Characterization of the cyclic AMP-independent actions of somatostatin in GH cells. I. An increase in potassium conductance is responsible for both the hyperpolarization and the decrease in intracellular free calcium produced by somatostatin

Interaction of opiate peptides with dopamine effects on prolactin secretion and membrane electrical properties in anterior pituitary cells from lactating rats

Abstract Met-enkephalin and beta-endorphin induced a partial reversion of the dopamine inhibition of prolactin release from pituitary cells of lactating rats in primary culture. This effect of opiate peptides was dose-dependent with an EC50 of 40 +/- 8 nM and 45 +/- 7 nM and maximal blockade of dopamine inhibition of 60% and 68% for Met-enkephalin and beta-endorphin, respectively. Naloxone antagonized the effect of Met-enkephalin with an EC50 of 22 +/- 12 nM. Furthermore, this Met-enkephalin effect on dopamine inhibition of prolactin secretion appeared non-competitive since it reduced maximal inhibition without affecting the apparent affinity of dopamine. Finally, it should be noted that the two opiate peptides had no effect on spontaneous prolactin release. In electrophysiological experiments, local ejection of dopamine on tested cells induced an hyperpolarization concomitant with an increase of the membrane conductance. Ejection of Met-enkephalin or beta-endorphin alone did not modify the electrical properties of the cells (resting potential, membrane conductance and excitability). In contrast, both peptides blocked in a reversible manner the dopamine-induced electrical responses. These effects were antagonized by naloxone. However, this interaction of opiatepeptides with dopamine electrical response was not observed on all cells tested. We conclude that the blocking effect of opiates on dopamine-induced hyperpolarization may account, at least in part, for the ability of these peptides to interact with dopamine inhibition of prolactin release.

Somatostatin pretreatment facilitates GRF-induced GH release and increase in free calcium in pituitary cells

Somatostatin pretreatment sensitizes rat anterior pituitary to hGRF stimulation in vitro. The pretreatment (1 nM for 10 min) facilitated GH release response of dispersed rat anterior pituitary cells to hGRF (1 nM for 3 min) 2.04-fold in a perifusion system. The effect lasted even 20 min after the pretreatment. SRIF pretreatment decreased cAMP content in the cells after hGRF stimulation to 61% of the control value. When hGRF was replaced by 1 mM DBcAMP and 15 mM KCl, the pretreatment increased GH secretion 1.69- and 1.67-fold respectively. SRIF pretreatment (1 nM for 10 min) caused a larger increase in (Ca2+)i by hGRF than that of control. The effect of SRIF pretreatment facilitates GRF-induced increase in GH secretion probably through the stimulation of increase in (Ca2+)i.

Differential coupling with pertussis toxin-sensitive G proteins of dopamine and somatostatin receptors involved in regulation of adenohypophyseal secretion

D2 dopamine receptors and somatostatin receptors in adenohypophyseal cells are coupled through G proteins to various transduction mechanisms. To study the involvement of these different transduction mechanisms and of various G proteins in the dopamine and somatostatin regulation of prolactin (PRL), growth hormone (GH) and thyroid-stimulating hormone (TSH) secretions, we have pretreated the adenohypophyseal cells in primary culture with increasing doses of pertussis toxin. The guanosine triphosphate (GTP) dependency of the negative coupling of dopamine and somatostatin receptors with adenylate cyclase in the same membrane preparation from anterior pituitary cells was different. In fact, higher GTP doses were requested to obtain dopamine inhibition, suggesting that different G proteins were involved in the coupling of these two receptors with adenylate cyclase. However, the inhibition of adenylate cyclase activity by both neurohormones was fully sensitive to pertussis toxin pretreatment with a similar IC50 for the toxin. The IC50 for the toxin was also similar for the blockade of dopamine or somatostatin inhibition of the three-hormone secretion as well as for the stimulation on basal PRL or GH secretion or the reduction of thyrotropin-releasing hormone (TRH)-stimulated prolactin secretion, suggesting that the toxin acts through similar mechanisms on these different phenomena. Pretreatment of the cells with Bordetella pertussis toxin differentially affected the effects of both neurohormones on the three cell types. A complete reversion of the inhibition of secretion was observed only in the case of somatostatin on PRL and TSH cells. In contrast, the somatostatin inhibition of GH secretion was only partially reversed by the pertussis toxin pretreatment. This was also the case of dopamine inhibition of PRL secretion. It can be concluded that: (1) On PRL secretion dopamine and somatostatin do not share all the mechanisms since the intensity of their inhibition and the reversibility of their effects by pertussis toxin were differential. (2) Different mechanisms of action are implicated in the effect of somatostatin on PRL, GH and TSH secretions. (3) Different G proteins might be involved in the coupling of dopamine and somatostatin receptors with adenylate cyclase.

