Islet cyclic AMP levels are not lowered during alpha 2-adrenergic inhibition of insulin release

Epinephrine-induced changes in insulin release and cyclic AMP levels were measured simultaneously in isolated rat islets. Forskolin was used to enhance islet cyclic AMP levels. Forskolin (30 microM) stimulated adenylate cyclase activity 10-fold in islet homogenates and raised cyclic AMP levels 5-fold in intact islets (both at low and high glucose). Insulin release was enhanced by forskolin only at high glucose. Epinephrine (0.1 microM) inhibited glucose- and forskolin-induced insulin release to basal rates. At the same time epinephrine potentiated forskolin-elevated cyclic AMP levels. In contrast epinephrine attenuated forskolin-stimulated adenylate cyclase activity in islet homogenates. At low glucose, both alpha 2- and beta-adrenergic blockade counteracted the epinephrine potentiation, each by 50%. At high glucose the effect was mainly beta-adrenergic in nature. The actions of epinephrine in the presence of a beta-blocker were mimicked by the alpha 2-agonist clonidine. Despite the variations in cyclic AMP levels stimulated insulin release was always inhibited by activation of alpha 2-receptors. Finally, insulin release stimulated by exogenous cyclic AMP was abolished by epinephrine. These results suggest that epinephrine inhibits insulin release at a step distal to the generation of cyclic AMP.     

Hormonal activation of adenylate cyclase and its role in cyclic AMP mediated regulation of RNA synthesis in the mouse mammary gland.

The activity of adenylate cyclase (EC 4.6.1.1) in the mouse mammary gland increases during late pregnancy and reaches its maximum value at one day pre partum. In the mouse mammary gland explant culture the adenylate cyclase activity is stimulated by a cooperative action of insulin, prolactin and hydrocortisone. The effect of these hormones can be demonstrated in intact cells, but not in a cell-free system. In the explants, RNA synthesis is stimulated by dibutyryl cyclic AMP, insulin and prolactin. The effects of both protein hormones and cyclic AMP are additive. The results obtained suggest that insulin and prolactin in cooperation with hydrocortisone are involved in the regulation of RNA synthesis in the mammary gland by activation of the adenylate cyclase system, independently of their effect on this process not mediated by cyclic AMP.     

Pertussis toxin blocks somatostatin's inhibition of stimulated cyclic AMP accumulation in anterior pituitary tumor cells

Somatostatin inhibits both forskolin and (-) isoproterenol-stimulated cyclic AMP accumulation in AtT-20 cells. Pretreatment of these cells with pertussis toxin prevents somatostatin's inhibitory effects on cyclic AMP production. This pretreatment also enhances the cyclic AMP response to forskolin and (-) isoproterenol without affecting basal cyclic AMP levels. The blockade of somatostatin's inhibitory effect was dependent both on the time of preincubation and concentration of pertussis toxin used. The rise in forskolin-stimulated cyclic AMP formation following pertussis toxin treatment preceded the blockade of somatostatin's inhibitory actions. The results suggest that somatostatin acts through an inhibitory guanine nucleotide regulatory protein to affect adenylate cyclase activity.     

Changes in brain cyclic AMP metabolism and acetylcholine and dopamine during narcotic dependence and withdrawal.

Effects of morphine administration were studied on cyclic AMP metabolism in several regions of rat brain. In the cortex, cerebellum and thalamus-hypothalamus, morphine dependence did not alter the activity of either adenylate cyclase or phosphodiesterase. However, during withdrawal from the opiate treatment, adenylate cyclase activity declined in all three regions studied. In contrast, the striatal cyclic AMP metabolism was enhanced during morphine treatment as reflected by elevated endogenous cyclic AMP and increased adenylate cyclase. Furthermore, narcotic dependence produced significant increases in acetylcholinesterase activity of rat striatum. Whereas morphine withdrawal reversed the changes in striatal acetylcholine levels and acetylcholinesterase activity, the enhanced striatal dopamine remained unaltered. Although the activity of striatal adenylate cyclase was significantly reduced when compared to the morphine-dependent rats, the drop in cyclic AMP levels was not significant. Methadone replacement did not affect the changes in striatal dopamine seen in morphine-withdrawn rats. Whereas dopamine stimulated equally well the striatal adenylate cyclase from control or morphine-dependent animals, it failed to stimulate the striatal enzyme from rats undergoing withdrawal. The crude synaptosomal fraction of the whole brain from morphine-dependent rats exhibited an increase in cyclic AMP which was accompanied by elevated adenylate cyclase and protein kinase activity. Naloxone administration suppressed this rise in cyclic AMP and reversed the morphine-stimulated increases in the activities of adenylate cyclase and protein kinase. Following the withdrawal of morphine treatment, alterations in cyclic AMP metabolism were similar to those noted in morphine-naloxone group. Furthermore, substitution of morphine with methadone antagonized the observed alterations in cyclic nucleotide metabolism during withdrawal.     

