Inhibition of adenosine 3',5'-monophosphate accumulation in anterior pituitary gland in vitro by growth hormone-release inhibiting hormone

Angiopeptin inhibits thymidine incorporation by explants of porcine coronary arteries

Angiopeptin, a stable octapeptide analog of somatostatin, inhibits proliferation in a variety of cancer cell lines. We studied the effect of angiopeptin on 3H-thymidine uptake into ring segments from the porcine coronary tree. The incorporation of 3H-thymidine into segments of porcine left anterior descending (LAD) coronary artery was time dependent and reached a plateau after 48 h. The addition of angiopeptin (48.1 and 96.2 nM) to the culture medium significantly inhibited 3H-thymidine incorporation into the segments by 36.7 +/- 10.1% and 48.3 +/- 2.3% of the control, respectively. Forskolin (100 microM), inhibited 3H-thymidine incorporation (52.7 +/- 10.1%) to the same degree as did angiopeptin (96.2 nM). Incubation of the segments with 125I-labeled angiopeptin, for 2 h at 37 degrees C, showed angiopeptin uptake to be time dependent and exhibited a first-order kinetics, reaching equilibrium after 30 min. Autoradiographic studies showed a uniform distribution of angiopeptin within the endothelium, media, and adventitia. Most of the labeling was associated with the nuclei of the cells. Angiopeptin, after 30-min incubation, did not significantly modify the basal levels of cyclic adenosine monophosphate (cAMP). In contrast, forskolin (100 microM) elicited a 50-fold increase of the basal levels of cAMP. These results indicate that in addition to its endocrine effects, angiopeptin reduces the rate of proliferation by acting directly on the vessel wall.

Somatostatin receptors in the central nervous system

Somatostatin was first identified chemically in 1973, since when much has been established about its synthesis, storage and release. It has important physiological actions, including a tonic inhibitory effect on growth hormone release from the pituitary. It has other central actions which are not well understood but recent cloning studies have identified at least five different types of cell membrane receptor for somatostatin. The identification of their genes has allowed studies on the distribution of the receptor transcripts in the central nervous system where they show distinct patterns of distribution, although there is evidence to indicate that more than one receptor type can co-exist in a single neuronal cell. Receptor selective radioligands and antibodies are being developed to further probe the exact location of the receptor proteins. This will lead to a better understanding of the functional role of these receptors in the brain and the prospect of determining the role, if any, of somatostatin in CNS disorders and the identification of potentially useful medicines.

Growth hormone-releasing hormone stimulates cAMP release in superfused rat pituitary cells

The release of growth hormone (GH) and cAMP was studied in superfused rat pituitary cells by infusing growth hormone-releasing hormone (GHRH) at different doses or a combination of GHRH and somatostatin 14 (SS-14). Three-minute pulses of GHRH caused a dose-dependent GH and cAMP release (effective concentration of 50% of the maximal biological effect is 0.21 nM and 52.5 nM, respectively). The lowest effective doses of GHRH in the superfusion system were 0.03 nM for GH release and 0.3 nM for cAMP discharge when 3-min pulses were applied. The amount of cAMP liberated from the cells was not proportional to GH release: cAMP responses to low doses of GHRH were disproportionally small, and the gradual increase in the release of cAMP after high doses of GHRH was not followed by a parallel rise in GH release. The desensitization induced by repeated pulses or prolonged infusion of GHRH resulted in a greater reduction in GH release than in cAMP liberation. A simultaneous infusion of SS-14 completely blocked GH release stimulated by GHRH but did not inhibit the immediate release of cAMP caused by GHRH. An abrupt decrease in GHRH-stimulated GH release induced by SS-14 was followed by only a minimal reduction in cAMP liberation 9 min later. Our findings indicate that a discharge of cAMP is stimulated after a GHRH pulse, but this effect alone cannot maintain the release of GH. Other steps of the signal transduction mechanisms that are independent of the cAMP route may participate in the process of GH release. The nature of the mechanisms involved in the mediation of GH release may vary with the doses of GHRH used.

Somatostatin and the immune and haematopoetic system; a review

SS-R and SS have been demonstrated in non-pathological and pathological lymphoid tissue and may play a regulatory, mostly inhibitory, role in the immune response. SS is produced by lymphocytes and monocytes which suggests an autocrine or paracrine regulatory role, but SS may be released as well by nerve endings. It can therefore be hypothesized that apart from a role of SS in local, immunomodulation of cells belonging to the immune system, a second pathway exists in which this peptide exerts its effects via neuroendocrine modulation of the immune response which might represent a direct regulatory relation between the nervous and the immune system. The presence of SS-R in lymphoid neoplasm cells and in diseases involving activated mononuclear leukocytes allows the visualization of these diseases using octreotide scintigraphy. This technique may be used in the evaluation of the spread of these diseases and to monitor their response to therapy. Moreover, the presence of SS-R in immune diseases warrants studies on the efficacy of treatment with octreotide of patients suffering from these diseases.

Growth hormone secretion: the role of glucocorticoids

Glucocorticoids (GCs) modulate the somatotropic axis at a genomic and a non-genomic level. Critical concentrations of steroids not only determine somatotrope differentiation but also enhance growth hormone (GH) gene expression. At a cellular level GH-releasing hormone (GHRH) and somatostatin (SS) are the two principal neuropeptides involved in the release of GH. In vitro data indicates that steroids enhance GH release by altering the affinity and the density of GHRH receptors. In addition, they reduce the sensitivity of the somatotrope to SS and decrease IGF-1 induced negative feedback on GH secretion. The net effect is an enhancement of GH release.

