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S A Tait

Publications and source records attributed to S A Tait.

At least 19 recordsLinked to original sources
PubMed1999

Role of cAMP in the effects of K+ on the steroidogenesis of zona glomerulosa cells.

1. Investigations of the role of cAMP in the stimulation of the steroidogenesis of zona glomerulosa (ZG) cells by increased extracellular K+ concentration are reviewed. 2. Possible reasons for discrepancies in the results of different investigators on whether K+ increases the cAMP content or output of ZG tissue or dispersed cells are discussed. 3. The concentration of cAMP in the incubating media of ZG tissue or cells, rather than their cAMP content, seems to respond more sensitively to stimulation by extracellular K+, as was also found for adrenocorticotropic hormone stimulation of zona fasciculata-reticularis cells. 4. The addition of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (IBMX) to incubations with the aim of increasing the sensitivity of the response in cAMP to extracellular K+ in ZG cells may give rise to effects, probably nonspecific, which actually inhibit the measured response. 5. The immediate stimulation in the steroidogenesis of ZG cells with raised extracellular K+ is probably mostly due to the direct effect of increases in cytoplasmic Ca2+ (arising from increases in Ca2+ influx) on mitochondrial processes. However, increases in cAMP may prolong the stimulation of steroidogenesis by increased extracellular K+. This increased cAMP is probably due to stimulation of adenylyl cyclase activity. 6. It has been concluded that the increase in Ca2+ influx output after rises in the extracellular K+ concentration of ZG cells is responsible for most of the increase in cAMP. 7. According to one group of investigators, there is weak stimulation of phospholipase C (PLC) activity after increasing the extracellular K+ concentration of rat ZG cells. 8. If there is such a stimulation of PLC activity, it seems that the action of increased extracellular K+ can potentially involve all known mechanisms for the stimulation of steroidogenesis in endocrine cells. The common primary event is probably the increase in Ca2+ influx. The relative importance of these various potential mechanisms may depend on the particular in vitro conditions used.

Animals↗
PubMed1991

Tonography, Valsalva manoeuvre and central visual fields. A study of 429 patients.

Four hundred and twenty nine patients (846 eyes) referred to the glaucoma clinic at Concord Hospital between 1979 and 1989, were included in a prospective study. This study investigated the circulatory status of each eye during tonography, by using a modified Valsalva manoeuvre to induce changes in intraocular pressure (IOP) and amplitude of tonography pulse pressure (TPP). Of the large falls in IOP, 85% of the eyes had a Po/C greater than 150. The average IOP without medication, on the morning of the test, was 20 mmHg (2.67 kPa). Central visual field defects were found in 495 eyes, and these were far more prevalent in the Po/C greater than 150 category. An unexpected finding was the ratio of 67:33 right to left eyes in the Po/C less than 100 category. The commonest reasons for referral were intermittent rises of IOP, suspicious optic disc cupping, and the need to confirm a previously made diagnosis.

Adult↗
PubMed1991

The effect of plasma protein binding on the metabolism of steroid hormones.

