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Biomedical subjects

C H Irvine

Publications and source records attributed to C H Irvine.

At least 55 records · Page 3Linked to original sources
PubMed1986

Effect of graded doses of gonadotrophin-releasing hormone on serum LH concentrations in mares in various reproductive states: comparison with endogenously generated LH pulses.

Luteinizing hormone release induced by a range of small (3.3-33 micrograms) and large (300-500 micrograms) i.v. doses of gonadotrophin-releasing hormone (GnRH) was measured in acyclic (n = 4), luteal phase (n = 3) and follicular phase (n = 5) mares and compared with endogenously generated LH pulses in the same reproductive states. Extrapolation from log-linear dose-response curves showed that an LH pulse comparable to an endogenous one would be simulated by i.v. injection of 7.0 (n = 4) and 4.1 (n = 6) micrograms GnRH in luteal and follicular phase mares respectively; a much smaller dose than the 500 micrograms usually given clinically or experimentally. In acyclic mares (n = 4), LH pulses occurred too infrequently to be characterized. At small doses of GnRH the amount of LH released by the same dose was similar in all three reproductive states, although the steroid hormone milieu differed markedly. This implies that observed differences between states in mean (+/- S.E.M.) serum LH concentrations (0.7 +/- 0.01, 1.2 +/- 0.03 and 11.6 +/- 0.33 (microgram/l) in acyclic, luteal and follicular phase mares respectively) were produced by differences in GnRH pulse frequency and/or amplitude and not by steroid-mediated changes in pituitary response to GnRH. In acyclic, luteal and follicular phase mares, LH pulse frequency was: immeasurably low, 0.09 and 1.14 pulses/h respectively, which supports the important contribution of pulse frequency to determining mean LH concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Sexual behavior and serum concentrations of reproductive hormones in normal stallions.

Recently, it has been reported that impotence in the stallion has a physiological basis that involves decreased serum concentrations of luteinizing hormone (LH) and estradiol-17beta, but not testosterone. We have found such a hormonal profile in two of nine stallions studied during an ongoing investigation of the endocrinology of the normal stallion. Nevertheless, both of these stallions possessed vigorous libido and normal seminal characteristics. We conclude that the hormonal profile of low LH, low estradiol and normal testosterone, although it may accompany impotence in the stallion, is not predictive of, or causally related to, abnormalities in sexual behavior.

Journal Article↗
PubMed1984

Hypothyroidism in the foal.

Hypothyroidism in the foal occurs as two entities because of the separate actions of thyroid hormones in regulation of metabolic rate and in cell differentiation. The hypometabolic state which results in inadequate thermogenesis and lethargy, occurs concurrently with a period when thyroid hormone secretion is inadequate. Also the severity of the concurrent symptoms is related to the degree of hormone inadequacy as measured by plasma concentrations of free T4 and T3. By contrast, the developmental lesions caused by hypothyroidism are often observed during periods when plasma thyroid hormone concentrations are normal. This is because during the development of most tissues there is a period during which deprivation of thyroid hormones leads to developmental defects which may first appear weeks or months later, by which time thyroid hormone levels may have returned to normal. In the foal the critical period for some developmental processes, eg, myelination, is before birth so it is difficult to confirm a pre-natal hypothyroid state as the cause of neonatal neuromuscular incompetence. Post natal developmental lesions of the epiphyses or ossification centres, for example, may also manifest themselves some weeks subsequent to the period during which hypothyroidism existed. Because confirmation of diagnosis using plasma hormone measurements is very difficult in the foal, and because the symptoms of hypothyroidism are not specific, the incidence of confirmed hypothyroidism in foals is low. However there is suggestive evidence that where awareness of the condition exists it is diagnosed frequently. The aetiology is obscure although diet has been implicated.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1982

Effect of prolonged administration of anabolic and androgenic steroids on reproductive function in the mare.

