Platelet Adenylate Cyclase Activity Research Articles

Extraction of the adenylate cyclase-activating factor of bovine sperm and its identification as a trypsin-like protease.

We previously reported the activation of adenylate cyclases from rat brain (Johnson, R. A., Awad, J. A., Jakobs, K. H., and Schultz, G., (1983) FEBS Lett. 152, 11-16) and from human platelets (Jakobs, K. H., Johnson, R. A., and Schultz, G. (1983) Biochim. Biophys. Acta 756, 369-375) by a factor derived from bovine sperm. In this report we describe the conditions for the extraction of the factor from bovine sperm and characteristics of its effects on adenylate cyclase which are consistent with its being a protease. The activating capacity of sperm particles was extracted from previously washed and frozen sperm into a 30,000 X g supernatant fraction by various salts, but not by the nonionic detergent Lubrol-PX. The amount of extracted factor: (a) was greatest with NH4HCO3 greater than NaCl greater than Na acetate; (b) was optimal with 0.5 M salt; (c) was not appreciably affected by the pH of the extraction buffer between pH 5.0 and 8.5; and (d) exhibited the greatest specific activity at the lower pH. The extracted sperm factor could be concentrated without loss by ultrafiltration on Amicon PM-10 membranes. The effect on adenylate cyclase of concentrated and desalted sperm extracted was inhibited 50% by various salts at 10 to 30 mM. The effects of the sperm factor to activate platelet adenylate cyclase, to block its inhibition via the alpha-adrenoceptor, and to block inhibition of stimulated forms of the enzyme by stable guanine nucleotides were prevented by protease inhibitors. A 50% reduction in the sperm factor's activation of platelet adenylate cyclase was caused by 30 nM soybean trypsin inhibitor, 30 nM alpha 2-macroglobulin, 300 nM leupeptin, 1 microM antipain, 15 microM aprotinin, and 100 microM benzamidine. Up to 3 mM phenylmethanesulfonyl fluoride was without effect on activation of the platelet cyclase by the sperm factor. The effects of the sperm factor persisted after its removal by the washing of pretreated platelet membranes and after its inactivation by the subsequent addition of leupeptin. The data strongly support the conclusion that the bovine sperm factor is a trypsin-like protease. alpha-Chymotrypsin, trypsin, and sperm acrosin were comparably effective in stimulating the platelet adenylate cyclase 5- to 8-fold, with concentrations eliciting maximal stimulation being: 200 ng trypsin/ml; 2 micrograms alpha-chymotrypsin/ml; and 2 micrograms acrosin/ml.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced signal transduction by adenylate cyclase in platelet membranes of patients showing antidepressant responses to alprazolam: Preliminary data

The triazolobenzodiazepine, alprazolam, was administered to 11 depressed patients over a period of six weeks, and six patients showed a favorable antidepressant response. There were no significant differences between responders and nonresponders in age, pretreatment Hamilton Depression Rating Scores, 4 p.m. postdexamethasone plasma cortisol levels, or platelet monoamine oxidase activities. Blood levels of alprazolam were not meaningfully different in responders and nonresponders when measured on treatment day 8. However, on treatment day 8, significantly enhanced prostaglandin D 2-stimulated platelet adenylate cyclase activity, greater suppression of prostaglandin D 2-stimulated platelet adenylate cyclase activity by epinephrine, and enhanced sodium fluoride-stimulated platelet adenylate cyclase activity were seen in the six patients who went on to respond to alprazolam, but not in the five nonresponders. In contrast, there were no significant changes in prostaglandin D 2, (-)-isoproterenol, or fluoride ion-stimulated leukocyte adenylate cyclase activity in responders or nonresponders. No meaningful changes were observed in the mean densities of either the high-affinity platelet α 2-adrenergic receptor (for 3H-p-aminoclonidine) or the leukocyte β-adrenergic receptor (for 3H-dihydroalprenolol) in responders or nonresponders. The present findings, taken in conjunction with findings from other recent studies, suggest that enhanced coupling between certain neurotransmitter or hormone receptors and adenylate cyclase through the guanine nucleotide regulatory proteins may help explain the antidepressant effects of alprazolam and possibly other forms of antidepressant treatment.

ADP-dependent common receptor mechanism for binding of von Willebrand factor and fibrinogen to human platelets.

