Somatostatin pretreatment desensitizes somatostatin receptors linked to adenylate cyclase and facilitates the stimulation of cyclic adenosine 3':5'-monophosphate accumulation in anterior pituitary tumor cells

Somatostatin-14 (SRIF) inhibits both hormone- and forskolin-stimulated cyclic adenosine 3':5'-monophosphate (cyclic AMP) formation in tumor cells of the mouse anterior pituitary (AtT-20/D16-16). However, long-term pretreatment of cells with SRIF modifies the responsiveness of this system in two ways: The response of adenylate cyclase to stimulatory agents is enhanced, whereas the ability of SRIF to inhibit stimulated cyclic AMP formation is reduced. The supersensitive adenylate cyclase response and the SRIF desensitization were dependent on the concentration and duration of SRIF pretreatment. Enhancement of forskolin-stimulated cyclic AMP formation occurred within 4 hr, whereas that of corticotropin-releasing-factor-, (-)-isoproterenol-, and vasoactive intestinal peptide-induced cyclic AMP accumulation required 16 hr of pretreatment. The elevated responses to each of these stimulants were due to increases in their maximal ability to stimulate cyclic AMP formation. Cycloheximide treatment blocked the enhanced cyclic AMP response induced by SRIF pretreatment, suggesting a requirement for protein synthesis. In membrane preparations, SRIF pretreatment facilitated activation of adenylate cyclase by forskolin, sodium fluoride, and guanosine 5'-(beta,tau-imido)-triphosphate without affecting basal activity. These results suggest that desensitization of an inhibitory input to adenylate cyclase is accompanied by a supersensitivity of adenylate cyclase to stimulatory agents through a process requiring protein synthesis.     

The accumulation of adenosine 3',5'-monophosphate in cerebral cortical slices of the quaking mouse, a neurologic mutant

Norepinephrine, adenosine, veratridine, and adenosine-biogenic amine combinations elicit an accumulation of cyclic AMP in cerebral cortical slices from C57B1/6J control (+/+), quaking (qk/qk) and heterozygous (qk/+) mice. The percent conversion of radioactive adenine nucleotides to cyclic AMP in slices previously incubated with radioactive adenine or adenosine is markedly lower in slices from quaking and heterozygous mice compared to controls with all stimulatory agents except an adenosine-norepinephrine combination. Total incorporation of radioactive adenine or adenosine into adenine nucleotides is, however, significantly higher with slices of quaking and heterozygous mice. The absolute amount of radioactive cyclic AMP and the levels of endogenous cyclic AMP are nearly identical in the 3 groups of mice following incubation with all stimulatory agents except adenosine. The absolute accumulation of both radioactive and endogenous cyclic AMP was significantly lower in quaking and heterozygous mice after incubation with adenosine. The ratio of [¹⁴C]- to [³H]cyclic AMP in slices previously incubated with [¹⁴C]adenine and [³H]adenosine is dependent upon the stimulatory agent. The ratio with each agent is consistently lower in quaking mice compared to controls. The data provide evidence for biochemical alterations in nucleotide metabolism in cortical slices ofboth quaking and heterozygous mice. These effects would not appear to be directly associated with hypomyelination and tremor of the quaking mouse, since the heterozygous mouse, with one quaking gene does not show the latter gross abnormalities.     

Ontogeny of beta-adrenergic receptor-mediated cyclic AMP generating system in primary cultured neurons.

The developmental changes in the beta-adrenergic receptor/cyclic AMP generating system were examined using mouse cerebral cortical neurons in primary culture. During neuronal growth in vitro, the number of binding sites for [3H]dihydroalprenolol (DHA) showed a tendency to increase (Bmax), while the affinity (Kd) for [3H]DHA did not show any noticeable changes. Basal and isoproterenol-stimulated adenylate cyclase activities as well as the activation of adenylate cyclase by 5'-guanylylimidodiphosphate (GppNHp), NaF and forskolin showed progressive and parallel increases during neuronal growth on a polylysine-coated surface. The treatment of primary cultured neurons with islet-activating protein (IAP), one of the pertussis toxins, attenuated the inhibitory effect of carbachol, a muscarinic agonist, on isoproterenol-induced activation of adenylate cyclase activity. These results indicate that primary cultured neurons possess a cyclic AMP generating system coupled with beta-adrenergic and muscarinic receptors, which is regulated via stimulatory and inhibitory GTP-binding proteins, respectively. The results described above also suggest that the beta-adrenergic receptor, stimulatory and inhibitory types of GTP-binding proteins and adenylate cyclase may develop in a parallel fashion during neuronal growth on a polylysine-coated surface.     

