACTH release both before and during hypoglycaemia; however, the increasing magnitude of ACTH increments induced by greater degrees of hypoglycaemia is determined largely by selective CRH release, which then augments corticotroph responses to AVP.

Adrenocorticotropic Hormone↗

PubMed1996

Dynamics of the regulation of the hypothalamo-pituitary-adrenal (HPA) axis determined using a nonsurgical method for collecting pituitary venous blood from horses.

Since 1985, we have applied our nonsurgical technique for collecting pituitary venous (PitVen) blood from ambulatory horses to investigate the regulation of adrenocorticotropic hormone (ACTH) secretion. This method offers particular advantages for studying the hypothalamo-pituitary-adrenal axis since its benign nature enables hypothalamic and pituitary interactions to be monitored without disturbing the animal, and the horse's large blood volume allows 3- to 4-ml samples to be collected as frequently as every 20s for prolonged periods so that the secretion patterns of ACTH and its secretagogues can be precisely defined. When PitVen blood was sampled every 20 or 30s during the circadian maximum, arginine vasopressin (AVP) and ACTH secretion patterns were complex and irregular, with mean interpeak intervals of approximately 5 min. Despite their erratic patterns, AVP and ACTH secretions were closely coupled on cross-correlation analysis. By contrast, PitVen corticotropin-releasing hormone (CRH) concentrations were low, relatively stable, and not consistently related to ACTH secretion. However, when cortisol negative feedback was reduced acutely by metyrapone infusion, CRH and AVP secretion were stimulated. Mathematical modeling suggested that CRH had become the more effective secretagogue and that much of the ACTH response was mediated by increased pituitary responsiveness to CRH. Elevated blood osmolality triggered synchronous AVP and ACTH secretion, without altering PitVen CRH. In this case, the source of PitVen AVP was presumably the magnocellular/neurohypophysial pathway, which is thought to respond primarily to changes in blood osmolality and pressure. Our results suggest that this pathway also participates in ACTH regulation. We have studied the effect of several perturbations and found, as have others, that the secretagogues released vary with the stimulus given. For example, vigorous exercise promptly raised PitVen AVP and ACTH, but not PitVen CRH. Hypoglycemia provoked both CRH and AVP secretions, with the CRH increment being inversely proportional to the glucose nadir. Administration of the opioid antagonist, naloxone, increased PitVen ACTH; however, changes in AVP and CRH were variable and overall could not account for the ACTH response. This suggests that endogenous opioids inhibit a third ACTH secretagogue, stimulate an inhibitory factor, or also act at the pituitary. Chronic social stress, induced by confining newcomers with aggressive, resident mares, caused most introduced horses to become submissive. In such horses, plasma cortisol declined to levels similar to those during metyrapone infusion. Despite hypocortisolemia, PitVen ACTH was low, whereas PitVen CRH tended to be elevated. Moreover, chronically stressed horses did not respond to exogenous CRH. We conclude that at rest and during some perturbations AVP is the immediate stimulus for ACTH release. Even ACTH micropulses, previously thought to occur spontaneously, appear to be regulated by AVP in horses. On the other hand, CRH secretion and pituitary responsiveness to CRH rise when cortisol falls, suggesting that a major role for CRH is to fix the cortisol setpoint. However, during chronic stress, these relationships become disturbed, with results to date pointing toward the existence of an ACTH-release inhibiting factor.

Adrenal Glands↗

PubMed1995

The effect of naloxone administration on the secretion of corticotropin-releasing hormone, arginine vasopressin, and adrenocorticotropin in unperturbed horses.

