Mechanisms involved in the effects of TRH on GHRH-stimulated growth hormone release from ovine and bovine pituitary cells

Incubation of cultured ovine pituitary cells with growth hormone-releasing hormone (GHRH) (10(-12)-10(-7) M) stimulated growth hormone secretion up to 3-fold. At a maximal stimulatory concentration of GHRH (10(-10) M), thyrotropin-releasing hormone (TRH) (10(-7) M) caused an inhibition of growth hormone release to approx. 50% of the response obtained with GHRH alone (during a 15 min incubation period). TRH also caused a small inhibition of the GHRH-stimulated cellular cyclic AMP level but this effect was only significant at a relatively high concentration of GHRH (10(-9) M). Incubation of cultured bovine pituitary cells with GHRH (10(-11)-10(-8) M) plus TRH (10(-7) M) caused a significant stimulation of growth hormone release by up to 40%, compared with the response obtained with GHRH alone (at all concentrations of GHRH). TRH (10(-7) M) had no effect on GHRH (10(-8) M)-stimulated cellular cyclic AMP levels in a partially purified bovine pituitary cell preparation. The effects of varying extracellular [Ca2+] (0.1-10 mM) on intracellular [Ca2+] and on the responsiveness to releasing hormones were also determined using ovine pituitary cells. GHRH (10(-10) M)-stimulated growth hormone release was inhibited when cells were incubated at both high (10 mM) and low (0.1 mM) [Ca2+] (compared with 1 mM or 3 mM Ca2+) with or without TRH (10(-7) M). At 1 mM Ca2+, TRH produced a synergistic effect with GHRH to stimulate growth hormone release. However, at 3 mM Ca2+ TRH inhibited GHRH-stimulated growth hormone release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopamine and somatostatin inhibit forskolin-stimulated prolactin and growth hormone secretion but not stimulated cyclic AMP levels in sheep anterior pituitary cell cultures

Forskolin, an activator of adenylate cyclase, has been used to investigate the effects of raising pituitary cell cyclic AMP concentrations on prolactin and growth hormone secretion and to examine the role of cyclic AMP in the inhibitory actions of dopamine and somatostatin. Incubation of cultured ovine pituitary cells with forskolin (0.1-10 microM; 30 min) produced a modest dose-related increase in prolactin release (120-140% of basal) but a much greater stimulation of growth hormone secretion (170-420% of basal). Cellular cyclic AMP concentrations were only increased in the presence of 1 and 10 microM forskolin (2-5.5 times basal). A study of the time course for forskolin (10 microM) action showed that stimulation of prolactin (1.5-fold) and growth hormone (4.7-fold) secretion occurred over 15 min; subsequently (15-60 min) the rate of prolactin secretion from forskolin-treated cells was equivalent to that measured in controls, while growth hormone release remained elevated. Cellular cyclic AMP concentrations were also rapidly stimulated by forskolin (10 microM); they reached a maximum (12 times control) within 15 min, and then declined (15-60 min) but remained elevated relative to those in untreated cells (4.9 times control at 60 min). Dopamine (0.1 microM) inhibited basal secretion of both prolactin and growth hormone. In the presence of forskolin (0.1-10 microM), dopamine (0.1 microM) inhibited prolactin secretion to below the basal level and considerably attenuated the stimulation of growth hormone secretion. Similarly, somatostatin suppressed both basal and forskolin-induced prolactin and growth hormone secretion. However, neither dopamine nor somatostatin significantly decreased the stimulatory effect of forskolin on cellular cyclic AMP accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of dopamine and somatostatin on phorbol ester-stimulated prolactin and growth hormone secretion

