Tissue Cyclic AMP Research Articles

Direct inhibition of chicken gizzard smooth muscle contractile apparatus by caffeine

The mechanism of the inhibitory effect of caffeine on smooth muscle contraction was examined using chicken gizzard. Caffeine (0.1-5 mmol/l) inhibited the KCl-induced contraction of the muscle with an IC50 of 1.1 mmol/l. Forskolin (0.01-10 mumol/l) also inhibited KCl-induced contraction. The inhibitory effect of caffeine was potentiated by a low concentration of forskolin (0.3 mumol/l) and the inhibitory effect of forskolin was potentiated by a low concentration of caffeine (0.1 mmol/l). Although caffeine and forskolin increased tissue cyclic AMP levels, caffeine inhibited the KCl-induced contraction more strongly than forskolin at a given cyclic AMP level. Caffeine (1-40 mmol/l) inhibited the contractions induced by 3 mumol/l Ca2+ in Triton X-100-permeabilized muscle. Caffeine (1-40 mmol/l) inhibited the phosphorylation of 20 kDa myosin light chain (MLC) in native actomyosin preparation and the inhibition was enhanced by decreasing the ATP concentration in the reaction medium. Calmodulin (CaM) activity, as monitored by Ca2+/CaM-dependent erythrocyte membrane (Ca2+ + Mg2+)-ATPase, was not affected by 20 mmol/l caffeine. Time-dependent dephosphorylation of MLC upon removal of Ca2+, an indicator of phosphate activity, was not affected by caffeine. Caffeine also inhibited the Ca2(+)-independent contraction in thiophosphorylated permeabilized muscle. These results indicate that caffeine inhibits smooth muscle contraction by a direct inhibition of MLC kinase and actin-myosin interaction. A part of the inhibitory effect may be mediated by cyclic AMP-dependent mechanism(s).

Human Leukocyte β‐Adrenergic Stimulated Cyclic AMP in Ethanol Intoxication and Withdrawal

The effects of ethanol were examined on cyclic AMP production in intact human leukocytes in vitro. In leukocytes isolated from alcohol abstinent subjects, ethanol enhances isoproterenol-stimulated and prostaglandin E1-stimulated cyclic AMP formation. Without agonist, ethanol is able to stimulate cyclic AMP production at concentrations of 160 mM and above. Leukocytes isolated from subjects undergoing alcohol detoxification showed increases in basal levels of leukocyte cyclic AMP, in isoproterenol-stimulated cyclic AMP, and in ethanol-stimulated cyclic AMP on the 2nd or 3rd day of withdrawal in comparison to the first day of withdrawal and in comparison to abstinent subjects. The data suggest that increased beta-adrenergic activity seen in alcohol withdrawal may be in part due to an increased responsiveness of beta-adrenergic stimulated cyclic AMP in tissues.

Possible mechanisms of spasmolytic action of bile salts on the isolated guinea-pig gallbladder.

The spasmolytic action of bile salts on gallbladder smooth muscle could explain the alleged relief of biliary colic seen during bile acid therapy. The mechanisms of spasmolytic action of bile salts, ursodeoxycholate and deoxycholate were studied in the isolated gallbladder of guinea-pigs. The bile salts accelerated the 45Ca-efflux from the gallbladder with synchronous relaxation and inhibited the cellular 45Ca-uptake by the depolarized muscle preparation. Further, they sensitively inhibited CaCl2-induced contraction of the depolarized muscle. The tissue cyclic AMP content of the gallbladder was significantly elevated by the bile salts. Dibutyryl cyclic AMP mimicked the effects of bile salts on the Ca-efflux and the muscle relaxation, but showed no effect on the cellular Ca-uptake. From these results, it is suggested that the bile salts produce the relaxant action through accelerating Ca-efflux, which is probably coupled with the elevation of the cellular cyclic AMP level, and through suppressing the Ca-influx across the cell membrane.