Role of cyclic AMP and related enzymes in rat lung growth and development.

Cyclic AMP levels in rat lungs showed phasic elevations which peaked during fetal, neonatal and late postnatal periods of development. Lung phospholipids showed major alterations in their levels during fetal and early neonatal life. Alterations in glycogen levels were accompanied by parallel changes in phosphorylase a/total phosphorylase activity which may be related to changes in cyclic AMP during development. Cyclic AMP levels were dependent on the relative activities of adenylate cyclase and cyclic AMP phosphodiesterase which also changed with age. Activation of adenylate cyclase by norepinephrine and NaF, and of cyclic AMP phosphodiesterase by calcium, was maximum neonatally and declined variably thereafter. These data suggest a relationship between cyclic AMP, glycogen and phospholipids during rat lung development.     

Cholinergic inhibition of catecholamine-stimulable cyclic AMP accumulation in murine atria.

Carbachol antagonizes isoproterenol-stimulable cyclic AMP accumulation in mouse atria by direct activation of cardiac muscarinic receptors. Inhibition by carbachol occurs rapidly and is completely reversed when the drug is removed. Neither nitroprusside nor 8-bromo-cyclic GMP mimics the actions of carbachol and low concentrations of carbachol block cyclic AMP accumulation without increasing the intracellular cyclic GMP content. Carbachol does not block cyclic AMP accumulation by activating phosphodiesterase since it is fully effective in the face of marked phosphodiesterase inhibition, nor does it appear to inhibit the catalytic activity of adenylate cyclase since it does not decrease either basal or cholera toxin-stimulated cyclic AMP accumulation. The interaction between carbachol and isoproterenol is not competitive, since cholinergic inhibition cannot be surmounted by increasing concentrations of isoproterenol. The site of muscarinic action therefore appears to involve the mechanisms coupling the hormone-receptor complex to adenylate cyclase. This site is distinct from that of cholera toxin action since there is no antagonism between the effects of cholera toxin and carbachol on cyclic AMP metabolism in the atrium.     

Effects of activation of protein kinase C on the agonist-induced stimulation and inhibition of cyclic AMP formation in intact human platelets.