Incubation of cultured ovine pituitary cells with the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA) (0.1-100 nM), caused a dose-related stimulation of both growth hormone (ED50 approximately 4 nM) and prolactin (ED50 approximately 14 nM) secretion. Stimulation by TPA (100 nM) produced a substantial 10-fold increase in growth hormone with a smaller, 2-fold rise in prolactin secretion over 30 min; significant effects on the release of both hormones occurred within 2 min. Treatment with TPA also produced a small, time- and concentration-dependent rise in cellular cyclic AMP content which reached, at maximum, a level 20-30% over basal values. Non-tumor-promoting phorbol esters did not stimulate the secretion of either growth hormone or prolactin. In the presence of TPA (10 nM), dopamine (1-1000 nM) suppressed prolactin secretion to a level close to that observed for maximal inhibition of unstimulated cells. At high concentrations (0.1-1.0 microM) dopamine also partially attenuated (by 43%) the TPA-induced stimulation of growth hormone secretion. Somatostatin (0.01-1.0 microM) completely inhibited the substantial (approximately 9-fold) TPA-induced stimulation of growth hormone secretion (inhibitory ED50 approximately 47 nM), and also suppressed TPA-stimulated prolactin secretion to the control level. Our results suggest that activation of protein kinase-C may be involved in the stimulatory regulation of both growth hormone and prolactin secretion in sheep pituitary cells. Failure of TPA to attenuate the inhibitory activity of dopamine and somatostatin suggests that inhibitory regulation occurs at, or beyond, the point in the secretory process regulated by protein kinase-C.     

Actions of dopamine on prolactin secretion and cyclic AMP metabolism in ovine pituitary cells

Prolactin secretion from cultured sheep pituitary cells was inhibited by low concentrations of dopamine (0.1 nM-0.1 microM) with a half-maximal effect at 3 nM. At a maximally effective dose (0.1 microM) dopamine significantly inhibited prolactin secretion within 5 min. with an 80% inhibition of basal secretion over 2 h. Basal prolactin secretion was stimulated by the addition of methylisobutylxanthine (MIX) (0.3-1.0 mM) and 8-bromo-cyclic AMP (2 mM), but cholera toxin (3 micrograms/ml) and prostaglandin E2 (0.1-1.0 microM), which also raised cellular cyclic AMP levels, had no effect on prolactin release. The inhibition of prolactin release by dopamine (0.1 microM) was not affected by any of these compounds. Dopamine inhibited MIX-induced cyclic AMP accumulation over a similar concentration range to the inhibition of secretion, but had no effect on the changes in cyclic AMP concentration produced by cholera toxin and prostaglandin E2. Overall the results with sheep pituitary cells suggest that lowered cyclic AMP levels do not mediate the inhibitory effects of dopamine on basal prolactin secretion, but that changes in cellular cyclic AMP levels may alter the secretion of this hormone, and dopamine may affect pituitary cell cyclic AMP concentrations in some circumstances.     

Effects of theophylline infusion on the growth hormone (GH) and prolactin response to GH-releasing hormone administration in acromegaly

Since theophylline has been shown to blunt the GH response to growth hormone-releasing hormone (GHRH) in normal subjects, we investigated whether the same effect of theophylline administration could be reproduced in patients with active acromegaly. Ten acromegalic patients received on two different days 100 micrograms GHRH iv alone and the same GHRH dose during a constant infusion of theophylline (3.56 mg/min), beginning 2 h before GHRH administration. In the whole group theophylline did not affect basal GH secretion significantly (from a mean of 44.6 +/- 14.4 at 0 min to 41.8 +/- 13.5 ng/ml at 120 min). However, the amount of GH released after GHRH stimulation was lower when theophylline was concomitantly infused (7525 +/- 3709 ng min/ml vs. 12038 +/- 6337 ng min/ml; p less than 0.05). The inhibitory effect of theophylline was not homogeneous, since either marked or minimal reductions of the GHRH-stimulated GH secretion occurred. Serum PRL levels increased after GHRH administration in 6 patients and theophylline infusion had no influence upon this response. Peak GHRH levels were not different in both studies (14.9 +/- 1.7 and 17.1 +/- 4.0 ng/ml, respectively). Free fatty acid levels rose progressively during theophylline administration (from 0.66 +/- 0.10 at 0 min to 1.04 +/- 0.10 mEq/l at 240 min) and were significantly higher than after GHRH stimulation alone from 180 min up to the end of the test. Our results demonstrate that in active acromegaly theophylline blunts the GH response to GHRH, though this effect is not uniformly seen in all patients.(ABSTRACT TRUNCATED AT 250 WORDS)     

On the role of the peptide galanin in regulation of growth hormone secretion.