Regulation of force and intracellular calcium transients by cyclic AMP generated by forskolin, MDL 17,043 and isoprenaline, and its modulation by muscarinic receptor agents: a novel mechanism for accentuated antagonism

The relation of changes in intracellular calcium transients and force of isometric contractions in response to an elevation or reduction of cyclic AMP levels was investigated in isolated dog ventricular trabeculae and rabbit papillary muscles, in which multiple superficial cells have been microinjected with the calcium sensitive bioluminescent protein aequorin. Forskolin, MDL 17,043 and isoprenaline elevated the tissue cyclic AMP level, increased consistently the peak aequorin signals and force, and abbreviated the duration of both signals in a concentration-dependent manner. When the effect of isoprenaline was compared with that of alteration of extracellular calcium concentration [( Ca2+]0), the increase in force by isoprenaline was associated with higher peak aequorin signals than that by alteration of [Ca2+]0 for a given increase in force, indicating the decrease in calcium sensitivity of myofibrils by cyclic AMP generated by beta-adrenoceptor stimulation. Carbachol, which did not affect significantly the basal force and cyclic AMP levels, lowered the cyclic AMP levels elevated previously by forskolin, MDL 17,043 or isoprenaline in the isolated dog ventricular trabeculae. It antagonized the increase in peak aequorin signals and force caused by these agents in a concentration-dependent manner. When carbachol had been administered prior to isoprenaline and the concentration-response curve for isoprenaline was determined in the presence of carbachol, the relation of force peak aequorin signals was not modified by carbachol in the rabbit papillary muscle. Carbachol, when administered during induction of the positive inotropic action by forskolin, MDL 17,043 and isoprenaline, decreased the force more than peak aequorin signals in a concentration-dependent manner in the dog ventricular trabeculae. Therefore, the relation of force to peak aequorin signals was shifted downwards during the carbachol-induced inhibition, indicating a further decrease of calcium sensitivity of myofibrils by carbachol. This effect of carbachol appears to be specific to the cyclic AMP-mediated positive inotropic action, since the alpha-adrenoceptor-mediated (cyclic AMP-independent) action was unaffected by carbachol. This mechanism may play an important role for "accentuated antagonism" in the mammalian ventricular myocardium.

Pacemaker activity is modulated by tissue levels of cyclic adenosine 3′,5′-monophosphate in human atrial fibers

We studied the role of tissue cyclic AMP levels in the chronotropic effects of theophylline on automatic human atrial fibers obtained from the hearts of 17 patients undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and transmembrane action potentials were recorded with a conventional microelectrode technique. In normal Tyrode solution, theophylline (0.1–1 mM) often decreased the late diastolic slope and the spontaneous rate. In the presence of 0.3–1 μM epinephrine, however, theophylline dose-dependently increased the diastolic slope, the rate of spontaneous discharges and the force of contraction. The increase in tissue level of cyclic AMP (+288 ± 69%) induced by 0.3 mM theophylline in the presence of epinephrine was much greater than the increase (+73 ± 19%) in the absence of epinephrine. It is concluded that pacemaker activity in human atrial fibers is modulated by tissue levels of cyclic AMP and theophylline may induce atrial tachycardia through an increase in the diastolic slope and the rate of discharges of automatic atrial fibers.

Cyclic-AMP in human lung preparations

Cyclic-AMP levels in unaffected and affected cancerous portions of human lung preparations were determined by measuring displaced 3H-cyclic-AMP from the specific binding protein by unlabeled ligand. The levels of cyclic-AMP in unaffected portions of human lungs ranged between 52–116 pmoles/g protein. Significantly higher levels of cyclic-AMP were found in lung samples affected with squamous cell carcinoma, adenocarcinoma and malignant melanoma with the mean values being 251, 290, and 509 pmoles/g protein, respectively. On the other hand, a decrease in the level of cyclic-AMP to 39 pmoles/g protein was observed in portions of lungs affected with granuloma. These results suggest that the level of cyclic-AMP in lung tissue may reflect the malignant or benign nature of the pulmonary disease.

Further characterization of the 5‐HT receptor mediating vascular relaxation and elevation of cyclic AMP in porcine isolated vena cava

1. 5-Hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) produce both smooth muscle relaxation and elevation of tissue adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels in isolated rings of neonatal porcine vena cava. We now present studies attempting to characterize in more detail the 5-HT receptor mediating these responses. 2. Both 5-HT and 5-CT relaxed porcine isolated vena cava rings (EC50 values 200 nM and 4 nM respectively) and elevated tissue cyclic AMP levels (EC50 values 1500 nM and 16 nM respectively). For both responses 5-CT was approximately 50-100 fold more potent than 5-HT. 3. Both 5-CT-induced smooth muscle relaxation and cyclic AMP elevation were potently and specifically antagonized to a similar extent by methiothepin, methysergide and spiperone. 4. At concentrations up to 1 microM, 8-hydroxy-2-(di-n-propylamino) tetralin, buspirone, ipsapirone, n,n-dipropyl-5-CT, cyanopindolol, RU24969, ketanserin, GR38032 and GR43175 were devoid of both agonist and antagonist activity for both responses. 5. These findings suggest that the same 5-HT1-like receptor mediates both smooth muscle relaxation and elevation of cyclic AMP. This receptor is unlike any known 5-HT1 ligand binding site or adenylate cyclase-coupled 5-HT receptor in brain tissues.