Human von Willebrand factor (vWF) and fibrinogen are adhesive plasma glycoproteins essential for formation of a platelet hemostatic plug. We investigated the role of ADP and fibrinogen in binding of vWF to platelets in vitro. Binding of 125I-labeled vWF to human platelets separated from plasma proteins and treated with ADP was specific, and time and concentration dependent, reaching equilibrium at 20 min and approaching saturation at 12 micrograms/ml. The binding was inhibited by EDTA and by prostaglandin I2, a known activator of platelet adenylate cyclase. A purine nucleotide affinity analog, 5'-p-fluorosulfonylbenzoyl adenosine (FSBA), which covalently modifies the ADP binding sites on the human platelet membrane, prevented binding of vWF induced with ADP, as well as with human thrombin and with ionophore A23187, agents known to cause platelet ADP secretion. By comparison, FSBA did not inhibit binding of vWF induced by ristocetin, indicating that the ristocetin mechanism is not dependent on ADP. Human fibrinogen inhibited in a competitive manner the ADP-induced binding of 125I-labeled vWF (9 micrograms/ml) with an IC50 of 25 micrograms/ml. Conversely, unlabeled vWF inhibited ADP-induced binding of 125I-labeled fibrinogen (60 micrograms/ml) with an IC50 of 16 micrograms/ml. A synthetic dodecapeptide (Mr, 1188), analogous with the specific platelet receptor recognition site of human fibrinogen gamma chain (gamma 400-411), inhibited binding of both 125I-labeled vWF and 125I-labeled fibrinogen to ADP-treated platelets, whereas it was without effect on binding of 125I-labeled vWF to ristocetin-treated platelets. These data indicate that vWF and fibrinogen have a common receptor mechanism for their interaction with human platelets that is dependent on ADP occupancy of its binding sites and is recognized by the sequence of 12 amino acid residues at the carboxyl terminus of the human fibrinogen gamma chain.

The inhibitory guanine nucleotide-binding regulatory component of adenylate cyclase. Properties and function of the purified protein.

Treatment of membranes with islet activating protein (IAP), a toxin from Bordetella pertussis, results in abolition of GTP-dependent, receptor-mediated inhibition of adenylate cyclase. This appears to result from IAP-catalyzed ADP-ribosylation of a 41,000-Da membrane-bound protein. A protein with 41,000- and 35,000-Da subunits has been purified from rabbit liver membranes as the predominant substrate for IAP. This protein has now been shown to be capable of regulating membrane-bound adenylate cyclase activity of human platelets under various conditions. The characteristics of the actions of the IAP substrate are as follows. 1) Purified 41,000/35,000-Da dimer is capable of restoring the inhibitory effects of guanine nucleotides and the alpha 2-adrenergic agonist, epinephrine, on the adenylate cyclase activity of IAP-treated membranes. 2) The subunits of the dimer dissociate in the presence of guanine nucleotide analogs or A1(3+), Mg2+, and F-. The 41,000-Da subunit has a high affinity binding site for guanine nucleotides. 3) The resolved 35,000-Da subunit of the dimer mimics guanine nucleotide- and epinephrine-induced inhibition of adenylate cyclase. 4) The resolved (unliganded) 41,000-Da subunit stimulates adenylate cyclase activity and relieves guanine nucleotide- +/- epinephrine-induced inhibition of the enzyme. In contrast, the GTP gamma S-bound form of the 41,000-Da subunit inhibits adenylate cyclase activity, although with lower apparent affinity than does the 35,000-Da subunit. 5) The 35,000-Da subunit increases the rate of deactivation of Gs, the stimulatory regulatory protein of adenylate cyclase. In contrast, the 41,000-Da subunit can interact with Gs and inhibit its deactivation. These data strongly suggest that the IAP substrate is another dimeric, guanine nucleotide-binding regulatory protein and that it is responsible for inhibitory modulation of adenylate cyclase activity.