Earlier views indicated that globulin (corticosteroid-binding globulin (CBG) or sex hormone-binding globulin (SBG)) but not albumin binding in plasma, protects steroids from splanchnic metabolism in man. Also, the splanchnic extraction (HE) of a steroid seemed to be highly dependent on the rate of disassociation of the steroid-protein complex. However, the faster rate of disassociation (tau 1/2 = 0.9 s) of cortisol-CBG, as determined by later accurate fluorescence methods, intuitively meant that this complex must disassociate completely in a single 9 s passage through the liver. The low HE of total cortisol was then a puzzling anomaly. Using a differential equation solver (TUTSIM) and a model with unbound, albumin- and globulin-bound pools of steroid (with metabolism of unbound and also possibly albumin-bound steroid), the mechanism of splanchnic metabolism has been studied. The 'complex', probably most realistic, model includes 13 steroids, which can simultaneously bind to plasma albumin, CBG and SBG. The steroid concentration and numbers of occupied binding sites of the globulins decrease during the time of metabolism. The experimental data used are the in-vitro binding characteristics of the steroid-protein complexes, including the equilibrium constants and rates of disassociation and the in-vivo HE of nine steroids, usually measured by direct analysis of hepatic venous blood. However, the HE of cortisol had to be calculated from the metabolic clearance rate/splanchnic blood flow, giving a maximum value of 12%. The fractional metabolic rate of unbound steroid is generally represented by e. A certain value of e (RE) is required to give a remaining steroid concentration after 9 s of metabolism, which is made equal to (1-HE) in the model to simulate splanchnic extraction. If the fractional rate of metabolism of albumin-bound steroid is h (f = h/e), then RE will depend on the value of f. The maximum RE for cortisol is RE0 = 0.42 and RE1 = 0.16 for f = 0 and 1 respectively. For either value of RE, there will be the appreciable reassociation of cortisol to CBG after disassociation of the cortisol-CBG complex. With such reassociation, the total cortisol remaining after 9 s metabolism is fairly independent of the rate of dissociation of the cortisol-CBG complex. This explains the low total HE of cortisol in spite of the high rate of disassociation of cortisol-CBG.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Cortex Hormones↗
PubMed1986

The mechanism of the effect of K+ on the steroidogenesis of rat zona glomerulosa cells of the adrenal cortex: role of cyclic AMP.

The effects of various concentrations of extracellular K+ (3.6-13 mM) on the steroid (corticosterone and aldosterone) and cyclic AMP outputs of capsular cells (95% zona glomerulosa) of the rat adrenal cortex were studied at different concentrations of extracellular Ca2+. Small amounts of EGTA (50 microM) were added to reduce the free Ca2+ concentrations effectively to zero at the lowest possible total Ca2+ concentration. At a total extracellular concentration of 2.5 mM Ca2+, in 27 experiments the mean values of the steroid and cAMP outputs showed a maximum at 8.4 mM K+. The increase in steroid and cAMP outputs at 5.9, 8.4 and 13 mM K+ compared with that at 3.6 mM were highly significant (p less than 0.01). The overall correlation of either corticosterone or aldosterone with cAMP outputs was also highly significant and was even better from 3.6 to 8.4 mM K+. Lowering the effective free concentration of Ca2+ to zero decreased the steroid and cAMP outputs significantly at all K+ concentrations, and no output was then significantly higher than at 3.6 mM. With the pooled data on outputs at all total Ca2+ (2.5, 0.5, 0.25, 0.10, 0.05 and 0.0 mM) and K+ (3.6, 5.9, 8.4 and 13 mM) concentrations, the correlation of either steroid with cAMP outputs was highly significant (but again optimally from 3.6 to 8.4 mM K+). Nifedipine (10(-6) to 10(-4) M) was added to the incubations with the aim of specifically inhibiting Ca2+ influx at total extracellular Ca2+ concentrations of 2.5, 1.25 and 0.25 mM and with the usual K+ concentrations. The cAMP outputs were reduced at all K+ concentrations above 3.6 mM K+. The effect was highly significant at 10(-4) M nifedipine and a total Ca2+ of 1.25 mM, which with the incubation conditions used, corresponds to the free Ca2+ concentrations in vivo. These results indicate that cAMP plays a significant role in the stimulation of steroid output by K+ particularly between 3.6 and 8.4 mM K+. In this range of K+ concentrations the stimulation of cAMP seems to be controlled by increases in Ca2+ influx. The correlation of steroid and cAMP output at the higher K+ concentrations (between 8.4 and 13 mM K) and at the various total Ca2+ concentrations is less significant. Also, with all concentrations of added nifedipine there is an 'anomalous' increase in steroid output at 13 mM K+ and at total Ca2+ concentrations of 2.5 and 1.25 mM. However, at the same K+ concentrations and at 0.25 mM Ca2+, nifedipine decreases steroid outputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Cortex↗
PubMed1986

Effects of alpha-melanocyte-stimulating hormone on the cyclic AMP and phospholipid metabolism of rat adrenocortical cells.