Inotropic responses to isoproterenol and phosphodiesterase inhibitors in intact guinea pig hearts: comparison of cyclic AMP levels and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins.

The influence of selective (milrinone: 10, 50, 100 microM) and nonselective phosphodiesterase (isobutylmethylxanthine: 0.1, 10, 100 microM) inhibitors and beta-adrenergic stimulation (isoproterenol: 0.01, 0.1 microM) on phospholamban and myofibrillar protein phosphorylation was studied in guinea pig hearts perfused with [32P]orthophosphate. Changes in protein phosphorylation were compared to alterations in tissue cyclic AMP (cAMP) levels and positive inotropic effects induced by these agents. Isoproterenol (0.01 microM), milrinone (50 microM), and isobutylmethylxanthine (100 microM) all produced similar, twofold increases in dP/dt and -dP/dt but only stimulation with isobutylmethylxanthine and isoproterenol was associated with significant increases in phospholamban phosphorylation. At these equipotent doses, the effects of isobutylmethylxanthine were associated with higher increases (3.1-fold) in cAMP than those observed with isoproterenol (twofold). Milrinone (50 microM) produced a 2.5-fold increase in cAMP levels but failed to change phospholamban phosphorylation. Higher doses of milrinone (100 microM) resulted in relatively high (4.1-fold) cAMP levels, and this was associated with increased (1.5-fold) phosphorylation of phospholamban. Phosphorylation of troponin I was significantly increased at 0.01 microM and 0.1 microM isoproterenol, while phosphorylation of C protein was observed only at 0.1 microM isoproterenol. Isobutylmethylxanthine and milrinone did not significantly increase phosphorylation of either troponin I or C protein at any of the doses studied. These findings indicate that cardiotonic agents acting via the cAMP pathway may produce similar inotropic responses at different levels of cAMP and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins.

Modification of the positive inotropic effects of catecholamines, cardiac glycosides and Ca 2+ by the orally active male contraceptive, gossypol, in isolated guinea-pig heart

Gossypol is an orally active male contraceptive with cardio-depressant side effects. To understand the mechanism of its cardiac actions, the interaction of gossypol with positive inotropic grugs was examined in isolated atrial muscle preparations obtained from guinea- pig heart. Gossypol delayed the onset of arrhythmias caused by digoxin. In the presence of gossypol, the positive inotropic effect of isoproterenol declined rapidly, and the effect of isoproterenol to increase tissue cyclic AMP concentrations was smaller. Pretreatment of atrial muscle with the combination of gossypol and isoproterenol markedly reduced effects of isoproterenol on developed tension and cyclic AMP concentrations when these effects were tested after the washout of the first dose of isoproterenol. These effects, however, were not specific to isoproterenol. The gossypol-isoproterenol pretreatment reduced the positive inotropic effect of ouabain or extracellular Ca 2+ These results indicate that gossypol has pharmacodynamic interactions with several positive inotropic agents that are known to enhance developed tension by increasing intracellular Ca 2+ transients.

Renin release responses to in vitro prostaglandin challenge in the summer-active ground squirrel, Spermophilus lateralis

1. 1. Using a renal cortical slice preparation from the summer-active ground squirrel, Spermophilus lateralis, resting renin release (RR) and tissue cyclic AMP content (TcAMPC) levels were found to be significantly higher and lower, respectively, than those previously reported for its hibernating counterpart. 2. 2. At a 10 −5 M dose, PGE2 but not PGE1, PGF2-alpha or PGA1, significantly stimulated RR in the summer-active ground squirrel (SAGS). 3. 3. Addition of agents which normally increase TcAMPC significantly potentiated the effect of PGE1, while preventing that of PGE2, on RR and TcAMPC. 4. 4. Opposite TcAMPC changes may mediate the in vitro RR responses to PGE1 and PGE2 administration in the SAGS.

"In vitro" study on release of cyclic AMP and thyroid hormone in autonomously functioning thyroid nodules.