We used our nonsurgical method for collecting equine pituitary venous blood to study the role of endogenous opioids in the basal regulation of the hypothalamo-pituitary-adrenal axis. We gave mares the opioid antagonist, naloxone (NAL), at either a high (0.5 mg/kg i.v. bolus, followed by infusion of 0.25 mg/kg.h; n = 4) or low (0.2 mg/kg i.v. bolus; n = 6) dose rate. Pituitary venous blood was collected continuously, divided into 0.5- or 1-min segments for 15-30 min before and 1 h after the NAL bolus, and assayed for CRH, arginine vasopressin (AVP), and ACTH. The mares tolerated NAL administration well, with little difference between dose rates in the mild transient side-effects. Both NAL doses increased jugular cortisol concentrations (high, P = 0.0022; low, P = 0.0001) and the ACTH secretion rate (high, P = 0.0056; low, P = 0.0103). High dose NAL raised the secretion rates of AVP (P = 0.0252) and CRH (P = 0.0106); however, the magnitude of ACTH responses exceeded those in AVP and CRH, as shown by increased ratios between ACTH and AVP (P = 0.0246) or CRH (P = 0.0122) secretion rates. After low dose NAL, neither CRH nor AVP secretion was altered. Indeed, CRH declined as ACTH rose in 4 mares and was unchanged in a fifth mare. When data from the 10 mares were pooled, mean secretion rates of ACTH and CRH were correlated after (P < 0.05), but not before, NAL treatment. Overall, mean ACTH and AVP secretion rates were not correlated during any 30-min period, but in individual mares, minute to minute AVP and ACTH secretion patterns were always correlated. We conclude that endogenous opioids inhibit the equine hypothalamo-pituitary-adrenal axis under basal conditions; however, their sites of action do not appear to lie solely on CRH and/or AVP neurons. It seems likely that endogenous opioids also inhibit the release of a third ACTH secretagogue or promote the secretion of an ACTH release inhibitory factor.

Adrenocorticotropic Hormone↗

PubMed1994

Factors affecting the circadian rhythm in plasma cortisol concentrations in the horse.

In horses, a circadian rhythm in plasma cortisol concentrations has been reported in some but not all studies. When a rhythm occurred, horses were accustomed to a management routine, comprising stabling, feeding and sometimes exercise, which may entrain a circadian pattern. In this work, we monitored plasma cortisol by collecting jugular blood through indwelling cannulae from four groups: 1): 10 untrained, unperturbed mares grazing excess pasture, bled hourly for 26 hr; 2) 4 mares housed in a barn for 48 hr before sampling every 15 min for 20-24 hr; 3) 5 mares placed in an outdoor yard for sampling every 30 min from 0930-2100 hr; and 4) 4 stabled racehorses in training, bled every 30 min from 0730-2000 hr and once the following morning at 0830 hr. Plasma cortisol showed a similar-timed circadian rhythm (P < 0.0001) in all Group 1 horses, with a peak at 0600-0900 hr, and a nadir at 1800-2100 hr. By contrast, cortisol concentrations did not vary with time in either Group 2 or 3. Neither daily mean nor peak cortisol values differed in Group 1 and 2 (i.e. bled for > or = 20 hr); however nadir values were higher (P < 0.05) in Group 2. In Group 4, cortisol declined (P = 0.004) during the sampling period but had returned to initial concentrations the next morning. Values did not differ from those for Group 1, except between 1000 and 1300 hr when cortisol in Group 4 was lower (P < 0.05). We conclude that a circadian cortisol rhythm exists in horses in the absence of any known cues imposed by humans. However, this rhythm can be obliterated by the minor perturbation of removing the horse from its accustomed environment. By contrast, the rhythm occurs in trained racehorses, suggesting either that they have adapted to their environment thereby allowing an endogenous rhythm to emerge, or that the rhythm is entrained by their daily routine. These observations highlight the difficulties in determining the cortisol status of a horse, since measurements will be affected by time of day, the occurrence of short-term fluctuations, and how accustomed the horse is to its environment.

Adaptation, Physiological↗

PubMed1994

Short-term secretion patterns of corticotropin-releasing hormone, arginine vasopressin and ACTH as shown by intensive sampling of pituitary venous blood from horses.