Results on the effects of peptides on the phospholipid metabolism and steroid and cyclic AMP (cAMP) outputs of rat adrenal capsular cells (96% zona glomerulosa, 4% zona fasciculata) were obtained in a series of three batch experiments. Their significance was examined by analysis of variance. Incorporation of [32P] into phosphatidylcholine, phosphatidic acid and phosphatidylinositol was measured. Production of [3H]inositol-1 monophosphate, inositol-1,4 bisphosphate and inositol-1,4,5 tris-phosphate was estimated after prelabelling with [3H]inositol followed by 1 min incubation with a steroidogenic stimulus. Angiotensin II (0.25 nmol/l to 0.25 mumol/l) highly significantly (P less than 0.01) stimulated aldosterone and corticosterone outputs, [32P] incorporation into phosphatidic acid and phosphatidylinositol (but not into phosphatidylcholine) and the production of the three [3H]inositol phosphates. Aldosterone and corticosterone outputs were stimulated by alpha-MSH (above 0.1 nmol/l). However, incorporation of [32P] was not significantly increased until 10 mumol alpha-MSH/l but, unlike with angiotensin II, incorporation into phosphatidylcholine was also then stimulated. Also, the production of the inositol phosphates was not increased significantly (P greater than 0.05) by any dose of alpha-MSH (10 nmol/l, 1 mumol/l and 0.1 mmol/l) used. Therefore, it can be concluded that alpha-MSH does not stimulate phospholipase C in rat zona glomerulosa cells. In further experiments, it was also found that there were significant increases in cAMP as well as in steroid outputs above 1 nmol alpha MSH/l (highly significant above 10 nmol alpha-MSH/l). There were plateaux of the outputs of both steroids and cAMP from 0.1 to 1 mumol alpha-MSH/l. However, there were further increases in steroid and cAMP outputs of the capsular cells at higher doses. Concomitant results on the stimulation of corticosterone output by zona fasciculata-reticularis cells indicate that this additional increase was mostly due to the stimulation of the contaminating zona fasciculata cells. It was also confirmed that alpha-MSH preferentially stimulates steroidogenesis by the zona glomerulosa. However, under our conditions, alpha-MSH highly significantly increased the output of cAMP by both zona fasciculata and glomerulosa cells.

Adrenal Cortex↗
PubMed1984

The effects of ACTH, serotonin, K+ and angiotensin analogues on 32P incorporation into phospholipids of the rat adrenal cortex: basis for an assay method using zona glomerulosa cells.

The effects of various concentrations of serotonin, ACTH, K+, angiotensin II (AII), angiotensin III (AIII) and [Sar1]angiotensin II (SAII) on steroidogenesis and the incorporation of 32P (after preincubation to near equilibrium with the ATP pool) into phosphatidylinositol (PI), phosphatidic acid (PA) and phosphatidylcholine (PC) in a preparation of capsular cells from rat adrenals, consisting of 95% zona glomerulosa (z.g.) and 5% zona fasciculata plus reticularis (z.f.r.) cells, were investigated. Serotonin and ACTH stimulated steroidogenesis in the usual manner but had little or no effect on 32P incorporation into any of the three phospholipids. However, AII, AIII and SAII stimulated steroidogenesis and also 32P incorporation into PA and PI (maximally to about 280% of control values) but not into PC. These results taken together with other data on effects on the cAMP output and Ca2+ fluxes of z.g. cells suggest that stimulation by ACTH and serotonin is mediated by cAMP as second messenger. However, the angiotensins probably act through Ca2+, with associated changes in phospholipid metabolism. The 32P incorporation into PA as a function of lg concentration of AII was linear and showed a reasonable index of precision (0.36 +/- 0.03, eight experiments, 0.23 +/- 0.02 for a further eight experiments) and correlation with steroidogenesis. The corresponding incorporation into PI showed a maximum effect and a much poorer index of precision (1.02 +/- 0.30 (4.69 +/- 3.7] over the same full range of AII concentration used. The effects of AIII and SAII showed similar characteristics for 32P incorporation into both PA and PI, but, as for stimulation of steroidogenesis, at higher concentrations for AIII than for AII. The effects of different doses of AII, AIII and ACTH on the corticosterone output and 32P incorporation into PA, PI and PC of a preparation of cells, consisting of more than 98% z.f.r. cells, from rat decapsulated adrenals were also studied. ACTH, at low doses, which nevertheless markedly stimulated corticosterone output, had a small (maximally to about 125% of control values) but significant effect on 32P incorporation into PA, PI and PC. The maximum effect was usually at about 10(-10) M ACTH and was not significant at 10(-8) M.