Somatostatin inhibition of hormone release: effects on cytosolic Ca++ and interference with distal secretory events

In normal pituitary cells somatostatin (SRIF) blocks the spontaneous oscillations in [Ca++]i by inhibiting the generation of action potentials. This is sufficient to explain the inhibitory effect on basal, but not entirely that on stimulated pituitary hormone secretion. In insulin secreting cells, which, in contrast to pituitary cells, only fire action potentials on stimulus-evoked depolarization, SRIF hyperpolarization and lowering of [Ca++]i is only transient. The marked inhibition of insulin secretion is suggested to be due to a coordinated action of SRIF on membrane potential and [Ca++]i as well as a direct interference with late secretory events.

Mechanism of action of somatostatin: an overview of receptor function and studies of the molecular characterization and purification of somatostatin receptor proteins

To determine whether somatostatin receptor subtypes arise from molecular heterogeneity of the receptor protein, we have cross-linked the putative receptor in normal rat tissues and in AtT-20 and GH3 cells, both chemically with SS-14, SS-28 and Tyr3 SMS ligands, as well as by photoaffinity labeling with an azido derivative of Tyr3 SMS (EE 581). Three prominent somatostatin receptor proteins of 58-kDa, 32-kDa, and 27-kDa size have been identified. These proteins exhibit a tissue-specific distribution, ligand selectivity, and relative preference for SS-14 and SS-28 binding, and thus qualify as somatostatin receptor subtypes. Using EE 581 as a photoaffinity probe, the 58-kDa and 32-kDa proteins have been purified to homogeneity from brain and AtT-20 cells by successive SDS-PAGE. The 58-kDa form has been trypsinized and amino acid sequence data obtained from four tryptic fragments. With the help of synthetic oligonucleotides derived from these sequences, work is currently in progress to clone the 58-kDa protein to elucidate its complete sequence, its expression, and its functional relationship to the somatostatin receptor and its pharmacological subtypes.

Forskolin and prostaglandin E2 regulate the generation of human cytolytic T lymphocytes

In this paper we examine the characteristics of human cytolytic T lymphocytes (CTL) generated in the presence of forskolin and PGE2. Forskolin and PGE2 suppressed the generation of class-I-specific CTL. The CTL generated in the presence of forskolin and PGE2 had different characteristics which included their ability to proliferate in response to the alloantigen and their lectin-mediated cytolytic activity. The CTL generated in the presence of forskolin had normal proliferative response to the alloantigen, whereas the CTL generated in the presence of PGE2 showed a suppressed proliferative ability to the alloantigen. The two groups of CTL were then tested for their activity in the process of lectin-dependent cell-mediated cytotoxicity. After the addition of PHA into the chromium release assay the CTL generated in the presence of forskolin normally lysed the nonspecific targets, whereas the CTL generated in the presence of PGE2 did not show the normal response in lysing the nonspecific targets. The results suggest that the cytolytic machinery was intact when the CTL were generated in the presence of forskolin but CTL were not able to either recognize or lyse the target cell. However, the CTL generated in the presence of PGE2 did not share the same characteristics as the CTL generated in the presence of forskolin because the CTL generated in the presence of PGE2 were unable to kill even in the presence of lectin. It appears that the inhibitory effects of forskolin were mediated by cAMP and not by its effects on the potassium channels because the 1,9-dideoxy derivative of forskolin which did not activate adenylate cyclase also did not suppress the generation of CTL. However, it was not established whether the diverse effects of PGE2 on the generation of CTL were mediated by cAMP-dependent, -independent or by both mechanisms.