SQ 22536, an Adenylate-Cyclase Inhibitor, Prevents the Antiplatelet Effect of Dazoxiben, a Thromboxane-Synthetase Inhibitor

This study shows that dazoxiben, a selective inhibitor of thromboxane A2-synthetase in human platelets, inhibited arachidonic acid-induced platelet aggregation in platelet-rich plasma samples from four out of 16 healthy volunteers. In these four "responder" samples, the anti-aggregating effect of dazoxiben was prevented by the compound SQ 22536, a 9-substituted adenine analogue, endowed with an inhibitory activity on adenylate-cyclase. The compound SQ 22536 also counteracted the antiaggregating effect of prostaglandin D2, a known activator of platelet adenylate-cyclase. When platelet thromboxane A2-synthetase was blocked by dazoxiben, a marked increase of prostaglandin D2 was concomitantly observed both in "responder" and "non responder" samples. The compound SQ 22536 blunted the increase in platelet cAMP caused by either dazoxiben and sodium arachidonate or prostaglandin D2. It is suggested that the antiaggregating effect of dazoxiben is mediated by newly synthesized prostaglandin D2. The latter acts by stimulating adenylate-cyclase and increasing cAMP levels. The compound SQ 22536 prevents both phenomena. In "non responder" samples some factors--still to be defined--might counteract similarly to the compound SQ 22536 the antiaggregating activity of PGD2.

Platelet enzyme activities in diabetes mellitus in relation to endothelial damage.

Increased platelet reactivity has been suggested in the pathogenesis of both arteriosclerosis and diabetic microangiopathy. Therefore, platelet function and platelet enzyme activities were assessed in a large group of 357 diabetics (256 patients with IDDM, aged 16-49 and 101 patients with NIDDM, aged 50-78) and 163 matched controls, and related to photographically documented retinopathy (Rd) and to peripheral vascular disease (PVD) as well as to plasma levels of von Willebrand factor (VIII R:Ag) as an indicator of endothelial damage. Patients with IDDM had increased platelet aggregation (PA, expressed as microM ADP threshold concentration) before Rd was detectable in comparison to control subjects (P less than 0.01). PA was further increased in patients with advanced Rd (P less than 0.01), whereas 20 newly diagnosed diabetics with IDDM exhibited normal PA. Patients with minimal Rd did not differ from patients without Rd. Plasma beta-thromboglobulin (reflecting platelet consumption in vivo) was enhanced significantly in patients with Rd only (P less than 0.05), as was malondialdehyde (MDA) production of platelets (as a measure of platelet endoperoxide formation). Factor VIII-related antigen in plasma was already increased in patients without Rd (P less than 0.05), yet more so in patients with Rd (P less than 0.01). Prostacyclin-stimulated adenylate cyclase activity (ACA) of platelets (as an antiaggregatory enzyme system) was twice as high in diabetics with advanced Rd compared with patients without Rd and with controls (P less than 0.01). Significant correlations were found between PA and plasma F VIII R: Hg, MDA production, and ACA of platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of ristocetin and factor VIII/von Willebrand factor protein on human platelet adenylate cyclase activity

Inhibitory effect of ristocetin and factor VIII/von Willebrand factor protein on human platelet adenylate cyclase activity

Activation of human platelet adenylate cyclase by a bovine sperm component

The interaction of bovine sperm particles and human platelet membranes was studied with regard to adenylate cyclase activity. When sperm and platelet membrane preparations were combined, greater than additive adenylate cyclase activity was measured. Data obtained after inactivation of the sperm enzyme indicated that the observed increase in adenylate cyclase activity was not due to an activation of the sperm cyclase but that a sperm component, which could be extracted from the sperm particles, activated the platelet adenylate cyclase. Platelet cyclase activation by the sperm particles was a saturable and time-dependent process. The extent of activation was particularly high (up to 10-fold) in the presence of a stable GTP analog. Under this condition, the apparent affinity of the platelet enzyme for Mg 2+ was increased by about one order of magnitude, whereas with Mn 2+ only a minor effect of the sperm particles was observed. Platelet adenylate cyclase stimulation by prostaglandin E 1 (up to 20-fold) was more than doubled by the sperm particles, whereas the inhibition of the platelet enzyme by epinephrine was abolished. The sperm factor was trypsin-sensitive and was inactivated by boiling. The data indicate that bovine sperm particles contain an extractable protein, which activates platelet adenylate cyclase probably by inactivating an inhibitory site.

The activation of human platelet adenylate cyclase by vanadate

The activation of human platelet adenylate cyclase by vanadate

Studies on aspirin derivatives with very little side effect. IV. Inhibitory effect of aspirin-isopropylantipyrine (AIA) on several experimental thromboses.