Adrenal Cortex↗
PubMed1984

The use of the superfusion approach with rat adrenal capsular cells to compare the steroidogenic potencies of angiotensin analogues, without the effects of peptide degradation.

The superfusion approach was used to assay the relative potencies of (Asp1)angiotensin II (A II), (des-Asp1)angiotensin II (A III) and (Sar1)angiotensin II (SA II) in stimulating the steroidogenesis (corticosterone) of rat adrenal capsular (zona glomerulosa) cells without the effects of peptide degradation; an example of the advantages of such a procedure. At the rate of superfusion employed, there was no significant reduction in the concentration of A II or A III through the cell chamber. However, in the in vitro closed system A III was degraded more rapidly than A II. SA II would be expected to be more protected than A II. The potency ratios were found to be 0.30 +/- 0.05 (s.e.), four experiments for A III/A II and 2.23 +/- 0.16 (s.e.), six experiments for SA II/A II in the usual closed system with 1 h incubations. In the superfused system, however, the corresponding ratios were 0.82 +/- 0.11, five experiments and 1.19 +/- 0.23, four experiments. These results indicated that, as shown by the relative potencies in the superfusion system and therefore without the effects of degradation. A II, A III and SA II have about the same intrinsic activity. The reason for the relatively high activity of SA II in the closed system is its lower rate of degradation compared with A II and the low activity of A III is owing to its relatively high rate of degradation. It is concluded that, although the intrinsic activities of A III and A II are similar, because of the equivalent activity of SA II, it is probably not necessary for SA II, or therefore also A II, to be converted to A III for it to stimulate steroidogenesis in the system.

Adrenal Cortex↗
PubMed1981

Lack of effect of angiotensin on levels of cyclic AMP in isolated adrenal zona glomerulosa cells from the rat.

The effects of pure [Asp1,Val5]- and [Asn1,Val5]-angiotensin II and also [des-Asp1,Ile5]-angiotensin II (angiotensin III) on cyclic AMP and steroid outputs by dispersed rat capsular cells, comprising 95% zona glomerulosa and 5% zona fasciculata cells, have been studied. The results showed that [Asp1, Val5]-and [Asn1, VAl5]-angiotensin II, at doses between 2.5 X 10(-11) and 2 X 10(-4) mol/l, which produced typical increases in steroidogenesis, failed to increase output of cyclic AMP. This lack of effect was observed whether the nucleotide was measured by radioimmunoassay or by adrenal binding protein and under the same conditions in which 8.4 mM-K+ consistently increased the output of cyclic AMP. Instead the results showed a small but significant decrease in cyclic AMP output with angiotensin II. Similar results were obtained with incubations for 60 rather than 120 min and with medium containing a concentration of 5 or 40 g bovine serum albumin/l. Although the levels of cyclic AMP were generally higher in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, the same decrease relative to basal outputs was observed with angiotensin II which increased steroidogenesis. Angiotensin III also failed to increase output of cyclic AMP at doses (2.5 X 10(-9) to 2.5 X 10(-6) mol/l) which produced increases in steroid output equivalent to those with angiotensin II. These results indicate that angiotensin II and III can act through a cyclic AMP-independent mechanism.

Adrenal Glands↗
PubMed1980

Cyclic AMP levels in purified rat adrenal zonae fasciculata and reticularis cells and the effect of adrenocorticotrophic hormone.