Beta-adrenoceptor activity change after prolonged treatment with growth hormone and somatostatin

1. The effect of 10 days treatment with growth hormone (GH) (1 mg/kg body wt/day) and somatostatin (SRIF) (0.25 mg/kg body wt/day) subcutaneously on the activity of beta-adrenoceptors in rat hypothalamic, pituitary and cerebral cortical membrane fractions was studied using [3H]dihydroalprenolol ([3H]DHA) as radioligand. 2. The administration of GH significantly increased the beta-adrenoceptor binding affinity and the administration of SRIF decreased the beta-adrenoceptor binding capacity in the hypothalamus. 3. In the pituitary the beta-adrenoceptor binding affinity was significantly decreased after both hormonal applications. 4. In the cerebral cortex the beta-adrenoceptor binding affinity was significantly decreased after the GH treatment and increased after the SRIF treatment. 5. The present study provides direct evidence for GH and SRIF effects on the activity of rat beta-adrenoceptors and supports the view about the involvement of beta-adrenergic mechanisms in the neurotransmitter regulation of GH secretion in the rat.

Identification and characterization of somatostatin receptors in neonatal rat long bones

Somatostatin (somatotropin release inhibiting factor; SRIF) has widespread functions as a modulator of neural activity as well as of endocrine and exocrine secretion. In the present paper, the binding characteristics of somatostatin receptors have been investigated in rat long bones using the stable analogue, 125I-SDZ 204-090, as a ligand. Binding studies revealed the presence of a single class of high-affinity binding sites for 125I-SDZ 204-090 on cells prepared from neonatal rat long bones with an equilibrium dissociation constant (KD) of 70.1 +/- 8.2 pM (n = 3). An excellent correlation was found between the ability of various somatostatin analogues to inhibit growth hormone in pituitary cells and to displace the binding of 125I-SDZ 204-090 to the bone cell preparation, indicating that the receptors are very similar, if not identical. The localization of the somatostatin-binding sites was examined by autoradiography after labelling in vitro and in vivo. The binding sites were shown by both procedures to be selectively localized to the metaphysis of rat long bones. The labelling experiments in vivo indicate that these receptors can be reached in the living animal by circulating somatostatin analogues. In addition, the analogue SMS 201-995 inhibited the forskolin-stimulated adenylate cyclase activity in bone cell suspensions. These results suggest that somatostatin could be an important regulatory factor in bone metabolism.

Effect of high doses of somatostatin on adenylate cyclase activity in peripheral mononuclear leukocytes from normal subjects and from acute leukemia patients

In normal lymphocytes somatostatin non-competitively inhibited basal (ID50 5 x 10(-4) M) and isoproterenol- and forskolin-stimulated adenylate cyclase activity (Ac). In acute leukemia blasts, non-responsive to isoproterenol, forskolin, which activates the catalytic subunit, stimulated and somatostatin inhibited Ac, thus indicating the leukemic enzyme, though defective, retains the inhibitory pathway and catalyst function.

Somatostatin inhibits prolactin secretion by multiple mechanisms involving a site of action distal to increased cyclic adenosine 3',5'-monophosphate and elevated cytosolic Ca2+ in rat lactotrophs

The release of prolactin (PRL) from a clonal cell-line of anterior pituitary cells (GH4C1) was inhibited by somatostatin (SRIH) in a dose-dependent manner (ED50 nM). The inhibition (20% of control levels) was detectable within 50 s and maximal within 90 s. Thyroliberin (TRH) enhancement of PRL secretion was biphasic. SRIH inhibited both phases equally. Ionomycin in combination with the phorbol ester, TPA, mimics the TRH-elicited PRL release, and SRIH partly inhibited this effect. SRIH had no effect on TRH-stimulated formation of inositol trisphosphate, and only small effects on TRH-activated adenylate cyclase. Vasoactive intestinal peptide (VIP) and forskolin stimulated cAMP formation and PRL release potently. SRIH inhibited both effects of VIP and forskolin, and there was a close correlation between the inhibition of PRL secretion and cAMP accumulation. 8-Bromo-cAMP enhanced PRL release, an effect that was also partly reduced by SRIH. The Ca2+ channel activator, BAY-K-8644 and high extracellular K+ increased PRL release, and SRIH caused a partial reduction in the release response to both secretagogues. SRIH lowered [Ca2+]i, and markedly reduced the rise in [Ca2+]i elicited by TRH, VIP and K+. SRIH did not influence the Ca2+ spikes recorded in Na+-free solution, and had no effect on the TRH-induced membrane potential changes. Our results demonstrate that SRIH may inhibit PRL release from GH4C1 cells by (1) inhibiting hormone-sensitive adenylate cyclase, (2) blocking the effect of cAMP and (3) lowering [Ca2+]i. None of these effects is, however, sufficient to explain all the effects of SRIH, suggesting that SRIH also exerts a major action at a step subsequent to cAMP accumulation and [Ca2+]i elevation. Since the GH4C1 cells possess one single class of binding sites, this implies that the same SRIH receptor is coupled to several cellular signalling systems.