Administration to mares of the anabolic steroid, methandriol, at the maximum recommended dose (300 mg every 3 weeks) for 1 1/2 years had no effect on reproductive characteristics except for suppression of GnRH-induced LH release and a tendency to suppress basal LH levels and the height of the ovulatory LH surge. A 4-fold increase in dosage caused marked suppression of basal LH, the LH surge, and GnRH-induced LH release. Other reproductive responses were minimally affected. There were no behavioural effects, and no changes in weight occurred when mares were compared with matched controls. Small and moderate doses of testosterone induced behavioural changes without affecting reproductive function. However, large doses, which raised plasma testosterone to levels similar to those of stallions, eventually caused total suppression of all reproductive activity within 1 month and the development of markedly vicious stallion-like behaviour. Apart from the aggression all changes disappeared within 1 month after the end of treatment.

Aggression↗
PubMed1982

Radioimmunoassay and in-vitro bioassay of serum LH throughout the equine oestrous cycle.

Mares were bled once daily throughout a cycle, or 3 times daily from the first day of oestrus to the 2nd day after ovulation. LH was measured by heterologous radioimmunoassay and by an in-vitro bioassay based on LH-stimulated testosterone production by mouse Leydig cells. The patterns of bio- and immuno-active LH during the oestrous cycle were similar but not identical, so that in both groups of mares the ratio of biological: immunological (B:I) activity during the LH surge was significantly higher before than after ovulation (P less than 0 . 001). Considerable individual variation in cycle mean B:I ratio, and extent and duration of the peri-ovulatory change in ratio was observed. While the similarity between bio- and immuno-assay LH profiles indicates that the RIA system used can provide useful measurements of LH in the normal cycle, the significant differences between the two profiles suggest that there are circumstances in which RIA alone may not give adequate information on the level of LH stimulation at target tissues. These changes in B:I ratio during the LH surge suggest qualitative differences in the circulating molecule which may be physiologically important in the control of ovulation. The in-vitro bioassay appears to be a useful method to study the biological role of LH in the mare and investigate the roles of the various forms of this polymorphic hormone.

Animals↗
PubMed1982

Importance of testicular hormones in maintaining the annual pattern of LH secretion in the male horse.

Serum LH was measured by radioimmunoassay in 5 long-term (greater than or equal to 3 years) castrated male horses bled at 30-min intervals for 2 h twice a month from March to February. There was no significant effect of month on LH levels; however, mean levels were significantly lower in spring than autumn (P less than 0.05). By contrast, stallions in the same environment showed a markedly seasonal pattern of LH secretion, with LH rising at the onset of the breeding season to reach levels in late spring 3-4 times those in early winter. Despite differences in seasonal patterns of secretion, annual mean LH levels in geldings (19.6 ng/ml) and stallions (20.4 ng/ml) were similar. In mid-summer, LH was measured in 6 geldings castrated 4-6 weeks previously, 25 geldings castrated 3-25 years previously, and 8 stallions; mean +/- s.e.m. LH values were 62.2 +/- 8.0, 14.0 +/- 1.6, and 13.0 +/- 2.2 ng/ml respectively. In the long-term geldings, LH was not affected by age, indicating that, after the rapid 4-fold post-castration rise, LH falls to within the normal range of pre-castration values and stabilizes. These results show that testicular hormones are necessary to maintain the normal seasonal pattern of LH secretion in the male horse, including the increase in LH at the onset of the breeding season. It is suggested that the effects of testicular hormones on LH secretion include (1) a negative feedback at the hypothalamus/pituitary as observed in other species, and (2) a positive component in the pathway by which LH is increased at the onset of the breeding season.

Androgens↗
PubMed1979

Differential release of LH and FSH in cyclic mares in response to synthetic Gn-RH.

Mares at different stages of the oestrous cycle were given a single intravenous injection of 0.5 mg synthetic Gn-RH. The mean area of the induced LH peak was significantly less at mid-cycle (Day 10-11) than at any other time. The mean height of the LH peak above preinjection concentration was greater at late oestrus and early cycle (Day 5-6) than at mid-cycle and early oestrus. There were no significant different in mean FSH responses. The LH:FSH ratio for both height and area of induced peaks was significantly less at mid-cycle than at other times of injection. These results suggest that one releasing hormone could cause the release of both FSH and LH in the normal cyclic mare.

Animals↗
PubMed1979

Kinetics of gonadotrophins in the mare.