Glycine stimulates calcium-independent release of 3H-GABA from isolated retinas of Xenopus laevis

A perfusion system was used to monitor the release of [3H]-GABA from isolated retinas of Xenopus laevis. Measurable release was stimulated by glycine at concentrations as low as 200 microM. Glycine-stimulated release was blocked by strychnine, and was not reduced in "calcium-free" Ringer's solution (0 Ca2+/20 mM Mg2+). Glutamate also stimulated calcium-independent release, using concentrations as low as 100 microM. In contrast, release stimulated by 25 mM potassium was reduced by 80% in calcium-free medium. In most experiments, agonists were applied in six consecutive 4-min pulses separated by 10-min washes with Ringer's solution. Under these conditions, the release stimulated by 0.5 mM glutamate or 25 mM potassium decreased by at least 50% from the first to the second pulse, and then gradually decreased with successive applications. In contrast, the response to 0.5 mM glycine at first increased and then only gradually decreased with successive pulses. These patterns of response to different agonists were similar in calcium-free medium. Somatostatin (-14 or -28) also stimulated release, and this effect was inhibited by AOAA, an inhibitor of GABA degradation. In the presence of AOAA, somatostatin had little effect, except at high concentrations of somatostatin (5 microM), which increased both basal and glycine-stimulated release. In contrast to somatostatin, glycine-stimulated release was much larger in the presence of AOAA. Autoradiography was used to investigate which cell types released [3H]-GABA under our conditions. Autoradiograms showed that horizontal cells and a population of apparent "off" bipolar cells were well-labeled by [3H]-GABA high-affinity uptake. In addition, light labeling was seen over numerous amacrine cells. After application of glycine, glutamate, or potassium, there was a decrease in label density over horizontal cells.

Pertussis toxin lesioning of the nucleus caudate-putamen attenuates adenylate cyclase inhibition and alters neuronal electrophysiological activity

Receptor-mediated inhibition of brain adenylate cyclase activity has been well characterized at the biochemical level. However, less understood is how these, typically modest, inhibitory effects on cyclase activity correlate with the electrophysiological activity of brain preparations. In addressing this question, we injected pertussis toxin (PT) into the nucleus caudate-putamen of intact rats, and observed a subsequent attenuated inhibition of adenylate cyclase activity in caudate membranes, which correlated with altered electrophysiological activity in this nucleus. PT completely abolished and electrophysiological activity in this nucleus. PT completely abolished and significantly reduced, respectively, dopamine D2 and opioid receptor-mediated inhibition of adenylate cyclase. In addition, pretreatment of rat caudate nuclei with PT attenuated the amount of in vitro ADP-ribosylation of 41,000 and 39,000 Da PT substrates measured in caudate membranes. Extracellular recording of the spontaneous activity of caudate neurons revealed that PT pretreatment significantly increased firing rates above those of cells recorded from sham-operated and unoperated controls. Furthermore, a significantly greater number of cells pretreated with PT displayed interspike intervals less than 50 ms, reflecting 'burst-like' activity. In short, the inactivation of G-proteins serving as PT substrates in rat caudate-putamen renders caudate cells more likely to fire spontaneously, and to fire in bursting, rather than uniform, patterns of activity. These observations suggest that PT substrates identical or similar to those which regulate adenylate cyclase, play a significant role in governing the electrophysiological behavior of intact caudate neurons.

Somatostatin and lanthanum discriminate two Ca2+ mobilizing mechanisms regulated by bombesin receptors in peptic cells

Bombesin (BB)-stimulated pepsinogen secretion from frog esophageal peptic acini was inhibited 40% by somatostatin (SS) (IC50 = 1 nM), and by 30-50% by low concentrations (10(-7)-10(-4)M) of lanthanum chloride. SS inhibited basal secretion as well as both early (0-2 min) and late (2-30 min) BB-stimulated secretory phases. By contrast, LaCl3 selectively inhibited the late secretory phase and was without effect on basal secretion. SS (100 nM) and LaCl3 (30 microM) attenuated BB-stimulated 45Ca2+ uptake, and the combination resulted in additive inhibition. High K+ media decreased basal secretion, abolished SS but not LaCl3 inhibition of BB-stimulated secretion, and blocked SS inhibition of BB-mediated 45Ca2+ uptake. These findings suggest the existence on peptic cells of distinct La3+-sensitive, and somatostatin-sensitive, K+ dependent, Ca2+ mobilizing mechanisms which contribute to BB receptor-mediated secretion.