The effects of the peptide galanin on growth hormone secretion were studied in vitro using cultured rat and human anterior pituitary cells, and in vivo by iv administration of galanin in both rats and humans. Galanin in concentrations from 10 nmol/l to 1 mumol/l did not alter basal GH release, but slightly inhibited GHRH-stimulated GH release from cultured rat anterior pituitary cells. Galanin (1 mumol/l) did not significantly change basal or GHRH-stimulated GH secretion from cultured human anterior pituitary cells. In contrast, iv injection of 1 microgram (300 pmol) galanin to rats induced an increase in plasma GH that was reproducible at repetitive injections. The galanin-induced GH release in rats was of a lower magnitude than the increase in plasma GH after iv injections of GHRH, and was seen with a 5-15 min delay in comparison to iv administered GHRH. In man, iv infusions of galanin (40 pmol.kg-1.min-1.(40 min)) also caused a significant increase in plasma GH, but it occurred 25-30 min after the beginning of the infusion. These results suggest an indirect action of galanin on GH release in both rats and humans, i.e. galanin does not directly affect the somatotropes. In agreement with a central action, no binding sites for galanin could be demonstrated in the rat anterior pituitary by autoradiography. Since galanin did not affect somatostatin release from fragments of rat mediobasal hypothalamus, the stimulatory effects of galanin on GH release are most likely mediated via a stimulatory effect on GHRH neurons.     

Actions of growth hormone-releasing factor and somatostatin on adenylate cyclase and growth hormone release in rat anterior pituitary

The interaction of growth hormone-releasing factor (GRF) and somatostatin (SRIF) on adenylate cyclase activity and growth hormone release was investigated in pituitary homogenates and 2-day cultured rat anterior pituitary cells. GRF stimulated growth hormone release by about 3-fold (ED50 1.6 X 10(-12) M) and caused a rapid 15-fold increase in cyclic AMP production (ED50 6.0 X 10(-12) M). The increase in cyclic AMP was due to direct stimulation of adenylate cyclase by GRF, which caused a 4-fold increase in the activity of the enzyme measured in anterior pituitary homogenates. GRF-induced cyclic AMP formation and GRF-stimulated adenylate cyclase activity were maximally inhibited to the extent of about 50% by 10(-8) M somatostatin. In contrast, GRF-stimulated growth hormone release was completely inhibited by somatostatin (ID50 3.2 X 10(-11) M), suggesting a second site of action of somatostatin. These studies demonstrate that GRF stimulates growth hormone release via activation of adenylate cyclase and a rise in intracellular cyclic AMP. In addition, these findings indicate that the inhibitory action of somatostatin on growth hormone release is exerted at two levels, one at the level of adenylate cyclase affecting the production of cyclic AMP, and the other beyond the formation of the nucleotide, at a site which modulates the release of growth hormone from the cell.     