Somatostatin inhibition of VIP- and isoproterenol-stimulated cyclic AMP production in rat peritoneal macrophages.

The dual regulation of cyclic AMP levels in rat peritoneal macrophages incubated with somatostatin, vasoactive intestinal peptide (VIP), and isoproterenol was studied. Somatostatin exerted a non-competitive inhibition of the stimulatory effect of VIP and isoproterenol on cyclic AMP production. In addition, somatostatin inhibited basal cyclic AMP levels. Our results suggest that somatostatin and VIP may modulate the immune response acting, through cyclic AMP, on macrophage functions.     

Peptidergic modulation of G-protein coupled cyclic-AMP accumulation in the rat caudate nucleus.

Somatostatin, substance P, and vasoactive intestinal polypeptide were incubated in an adenylate cyclase assay with a particulate fraction of caudate-putamen tissue of the rat in order to examine the effect of the neuropeptides on G-protein coupled adenylate cyclase in vitro. Somatostatin induced an enhancement of cyclic AMP formation in presence of guanine nucleotides and cholera toxin but inhibited pertussis toxin and forskolin enzyme stimulation. Pertussis toxin and cholera toxin also depressed forskolin-induced stimulation as described previously. Somatostatin was able to antagonize these inhibitory effects of both toxins. On the contrary, substance P reduced GTP and cholera toxin stimulated striatal adenylate cyclase, without affecting forskolin activation. In our preparation, VIP did not influence basal adenylate cyclase activity or the stimulation by guanine nucleotides, cholera toxin, and pertussis toxin. VIP potently inhibited the enhancement of cyclic AMP formation by forskolin and completely antagonized the inhibitory effect of cholera toxin on forskolin activation. These results suggest that neuromodulatory effects of somatostatin, substance P, and VIP are mediated by the inhibitory as well as stimulatory guanine nucleotide proteins G-i and G-s coupled to an adenylate cyclase system.     

Chronic infusion of agents that increase cyclic AMP concentration enhances the regeneration of mammalian peripheral nerves in vivo

Our previous investigation indicates that forskolin, a robust activator of adenylate cyclase, promotes sensory nerve regeneration in amphibians. The present study was designed to determine if forskolin had a similar effect in mammals. We also wished to test the hypothesis that cyclic AMP modulates nerve regeneration by comparing the effects of chronically infused forskolin with the effects of infused dibutyryl cyclic AMP, 8-bromo cyclic AMP, and the phosphodiesterase inhibitor, theophylline. Our results indicated that all agents promoted some aspect of regeneration. The two which presumably generated the largest increase in cyclic AMP concentration, forskolin and 8-bromo cyclic AMP, had the most profound effect on axonal elongation. All agents decreased the time to sprout initiation, but theophylline produced the largest decrease and its effect was mimicked by caffeine, a methylxanthine with limited ability to inhibit phosphodiesterase. This suggests that sprout formation may be triggered by an increase in intraaxonal free Ca2+, possibly modulated by cyclic AMP. The role of cyclic AMP in axonal elongation remains to be determined, but may be associated with stimulation of protein synthesis in the nerve cell body.     

Agents that affect calcium influx can change cyclic nucleotide levels in cultured chick pineal cells.

Previous experiments examined interactions among the effects of cyclic AMP and calcium-related agents on melatonin output by cultured chick pineal cells, and suggested that changes in calcium influx might act through cyclic AMP. Here, effects of calcium-related agents and manipulations on cyclic AMP (and cyclic GMP) levels are demonstrated directly. These effects support a role for cyclic AMP (but not cyclic GMP) in the effects of changes in calcium influx on melatonin production by these cells.     

α2-Adrenergic receptors mediate inhibition of cyclic AMP production in neurons in primary culture

The actions of adrenergic agents on the intracellular production of cyclic adenosine monophosphate (AMP) was examined in intact cortical and striatal neurons in primary culture, generated from the fetal mouse brain. Exposure of striatal neurons to the β-adrenergic agonist isoproterenol (10 μM) resulted in a 5-fold increase in intraneuronal cyclic AMP; norepinephrine (100 μM), alone or in combination with isoproterenol, produced only a 3-fold increase in cyclic AMP levels. However, in the presence of yohimbine (10 μM), cyclic AMP productions due to norepinephrine or isoproterenol plus norepinephrine were identical to isoproterenol alone. When striatal or cortical neurons were exposed to pertussis toxin (100 ng/ml) overnight, there was no detectable difference between isoproterenol- and norepinephrine-stimulated cyclic AMP production. These data suggest thatα₂-adrenergic receptors mediate the attenuation of cyclic AMP production in neurons and do so via the inhibitory guanine nucleotide regulatory protein of adenylate cyclase.     