To characterize the short-term ACTH secretion pattern and to investigate factors regulating it, pituitary venous (PV) blood was collected using our nonsurgical method from 8 unperturbed horses every 20 or 30 s for approximately 1 h. In all but 1 horse, sampling occurred during the broad circadian maximum in plasma cortisol concentrations. Concentrations of corticotropin-releasing hormone (CRH; n = 7 horses), arginine vasopressin (AVP), ACTH and cortisol were measured by radioimmunoassay. In all horses, CRH, AVP and ACTH secretion patterns appeared irregular in time and amplitude. The mean (+/- SEM) numbers of peaks per hour detected by the cluster program were 2.8 +/- 1.2, 10.1 +/- 1.9 and 10.2 +/- 1.4 for CRH, AVP and ACTH, respectively. However, when 2- and 5-min sampling frequencies were simulated by meaning consecutive values, significantly fewer peaks were detected in each hormone. There was no correlation between the prevailing cortisol concentration and peak frequencies of CRH, AVP or ACTH. Secretion patterns of ACTH and AVP were closely related in all horses as assessed by cross correlation analysis and coincidence of peaks, although the ratio between PV ACTH and AVP concentrations fluctuated markedly within each horse. In contrast, the relationship between CRH and ACTH secretion was variable. Bivariate spectral analysis showed only a modest degree of underlying periodicity in CRH, AVP and ACTH secretion during the very short term studied. Nevertheless, distinct peaks exceeding the 95% confidence limits of white noise were observed at periods between 2 and 30 min in 5 of 7 CRH, 6 of 8 AVP and 5 of 8 ACTH spectra. Furthermore, the slope of the regression line through each spectrum did not become indistinguishable from zero, i.e. the flat white noise continuum, until mean (+/- SEM) periods of 2.6 +/- 0.8, 1.6 +/- 0.2, and 2.0 +/- 0.2 min, for CRH, AVP and ACTH spectra, respectively. At the ACTH spectral maximum, the coherence coefficient, which is analogous to the squared correlation coefficient, exceeded 0.5 in comparisons of all ACTH and AVP spectra and of 5 of 7 ACTH and CRH spectra.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenocorticotropic Hormone↗

PubMed1994

The dynamics of gonadotrophin-releasing hormone, LH and FSH secretion during the spontaneous ovulatory surge of the mare as revealed by intensive sampling of pituitary venous blood.

Conflicting views exist on the mode of gonadotrophin-releasing hormone (GnRH) secretion during the ovulatory LH surge and the relative importance of changes in pituitary responsiveness to GnRH in generating the LH surge. This disagreement may stem from species differences and/or methodological problems. To provide data on the exact relationship between GnRH and gonadotrophin secretion during the spontaneous LH surge, we collected pituitary venous (PV) blood every 30 s for 3-4 h from eight mares and then assayed GnRH (in six of the mares), FSH and LH. Jugular blood was also collected from twelve mares without PV cannulae either thrice daily during the surge (n = 8) or hourly for 24 h when close to ovulation (n = 4) and assayed for LH. Hormone peaks in PV blood were detected by the Cluster program and PV hormone patterns were scanned for underlying periodicity using spectral analysis. Jugular LH concentrations rose slowly and steadily without abrupt increase during the prolonged ovulatory surge, suggesting that hormone secretory patterns seen during the periods of rapid sampling were typical of the surge. Jugular LH concentrations were similar in mares with and without PV cannulae. Intensive sampling of PV blood showed that GnRH, FSH and LH were secreted in frequent (two to five per h) brief (5-7 min) peaks. Secretion was not detectable in 24%, 28% and 57% of the total sampling time for GnRH, LH and FSH respectively. GnRH and LH peaks appeared to be irregular in time and amplitude in most mares. However, spectral analysis of the data revealed an underlying periodicity in the secretion of all three hormones, with the dominant period ranging from 20 to 65 min in individual mares. The spectra of GnRH, FSH and LH were highly coherent at this dominant frequency, and 90% of GnRH peaks were concurrent with LH peaks, which is consistent with the dogma that GnRH is the primary secretagogue for both FSH and LH. Although PV FSH and LH concentrations were closely correlated, PV GnRH and gonadotrophin concentrations were only weakly correlated, implying that there was no consistent relationship between the magnitudes of changes in GnRH and gonadotrophin secretion. When compared with our published mid-luteal phase values, the daily GnRH secretion rate during the LH surge was trebled, while the LH responsiveness to endogenous GnRH, as assessed by the ratio between newly secreted LH and PV GnRH concentrations, was four times greater.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Secretory patterns and rates of gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone revealed by intensive sampling of pituitary venous blood in the luteal phase mare.