Effects of sex and age on growth hormone response to growth hormone-releasing hormone in healthy individuals

The influence of age and sex on human GH secretion is controversial. In previous studies, serum GH responses to arginine and insulin-induced hypoglycemia were significantly higher in pre- and postovulatory women than in men. In contrast, recent studies suggest that GH responsiveness to GHRH is higher in normal young men than in age-matched women. To clarify the question of sex and age influence on GHRH-(1-44)-stimulated GH secretion, we studied 116 normal women and men (with body mass indexes of 18-25 and 19-26, respectively) between the ages of 18 and 95 yr. The peak serum GH increments after GHRH administration were significantly higher in premenopausal women than in age-matched men (P less than 0.003 for the age group 18-30 yr and P less than 0.03 for the age group 30-50 yr, as assessed by analysis of variance). The responses were not different in postmenopausal women and age-matched men. Multiple regression analysis revealed a significant negative correlation between GHRH-induced GH responses (integrated area under the curve) and age in both women (P less than 0.002) and men (P less than 0.001). In addition, we determined basal serum testosterone, estradiol, cortisol, and PRL levels in all subjects. Multivariate regression analysis of the GH responses to GHRH administration revealed a significant positive correlation (P less than 0.01) between serum estradiol and both GH increase and the area under the GH response curve. No correlation was found between GHRH-stimulated GH secretion and basal serum cortisol, testosterone, or PRL concentrations. Our data clearly demonstrate a marked influence of both sex and age on GHRH-stimulated GH secretion. We found a higher GH increase in premenopausal women compared with age matched-men and an age-dependent decrease in GHRH-stimulated GH secretion in both sexes. Furthermore, in women a significant influence of estradiol on GH secretion after GHRH administration could be demonstrated.     

Insulin-like growth factor I regulates growth hormone secretion and messenger ribonucleic acid levels in human pituitary tumor cells

GH secretion and mRNA levels were measured in cultured human GH adenoma cells incubated in serum-free medium for up to 48 h. A human recombinant insulin-like growth factor I (IGF-I) analog, Thr-59-IGF-I (6.5 nM), inhibited basal GH secretion by up to 60% in tumor cell cultures. The 30-50% stimulation of GH secretion by GH-releasing hormone (GHRH) was prevented by simultaneous exposure of the cells to IGF-I (6.5 nM). Gel electrophoresis of total RNA derived from GH cell adenoma tissue, followed by transfer and hybridization with 32P-labeled human GH cDNA, revealed a distinct mRNA species of about 1.0 kilobases. Using cytoplasmic dot blot hybridization, IGF-I inhibited the levels of human GH mRNA sequences in these cells and also prevented the GHRH-induced stimulation of GH mRNA. A monoclonal antibody to the type I IGF-I receptor (alpha IR3) prevented the inhibitory effects of IGF-I on basal and GHRH-stimulated GH secretion. This antibody also prevented the IGF-I-induced suppression of GH mRNA sequences. PRL secretion in these cells was not altered by IGF-I. Furthermore, relative levels of beta-actin mRNA were unaltered by IGF-I. Thus, IGF-I suppresses basal and GHRH-stimulated GH secretion and GH mRNA levels in pituitary adenoma cells, indicating that IGF-I acts selectively on the somatotroph to directly regulate GH gene expression.     

Regulation of growth hormone release from cultured human pituitary adenomas by somatomedins and insulin

GH secretion is stimulated by hypothalamic GH-releasing factor (GHRH) and inhibited by somatostatin. Since GH induces the production of insulin-like growth factors (IGF) in liver and other tissues, it is of interest to learn whether IGF alters GH release through long loop feedback inhibition. Pituitary adenomas which had been removed from six acromegalic patients were processed for dispersed cell cultures and/or cell membrane preparations. Binding studies using 125I-labeled IGF-I, IGF-II, and insulin revealed specific hormone binding for each ligand to cell membranes derived from four somatotropinomas. A partially purified somatomedin preparation inhibited basal and/or GHRH-stimulated GH release from cultured pituitary cells derived from three of four adenomas; there was no effect of somatomedin in one tumor. In a single tumor, insulin also partially inhibited GHRH-stimulated GH release. Additionally, in one nonadenomatous pituitary removed from a patient with diabetes mellitus, insulin and somatomedin inhibited GHRH-stimulated GH release, and insulin inhibited basal GH secretion. These results indicate that specific cell membrane receptors for somatomedin peptides and insulin may be found on cell membranes from GH-secreting tumors, and that somatomedins and insulin can inhibit GH release in cultured human somatotropinoma cells. Thus, these data suggest that somatomedins may exert feedback inhibition of GH secretion in some patients with acromegaly.     