A comparison of the effects of bromocriptine and somatostatin on growth hormone gene expression in the rat anterior pituitary gland in vitro

The effects of the dopamine agonist bromocriptine (BCR) have been compared with those of somatostatin (SS) on growth hormone (GH) synthesis and secretion by rat anterior pituitary cells in vitro. Both BCR and SS produced a dose-related reduction in GH release. Cytoplasmic GH mRNA levels were unchanged by BCR treatment and this finding was associated with an increase in total intracellular GH content. The reduction in GH release seen following SS treatment was accompanied by a fall in cytoplasmic GH mRNA levels and no significant change in intracellular GH content. These results suggest that the effects of BCR are predominantly on GH release mechanisms, whereas SS appears not only to regulate hormone release but also to regulate GH gene expression at a pre-translational level.

Growth hormone neurosecretory dysfunction

The basis for understanding clinical disorders in the neuroregulation of GH secretion is derived from the complexity of the CNS-hypothalamic-pituitary axis. Studies in animals and humans demonstrate an anatomic, physiological and pharmacological evidence for neurosecretory control over GH secretion including neurohormones (GRH, somatostatin), neurotransmitters (dopaminergic, adrenergic, cholinergic, serotonergic, histaminergic, GABAergic), and neuropeptides (gut hormones, opioids, CRH, TRH, etc). The observation of a defect in the neuroregulatory control of GH secretion in CNS-irradiated humans and animals led to the hypothesis of a disorder in neurosecretion, GHND, as a cause for short stature. We speculate that in this heterogeneous group of children a disruption in the neurotransmitter-neurohormonal functional pathway could modify secretion ultimately expressed as poor growth velocity and short stature.

Growth hormone release: interaction of increased extracellular calcium and somatostatin

These studies were designed to examine the effects of extracellular calcium ion (Ca++) concentration upon basal and dibutyryl (db) cAMP or potassium ion (K+)-stimulated release of growth hormone (GH) and to determine whether increased extracellular Ca++ can overcome somatostatin (SRIF)-inhibited release of stored rGH in parallel with its reported effect upon SRIF inhibition of stimulated insulin and glucagon release. Experiments were performed in vitro using prelabeled rat pituitary fragments in a perifusion, specific immunoprecipitation system designed to limit observations to release of stored hormone from viable cells. Increased (up to 5.4 mM) extracellular Ca++ inhibits basal and dbcAMP-stimulated release of stored, prelabeled [3H]rGH in parallel with the effects of SRIF: post-inhibition rebound, dose responsivity, and differential effect upon early and late dbcAMP-stimulated release of stored [3H]rGH. Increased (21 mM) extracellular K+ interferes with both Ca++- and SRIF-inhibited early dbcAMP-stimulated release of stored [3H]rGH. The combination of increased extracellular Ca++ and SRIF inhibits basal release of stored [3H]rGH more than either agent alone and during dbcAMP stimulation, rebound release of stored [3H]rGH follows withdrawal of either inhibitor despite continuation of the other. This rebound release is enhanced when both inhibitors are withdrawn simultaneously.

CONCLUSIONS

(a) the inhibition of stored rGH release induced by increased extracellular Ca++ and SRIF occurs through at least partially independent mechanisms, and (b) increased extracellular Ca++ does not reverse SRIF inhibition of stimulated rGH release from prelabeled intracellular storage, in contrast with observations in the pancreatic islet.

Somatostatin and regulation of prolactin secretion

In addition to its classical growth hormone (GH) inhibiting action, somatostatin (SRIF) inhibits prolactin (PRL) secretion in man and rat under specific endocrine conditions. Furthermore, SRIF counteracts the thyrotropin releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulated prolactin release from rat adenohypophysis in vitro. Two criteria are needed to demonstrate a physiological role of SRIF in PRL control: specific receptors must be present on prolactin secreting cells, and antagonization of endogenous SRIF must affect PRL secretion in vitro. In fact [125I]N--Tyr--SRIF binds to membranes not only of human GH-secreting adenomas, but also of prolactinomas. Specific binding characteristics are comparable in both cell types, but the density of sites in PRL-secreting adenomas is only one-quarter that in GH-secreting adenomas. In contrast, non-PRL-secreting chromophobe adenomas are devoid of specific binding. On the other hand, administration of SRIF antisera (SRIF-AS) affects both GH and PRL secretion in starved rats (a model in which pulsatile GH secretion is abolished); a marked increase in PRL plasma levels occurs, but the needed SRIF-AS concentration is higher than that for GH disinhibition. This demonstrates that endogenous SRIF may exert a negative control over PRL secretion, although lactotroph cells appear less sensitive to SRIF than somatotrophs. Since the apparent affinity of SRIF binding sites is similar on both GH and PRL secreting cells, at least in human tumor tissues, a lower density of SRIF receptors on PRL cells could account for this reduced responsiveness. Alternatively, different coupling mechanisms may be involved in the two cell types.