Jakobs, Bauer & Watanabe [(1985) Eur. J. Biochem. 151, 425-430] reported that treatment of platelets with phorbol 12-myristate 13-acetate (PMA) prevented GTP- and agonist-induced inhibition of adenylate cyclase in membranes from the platelets. This was attributed to the phosphorylation of the inhibitory guanine nucleotide-binding protein (Gi) by protein kinase C. In the present study, the effects of PMA on cyclic [3H]AMP formation and protein phosphorylation were studied in intact human platelets labelled with [3H]adenine and [32P]Pi. Incubation mixtures contained indomethacin to block prostaglandin synthesis, phosphocreatine and creatine kinase to remove ADP released from the platelets, and 3-isobutyl-1-methylxanthine to inhibit cyclic AMP phosphodiesterases. Under these conditions, PMA partially inhibited the initial formation of cyclic [3H]AMP induced by prostaglandin E1 (PGE1), but later enhanced cyclic [3H]AMP accumulation by blocking the slow decrease in activation of adenylate cyclase that follows addition of PGE1. PMA had more marked and exclusively inhibitory effects on cyclic [3H]AMP formation induced by prostaglandin D2 and also inhibited the action of forskolin. Adrenaline, high thrombin concentrations and, in the absence of phosphocreatine and creatine kinase, ADP inhibited cyclic [3H]AMP formation induced by PGE1. The actions of adrenaline and thrombin were attenuated by PMA, but that of ADP was little affected, suggesting differences in the mechanisms by which these agonists inhibit adenylate cyclase. sn-1,2-Dioctanoylglycerol (diC8) had effects similar to those of PMA. The actions of increasing concentrations of PMA or diC8 on the modulation of cyclic [3H]AMP formation by PGE1 or adrenaline correlated with intracellular protein kinase C activity, as determined by 32P incorporation into the 47 kDa substrate of the enzyme. Parallel increases in phosphorylation of 20 kDa and 39-41 kDa proteins were also observed. Platelet-activating factor, [Arg8]vasopressin and low thrombin concentrations, all of which inhibit adenylate cyclase in isolated platelet membranes, did not affect cyclic [3H]AMP formation in intact platelets. However, the activation of protein kinase C by these agonists was insufficient to account for their failure to inhibit cyclic [3H]AMP formation. Moreover, high thrombin concentrations simultaneously activated protein kinase C and inhibited cyclic [3H]AMP formation. The results show that, in the intact platelet, the predominant effects of activation of protein kinase C on adenylate cyclase activity are inhibitory, suggesting actions additional to inactivation of Gi.     

ATP enhances cyclic AMP accumulation by intact P388 mouse lymphoma cells

One of the characteristics of malignant cells is a poor response to hormones and a low level of cyclic AMP. Whilst this is true of intact P388 mouse lymphoma cells, high levels of adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) activity can be measured in particulate preparations of these cells. When ATP is added to the incubation medium of intact lymphoma cells, the cyclic AMP level is enhanced. This effect of ATP is not mediated by adenosine, nor is it enhanced by NaF. The ATP content of the lymphoma cells is much lower than that of CH23 Chinese hamster fibroblast and PCM3 hybrid cells, whose cyclic AMP levels are not affected by the presence of ATP. This suggests that adenylate cyclase, in the lymphoma cells, is bathed in a pool which is deficient in substrate. The substrate concentration of this pool is thought to be elevated by addition of ATP to the incubation medium with ATP, itself, crossing the plasma membrane.     

Short-term changes in levels of cyclic AMP, adenylate cyclase, and phosphodiesterase during the initiation of sperm motility in rainbow trout

In order to clarify the role of the system that generates and degrades cyclic AMP during the initiation of motility of trout sperm, short-term changes in levels of intraspermatozoal cyclic AMP, adenylate cyclase, and phosphodiesterase were measured. Levels of cyclic AMP and the activity of adenylate cyclase increased and reached a maximum level 1 sec after transfer of sperm to K+-free medium, where they became motile, and then decreased rapidly. However, there were no changes in either parameter in sperm which remained immotile in K+-rich medium. In addition, an increase in the activity of phosphodiesterase was observed 4 sec later than the increase in levels of cyclic AMP and adenylate cyclase. These findings suggest that a very rapid change in the level of intracellular cyclic AMP occurs within 1 sec, at the moment of spawning, by the activation of adenylate cyclase and phosphodiesterase, and regulates the initiation of trout sperm motility.     

Resensitization of hepatocyte glucagon-stimulated adenylate cyclase can be inhibited when cyclic AMP phosphodiesterase inhibitors are used to elevate intracellular cyclic AMP concentrations to supraphysiological values.