Suppressing actions of butyrate on growth hormone (GH) secretion induced by GH-releasing hormone in rat anterior pituitary cells

We assessed the inhibitory effects of butyrate on the growth hormone (GH) secretion in order to investigate the cellular mechanisms in rat somatotrophs. Isolated anterior pituitary cells were cultured in DMEM for several hours, either in the presence (1, 3, or 10mM) or absence of butyrate, and then stimulated with 10(-7)M GHRH for 30 min, in the presence of butyrate at the concentrations used for the previous culture. The increase in GHRH-induced GH release was significantly reduced in a time-dependent and concentration-dependent manner in the cells previously cultured with butyrate. GH content (the sum of GH released into the medium induced by GHRH stimulation and the GH remaining in the cells after stimulation) was reduced by the culture of cells in the presence of butyrate, which was also inversely dependent on the concentrations used for the culture. Simultaneous addition of an L-type Ca(2+) channel blocker, nifedipine (10 pM), to the medium during 10(-9)M GHRH stimulation significantly reduced the stimulated GH release, which was further significantly decreased by a simultaneous addition of 10 mM butyrate. Butyrate blunted the GHRH (10(-9)M)-induced increase in cellular cyclic AMP and calcium ion concentrations, the activity of protein kinases (A and C), and GHmRNA expression. The expression of mRNA for GPR 41 and 43, known as receptors for short-chain fatty acids, was confirmed in the anterior pituitary cells. These findings suggest that butyrate inhibits GHRH-induced GH release as well as GH production, and the cellular inhibitory actions of butyrate occur in diverse cellular signaling pathways of rat somatotrophs.     

Studies on the mechanism of the antilipolytic effects of growth hormone.

Ovine growth hormone (1 mug/ml) antagonized the lipolytic action of epinephrine (0.25 mug/ml) in segments of adipose tissue obtained from hypophysectomized rats, but a lag period of about 10 min was required. When added simultaneously with epinephrine, growth hormone neither reduced the maximal accumulation of cyclic AMP which occurred at 3 min nor accelerated the return to basal levels. Only when tissues were exposed to epinephrine 15 min after preincubation with growth hormone was cyclic AMP accumulation compromised. Growth hormone also produced a delayed increase of about 20% in the activity of a low Km cyclic nucleotide phosphodiesterase, which might have contributed to the decrease in cyclic AMP accumulation. The increase in phosphodiesterase activity probably did not account for the antilipolytic effect, however, since antilipolysis was evident before the increase in phosphodiesterase activity could be detected. The antilipolytic effects of growth hormone similarly could not be attributed to the decrease in cyclic AMP concentrations, for when added simultaneously with epinephrine the antilipolytic effects did not occur until after the evanescent changes in cyclic AMP had passed. Growth hormone added simultaneously with epinephrine or 30 min later significantly decreased the activity of protein kinase assayed in the absence of exogenous cyclic AMP, but did not change total protein kinase activity as measured in the presence of a saturating concentration of cyclic AMP. This effect of growth hormone was evident as early as 3 min after addition of the hormone and may at least partially account for the antilipolytic effect.     