Effect of heparin on platelet aggregation inhibited by PGI2, trifluoperazine and verapamil

The enhancement of platelet aggregation by heparin in the presence of certain inhibitors of aggregation was investigated in an attempt to discern the mechanism through which heparin alters platelet function in plasma. These studies were performed by adding prostaglandin I2 (PGI2), verapamil, or trifluoperazine to platelet-rich plasma (PRP) in the presence or absence of heparin. Adenosine diphosphate (ADP), collagen, or arachidonic acid were used for induction of platelet aggregation. The inhibitory agents reduced platelet aggregation to 5 to 20% of control in the absence of heparin. When present in the reaction mixture along with the inhibitor, heparin restored aggregation to approximately 57 to 92% of control depending on the inhibitor and aggregating agent. This proaggregatory action of heparin was observed when heparin and PGI2 were preincubated together or separately for 20 min prior to the addition of PRP and ADP. Results were similar regardless of the sequence in which PGI2 and heparin were added to PRP, and irrespective of the time of incubation of platelets with PGI2. No suppression of platelet cyclic AMP concentration was observed with heparin alone. Heparin also failed to reduce the magnitude of platelet cyclic AMP accumulation promoted by PGI2, forskolin, or a mixture of PGI2 and forskolin. These observations suggest that heparin promoted platelet aggregation and partially overcame the effect of certain inhibitory agents by mechanism(s) that did not involve a reduction of platelet cyclic AMP.     

Serotonin-induced protein phosphorylation in a lobster neuromuscular preparation

Three substances believed to be neurohormones in the lobster--serotonin, octopamine, and proctolin--can modulate the physiology of the dactyl opener muscle of the lobster walking leg. All three act directly on muscle fibers to cause long-lasting contractures. In addition, serotonin enhances the release of transmitter from the excitatory and inhibitory axons that innervate the muscle. We now report that serotonin, applied to intact nerve-muscle preparations, increases the phosphorylation of a soluble 29,000-dalton protein by as much as 20-fold. In two respects the phosphorylation parallels the physiological effects of serotonin: both have a slow time course, and both are caused by low concentrations of serotonin (thresholds of approximately 10(-8) M). The 29,000-dalton protein is present in many lobster tissues, including thoracic ganglia, gills, and several different muscles. This suggests that the phosphorylation participates in some aspect of metabolism common to many types of cells, rather than in an activity unique to nerve or muscle. The function of the phosphorylated protein remains unknown. Several lines of evidence suggest that the phosphorylation is mediated by a cyclic nucleotide, probably cyclic AMP. (1) Adding cyclic AMP or cyclic GMP to nerve-muscle homogenates leads to the phosphorylation. (2) Adding 8-Br-cyclic AMP or 8-Br-cyclic GMP to intact preparations leads to the phosphorylation. (3) In intact tissue, forskolin, an adenylate cyclase activator, and isobutylmethylxanthine, a phosphodiesterase inhibitor, increase cyclic AMP and cyclic GMP and also cause the phosphorylation. (4) Serotonin increases cyclic AMP in intact preparations. (5) RMI 12,330A, which blocks the serotonin-induced cyclic AMP increase, also blocks the phosphorylation. (6) Serotonin has no detectable effect on cyclic GMP. This suggests that cyclic AMP, and not cyclic GMP, is the second messenger that mediates phosphorylation in vivo. In contrast, calcium is not likely to mediate the phosphorylation. Agents that block calcium channels do not block the serotonin-induced phosphorylation, and agents that increase sarcoplasmic calcium do not cause it. Furthermore, calcium does not induce the phosphorylation in a homogenate. One of the other modulators, proctolin, does not increase cyclic AMP or cause the phosphorylation even at concentrations 10(6)-fold greater than the hormone's threshold for producing contractures. The third modulator, octopamine, causes small increases in cyclic AMP and is a weak agonist of the phosphorylation. Neither proctolin nor octopamine alters cyclic GMP levels.     