Cyclic AMP levels were measured in combined cells and supernatant fraction from incubations of dispersed rat adrenal zona fasciculata and zona reticularis cell preparations purified by unit gravity sedimentation. These measurements were correlated with deoxycorticosterone (DOC) and corticosterone outputs from the cells in the presence or absence of ACTH. Similar measurements of cyclic AMP outputs were made for unpurified dispersed, decapsulated rat adrenal cell preparations and they were found to correspond to previously reported measurements made by other workers on such preparations. The response of the purest zona reticularis cells to ACTH in terms of cyclic AMP output was 28-fold lower than that of the purest zona fasciculata cells (compared with a fivefold lower DOC output and a 20-fold lower corticosterone output) and the response to ACTH of the mixed-cell preparations was related to the number of zona fasciculata cells in the preparation, i.e. the greater the proportion of zona fasciculata cells in the preparation the greater the response in terms of both outputs of cyclic AMP and of either of the two steroids measured. This correlation is in accordance with the theory that cyclic AMP may be the secondary messenger for both zona fasciculata and zona reticularis cells of the rat adrenal cortex in mediating the response to an ACTH stimulus.

Adrenal Glands↗
PubMed1980

Further studies on the stimulation of rat adrenal capsular cells: four types of response.

Preparations of capsular rat adrenal cells consisting mainly of zona glomerulosa with less than 5% zona fasciculata contamination are described. The responses of the aldosterone and corticosterone outputs of these preparations to various stimuli were of four types. (1) Variations in K+ concentration gave a maximum aldosterone response at 5.9-8.4 mM-K+, about sixfold greater than the control output at 3.6 mmol/l. At higher K+ concentrations such as 13 mmol/l, the response decreased. (2) Serotonin (at a concentration of about 10(-4) mol/l) gave only a slightly lower maximal aldosterone response than did K+ but this did not decrease significantly at higher concentrations. Serotonin gave significant steroidogenic response at 10(-8) mol/l. (3) [Asp1,Val5]-Angiotensin II (10(-10) mol/l) with 3.6 mM-K+ gave a significant response and a constant maximal response at 2.5 x 10(-8) mol/l. This maximum response was about half that found for both aldosterone and corticosterone when stimulated maximally by K+ or serotonin: [des-Asp1,Ile5]- and [des-Asp1,Val5]-angiotensin II (angiotensin III) gave similar response characteristics but had a lower potency in this cell preparation. The initial maximum response could be further increased at a higher concentration (from 2.5 x 10(-5) mol/l) of a preparation of [Asn1,Val5]-amide angiotensin II (Hypertensin-Ciba) and might eventually be greater than with K+. This additional response was, to a major extent, due to stimulation of the contaminating zona fasciculata cells and was not seen with high concentrations of the free acid, angiotensin II. It was also not seen in two experiments with pure [Asn1]-amide angiotensin II and therefore it could have been due to some impurity in Hypertensin-Ciba. (4) Adrenocorticotrophin (Synacthen) at 3 x 10(-11) mol/l gave a significant steroidogenic response. Higher concentrations (3 x 10(-10) to 7.5 x 10(-9) mol/l) gave no constant maximum but the response could be much greater than for other stimuli such as K+, serotonin and [Asp1]-angiotensin II. This additional response was again due to steroid precursors, e.g. deoxycorticosterone and corticosterone from contaminating zona fasciculata cells. Similar results were obtained with ACTH (ACTHAR) in three experiments. Threshold sensitivity (a significant increase in steroidogenesis) for ACTH (Synacthen) was, in two experiments, greater for zona fasciculata-reticularis cells (3 x 10(-12) mol/l) than for zona glomerulosa cells (3 x 10(-11) mol/l). The data show that aldosterone output was approximately a function of the square of the corresponding corticosterone value. Specific effects on this pathway can be shown by values of aldosterone/corticosterone2 greater than one. Of all stimuli used, only K+ concentrations of 5.3, 5.9 and 13 mmol/l gave such effects. However, because of several considerations, only positive results with other stimuli may be meaningful. Calculation of this parameter might be useful as a screening test in bioassays for substances with aldosterone-stimulating activity.

Adrenal Cortex↗