The effect of a new aspirin derivative, aspirin-isopropylantipyrine (AIA), with potent platelet anti-aggregant activity, on several experimental thromboses was evaluated and compared with that of aspirin. AIA (50 mg/kg, s.c.) as well as aspirin (50 mg/kg, s.c.) significantly inhibited thrombus formation in extracorporeal shunt model of rats. AIA (50 mg/kg, s.c.) significantly shortened the duration of apnea and respiratory distress induced by a rapid injection of adenosine 5'-diphosphate in rats, while aspirin (50 mg/kg, s.c.) did not. Inhibitory effect of AIA (50 mg/kg, s.c.) on arachidonic acid-induced mortality in mice was less than that of aspirin (50 mg/kg, s.c.). AIA and aspirin (10 mg/kg/d X 10, s.c.) had no effect on laurate-induced arterial occlusive disease in rats. AIA (200 microM) showed weak and reversible inhibition of prostaglandin I2 generation in isolated rat aorta strip, while aspirin (200 microM) showed irreversible inhibition. AIA (50 and 200 microM inhibited Ca2+-, K+- or norepinephrine-induced contraction on isolated rat aorta strip. AIA (200 microM) had no effect on malondialdehyde formation, cyclic AMP level and adenylate cyclase activity in rat platelets. AIA (100 microM) inhibited arachidonic acid-induced contraction on rat fundus strip by about 50%, while aspirin (100 microM) did not. These results strongly suggest that anti-thrombotic activity of AIA was originated at least from its anti-aggregant effect on platelets, differing from aspirin.

Evidence that changes in platelet cyclic AMP levels regulate the fibrinogen receptor on human platelets.

Fibrinogen binds to human platelets after specific receptor sites are exposed by thrombin, ADP, epinephrine, and other stimuli. Since prostaglandin I2 (PGI2), a potent activator of platelet adenylate cyclase, prevents mobilization of the fibrinogen receptor by aggregating agents, we investigated the relationship between platelet cAMP levels and fibrinogen receptor status in thrombin-stimulated human platelets. A dose-dependent rise in platelet cAMP in response to two adenylate cyclase agonists, PGI2 and forskolin, correlated with progressive inhibition of fibrinogen binding. Moreover, the receptor inhibition produced by either agonist was sustained up to 2 h and was associated with a persistent increase in cAMP levels. The phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine, in the presence of a subthreshold concentration of PGI2 also raised cAMP and inhibited fibrinogen binding. In contrast, the effects of PGI2 on both cAMP and fibrinogen binding were markedly attenuated by 9-(tetrahydro-2-furyl) adenine, an adenylate cyclase inhibitor. These results indicate that the inhibition of fibrinogen binding by PgI2 is linked to its effect on cAMP levels and suggest that elevation of platelet cAMP levels from any cause prevents exposure of the fibrinogen receptor.

Normal adenylate-cyclase activity in platelets of patients with Huntington's disease

Adenylate-cyclase activity and cyclic-AMP concentration in platelets of 7 Huntington's disease patients were found to be similar to control values.

Adenylate cyclase activity and cAMP content of human platelets in uraemia.

We investigated the effects of uraemia and haemodialysis on the basal activity of adenylate cyclase and the cyclic-AMP content of human platelets in patients with end-stage renal insufficiency, patients receiving maintenance haemodialysis, and as controls healthy voluntary subjects. Basal adenylate cyclase activity in terminal renal disease (creatinine clearance less than 15 ml/min/1.73 m2) was 824 +/- SEM 57, in comparison to the healthy subjects with 453 +/-SEM 28 (P less than 0.001). We also found significant elevation (P less than 0 . 001) of platelet cAMP levels as compared to the controls. Basal adenylate cyclase activity and platelet cAMP levels were approximately normal in the dialysed patients. These results show that uraemic toxins adversely affect the platelet AC-cAMP system, possibly causing impaired platelet aggregation and the bleeding diathesis of uraemia.

Prostaglandin E1 action on adenylate cyclase activity and membrane lipids in platelets.