The TSH effect on slice and the incubation medium cyclic AMP levels and T3 and T4 released from 8 autonomously functioning thyroid nodules (AFTN) and their respective perinodular (PN) tissues were examined. The thyroid slices were incubated in Eagle's Medium containing TSH (5 to 100 mU/ml) for 60 min and 300 min for tissue cyclic AMP generation and for cyclic AMP, T3 and T4 release, respectively. Basal cyclic AMP levels were not different either in AFTN and in PN slices or into the incubation medium. In both tissues TSH produced a similar cyclic AMP generation. In contrast, cyclic AMP released into the incubation medium was significantly higher in AFTN than in PN tissues, after TSH stimulation. Basal T3 values and TSH-stimulated T3 release in AFTN were not different from PN tissue. However, basal T4 levels were significantly higher in AFTN than in PN tissue as well as T4 released in response to TSH. In addition, T3/T4 ratio was lower in AFTN than in PN tissues. The cyclic AMP released into the incubation medium correlated with both T3 and T4 release in PN tissue but in the AFTN tissue no correlations were found. These findings suggest that the adenylate cyclase-cyclic AMP system is more sensitive to TSH-stimulation in AFTN when compared with PN tissue and that AFTN tissue has a preferential T4 secretion.

Positive inotropic effects induced by carbachol in rat atria treated with islet-activating protein (IAP)--association with phosphatidylinositol breakdown.

1. To elucidate the functional consequences of phosphatidyl inositol (PI) breakdown produced by activation of the muscarinic receptor of the atrial muscle, and to clarify the subtypes of the muscarinic receptor involved, the effects of muscarinic agonists and antagonists on mechanical function were studied in atria isolated from rats given intravenous islet-activating protein (IAP; 50 micrograms kg-1 body weight) 48-72 h before the experiments. 2. The negative chronotropic and inotropic actions of carbachol (CCh) were attenuated and positive inotropic effects (62.5 +/- 5.8% above basal level) were observed with 10(-5) -10(-3) M CCh. Oxotremorine did not produce positive inotropic effects even in doses as high as 3 x 10(-4) M High doses of carbachol produced positive chronotropic effects, although the effects were weak. 3. Propranolol (10(-7) M) did not modify the positive inotropic effect of carbachol observed in IAP-treated atria, nor was there any change in the tissue cyclic AMP levels after carbachol. 4. High doses (10(-5)-10(-3) M) of carbachol produced PI breakdown in the absence and presence of IAP. Oxotremorine (3 x 10(-4) M) did not produce PI breakdown. In the presence of oxotremorine, the positive inotropic effects and PI breakdown by carbachol were abolished. 5. The positive inotropic effect of carbachol was readily antagonized by atropine but pirenzepine and gallamine exhibited only weak antagonist effects. 6. These results suggest that a muscarinic agonist such as carbachol can produce a positive inotropic effect in IAP-treated atria, in association with PI breakdown, through activation of a muscarinic receptor which shows some similarity to that previously identified in smooth muscles.

Effects of stimulants of electrogenic ion transport on cyclic AMP and cyclic GMP levels in locust rectum

AbstractA variety of agents stimulate the chloride‐dependent short‐circuit current (Isc) across isolated locust recta. In this study, the role of cyclic nucleotides in mediating this stimulation was examined by measuring levels of cyclic AMP and cyclic GMP in tissues exposed to these agents. After bathing recta in saline, there is a decline in Isc, and this is accompanied by a fall in cellular levels of cyclic AMP and cyclic GMP. Stimulants of Isc across locust recta, corpus cardiacum (CC) extracts containing chloride transport‐stimulating hormone (CTSH) and forskolin, also elevate cellular levels of cyclic AMP. Between 5 and 10 min after addition of CC extract, there is an increase in cyclic AMP levels. During this same time period, the Isc rises at its maximal rate. The Isc remains elevated for at least another 20 min, even though tissue cyclic‐AMP levels decline rapidly by 15 min. These results suggest that stimulation of Isc is induced by a rise in cyclic AMP, but the cessation of stimulated active ion transport is controlled by other biochemical processes. In contrast to the relatively rapid rise in cyclic‐AMP levels, a 60‐min exposure to CC extract was required to elicit a rise in cyclic GMP, and therefore it may not be involved in the early stages of Isc stimulation.