Isoelectric focussing of crude extracts of equine pituitaries was used to obtain fractions containing FSH and LH. By comparison with FSH, LH was distributed over a similar but wider pH range indicating more marked polymorphism as determined from their isoelectric point (pI). Molecules with more sialic acid showed lower pI consistent with the concept that sialic acid is the major factor in determining pI and polymorphism in FSH and LH. Appropriate fractions were labelled with 125I, purified further and used in kinetic studies. FSH and LH molecules of similar pI had similar kinetics; however, LH molecules of high pI disappeared from plasma more rapidly. This is attributed to the role of sialic acid in preventing hormone degradation by non-target tissues, thus increasing the half-life and therefore the biological potency of the hormone. Since the form in which gonadotrophins circulate is not known, data are presented using 2 forms of LH and one of FSH. While this provides information from which most kinetic parameters may be determined, meaningful production rates cannot be calculated until the circulating form is identified. Other experiments on gonadotrophin kinetics are reviewed critically in the light of these findings.

Animals↗
PubMed1979

Fertility of prostaglandin-induced oestrus compared to normal post-partum oestrus.

This study was undertaken to determine if fertility could be improved by increasing the interval from foaling to breeding. Forty-two mares, not bred during normal post-partum oestrus, were injected with a prostaglandin analogue on Day 6 or 7 following ovulation. Mares were mated artifically with antibiotic-treated semen during the resulting oestrus and, if necessary, for the following 4 cycles. Their fertility was compared, by cycles/pregnancy and rate of fetal loss, to mares bred by the same methods on 86 normal post-partum oestrous periods. The interval from foaling to the onset of breeding and the duration of oestrus were longer in the injected mares than the intervals observed in the untreated mares. Both groups averaged 1.3 breeding periods/pregnancy. The rate of fetal loss was not significantly different between the groups. Increasing the interval from foaling to breeding did not improve fertility.

Animals↗
PubMed1979

Induction of follicular development and ovulation in seasonally acyclic mares using gonadotrophin-releasing hormones and progesterone.

Deeply acyclic (seasonally anovulatory) mares were treated with GnRH or a GnRH analogue to induce follicular development and ovulation. Courses of GnRH (3--4) were administered at approximately 10-day intervals to reproduce the gonadotrophin surges which precede ovulation in the normal cycle. Exogenous progesterone was administered in an attempt to reproduce the luteal phase pattern. Induced serum FSH concentrations were comparable to those causing follicular development in the normal cycle, but induced LH levels were lower and of shorter duration than those of the periovulatory surge. Three of 4 mares treated with GnRH appeared to ovulate, but did not establish CL. Nine of 10 mares given GnRH analogue also developed follicles during the final treatment course, as did mares treated with progesterone only, while only 1 of 5 untreated control mares showed any ovarian development. Failure to induce final follicular maturation and CL development by this treatment regimen may be due to an inadequate LH surge at the time of the expected ovulation associated with the low preovulatory oestradiol-17 beta surge, possibly caused by the preceding FSH stimulation being inadequate or inappropriate. Progesterone treatment increased baseline FSH concentrations in GnRH-treated mares, and also stimulated follicular development in mares not treated with GnRH, indicating a possible role for progesterone in folliculogenesis and, indirectly, ovulation.

Animals↗
PubMed1979

Interaction of oestradiol and gonadotrophin-releasing hormone on LH release in the mare.

Acyclic mare given oestradiol for 3 days to simulate the preovulatory plasma oestradiol surge showed a non-significant 37% decrease in plasma LH during treatment. When GnRH analogue injections were given with oestradiol on Days 1--3, oestradiol had no effect on each GnRH-induced LH increase, but LH increases were more prolonged following subsequent GnRH injections on Days 4--7 when oestradiol was no longer being given. A much greater prolongation of LH release occurred when the course of GnRH injections was commenced after oestradiol treatment ceased; the LH response was almost identical to the prolonged periovulatory LH surge of the normal cycle. Therefore, it appears that the timing of the oestradiol surge, in relation to other hormonal events, is critical in inducing the uniquely prolonged periovulatory LH surge of the mare.

Animals↗