We used our unique nonsurgical technique for collecting pituitary venous (pit) blood to study GnRH, FSH, and LH secretion patterns in midluteal phase mares. This method does not perturb endocrine function and allows continuous monitoring of GnRH and gonadotropin (Gn) secretion, determination of the amount of GnRH perfusing gonadotropes, and direct measurements of the amounts of Gn secreted. In a total of 80 h of 5-min sampling in four mares, eight Gn peaks occurred; however, more frequent sampling was needed to define secretory events precisely. Therefore, pit blood was collected continuously and split into 30-sec segments in six mares. To ensure a peak during sampling, the opioid antagonist naloxone was given after 4-6 h of sampling to try to replicate a physiological signal for GnRH release. Naloxone induced Gn peaks in jugular blood that were indistinguishable in amplitude from spontaneous peaks. Intensive sampling of pit blood showed that jugular peaks reflected major episodes of GnRH and Gn secretion lasting 30-55 min, which were similar in profile whether naloxone induced or spontaneous and consisted of a train of three to six peaks of diminishing amplitude. Peaks of GnRH and, less often, Gn also occurred outside major episodes. Despite markedly variable size, GnRH peak maxima were correlated with the amount of LH and FSH secreted in concurrent peaks. Likewise, cross-correlation analyses (n = 960 samples/mare) showed close correspondence between patterns of GnRH and secreted FSH and LH. The delay (+/- SEM) between GnRH and Gn maxima was 0.62 +/- 0.18 min for LH and 0.18 +/- 0.22 min for FSH. The majority of GnRH and Gn peaks were concurrent; however, 34.7% of GnRH peaks occurred without Gn peaks. These peaks had a lower amplitude than those with Gn peaks (P < 0.001). For Gn, secretion (i.e. ratio between pit and jugular concentrations, > 1.5) continued at a low level for 40 +/- 9% (LH) or 64 +/- 14% (FSH) of the time between Cluster-defined peaks during the basal period. We conclude that in the luteal phase 1) the predominant mode of GnRH and Gn secretion is as concurrent, large amplitude, prolonged episodes that appeared to be the summation of a train of peaks; and 2) a GnRH dose-Gn response relationship operates endogenously. This along with the synchronicity of secretion patterns of the three hormones suggest that GnRH is the major secretagogue for both LH and FSH.

Animals↗

PubMed1993

The acute effect of lowering plasma cortisol on the secretion of corticotropin-releasing hormone, arginine vasopressin, and adrenocorticotropin as revealed by intensive sampling of pituitary venous blood in the normal horse.

The effect of an acute fall in plasma cortisol on the secretion of CRH, arginine vasopressin (AVP), and ACTH was studied using our nonsurgical technique for collecting pituitary venous (PV) blood from horses. PV blood from six mares was collected continuously and divided into 30-sec segments for 0.5 h before and during a 3-h infusion of metyrapone, an 11-beta-hydroxylase inhibitor. During treatment, plasma cortisol fell (P < 0.01) to a mean nadir of 15% of pretreatment levels, and 11-deoxy-cortisol rose (P < 0.02). Three mares became mildly agitated during treatment. Mean PV concentrations of CRH (P < 0.025), AVP (P < 0.05), and ACTH (P < 0.005) were higher during the second hour of treatment than before. For AVP (P < 0.05) and ACTH (P < 0.01), the amount secreted in peaks detected by CLUSTER analysis increased during treatment, whereas peak frequency did not. Responses, particularly in CRH and AVP, tended to be amplified during agitation. Increases in CRH, AVP, and ACTH secretion commenced when cortisol had fallen to 50-59% of the initial value (P < 0.005 for each). By contrast, the cortisol concentration at this point varied 3-fold among mares. The ratio between PV concentrations of ACTH and CRH, which was used as an index of pituitary responsiveness to endogenous CRH, also rose (P < 0.005) as cortisol fell. The increase in this ratio preceded any significant change in CRH secretion and was maintained to the end of the experiment. We suggest that the initial response to falling cortisol in the horse is at the pituitary, via increased responsiveness to CRH. If cortisol continues to fall, AVP and then CRH secretion are stimulated. However, the magnitude of the hypothalamic response to hypocortisolemia may be augmented by concurrent stress. Last, the hypothalamo-pituitary-adrenal axis of the horse appears to monitor changes in plasma cortisol and not concentrations, at least in the short term.

Adrenocorticotropic Hormone↗

PubMed1991

The effect of acute exercise on the secretion of corticotropin-releasing factor, arginine vasopressin, and adrenocorticotropin as measured in pituitary venous blood from the horse.