Relaxant effects of forskolin on guinea pig tracheal smooth muscle.

We investigated the relaxant effects of forskolin, a diterpene derivative isolated from the roots of Coleus forskohlii, on guinea pig airway smooth muscle by measuring the isometric tension of tracheal smooth muscle in vitro and transcutaneous Po2 during the histamine inhalation test (HIT) in vivo. Forskolin (10(-9)-10(-5) M) caused dose-dependent relaxant effects on resting tone and on leukotriene C4 (10(-7) M)-, leukotriene D4 (10(-7) M)-, and carbachol (3 X 10(-6) M)-induced contraction of tracheal smooth muscle. Moreover, with propranolol pretreatment the relaxant effect of forskolin on tracheal smooth muscle did not change, whereas with the same pretreatment the relaxant effect of isoproterenol diminished. Forskolin (10(-8)-10(-6) M) raised tissue cyclic AMP levels dose-dependently in tracheal smooth muscle (6.7-359.9 pmol/mg protein). Forskolin (1 mg/kg) administered subcutaneously raised the respiratory threshold of (RT-histamine in the HIT. The determination of the RT-histamine by measuring tcPo2 was possible without anesthesia. These results suggest that forskolin relaxes airway smooth muscle in guinea pigs in vitro and in vivo by raising tissue cyclic AMP levels and that its actions are independent of beta-adrenoceptors.

Hormonal regulation of hepatic glycogenolysis in the toad, Xenopus laevis, is mediated by cyclic AMP and not Ca 2+

Hepatic glycogenolysis and glycogen phosphorylase a activity were stimulated by arginine vasotocin (AVT) in liver pieces from Xenopus laevis cultured in vitro. In each case, the EC 50 was about 1 n M. The increased rate of glycogenolysis brought about by either AVT or adrenaline was maintained for at least 6 hr and was unchanged when Ca 2+ salts were omitted from the medium or when 2.5 m M EGTA was added. Neither the Ca 2+ ionophore, A23187, nor the Ca 2+ channel blocker, verapamil, had any effect on the rate of glycogenolysis in the presence or the absence of either hormone. Tissue cyclic AMP levels were unchanged by addition of AVT alone but were doubled in the presence of AVT plus either of the phosphodiesterase inhibitors, isobutylmethylxanthine or RO20–1724. These findings suggest that hormones regulating hepatic glycogenolysis in X. laevis use cyclic AMP, and not Ca 2+, as an intracellular messenger. We would argue that cytosolic Ca 2+ may not have become involved in regulation of hepatic glycogenolysis until after the ancestors of present day amphibians separated from those of present day mammals.

Calcium- and cyclic AMP-dependent chloride secretion in human colonic epithelia.

Three stable epithelial cell lines (HCA-7, HCA-7-Col 1 and HCA-7-Col 3) all derived from the same human adenocarcinoma have been cultured on collagen-coated Millipore filters. These epithelial monolayers have been used to record short circuit current (SCC) in response to of secretagogues. Similar monolayers, but grown on plastic dishes, were used for measurements of tissue cyclic AMP. Lysylbradykinin, applied to either side of the monolayers, increased SCC in HCA-7 cells but had little effect on the other two lines. The responses showed rapid desensitization, which could be prevented by cooling to 4 degrees C. Responses to kinin were not significantly attenuated by piroxicam, an inhibitor of cyclo-oxygenase. Other secretagogues, vasoactive intestinal polypeptide (VIP) and carbachol also increased SCC in monolayers. The responses to VIP were greatest in HCA-7-Col 1 monolayers while responses were virtually absent in HCA-7-Col 3. A similar profile was seen with carbachol except that responses of HCA-7 and HCA-7-Col 1 monolayers were more equal. With one exception the responses to VIP and carbachol showed sidedness, acting only from the basolateral side. The effects of the secretagogues were inhibited by piretanide, a loop diuretic, applied basolaterally. It is presumed that SCC responses represent electrogenic chloride secretion. Treatment with forskolin increased SCC in HCA-7 and HCA-7-Col 1 monolayers with little effect in HCA-7-Col 3. Nevertheless cyclic AMP levels were elevated most in HCA-7-Col 3 and least in HCA-7-Col 1 monolayers, in reciprocal relationship to the functional response. A23187 increased SCC when applied to HCA-7 and HCA-7-Col 3 monolayers with little effect on HCA-7-Col 1. The differential responses of the three human cell lines provide unique opportunities to discover the functional responsibilities of entities involved in the chloride secretory process. HCA-7-Col 3 cells which generate high levels of cyclic AMP in response to forskolin but which fail to show a substantial chloride secretory response may be a useful model of some disease conditions.