Treatment of intact hepatocytes with glucagon led to the rapid desensitization of adenylate cyclase, which reached a maximum around 5 min after application of glucagon, after which resensitization ensued. Complete resensitization occurred some 20 min after the addition of glucagon. In hepatocytes which had been preincubated with the cyclic AMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), glucagon elicited a stable desensitized state where resensitization failed to occur even 20 min after exposure of hepatocytes to glucagon. Treatment with IBMX alone did not elicit desensitization. The action of IBMX in stabilizing the glucagon-mediated desensitized state was mimicked by the non-methylxanthine cyclic AMP phosphodiesterase inhibitor Ro-20-1724 [4-(3-butoxy-4-methoxylbenzyl)-2-imidazolidinone]. IBMX inhibited the resensitization process in a dose-dependent fashion with an EC50 (concn. giving 50% of maximal effect) of 26 +/- 5 microM, which was similar to the EC50 value of 22 +/- 6 microM observed for the ability of IBMX to augment the glucagon-stimulated rise in intracellular cyclic AMP concentrations. Pre-treatment of hepatocytes with IBMX did not alter the ability of either angiotensin or the glucagon analogue TH-glucagon, ligands which did not increase intracellular cyclic AMP concentrations, to cause the rapid desensitization and subsequent resensitization of adenylate cyclase. It is suggested that, although desensitization of glucagon-stimulated adenylate cyclase is elicited by a cyclic AMP-independent process, the resensitization of adenylate cyclase can be inhibited by a process which is dependent on elevated cyclic AMP concentrations. This action can be detected by attenuating the degradation of cyclic AMP by using inhibitors of cyclic AMP phosphodiesterase.     

Regulation of cyclic AMP accumulation in lymphoid cells

We have examined several features of the regulation of cyclic AMP accumulation in lymphoid cells isolated from peripheral blood of human subjects and in the murine T-lymphoma cell line, S49, S49 cells are unique because of the availability of variant clones with lesions in the pathway of cyclic AMP generation and response. We found that human lymphoid cells prepared at 4 degrees C showed substantially greater cyclic AMP accumulation in response to histamine and the beta-adrenergic agonist isoproterenol than did cells prepared at ambient temperature. The muscarinic cholinergic agonist carbamylcholine and peptide hormone somatostatin failed to inhibit cyclic AMP accumulation in human lymphoid cells and treatment with pertussis toxin (which blocks function of Gi, the guanine nucleotide binding protein that mediates inhibition of adenylate cyclase) only minimally increased cyclic AMP levels in these cells. Thus the Gi component of adenylate cyclase appears to play only a small role in modulating cyclic AMP levels in this mixed population of lymphoid cells. Incubation of whole blood with isoproterenol desensitized human lymphocytes to subsequent stimulation with beta agonist. This desensitization was associated with a redistribution of beta-adrenergic receptors such that a substantial portion of the receptors in intact cells could no longer bind a hydrophilic antagonist. Wild-type S49 lymphoma cells showed a similar redistribution of beta-adrenergic receptors after a few minutes' incubation with agonist. Based on studies in S49 variants, this redistribution is independent of components distal to receptors in the adenylate cyclase/cyclic AMP pathway. By contrast, a more slowly developing, agonist-mediated down-regulation of beta-adrenergic receptors was blunted in variants with defective interaction between receptors and Gs, the guanine nucleotide binding protein that mediates stimulation of adenylate cyclase. Unlike results in human lymphoid cells, S49 cells show a prominent inhibition of cyclic AMP accumulation mediated by Gi; this inhibition is promoted by somatostatin and blocked by pertussis toxin. Inhibition by Gi is unable to account for the marked decrease in ability of the diterpene forskolin to maximally stimulate adenylate cyclase in S49 variants having defective Gs. These results emphasize that both Gs and Gi component are important in modulating cyclic AMP accumulation and receptors linked to adenylate cyclase in S49 lymphoma cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Negative control of norepinephrine on the thyroid cyclic AMP system

Negative control on the thyroid cyclic AMP system has been studied. The increase of cyclic AMP levels induced by TSH in dog thyroid slices and its consequent secretion were inhibited by norepinephrine. This effect was different from the previously described activation of cyclic AMP disposal by acetylcholine: it was not calcium-dependent, was observed in the presence of isobutyl methylxanthine and was not inhibited by atropine. The inhibitory action of norepinephrine was abolished by phentolamine but not by L-propranolol. Clonidine and epinephrine also markedly inhibited the elevation of cyclic AMP levels, but phenylephrine did not. The inhibitory effect of norepinephrine and clonidine was abolished by yohimbine but not by prazosin. These results suggest that the effect of adrenergic agents on dog thyroid follicular cells is mediated by alpha 2-receptors. Similar results were obtained on dog thyroid adenylate cyclase activity: norepinephrine diminished the activation of adenylate cyclase induced by TSH, in a sodium-dependent process. This inhibition was abolished by phentolamine and yohimbine, but not by L-propranolol and and prazosin. This shows that the negative alpha 2-adrenergic effect bears directly on adenylate cyclase.     