We have used the technique which we have developed for collecting pituitary venous blood from conscious, undisturbed horses to study the effect of acute vigorous exercise on the secretion of CRF, arginine vasopressin (AVP) and ACTH. Pituitary venous (pit) blood was collected every 1-5 min from nine trained racehorses at rest in the stable. The horses then trotted quietly for 10 min, after which they galloped as fast as possible for 4-6 min, before returning to the stable where sampling continued. In Exp 1 (n = 5) no blood samples were taken during exercise, whereas in Exp 2 (n = 4), pit blood was collected every 30 sec during exercise. Immediately after exercise, significant elevations in heart rate (P less than 0.001), body temperature (P less than 0.01) and hematocrit (P less than 0.001) were observed as compared with preexercise values. Jugular cortisol levels were higher after exercise (301.9 +/- 35.2 nmol/liter; mean +/- SEM) than before (187.3 +/- 34.8; P less than 0.01; n = 9). Likewise, jugular AVP levels increased with exercise (before, 0.65 +/- 0.11 pmol/liter; after 3.2 +/- 0.6; P less than 0.01; n = 6), whereas jugular CRF was not altered by exercise (before, 0.38 +/- 0.08 pmol/liter; after, 0.93 +/- 0.31; n = 6; NS). In Exp 1, no significant changes in pit ACTH, AVP, or CRF were observed after exercise. However in Exp 2 when pit blood was sampled during exercise all horses showed an immediate and dramatic rise in ACTH (P less than 0.01) and AVP (P less than 0.005) secretion which peaked during galloping with mean fractional changes above resting levels of 23.6 +/- 9.9 for ACTH and 51.7 +/- 24.0 for AVP. After exercise pit AVP levels were not different from resting, whereas ACTH remained elevated (11.4 +/- 6.9-fold above resting levels). By contrast, pit CRF levels were not altered by exercise. In both experiments together, pit AVP and ACTH concentrations were correlated in eight of the nine horses, whereas pit CRF and ACTH concentrations were positively correlated in only one of seven horses. We conclude that acute exercise causes a transient increase in ACTH secretion which occurs synchronously with an increase in AVP secretion. CRF does not appear to play a major role in mediating the initial ACTH response to exercise.

Adrenocorticotropic Hormone↗

PubMed1991

Comparison of the microheterogeneity of horse LH and FSH in the pituitary with that secreted into pituitary venous blood at oestrus.

For aqueous extracts of pituitary glands of oestrous mares, luteinizing hormone (LH) profiles were found to be similar to each other and to earlier work after chromatofocussing (CF) and isoelectricfocussing (IEF). After CF, both LH and follicle-stimulating hormone (FSH) in pituitary extracts focussed in multiple peaks in the acidic range, with 86% of LH and 80% of FSH found between pH 4 and 6. By contrast, in pituitary venous plasma, only 18% of the LH focussed in this range, whereas a significantly greater proportion (P less than 0.01) eluted above pH 7 than occurred in pituitary extracts (37% vs 2%, respectively). For pituitary venous FSH, there was only a slight shift in the distribution of isoforms compared with the pituitary extract, with a rise in the percentage of strongly acidic molecules in pituitary venous plasma (pH less than 3.65; 34% vs 16%). These results show that at oestrus, horse LH (which differs from that of other species because it has a heavily sialylated C-terminal extension to the beta-subunit, as does eCG), is much more alkaline when secreted as opposed to when it is stored in the pituitary. The authors of this report suggest that this modification is made after entry into a preferentially released pool of LH. Modulation of the forms of LH and FSH that are secreted may play a role in regulating target tissue responses.

Animals↗

PubMed1991

Effect of sexual arousal on gonadotrophin-releasing hormone, luteinizing hormone and follicle-stimulating hormone secretion in the stallion.

In an experiment conducted late in the physiological breeding season, 5 stallions were fitted with indwelling pituitary venous cannulae that permitted unobtrusive collection of blood coming from the pituitary and the hypothalamo-hypophyseal portal vessels. The next day, blood samples were collected at 5 min intervals for several hours while the stallions were resting. Pulses of gonadotrophin-releasing hormone (GnRH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) occurred approximately once per hour. After this, an oestrous mare was brought into contact with each stallion for 8-30 min. This exposure rapidly induced pulses of GnRH, FSH and LH secretion in all stallions, showing that sexual arousal stimulates the hormones of the reproductive axis.

Animals↗