Hormonal control of porcine adipose tissue fatty acid release and cyclic AMP concentration.

Compared with adipose tissue from other mammals, porcine adipose tissue has stringent specificity for stimulation of lipolysis by analogs of norepinephrine. This study was to ascertain whether the specificity for control was reflected in the concentration of tissue cyclic AMP (cAMP). Adipose tissue slices were incubated and concentrations of tissue cAMP and free fatty acids (FA) released to the medium were measured. It was necessary to include theophylline in the incubation medium to be able to measure changes in cAMP concentration. Fatty acid release and cAMP production were increased most effectively by the beta-adrenergic agonists; isoproterenol, fenoterol, dobutamine and the mixed alpha- + beta-adrenergic agonist, epinephrine. Isoproterenol-stimulated FA and cAMP production both inhibited by the beta-adrenergic antagonist, propranolol. There was no evidence for alpha 2-adrenergic inhibition of lipolysis in porcine adipose tissue because clonidine (alpha 2-adrenergic agonist) did not lower isoproterenol-induced FA or cAMP levels and phentolamine, an alpha-adrenergic antagonist, did not increase epinephrine-stimulated FA release or cAMP generation. These results imply that the stringent specificity observed for stimulation of swine adipose tissue lipolysis resides in the beta-adrenergic receptor coupled to cAMP production.

An increase of tissue cyclic AMP level by adenosine in the dog pancreas without stimulation of exocrine secretion.

In the vascularly isolated and self-hemoperfused dog pancreas, secretin (0.025 clinical units) and adenosine (1.0 mg) administered intra-arterially (i.a.) increased tissue cyclic AMP level by about 46% and 37%, respectively. Although the increases in the nucleotide were not statistically different, only secretin stimulated exocrine secretion. The increase in cyclic AMP induced by adenosine was significantly reversed by pretreatment with theophylline (0.3 mg, i.a.). These results suggest that the effect of adenosine on cyclic AMP formation occurs mainly in the non-exocrine system of the dog pancreas through A2/Ra-receptors.

Augmentation and inhibition of beta-adrenergic agonists-stimulated tissue cyclic AMP accumulation by alpha-adrenergic agonists in rat parotid gland.

It was found that phenylephrine and methoxamine had two effects (one was inhibitory and the other was augmentative) on isoproterenol-stimulated cyclic AMP in rat parotid slices. The augmentation was abolished by alpha-adrenergic antagonists or by omission of calcium in the medium. Cyclic AMP accumulation by norepinephrine (NE) was significantly decreased by omission of calcium in the medium. Calmodulin antagonists, trifluoperazine and W-7, decreased NE-induced cyclic AMP accumulation, but another calmodulin antagonist, carmidazolium, did not. Phorbol ester such as 4 beta-phorbol 12-myristate, 13-acetate and phorbol 12, 13-dibutyrate, did not augment the effect of isoproterenol. These results suggest that although the influx of calcium is required in the alpha-adrenergic agonists-induced augmentation, calmodulin and protein kinase C may not be intermediates in this process. Calcium ions (10(-7) and 10(-6) M) slightly increased the activity of adenylate cyclase, but calcium (10(-6)-10(-4) M) dose-dependently inhibited the effect of isoproterenol. Therefore, calcium ions do not participate in the augmentation by directly modulating the activity of adenylate cyclase. The inhibitory effect was not affected by alpha-adrenergic antagonists. The activation of adenylate cyclase by isoproterenol was inhibited by phenylephrine with higher inhibition being obtained in lower concentrations of isoproterenol. Phenylephrine in the presence of isobutylmethylxanthine increased the amount of cyclic AMP and this effect was inhibited by propranolol, but not by phentolamine. [3H]-CGP 12177 binding of the parotid membrane was inhibited by alpha-adrenergic antagonists. These results suggest that the inhibitory effect of phenylephrine and methoxamine may be mediated by beta-adrenergic receptor.

An Increase of Tissue Cyclic AMP Level by Adenosine in the Dog Pancreas without Stimulation of Exocrine Secretion

An Increase of Tissue Cyclic AMP Level by Adenosine in the Dog Pancreas without Stimulation of Exocrine Secretion