Metabolism of cyclic AMP in isolated renal tubules: effects of prostaglandins and parathyroid hormone.

Concentrations of cyclic AMP (cAMP) were increased in isolated renal cortical tubules from hamsters by both parathyroid hormone (PTH) and prostaglandin E1 (PGE1) with maximal effects of PGE1 being 6-8 fold greater than those of PTH during a 10 min period. However, cAMP concentrations in cells treated with 1-methyl-3-isobutylxanthine (MIX) were increased with maximal concentrations of either hormone to the same degree. Similar effects of both hormones were observed on adenylate cyclase activity in renal homogenates. Simultaneous addition of hormones produced changes in both cAMP concentrations in intact tubules as well as adenylate cyclase activity of homogenates which were not completely additive. Degradation of cAMP, estimated in intact tubules as the difference in cAMP levels in the presence and absence of MIX, was increased by both hormones, however, changes were 2-3 fold greater in tubules exposed to PTH than to PGE1. Neither hormone directly altered cAMP phosphodiesterase (PDE) activity in either 30,000 x g supernatant or pellets from renal cortical homogenates. The results suggest that both hormones increase the production of cAMP in renal cortical tubules and may share a common target cell type in this response. Degradation of cAMP, however, is differentially effected by the two hormones, probably reflecting differences exerted on intracellular mechanisms regulating the enzymatic hydrolysis of cAMP.     

Inhibition of a parathyroid hormone-stimulated renal adenylate cyclase by cyclic AMP.

Plasma membranes were isolated from bovine renal cortex. This particulate, adenylate cyclase-containing fraction was stimulated to produce cyclic AMP by parathyroid hormone and fluoride. When the time-course of adenylate cyclase activity was investigated, it was found that while PTH-stimulated cyclic AMP production comes to a halt in about 15 minutes after the initiation of the reaction, fluoride-stimulated activity continues unabated for at least an hour. Experiments to determine the cause of this showed that the cyclase enzyme is not degraded under our experimental conditions, but is inhibited by a soluble, unbound product of the reaction which requires ATP for its synthesis. In our experiments degradation of parathyroid hormone was relatively slow and could not account for the rapid inhibition of PTH-stimulated cyclase activity. Of the various agents tested, cyclic AMP was found capable of inhibiting PTH-stimulated cyclic AMP production by our purified membrane preparation. Half-maximal inhibition was observed at around 10(-6) M concentrations of the nucleotide. Pyrophosphate, adenosine, 5'-AMP and ADP had no effects. The significance of these results in relation to the regulation of adenylate cyclase activity is discussed.     

Pertussis toxin stimulates cholecystokinin-induced cyclic AMP formation but is without effect on secretagogue-induced calcium mobilization in exocrine pancreas