Ultrastructural identification of catecholamine neurones in the hypothalamic periventricular-arcuate nucleus-median eminence complex with special reference to quantitative aspects

Levels of cyclic AMP are elevated rapidly in rat superior cervical ganglia incubated in medium containing low concentrations of isoprenaline, adrenaline and noradrenaline. Phenylephrine and dopamine increase cyclic AMP when used at higher concentrations. The moderate rise induced by dopamine has a slow onset and long duration; the peak accumulation induced by beta-adrenergic agents is rapid and short-lasting. Preincubation with propranolol completely suppresses the elevation of cyclic AMP levels induced by all the agents tested.     

Development of adenosine-dependent cyclic AMP accumulation in the avian optic tectum

The present work shows the existence of adenosine-dependent cyclic adenosine monophosphate (AMP) accumulation in the chick optic tectum. When tecta from 18-day-old embryos were incubated with the phosphodiesterase inhibitor IBMX and RO 20-1724, the cyclic AMP level increased from 39.2 to 73.3 and 285.5 pmol/mg protein, respectively. The high level obtained with RO 20-1724 could be inhibited by increasing concentrations of IBMX or by adenosine deaminase, but not by dipyridamole. 2-Chloroadenosine promoted a dose-dependent cyclic AMP accumulation in tecta incubated with RO 20-1724 and adenosine deaminase. This effect was blocked by IBMX and varied substantially during the development of the tissue. The degree of stimulation increased after day 11 of incubation, attaining maximal levels on day 14. The effect of 2-chloroadenosine remained constant until day 18, a period when both the protein content and the basal cyclic AMP levels are increasing in the developing tectum. The cyclic AMP increase elicited by 2-chloroadenosine was greatly reduced in tecta from 20-day-old embryos and 2-day-old chicks. The putative transmitters glutamate and glycine and the neurotransmitter analogs isoproterenol and carbachol had no stimulatory effect on the cyclic AMP accumulation of tecta from 10- and 17-day-old embryos.     

Biochemical studies of taste sensation: II. Labelling of cyclic AMP of bovine taste papillae in response to sweet and bitter stimuli.

Labeling of cyclic AMP of tase papillae and its responsiveness to tast stimuli has been measured using whole papillae from bovine tongue prelabeled with [ 8-(14) C] adenine. Labeling was measured in circumvallate and fungiform papillae, both of which contain taste buds, and in filiform papillae and small blocks of tongue epithelium, which are devoid of taste buds. No differences were observed in the levels of activity. The labeling of cyclic AMP of circumvallate papillae showed only small increases (12-22%) in the presence of the taste stimulus sucrose (sweet), and the stimulatory effects were not statistically significant. The increase due to sucrose was not potentiated by theophylline. No stimulation by sucrose was observed with epithelium controls. Lactose, which is a poor taste stimulus, did not stimulate labeling of cyclic AMP in taste papillae. Theophylline, caffeine, and quinine (bitter) stimulated labeling of cyclic AMP by up to 2-fold, as did L-Epinephrine. Evidence for a specific role of cyclic AMP as a second messenger in taste sensation was not obtained. It is suggested that cyclic AMP might provide a mechanistic basis for studying some of the effects of sweet and bitter compounds in mixtures.     

The relationship between neurite extension and phospholipid metabolism in PC12 cells

NGF stimulates a variety of changes in PC12 cells. These include neurite extension and an increase in the metabolism of phosphatidylinositol and phosphatidic acid (the so-called 'PI effect'). Cyclic AMP has been postulated to mediate many of the effects of NGF on PC12 cells, and we recently found that a variety of agents which stimulate neurite extension also increase intracellular cyclic AMP. These agents included phospholipase A2 and phospholipase C. In an effort to clarify the relationship of the 'PI effect' to other NGF-induced events we investigated the effects of these agents and of cyclic AMP on phospholipid metabolism. All agents which elevate cyclic AMP stimulate phosphatidylinositol metabolism. In addition, elevated exogenous KCl, which does not stimulate cyclic AMP production, promotes neurite extension and also causes the 'PI effect'. These data suggest that an increase in the turnover of phosphatidylinositol may be a requisite event in neurite outgrowth.     