Altered activation by calcitonin and inhibition by somatostatin of human embryonal carcinoma cell adenylate cyclase with retinoic acid-induced differentiation

Human teratocarcinoma cells in culture offer an in vitro system for studying certain aspects of embryonic differentiation. To gain some insight into regulatory systems that might be operative during early human development, we have characterized the alterations that occur in the hormonal responsiveness of human embryonal carcinoma cell adenylate cyclase with differentiation in response to 10 microM retinoic acid. Two cell lines CL12 and CL13, cloned from Tera 2 cells by Dr. C. F. Graham, have been used in these studies. Adenylate cyclase of CL12 and CL13 cells is stimulated in the presence of 10 microM GTP by epinephrine and calcitonin, with calcitonin being the most potent stimulator of cyclic AMP production. Exposure of these cells to retinoic acid leads to an arrest in growth and within 6 days to a differentiated cell population with a stable nonreversible phenotype. No changes in basal, GTP- and fluoride-stimulated adenylate cyclase activities are observed with retinoic acid treatment, but the cyclase of differentiated cells exhibits a greater stimulation by calcitonin (7.5-fold) and the appearance of a somatostatin inhibitory effect. Somatostatin specifically inhibits, by 25%, the hormonal stimulation of adenylate cyclase of cells treated for 5 days with retinoic acid. The increase in calcitonin stimulation of adenylate cyclase activity of the differentiated cells is related to an increase (congruent to 3-fold) in the number of hormonal receptors and not to a significant change in receptor binding affinity (Kd 4.6 X 10(8) M-1). These alterations in calcitonin and somatostatin responsiveness suggest a possible regulatory role for these hormones during embryonic development. Furthermore, the results indicate that changes in adenylate cyclase hormonal responsiveness might serve as useful markers during early stages of human embryonal carcinoma cell differentiation.

Somatostatin: a metabolic regulator

Somatostatin, the hypothalamic release-inhibiting factor, has been found to stimulate gluconeogenesis in rat kidney cortical slices. Stimulation by somatostatin was linear and dose-dependent. Other bioactive peptides such as cholecystokinin, gastrointestinal peptide, secretin, neurotensin, vasoactive intestinal peptide, pancreatic polypeptide, beta endorphin and substance P did not affect the renal gluconeogenic activity. Somatostatin-induced gluconeogenesis was blocked by phentolamine (alpha adrenergic antagonist) and prazosin (alpha1 adrenergic antagonist) but not by propranolol (beta adrenergic antagonist) and yohimbine (alpha2 adrenergic antagonist) suggesting that the effect is via alpha1 adrenergic stimuli. Studies on the involvement of Ca2+ revealed that tissue depletion and omission of Ca2+ from the reaction mixture would abolish the stimulatory effect of somatostatin. Furthermore, somatostatin enhanced the uptake of 45calcium in renal cortical slices which could be blocked by lanthanum, an inhibitor of Ca2+ influx. It is proposed that the stimulatory effect of somatostatin on renal gluconeogenesis is mediated by alpha1 adrenergic receptors, or those which functionally resemble alpha1 receptors and that the increased influx of Ca2+ may be the causative factor for carrying out the stimulus.

Synergistic stimulation of luteinizing hormone (LH) release by protein kinase C activators and Ca2+-ionophore

When cultured pituitary cells were stimulated with synthetic diacylglycerol such as 1-oleoyl-2-acetylglycerol (OAG), or with a potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA), which are known stimulators of Ca2+-activated, phospholipid-dependent protein kinase (protein kinase C), enhanced release of luteinizing hormone (LH) was observed. Similarly, LH release was also stimulated by the Ca2+-ionophore, A23187. Simultaneous presence of A23187 and OAG or TPA resulted in a synergistic response that mimicked the full physiological response to gonadotropin releasing hormone (GnRH). Removal of extracellular Ca2+ only slightly affected the stimulatory action of TPA and OAG on LH release, but completely blocked the effect of GnRH. The results suggest that the stimulatory effect of GnRH on LH release may be mediated by two intracellular pathways involving Ca2+ and diacylglycerol as second messengers.

Effects of growth hormone-releasing factor and somatostatin on growth hormone secretion and cellular cyclic AMP levels. Cultured ovine and rat anterior pituitary cells show markedly different responses

Human pancreatic growth hormone-releasing factor (GRF-44-NH2) stimulated growth hormone (GH) secretion and intracellular cyclic AMP levels in cultured pituitary cells from both sheep and rat. Somatostatin (SRIF), over a wide range of doses and time, showed no significant effect on the elevated cyclic AMP levels in sheep cells, but did block the GH release in a dose-dependent manner. In rat cells, however, SRIF inhibited GRF-stimulated cyclic AMP levels by 75% maximum (still 8-fold greater than the basal levels) and GH release to almost half the basal value. We conclude that somatostatin inhibits GRF-elevated cyclic AMP levels in rat pituitary cells but not in sheep cells.

Mechanisms of somatostatin action in pituitary cells

We have examined the mechanisms by which S-14 inhibits pituitary hormone secretion in a homogeneous cell population: the clonal GH4C1 cell line. The S-14 receptor in GH4C1 cells is coupled to Ni, a guanine nucleotide binding protein which mediates S-14-induced inhibition of VIP-stimulated adenylate cyclase activity, cyclic AMP production and hormone secretion. In addition, a functional Ni is required for S-14 to inhibit basal hormone secretion, an action which appears to be independent of cyclic AMP concentrations. Accumulating evidence indicates that the mechanism of S-14 action in somatotrophs is similar to that in GH4C1 cells. Although S-14 consistently inhibits basal GH secretion, its effects on basal cyclic AMP levels in normal pituitary cells are variable and often not significant (10-14). In contrast, S-14 inhibits prostaglandin and growth hormone releasing factor (GRF) stimulated cyclic AMP accumulation and GH release in parallel. Furthermore, S-14 partially blocks prostaglandin and GRF stimulation of adenylate cyclase activity in rat anterior pituitary membranes. Finally, pretreatment of primary cultures of rat pituitary cells with IAP antagonizes S-14 inhibition of both basal and GRF-stimulated GH release.