Pyridostigmine does not reverse dexamethasone-induced growth hormone inhibition

Glucocorticoids inhibit the growth hormone (GH) response to a variety of stimuli, including GH-releasing hormone (GHRH) in vivo, but they increase GHRH-stimulated GH secretion when added, in vitro, to animal and human pituitary cells. This discrepancy has led to the hypothesis that glucocorticoids act in vivo by increasing somatostatin secretion from the hypothalamus. To examine this hypothesis, we used a cholinergic drug, pyridostigmine (PD), which reduces hypothalamic somatostatin secretion. Eight normal volunteers were studied. They underwent four tests: (1) GHRH test; (2) Dex + GHRH (GHRH test after treatment the night before, with dexamethasone (Dex)); (3) PD + GHRH; (4) Dex + PD + GHRH. Dex significantly inhibited the GH response to GHRH expressed as area under the GH/time curve (AUC, microgram/1/min) (mean +/- SEM = 895.2 +/- 196.6 vs 1970.9 +/- 600.1, P less than 0.05). PD significantly increased the AUC of GH secretion in PD + GHRH compared with GHRH alone (3541.2 +/- 571.3 vs 1970.9 +/- 600.1, P less than 0.01) but by no means restored completely the normal GH response to GHRH, when given to Dex-pretreated subjects. Furthermore, the mean AUC of Dex + PD + GHRH was significantly lower than that of PD + GHRH (1621.7 +/- 500.6 vs 3541.2 +/- 571.3, P less than 0.01), demonstrating that Dex continues to exert its inhibitory effect on GH secretion in the presence of PD. These results suggest that glucocorticoid-induced GH inhibition does not act solely through an increase in hypothalamic somatostatin secretion.     

Growth hormone therapy in normal short children induces a transitory decrease in plasma growth hormone releasing hormone levels and in human growth hormone responsiveness to exogenous growth hormone releasing hormone.

A three-month study of the effect of growth hormone (hGH) therapy (0.1 U/kg/day sc) on plasma levels of GH releasing hormone (GHRH), somatostatin and insulin-like growth factor I (IGF-I) and on the hGH responsiveness to exogenous GHRH was carried out in 32 prepubertal short-stature children with normal GH secretion. Blood samples were collected prior to initiation of therapy, and at 5, 30 and 90 days of onset of therapy, as well as 2 and 90 days after termination of therapy. The nonconventional hGH therapy induced an increase in serum IGF-I levels which lasted as long as therapy was continued. Plasma GHRH levels showed an early transitory decrease after five days of therapy, whereas plasma somatostatin levels were unaltered. A slight suppression in hGH responsiveness to exogenous GHRH was found at 2 but not at 90 days after termination of hGH therapy. It is concluded that nonconventional hGH treatment does not cause permanent changes in physiological hGH secretion.     

Effects of cytidine 5'-diphosphocholine administration on basal and growth hormone-releasing hormone-induced growth hormone secretion in elderly subjects

The basal and GH-releasing hormone-stimulated secretion of GH declines in the elderly. We tested the ability of cytidine 5'-diphosphocholine, a drug used in the treatment of stroke and Parkinson's disease, to alter GH secretion in 11 healthy elderly volunteers, aged 69-84. Each subject received an iv infusion of 2 g of cytidine 5'-diphosphocholine or normal saline. GHRH and TRH were also administered during cytidine 5'-diphosphocholine infusions. The infusion of cytidine 5'-diphosphocholine induced a 4-fold (p less than 0.05) increase in serum GH levels over basal values. A small increase in GH was seen after GHRH administration. However, the addition of GHRH to the cytidine 5'-diphosphocholine infusion resulted in a GH response which was significantly greater than that seen after GHRH alone; the integrated concentration of GH was more than 2-fold greater in the cytidine 5'-diphosphocholine treated group (706.85 +/- 185.1 vs 248.9 +/- 61.4 micrograms.l-1.(120 min)-1; p = 0.01). The PRL and TSH responses to TRH were not significantly affected by cytidine 5'-diphosphocholine infusion, indicating that dopaminergic mechanisms are not involved. These studies demonstrate that cytidine 5'-diphosphocholine can enhance basal and GHRH-stimulated GH release in the elderly, but the mechanism of action of the drug remains unclear.     