The effect of prostaglandin E1 (PGE1) on temperature-induced membrane lipid phase transitions of human blood platelets was investigated using excimer forming 1-pyrenedecanoic acid as fluorescent probe. PGE1 and PGF2 alpha at 10 microM concentrations had a small but reproducible effect on the melting curves between 20 degrees and 30 degrees C and lowered the temperature for the onset of the phase transition from 26 degrees to 21 degrees C. The adenylate cyclase of the platelet membrane was half maximally stimulated by PGE1 at 0.8 microM and maximally activated at 10 microM concentrations. PGF2 alpha at 100 microM concentrations produced only 40% of the maximal stimulation achieved with PGE1. Stimulation of adenylate cyclase by 10 microM PGE1 increased steeply with temperature between 20 degrees to 30 degrees C. Binding experiments with [3H]PGE1 indicated a high affinity and a low affinity site with KD 13nM and 2 microM, respectively. Non-radioactive PGE1 inhibited binding of [3H]PGE1 to both sites at concentrations corresponding to their respective dissociation constants. PGF2 alpha inhibited binding of PGE1 only at greater than or equal to 1 microM concentrations. It is assumed that only the high affinity binding sites represent specific PGE1 receptors. Binding of PGE1 to the low affinity sites and the observed effects on membrane structure may result from interactions of PGE1 with membrane lipids. Since these latter effects and also stimulation of adenylate cyclase required similar concentrations, it is proposed that local interactions of PGE1 with membrane lipids are a prerequisite for specific receptor-mediated adenylate cyclase activation.

Vasoactive intestinal polypeptide modulation of lymphocyte adenylate cyclase.

Vasoactive intestinal polypeptide (VIP) was found to be a potent stimulator of lymphocyte adenylate cyclase activity. VIP-induced activation of adenylate cyclase was specific for lymphocytes among peripheral blood cells; i.e., VIP did not stimulate the adenylate cyclase activity of neutrophils, monocytes, or platelets. The VIP-induced activation of lymphocyte adenylate cyclase was time, temperature, and concentration dependent. VIP and the GTP analog, Gpp(NH)p, acted synergistically to stimulate lymphocyte adenylate cyclase; stimulation by VIP and PGE1 was additive; and VIP activation was antagonized by somatostatin. VIP-mediated activation of lymphocyte adenylate cyclase was observed in normal human T cells, B cells obtained from a patient with chronic lymphocytic leukemia, and a human T cell culture line. The Raji human B cell culture line did possess adenylate cyclase activity, but this activity was not stimulated by VIP. These results suggest that lymphocytes possess functional receptors for VIP and that this peptide may play a role in modulation of lymphocyte function.

Inhibitory effects of sodium and other monovalent cations on human platelet adenylate cyclase.

Both basal and prostaglandin E1 (PGE1)-stimulated human platelet adenylate cyclase activity are inhibited by Na+ and other monovalent cations. Inhibition occurs when the cations are present in concentrations of 20 to 120 mM. Inhibition of basal activity by epinephrine requires the presence of GTP but not Na+. In the presence of epinephrine and GTP, Na+ further reduces basal activity, and the combined effects of all three ligands results in a 70% reduction of basal activity. Sodium, however, does not increase the fractional inhibition of basal activity attributable to epinephrine. Sodium increases the concentrations of epinephrine and of PGE1 required for half-maximal inhibition and stimulation of adenylate cyclase but does not alter the apparent km for Mg2+ or ATP. The rate and extent of adenylate cyclase activation by the GTP analog guanyl-5'-yl imidodiphosphate is reduced by Na+, although the cation does not prevent the acceleration of activation by guanyl-5'-yl imidodiphosphate induced by PGE1. Other monovalent cations also inhibit the platelet cyclase with an order of potency of Na+ greater than Li+ greater than K+ greater than choline+. In addition to demonstrating that Na+ is not required for hormonal inhibition of platelet adenylate cyclase, these studies demonstrate the multiple inhibitory effects which can be induced by Na+ and other monovalent cations. The pattern of these effects suggests that the cations interfere with functional coupling between receptors, guanine nucleotide binding units, and catalytic units of the adenylate cyclase complex.

Do human platelets have opiate receptors?