Inhibitory effect of thyroid hormones on pituitary cyclic amp phosphodiesterase activity in vitro

Thyroid hormones effectively inhibited partially purified cyclic AMP phosphodiesterase from the anterior and posterior lobes of bovine pituitary gland competitively with the substrate, but thyronine, diiodotyrosine, monoiodotyrosine, thyrotropin, thyrotropin releasing hormone, dexamethasone, and luteinizing hormone releasing hormone did not inhibit this enzyme activity. Ki values were found to be 1.0 and 2.5 microM for thyroxine, and 8.0 and 13.5 microM for triiodothyronine in the anterior and posterior lobes, respectively.

Somatostatin lowers the cytosolic free Ca2+ concentration in clonal rat pituitary cells (GH3 cells)

Changes in the cytosolic free Ca2+ concentration, [Ca2+]i, have been proposed to mediate the regulation of the secretion of pituitary hormones by hypothalamic peptides. Using an intracellularly trapped fluorescent Ca2+ probe, quin2, [Ca2+]i was monitored in GH3 cells. Somatostatin lowers [Ca2+]i in a dose dependent manner from a prestimulatory level of 120 +/- 4 nM (SEM, n = 13) to 78 +/- 9 nM (n = 5) at 10(-7)M; the effect is half maximal at 2 X 10(-9) M somatostatin. The decrease in [Ca2+]i occurs rapidly after somatostatin addition and a lowered steady state [Ca2+]i is maintained for several minutes. Somatostatin does not inhibit the rapid rise in [Ca2+]i elicited by thyrotropin releasing hormone (TRH) and can still cause a decrease in [Ca2+]i in the presence of TRH (10(-7)M). Concomitantly with its action on [Ca2+]i somatostatin causes hyperpolarization of GH3 cells assessed with the fluorescent probe bis-oxonol. The lowering of [Ca2+]i by somatostatin is however not only due to reduced Ca2+ influx through voltage dependent Ca2+ channels, since it persists in the presence of the channel blocker verapamil. These results suggest that somatostatin may exert its inhibitory action on pituitary hormone secretion by decreasing [Ca2+]i.

Inhibition by somatostatin of the vasopressin-stimulated adenylate cyclase in a kidney-derived line of cells grown in defined medium

LLC-PK1L cells, a kidney-derived cell line grown in defined medium, possess a vasopressin-sensitive adenylate cyclase. Somatostatin was able to inhibit the vasopressin-induced increase in adenylate cyclase activity, without affecting the basal enzyme activity. This inhibition was competitive. No effect of somatostatin could be detected on [3H]vasopressin binding suggesting an interaction of somatostatin with the vasopressin-sensitive system distal to the hormone-receptor interaction. At variance with N6-L-2-phenylisopropyladenosine (PIA), GTP did not potentiate the inhibition by somatostatin. The inhibition of the vasopressin stimulation by somatostatin and that by PIA were additive. Changing the composition of the cell growth medium increased the number of vasopressin receptors per cell. Cells with a high number of vasopressin receptors were less sensitive to inhibition by somatostatin. Such results suggested that somatostatin and vasopressin receptors and/or the inhibitory (Ni) and stimulatory (Ns) regulatory transducing components are regulated by different mechanisms.

Regulation of adenosine-3',5'-monophosphate levels in rat anterior pituitary by adrenal corticoids

The effect of adrenal cortical hormones on basal and prostaglandin-E2 (PGE2)-stimulated levels of cAMP was studied in incubated rat anterior pituitaries. It was found that oral administration of dexamethasone (DEX) at the concentration of 10 micrograms/mL of drinking saline to adrenalectomized (ADX) rats for four days decreases basal and PGE2-stimulated cAMP levels in the pituitary but not in the hypothalamus. Furthermore, a time-course study demonstrated that acute intraperitoneally (IP) DEX treatment had no effect; whereas, after one day of oral DEX, PGE2-stimulated levels, but not basal, were already reduced. The effect of DEX was also obtained with two natural rat adrenal hormones like corticosterone and deoxycorticosterone, whereas, progesterone and testosterone, which share with the corticoids a ring A-unsaturated-three-ketone structure but are devoid of corticoid activity, did not modify at physiological doses the levels of cAMP. More physiological validation of these results was given by experimental procedures that modified endogenous adrenal secretion. Thus, reduction of plasma corticoids by ADX significantly increased basal and PGE2-stimulated cAMP levels in the pituitary, whereas, increments of plasma corticosterone produced by ACTH had the reverse effect in intact rats. It is suggested that adrenal gland regulation of cAMP may reflect involvement of the nucleotide in the mechanism of action of corticoids at the pituitary level.