Actions of neuropoietic cytokines and cyclic AMP in regenerative conditioning of rat primary sensory neurons

A conditioning lesion to peripheral axons of primary sensory neurons accelerates regeneration of their central axons in vivo or neurite outgrowth if the neurons are grown in vitro. Previous evidence has implicated neuropoietic cytokines and also cyclic AMP in regenerative conditioning. In experiments reported here, delivery through a lentivirus vector of ciliary neurotrophic factor to the appropriate dorsal root ganglion in rats was sufficient to mimic the conditioning effect of peripheral nerve injury on the regeneration of dorsal spinal nerve root axons. Regeneration in this experimental preparation was also stimulated by intraganglionic injection of dibutyryl cyclic AMP but the effects of ciliary neurotrophic factor and dibutyryl cyclic AMP were not additive. Dibutyryl cyclic AMP injection into the dorsal root ganglion induced mRNAs for two other neuropoietic cytokines, interleukin-6 and leukemia inhibitory factor and increased the accumulation of phosphorylated STAT3 in neuronal nuclei. The in vitro conditioning action of dibutyryl cyclic AMP was partially blocked by a pharmacological inhibitor of Janus kinase 2, a neuropoietic cytokine signaling molecule. We suggest that the beneficial actions of increased cyclic AMP activity on axonal regeneration of primary sensory neurons are mediated, at least in part, through the induction of neuropoietic cytokine synthesis within the dorsal root ganglion.     

Pharmacological identification of the alpha-adrenergic receptor type which inhibits the beta-adrenergic activated adenylate cyclase system in cultured astrocytes

The adrenergic agonist norepinephrine can exert its influence on cell function by activating both alpha- and beta-adrenergic receptors. In astrocytes, the alpha-adrenergic receptor activity of norepinephrine is known to inhibit the cyclic AMP response elicited by its action at beta-adrenergic receptors. Pharmacological studies were conducted to identify the subtype of alpha-adrenergic receptor which mediates this inhibitory action. The alpha 2-adrenergic antagonist yohimbine potentiated the cyclic AMP response elicited by norepinephrine, whereas the alpha 1-adrenergic antagonist prazosin did not affect the response. The alpha 2-adrenergic agonist clonidine inhibited the cyclic AMP response elicited by the beta-adrenergic agonist isoproterenol and this inhibition could be blocked by yohimbine but not by prazosin. In contrast, the alpha 1-adrenergic agonist phenylephrine did not inhibit the cyclic AMP response to isoproterenol. These studies indicate that the inhibitory action of norepinephrine is mediated by its action at alpha 2-adrenergic receptors.     

Beta-adrenergic-control of cyclic AMP-generating systems in cerebellum: pharmacological heterogeneity confirmed by destruction of interneurons.

Rats subjected to neonatal x-irradiation selectively lose the vast majority of late-maturing granule, basket, and stellate cells, while the earlier-maturing Purkinje cells remain intact. These animals were used as a model to examine the electrophysiological and neurochemical interaction of neuroleptic compounds with the β-adrenergically linked cyclic AMP-generating systems of cerebellum. Intracisternal injection of 6-hydroxydopamine in x-irradiated animals resulted in an 80% increase in Purkinje cell mean discharge rate, suggesting an inhibitory adrenergic input comparable to that seen in normal animals. Iontophoresis of fluphenazine reversed the inhibitory effects of iontophoretically applied norepinephrine on Purkinje cell discharge rate in both x-irradiated animals and x-irradiated animals treated with 6-hydroxydopamine. The norepinephrine-stimulated formation of cyclic AMP was significantly reduced in cerebellar slices prepared from x-irradiated rats as compared with controls. Furthermore, while fluphenazine (10 μm) reduced the norepinephrine-stimulated formation of cyclic AMP by 50 to 60% in cerebellar slices from control rats, the same concentration of fluphenazine reduced norepinephrine-elicited accumulation of cyclic AMP to values which were not significantly different from basal values in slices from x-irradiated rats. These results indicate that at least two populations of cells containing catecholamine-sensitive cyclic AMP-generating systems exist in the cerebellum. One population, including the Purkinje cell, has β-receptors sensitive to the inhibitory effects of the clinically-active neuroleptic, fluphenazine. The other population of cells, destroyed by x-irradiation, has β-receptors which are resistant to the inhibitory effects of this neuroleptic. Moreover, the locus of the neuroleptic inhibition of catecholamine-sensitive cyclic AMP-generating systems in the cerebellum appears to be postsynaptic, since the electrophysiological effects of fluphenazine were qualitatively similar in both x-irradiated rats and x-irradiated rats treated with 6-hydroxydopamine.     