Cytosolic free calcium in normal somatotropes: effects of forskolin and phorbol ester

Digital imaging microscopy using the calcium-sensitive indicator probe fura-2 was combined with a reverse hemolytic plaque assay (RHPA) for growth hormone (GH) secretion. This technique allows dynamic measurements of the cytosolic free calcium concentration ([Ca2+]i) in individual pituitary somatotropes. Stimulation by growth hormone-releasing factor (GRF) increases, whereas somatostatin (SRIF) reduces [Ca2+]i in this cell type. [Ca2+]i increased in somatotropes when the cellular content of adenosine 3',5'-cyclic monophosphate (cAMP) was elevated by 1) activating cellular adenylate cyclase with forskolin (5 microM) and 2) treatment with the cAMP-analogues dibutyryl-cAMP (1 mM) or 8-bromo-cAMP (5 mM). The forskolin-induced calcium rise was abolished in the absence of extracellular calcium. This indicates that cAMP increases the influx of calcium into the cytosol and thereby stimulates hormone release. When forskolin was given in combination with SRIF (10 nM), [Ca2+]i decreased to the same level reached with SRIF treatment alone, indicating a site of action distal to the generation of cAMP. Activating protein kinase C with the phorbol ester 12,13-phorbol dibutyrate (PDB; 100 nM) increased [Ca2+]i as well. Again, this effect was dependent on extracellular calcium and blocked when PDB and SRIF were applied simultaneously. Combined stimulation with GRF plus PDB did not augment the response of [Ca2+]i over GRF treatment alone.

Inhibitory actions of somatostatin on cyclic AMP and aldosterone production in agonist-stimulated adrenal glomerulosa cells

Somatostatin (SRIF) is a potent inhibitor of angiotensin II (AII)-stimulated aldosterone production in rat adrenal glomerulosa cells. This inhibition can be prevented by pretreatment of the cells with pertussis toxin, but little else is known about either the specificity or the biochemical bases of SRIF action in this tissue. We therefore conducted detailed studies of the influence of SRIF on steroidogenesis elicited by AII and the other two physiological stimuli of aldosterone production, K+ and adrenocorticotropic hormone (ACTH), in rat adrenal glomerulosa cells. We also determined the effects of SRIF on cytosolic calcium concentration ([Ca2+]i) and cellular cAMP levels. In these studies, SRIF was found to inhibit the aldosterone responses elicited by low concentrations of all three stimuli, which are believed to promote steroid secretion via discrete but interacting cellular signalling mechanisms. In addition, SRIF consistently lowered cellular cAMP levels in the presence of each of the three agents. However, SRIF caused a small and transient increase rather than a decrease in basal ([Ca2+]i), and had no effect on the subsequent elevation of ([Ca2+]i) by AII and K+. These data indicate that activation of a Gi-like protein by SRIF influences steroid responses to all three major regulators of glomerulosa-cell function, and suggest that basal levels of cAMP play a facilitatory or permissive role in the control of aldosterone production by predominantly calcium-mobilizing regulators of mineralocorticoid secretion.

Blood pressure reduction in hypertensive-diabetic rats by the somatostatin analog MK-678

A hypotensive effect of an orally-administered cyclopeptide somatostatin analog, MK-678, has been demonstrated in a hypertensive diabetic rat model. Sustained blood pressure reduction failed to occur when the drug was administered to the spontaneously hypertensive rat. The mechanism of hypotension appears independent of effects on a variety of hormones including insulin, glucagon, growth hormone, and components of the renin-angiotensin system including renin activity, plasma angiotensin converting enzyme, and aldosterone.

Somatostatin activates glibenclamide-sensitive and ATP-regulated K+ channels in insulinoma cells via a G-protein

Somatostatin, an hyperglycemia-inducing hormone, was studied in rat insulinoma (RINm5F) cells using 86Rb+ efflux techniques. 86Rb+ efflux is stimulated by somatostatin in a dose-dependent manner. The half-maximum value of activation is 0.7 nM. Somatostatin-induced 86Rb+ efflux is abolished by the hypoglycemia-inducing sulfonylurea, glibenclamide, a known blocker of ATP-regulated K+ channels. Somatostatin activation is prevented by pretreatment of insulinoma cells with pertussis toxin. 86Rb+ efflux studies show that somatostatin activates an ATP-dependent K+ channel.