Interactions between growth hormone-releasing factor, prostaglandin E2 and somatostatin on cyclic AMP accumulation in rat adenohypophysial cells in culture

Synthetic human pancreatic growth hormone-releasing factor (hpGRF(1-40)-NH2) causes a 100% stimulation of cyclic AMP cell content in rat adenohypophysial cells in culture as early as 1 min after its addition while a maximal 33-fold increase is measured at 40 min. Somatostatin (10 nM) causes a 40-60% inhibition of GRF-induced cyclic AMP accumulation while GH release is inhibited by 90-95% at all time intervals. The inhibitory effect of somatostatin is exerted on the maximal effect of GRF while the ED50 values of GRF action on cyclic AMP cell content (approximately 1 nM) and GH release (approximately 0.1 nM) are not affected by the tetradecapeptide. Prostaglandin E2 causes a 2.5-fold increase in cyclic AMP levels within 1 min after its addition with a maximal 25-fold stimulation measured at 30 min. Although not completely additive, the stimulatory effects of PGE2 and GRF together on cyclic AMP cell content and GH release are more potent than when either substance is present alone. The inhibitory effects of somatostatin on PGE2 action are analogous to those observed in the presence of GRF. The present data suggest that the hypothalamic control of GH secretion by GRF and somatostatin results, at least to a large extent, from the balance between the stimulatory action of GRF and the inhibition of somatostatin on the adenylate cyclase system.

Growth hormone-releasing factor stimulates adenylate cyclase activity in the anterior pituitary gland

Synthetic human pancreatic growth hormone-releasing factor (hpGRF)(1-40)-NH2 stimulates adenylate cyclase activity in rat anterior pituitary particulate fraction at an ED50 value of approximately 150 nM. GTP more than doubles the stimulatory effect of hpGRF and PGE2 on [32P] cyclic AMP formation. The present data show that hpGRF as well as PGE2, another potent stimulus of GH secretion, act at least partly, through GTP-dependent mechanisms in their coupling with adenylate cyclase.

Growth hormone neuroregulation and the clinical relevance of somatostatin

The hypothalamus controls GH secretion from the anterior pituitary using two peptides; somatostatin inhibits GH, but physiologically the most important appears to GHRF, the structure of which has recently been discovered by two groups. This exciting development has not only given us further insight into the control of GH secretion, but also posed interesting questions as to the cause of the abnormal GH responses to various stimuli seen in patients with acromegaly. The other hypothalamic peptide controlling GH secretion, somatostatin has been the subject of intensive research in the last ten years. It is widely distributed and has important physiological actions including those involved in GH secretion and its action as a hypothalamic hormone. The secretion and synthesis of these two hypothalamic hormones is in turn modulated by a number of neurotransmitters, the most important of which appears to be dopamine. Knowledge gained in these studies has enabled the development of useful tools in the diagnosis of growth hormone deficiency as well as the only effective medical treatment for acromegaly. Much remains to be learnt of the physiology of growth hormone releasing factor and as a result further patients will benefit in the future.

Influence of somatostatin on plasma cyclic AMP and metabolic substrate responses to i.v. adrenaline and glucagon in humans

The influence of somatostatin (a bolus injection of 250 micrograms i.v. followed by 6 micrograms/min for 40 min) on the plasma cyclic AMP, insulin, glucose and non-esterified fatty acid (NEFA) responses to i.v. adrenaline (0.07 micrograms/kg body-weight/min for 20 min) and the plasma cyclic AMP, insulin and glucose responses to a bolus injection of i.v. glucagon (0.01 mg/kg BW) were studied in normal subjects. Somatostatin suppressed the insulin response to glucagon and inhibited the insulin rebound observed on termination of adrenaline infusion. The plasma glucose response to glucagon was exaggerated by somatostatin, reflecting insulin deficiency. Neither the plasma glucose or plasma non-esterified fatty acid responses to adrenaline were influenced by somatostatin. Adrenaline produced a three-fold and glucagon a twenty-fold rise in plasma cyclic AMP and 15 min. This was not influenced by concurrent somatostatin infusion, indicating that somatostatin is not a universal inhibitor of hormone stimulated adenylate cyclase activity. This is supported by the failure of somatostatin to inhibit the metabolic actions of glucagon and adrenaline thought to be mediated by cyclic AMP.

Inhibition by somatostatin of bovine growth hormone secretion following sodium channel activation

1. Growth hormone secretion, exchangeable cellular sodium and calcium concentrations measured by 22Na and 45Ca incorporation, and efflux of 45Ca were studied in dispersed bovine anterior pituitary cells. 2. Addition of veratridine (100 microM), an activator of sodium channels, increased exchangeable sodium and calcium concentrations in the cells, the efflux of 45Ca from prelabelled cells, and caused a biphasic stimulation of the rate of growth hormone secretion. Secretion of growth hormone was not stimulated when the extracellular calcium was decreased below 0.1 mM. 3. The increases in growth hormone secretion, exchangeable calcium concentration and 45Ca efflux from prelabelled cells caused by veratridine were abolished by addition of the calcium antagonist verapamil (20 microM). Verapamil also reduced the rise in exchangeable sodium caused by veratridine and increased the resting exchangeable sodium concentrations. 4. The peptide somatostatin (1 micrograms/ml) prevented veratridine-stimulated growth hormone secretion but did not inhibit the increases in exchangeable sodium and calcium caused by veratridine. The peptide itself elicited a transient increase in 45Ca efflux and subsequently partially inhibited veratridine-stimulated 45Ca efflux. 5. The data suggest that anterior pituitary cells possess voltage-sensitive sodium channels. Activation of these channels by veratridine may lead to depolarization and increased entry of calcium via potential-dependent calcium channels, which contributes to a rise in cytoplasmic calcium concentration and the subsequent stimulation of growth hormone secretion. We conclude that the calcium antagonist verapamil may also interact with sodium channels, and that the peptide somatostatin may act on growth-hormone-secreting cells either to prevent the rise in cytoplasmic calcium by hyperpolarizing the cells or to decrease the affinity of a population of calcium binding sites in the cells.