In their study of prostaglandin E1 (PGE1)-sensitive adenylate cyclase (AC) in rat brain homogenates, Collier and Roy claimed that the activity of this enzyme is inhibited by opiates. They also proposed that opiates exert their analgesic and allied effects by inhibiting AC of neurones that are normally stimulated by E prostaglandins. Studies using neuroblastoma x glioma hybrid cells supported this hypothesis. However, subsequent studies with mammalian brain and rat brain tissue slices yielded conflicting results. PGE1 also inhibits platelet aggregation, probably through activation of platelet AC. Gryglewski et al. showed that morphine inhibits the anti-aggregating effect of PGE1 on ADP- and adrenaline-induced platelet aggregation, and suggested that the inhibition by morphine is mediated through platelet AC activity. We report here our attempts to reproduce the results of Gryglewski et al. and our examination of the effect of morphine on PGE1-sensitive AC activity in platelet lysates and on PGE1-induced accumulation of cyclic AMP in intact platelets. The possible existence of opiate receptors in platelets was also assessed by direct binding studies with 3H-etorphine. In contrast to Gryglewski et al., we could not detect any effect of opiates on the aggregation of human platelets, nor did we find any other evidence supporting the presence of opiate receptors in these cells. Thus we conclude that the presence of opiate receptors in human platelets is unlikely.

Effects of Halothane on the Cyclic 3???,5???-Adenosine Monophosphate Enzyme System in Human Platelets

A study of the effects of halothane on the cyclic 3',5'-adenosine monophosphate (cAMP) system in human platelets was undertaken since cAMP has been implicated in the regulation of the process of platelet aggregation and this anesthetic has been reported to decrease platelet aggregation and, in other tissues, to increase adenylate cyclase activity. When exposed to halothane 0.5 to 10 vol%, adenylate cyclase activity was increased in the platelet preparation in a dose-dependent manner, reaching a maximum at 5 vol% (93% increase above basal activity). Platelet aggregation was also inhibited by halothane in a dose-dependent manner, with a maximum effect at about 5 vol% halothane (a decrease of 70%). Kinetic analysis of platelet cAMP-phosphodiesterase suggested two forms of activity, neither of which was altered by halothone. The results that the impairment of platelet aggregation observed with halothane may be brought about by the halothane-induced activation of platelet adenylate cyclase, which may result in a higher cAMP level, inhibiting platelet aggregation.

Effect of acute methadone withdrawal on prostaglandin E-stimulated 3H-cyclic adenosine monophosphate accumulation in human platelets

The effect of acute methadone withdrawal was studied on PGE1-sensitive platelet adenylate cyclase of 8 former heroin addicts who had been stabilized with methadone. During acute methadone withdrawal there was a significant increase in PGE1-sensitive platelet adenylate cyclase activity, which correlated with the severity of withdrawal symptoms. This suggests that PGE1-sensitive adenylate cyclase and cyclic adenosine monophosphate (cyclic AMP) are involved in the human addictive process.

Inhibition of platelet adenylate cyclase by ADP

The effect of adenosine 5′-diphosphate (ADP) was studied on adenylate cyclase [ATP:pyrophosphate lyase (cyclizing), EC 4.6. 1.1] activity in cell-free preparations of human platelets with adenylyl 5′-imidodiphosphate [AMP-P(NH)P] as enzyme substrate. ADP reduced the enzyme activity in platelet particles by maximally about 50 % without apparent lag phase. The inhibition by ADP was half-maximal at about 3 μM and maximal at about 100 to 300 μM. The inhibition was apparently not due to a competition by ADP or by possibly formed ATP with AMP-P(NH)P for the adenylate cyclase substrate binding site and also not due to the formation of adenosine, which can reduce platelet enzyme activity. ADP reduced the adenylate cyclase activity only in the additional presence of GTP (0.1 to 100 μM), which increased basal and prostaglandin E 1-stimulated activities by about 2- to 3-fold when AMP-P(NH)P was used as substrate. In the presence of cholera toxin, which in a GTP-dependent manner increased platelet adenylate cyclase activity, ADP largely counteracted the GTP-induced activation. However, pretreatment of the enzyme with the stable GTP analogue, guanosine 5′-O-(3-thiotriphosphate) (GTP-γ-S), which caused a persistent increase in adenylate cyclase activity, abolished the inhibitory effect of ADP. The data suggest that ADP inhibits platelet adenylate cyclase by a mechanism similar to that described for various hormonal factors causing adenylate cyclase inhibition in many different systems.