Beta-adrenergic stimulation of protein (arginine) methyltransferase activity in cultured cerebral cells from embryonic mice

Several adrenergic effectors and neurotransmitters were tested as potential regulators of myelin basic protein (MBP) and histone methyltransferase activities. Both enzymes were specifically activated by beta-adrenergic agonists in a stereospecific manner. Cyclic AMP (but not AMP) stimulated the enzymes to the same extent as did the beta-adrenergic agonist, (-) isoproterenol. The studies suggest that beta-adrenergic agonists stimulate adenylate cyclase thereby causing an increased production of cyclic AMP which stimulates the methyltransferases. Cycloheximide addition to the reaction mixture did not affect the stimulation due to cyclic AMP, indicating that new protein synthesis is not involved in the cyclic AMP stimulation of the methyltransferases. Thyroid hormone (T3) has been shown to stimulate MBP methyltransferase [Amur et al, 1984] and could exert its stimulatory effect through beta-adrenergic-dependent systems. But the beta-adrenergic antagonist, propranolol, did not block the stimulation by T3, suggesting that the effect of T3 is not mediated through beta-adrenergic-dependent systems. Thus, the methylation of MBP seems to be regulated both by T3 and by neurotransmitters and/or hormones mediating their effects through cyclic AMP production, whereas the methylation of histones seems to be regulated only by the latter.     

A biochemical and morphological study of the altered growth pattern of central catecholamine neurons following 6-hydroxydopamine

The ability of a series of adrenergic agents to displace the binding to brain membranes of [3H]WB 4101, a potent alpha-adrenergic antagonist (WB 4101 = 2-[2-(2,6-dimethoxyphenoxy)ethylaminomethyl]-1,4-benzodioxane hydrochloride), has been compared with the potency of these agents in stimulating or inhibiting the alpha-adrenergic component of cyclic AMP accumulation in rat cerebral cortex slices. [3H]WB 4101 rapidly bound to a high affinity site (KD = 2.7 nM) in membranes from cerebral cortex. Binding came to equilibrium by 2 min at 37 degrees C and was rapidly reversed in the presence of phentolamine. The potencies of adrenergic agents (WB 4101 greater than phentolamine greater than naphazoline) in displacing binding of [3H]WB 4101 were comparable to the potencies of these agents as inhibitors of the alpha-adrenergic component of norepinephrine-stimulated cyclic AMP accumulations. Phenoxybenzamine, clonidine, chlorpromazine and haloperidol were about 10--30 times more potent in inhibiting cyclic AMP accumulation than in displacing [3H]WB 4101 binding. The potency of classical alpha-adrenergic agonists in displacing WB 4101 (epinephrine greater than norepinephrine greater than methoxamine) correlated with the ability of these agonists to increase cyclic AMP levels. Overall a significant correlation (r = 0.87, P less than 0.005) was found between WB 4101 binding and alpha-adrenergically mediated cyclic AMP accumulation in brain. Several ligands bind to specific sites in brain membranes with alpha-adrenergic receptor properties. The identification of these binding sites as receptors depends on a correlation of binding with a known alpha-adrenergic receptor-mediated response in brain. These data demonstrating that WB 4101 correlates with norepinephrine-stimulated cyclic AMP accumulation suggest that WB 4101 may bind to the membrane receptor sites mediating the alpha-adrenergic accumulation of cyclic AMP in rat cerebral cortex.     

Vasoactive intestinal peptide stimulates cyclic AMP metabolism in choroid plexus epithelial cells

Some peptides of the glucagon-secretin family were found to stimulate intracellular cyclic AMP accumulation in cultured bovine choroid plexus epithelial cells. Vasointestinal peptide and porcine intestinal peptide at concentrations of 30 and 300 nM, respectively, evoked 50-fold elevations of cyclic AMP; half-maximal responses were obtained with concentrations of 15 and 102 nM for the two peptides, respectively. Secretin and glucagon each produced 25- to 50-fold elevations of cyclic AMP at 330 microM, but showed no effect below 3 microM. Gastric inhibitory peptide and prealbumin had little or no response at any concentration tested. Experiments measuring the cellular cyclic AMP accumulation in response to pairs of peptides suggested that vasointestinal peptide, porcine intestinal peptide and secretin act through a common receptor. Studies with antagonists to isoproterenol and histamine indicated that this receptor is distinct from the beta-adrenergic and H2-histamine receptors known to exist on choroidal cells.