Role of cyclic nucleotides in the inhibition of growth-hormone secretion by somatostatin

The effects of somatostatin on GH secretion, cyclic AMP and cyclic GMp concentrations in dispersed bovine anterior pituitary cells were studied following activation of adenylate cyclase with cholera toxin and inhibition of phosphodiesterase with isobutylmethylxanthine (IBMX). Cholera toxin (10(-5)M) increased intracellular cyclic AMP concentration 10-fold and cyclic GMP concentration 3-fold relative to control, and stimulated the secretion of GH. IBMX (10(-4) M) als increased intracellular concentrations of both cyclic AMP and cyclic GMP and the secretion of GH and potentiated the actions of cholera toxin particularly in raising intracellular cyclic AMP concentrations which were elevated 40-fold in the presence of cholera toxin and IBMX. Somatostatin (5 X 10(-7) M) completely prevented GH secretion elicited by cholera toxin and/or IBMX. Somatostatin was without effect on control cyclic AMP and cyclic GMp concentrations and on the increases in both cyclic AMP and cyclic GMP caused by cholera toxin and by IBMX alone, or in combination. The data suggest that bovine GH secretion is increased when concentrations of either or both cyclic nucleotides are elevated within the cells, although incubation of cells with extracellular concentrations of cyclic AMP and cyclic GMP derivatives up to 2 x 10(-3) M caused only small changes in GH release. We suggest that somatostatin inhibits cholera toxin-induced bovine GH secretion by preventing activation of the secretory process by either cyclic AMP or cyclic GMP.

The interrelationship of somatostatin and guanylate cyclase activity

Somatostatin has been shown to inhibit the release of various polypeptide hormones including insulin, glucagon, gastrin, thyroid stimulating hormone, and growth hormone. The mechanism by which somatostatin inhibits the release of these various polypeptide hormones has not been fully elucidated. It has been reported that somatostatin increases the level of the second messenger cyclic GMP in rat brain and in the anterior pituitary gland. The present investigation was designed to determine if these responses seen in the anterior pituitary gland and brain were due to activation of guanylate cyclase [GTP-pyrophosphate lyase (cyclizing), E.C.4.6.1.2.], the enzyme that catalyzes the formation of cyclic GMP. Somatostatin at a concentration of 2 pM enhanced guanylate cyclase activity two-fold in rat cerebrum and anterior pituitary gland. This enhancement of guanylate cyclase activity was also seen in rat liver, pancreas, stomach, and small intestine at the same concentration of somatostatin. Increasing the concentration of somatostatin to 20 microM, caused a marked inhibition of guanylate cyclase activity in all these tissues. Dose-reponse curves done on gastric guanylate cyclase activity revealed that over a concentration range of 2 pM to 0.2 microM, somatostatin had a stimulatory effect on guanylate cyclase activity while at concentrations above 10 microM somatostatin was inhibitory to guanylate cyclase activity. The biphasic pattern of enhancement of guanylate cyclase activity at lower concentrations of somatostatin and inhibition at higher concentrations may help to explain some of the discrepancies seen with previous investigations with somatostatin, hormone release, and cyclic nucleotide metabolism.

The relationship between somatostatin binding and cyclic AMP-stimulated protein kinase inhibition

Somatostatin binding and the ability to inhibit cyclic AMP stimulated protein kinase were investigated utilizing isolated pancreatic islets, anterior pituitary plasma membranes, adipocytes, erythrocyte ghosts, hepatic plasma membranes, and anterior pituitary secretion vesicles. Three types of response were observed. With type I response, somatostatin bound specifically to pancreatic islets and anterior pituitary secretion vesicles and inhibited cyclic AMP stimulated protein kinase. In type II response, adipocytes and anterior pituitary plasma membranes exhibited somatostatin binding but no effect of the ligand on the kinase. In erythrocyte membrane ghosts and hepatic plasma membranes, there was neither specific somatostatin binding nor protein kinase inhibition (type III response). The absence of somatostatin binding in erythrocytes or hepatic plasma membranes cannot be explained by degradation of the ligand per se. Secretory vesicles isolated from the anterior pituitary gland bind somatostatin with an average affinity which exceeds that observed in plasma membrane (for pituitary secretory vesicles Kd1 = 8.5 X 10(-8)M, Kd2 = 5.2 X 10(-7)M; for pituitary membranes Kd1 = 1.9 X 10(-8)M, Kd2 = 8.1 X 10(-7)M). The molar concentration of high affinity binding sites (Ro) for plasma membranes was 6.9 X 10(-10)M; for secretory vesicles 3.6 X 10(-9)M. Calculated in terms of somatostatin binding per U 5'nucleotidase activity, the binding for plasma membranes becomes 8.4 X 10(-14) mole/U 5'nucleotidase; secretory vesicles 4.4 X 10(-13) mole/U 5'nucleotidase. Thus, secretory vesicles are fivefold richer in high affinity receptor sites than plasma membranes. It is suggested that in order for somatostatin to act, both a receptor and an effector unit must be present. In the case of tissues secreting polypeptide hormones by granule extrusion, the secretory vesicle may possess both the receptor and the effector units. It is postulated that during the process of fusion of the plasma and secretory vesicle membranes, a high affinity binding site for somatostatin is incorporated into the plasma membrane, thereby allowing somatostatin to act at a specific locus in the cell in inhibiting hormone release.