Calcitonin suppresses growth hormone (GH) response to growth hormone-releasing hormone (GHRH) in man

Calcitonin (CT) receptors have been found in the hypothalamus, suggesting a neuroendocrine role for this peptide. We have recently shown that, in the rat, central administration of salmon calcitonin (sCT) suppresses basal and GHRH-stimulated GH secretion. To further investigate how sCT alters GH secretion, we studied the effects of sCT (100U MRC, im) or placebo on basal and GHRH (50 micrograms, iv)-stimulated GH secretion in 6 normal men. GHRH was administered 1 h after sCT injection. Basal GH levels were not altered by sCT administration. However, GH response to GHRH was markedly suppressed by sCT (area under the curve - sCT: 574.6 +/- 69.7 vs placebo: 1057.2 +/- 284.8 micrograms. min/L; p less than 0.02). Cortisol levels were higher in sCT-treated subjects compared to controls, from 45 to 105 min after sCT injection (p less than 0.05). However, no correlation was found between GH response to GHRH and cortisol levels. No changes in glucose, calcium and PTH levels were seen. These results demonstrate that sCT inhibits GHRH-induced GH secretion in man by a mechanism apparently independent of changes in peripheral cortisol, glucose, calcium and PTH levels.     

Tumor promoters as probes of protein kinase C in dog thyroid cell: inhibition of the primary effects of carbamylocholine and reproduction of some distal effects

The acute effects of phorbol esters, used as probes of protein kinase C activation, were studied on dog thyroid slices incubated in vitro. The derivatives used were: tetradecanoylphorbol acetate (TPA), phorbol-12,13, didecanoate (PDD), phorbol-12,13-diacetate (PDA), and phorbol dibutyrate (PDBu) and as inactive controls, phorbol itself, phorbol-12, myristate and phorbol-13, acetate, in concentrations ranging from 5.10(-8) to 5.10(-6) mol/L. The active phorbol esters had no effect on basal cyclic AMP concentrations; they inhibited cyclic AMP accumulation induced by prostaglandin E1 but not that induced by thyrotropin (TSH) 1 mU/mL and forskolin 10 mumol/L. Phorbol esters like carbamylcholine acutely stimulated iodide organification and inhibited the stimulation of hormone secretion resulting from TSH, Cholera Toxin, forskolin, and Bu2-cyclic AMP action. These metabolic effects did not require the presence of extracellular Ca++, and could not be antagonized by Ca++ depletion or manganese addition. The active phorbol esters abolished the cyclic AMP independent increased PI turnover induced by TSH 10 mU/mL or carbamylcholine (Cchol) 10(-6) mol/L but did not affect the basal incorporation of 32P into phosphatidylinositol. They reduced the 45Ca efflux from preloaded slices below basal levels and blocked the increased 45Ca release induced by TSH and Cchol. They also inhibited the increase in cyclic GMP concentrations resulting from Cchol action but not the effect of the ionophore A23187 (10(-5) mol/L) nor the basal levels of cyclic GMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of TRH and dopamine on cyclic AMP levels in enriched mammotroph and thyrotroph cells.

Populations of normal anterior pituitary cells enriched in thyrotrophs or mammotrophs prepared by velocity sedimentation were used to investigate the effect of modulators of TSH and prolactin secretion on cyclic AMP accumulation. In both thyrotroph-enriched and mammotroph-enriched fractions, IBMX increased cyclic AMP accumulation. In the presence of IBMX, TRH invoked an increase in cyclic AMP suggesting that TRH modulates cyclic AMP accumulation in both of these cell types from normal pituitary glands. In the mammotroph-rich fraction, dopamine inhibited the increase in cyclic AMP induced by TRH. In contrast however, in the thyrotroph-enriched fraction dopamine lowered neither cyclic AMP concentration nor TSH secretion. Thus the inhibiting effect of dopamine on cyclic AMP appears to be specific for prolactin-secreting cells.