Somatostatin: an endogenous peptide in the toad urinary bladder inhibits vasopressin-stimulated water flow

Somatostatin (somatotropin release-inhibiting factor; SRIF) is a tetradecapeptide present in brain, pancreas, gastrointestinal tract, and thyroid that inhibits the secretion or action of several hormones in these tissues. We observed that the toad urinary bladder contains concentrations of endogenous somatostatin (8.0 pg/micrograms of protein) comparable to those found in the mammalian pancreas and gastrointestinal tract. To determine if somatostatin directly alter the action of vasopressinn we studied the effects of this polypeptide on vasopressin-stimulated transport processes in the toad urinary bladder in vitro. Somatostatin produced a dose-dependent, reversible inhibition of vasopressin-stimulated osmotic water flow; it inhibited theophylline-stimulated osmotic water flow but not the water flow stimulated by 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate. These data are consistent with an inhibition of both basal and hormone-stimulated adenylate cyclase. Vasopressin-stimulated short circuit current was not inhibited by somatostatin. These studies provide direct evidence for an effect of somatostatin on hormone-modulated epithelial transport in tissues other than the gastrointestinal tract. We propose that endogenous somatostatin may function as a local regulator of the cellular action of vasopressin on osmotic water flow.

Interaction of somatostatin with PTH and AVP: renal effects

Six conscious intact dogs were studied to evaluate the interactions of somatostatin (SRIF) with exogenous antidiuretic hormone arginine vasopressin (AVP). SRIF administration caused a significant increase in free water clearance compared to a vehicle-treated group: -0.91 (+/- 0.41 SD) ml/min to 0.21 (+/- 0.32 SD) ml/min in the experimental group (P less than 0.01) versus 0.21 (+/- 0.81 SD) ml/min to -0.21 (+/- 0.68 SD) ml/min in the control (P greater than 0.5). Six conscious, thyroparathyroidectomized dogs were studied to test the interaction of SRIF and parathyroid extract (PTE). There were no significant changes in the phosphaturic and hypocalciuric effects of PTE with SRIF administration. We conclude that acute systemic SRIF administration interferes with the antidiuretic action of AVP, probably at the renal-tubular level, but does not antagonize the renal actions of PTE.

Inhibition of H+ secretion in frog gastric mucosa by somatostatin

Somatostatin added to the serosal bathing solution of the isolated gastric mucosa of Rana pipiens significantly inhibited pentagastrin- and histamine-stimulated H+ secretion. The decrease in H+ secretion rate was accompanied by an increase in the transmucosal potential difference and resistance. Somatostatin (10(-5) M) had no effect on the N6,O2-dibutyryl adenosine 3'-5'-cyclic monophosphate (DBcAMP)-stimulated H+ secretion rate. The mucosa exposed to somatostatin secreted H+ on stimulation by DBcAMP or histamine, but did not respond to 2.8 X 10(-7) M pentagastrin. However, pentagastrin added to the serosal solution stimulated H+ secretion after the somatostatin was washed away. Calcium inophore (3 X 10(-5) M) alone or 10(-2) M Ca2+ plus calcium ionophore temporarily increased the H+ secretion rate inhibited by somatostatin. The data suggest that somatostatin has a direct effect on the oxyntic cells in the gastric mucosa.

Effect of suppression of endocrine or exocrine pancreatic function in hemorrhagic shock

Somatostatin did not influence the pathologic consequences of hemorrhagic shock, but pancreatic duct ligation prevented the post-oligemic decline of arterial pressure and formation of toxic factors. These results indicate that pancreatic acinar cells release myocardial depressant factor and are important in the pathophysiology of shock.

Inhibition of prostaglandin-induced cyclic AMP accumulation in the rat anterior pituitary by alrestatin

Alrestatin at 25-1 X 10(-4) M inhibited the accumulation of cyclic AMP induced by prostaglandin E2, but not theophylline, in the rat anterior pituitary in vitro. Somatostatin, at lower concentrations, inhibited both; maximal inhibition of the prostaglandin effect was greater with alrestatin. As cyclic AMP is considered to be a mediator in induced-hormonal release, it appears from the present findings that alrestatin may be of potential use in altering hormonal release.

Somatostatin inhibits the mitogenic effect of thyroliberin

The effect of somatostatin and of thyroliberin on the mitotic incidence in the organ-cultured anterior pituitary lobe of the rat was investigated, using the colchicine metaphase-arrest technique. It was found that somatostatin added to the culture medium, together with thyroliberin, blocked the mitogenic effect of the latter.