The role of a pertussis toxin sensitive GTP-binding protein in mediating between cholecystokinin receptors and phosphatidylinositol 4,5-bisphosphate phosphodiesterase as well as in preventing cholecystokinin from increasing cellular cyclic AMP has been investigated using dispersed acini from rabbit pancreas. Pertussis toxin pretreatment (500 ng/ml, 2 h) did not affect cholecystokinin(octapeptide) (CCK-8)-induced increases in cytosolic free Ca2+ as judged from changes in fluorescence obtained from quin2-loaded acini. Although pretreatment with pertussis toxin was also without effect on resting acinar cell cyclic AMP levels, adenylate cyclase activity was increased, since inhibition of cyclic AMP phosphodiesterase activity by isobutylmethylxanthine (IBMX) resulted in an additional increase in cyclic AMP levels in toxin-treated acini, indicating that acinar cell adenylate cyclase activity is under some tonic inhibitory control by the pertussis toxin-sensitive inhibitory GTP-binding protein (Gi) of the adenylate cyclase system. CCK-8 gave an increase in cyclic AMP levels in both control (1.6-fold) and toxin-treated (2.3-fold) acini, leading to cyclic AMP levels in the toxin-treated acini 2-times as high as those in control acini. In the presence of IBMX, the cyclic AMP response to CCK-8 was again markedly enhanced in acini pretreated with the toxin (3.2- vs. 1.8-fold), resulting in cAMP levels in the toxin-treated acini 3.7-times those in the absence of IBMX, 2.5-times those in control acini in the presence of IBMX and 7.0-times those in control acini in the absence of IBMX. Neither the pretreatment with pertussis toxin, nor the presence of IBMX alone, nor the combination had an effect on basal amylase secretion. However, all three treatments potentiated the stimulatory effect of CCK-8 on amylase secretion and the amount of potentiation was proportional to the cyclic AMP levels reached. Our findings suggest that in the intact pancreatic acinar cell Gi inhibition of the catalytic subunit of the adenylate cyclase may largely be responsible for preventing cholecystokinin from increasing cellular cyclic AMP. They moreover show that cyclic AMP is a modulatory agent in rabbit pancreatic enzyme secretion, not able to stimulate secretion itself, but potentiating effects mediated by the phosphatidylinositol-calcium pathway.     

Modulation of adenylate cyclase/cyclic AMP response by thyrotropin and prostaglandin E2 in cultured thyroid cells. 2. Positive regulation.

Two different independent processes are operating in cultured thyroid cells to regulate adenylate cyclase/cyclic AMP responsiveness to thyroid stimulators (thyrotropin and prostaglandin E2): firstly, refractoriness or negative regulation [preceding paper], which is specific for each thyroid stimulator, is not mediated by cyclic AMP and is not accompanied by alteration of adenylate cyclase activity; secondly, positive regulation which is characterized by an augmentation of the cyclic AMP response stimulated by thyrotropin and prostaglandin E2. This process is not specific for each thyroid stimulator and is a state of increased susceptibility of cyclic AMP synthesis to stimulation, accompanied by increased activity of the catalytic subunit of adenylate cyclase. Positive regulation is apparently mediated by increased intracellular cyclic AMP levels. It is a time-dependent and dose-dependent process. Very low concentrations (5-50 micronU/ml) of thyrotropin augmented cyclic AMP synthesis stimulated by thyrotropin and prostaglandin E2 whereas higher concentrations (above 0.1 mU/ml) augmented prostaglandin E2 stimulation but induced refractoriness to thyrotropin. Prostaglandin E2 (0.1 to 10 micronM) augmented thyrotropin stimulation and dibutyryl adenosine 3':5'-monophosphate (0.3 to 2 mM) augmented thyrotropin and prostaglandin E2 stimulation. Positive regulation is a slow process which develops within days and increases up to day 5 in culture. Experiments using inhibitors suggested that protein synthesis is required for the full expression of the increase in adenylate cyclase activity induced by the studied thyroid stimulators.     

Theoretical analysis of the consequences of cyclic nucleotide phosphodiesterase negative co-operativity. Amplification and positive co-operativity of cyclic AMP accumulation.

Most tissues contain multiple forms of cyclic nucleotide phosphodiesterases (3':5'-cyclic-nucleotide 5' nucleotidohydrolase, EC 3.1.4.17). Consequently, in most, if not in all, tissues, substrate-velocity curves deviate from Michaelian kinetics and exhibit an apparent negative co-operativity. We have studied the possible theoretical consequences of this property on the quantitative features of cyclic AMP accumulation in response to activation of adenylate cyclase. Negative co-operativity of phosphodiesterases tends to generate a "positively co-operative" cyclic AMP accumulation curve. It amplifies the stimulation of cyclic AMP accumulation as compared with the stimulation of cyclic AMP synthesis. It enhances the sensitivity of cyclic AMP accumulation to slight variation of phosphodiesterase maximal velocity. It tends to shift the cyclic AMP accumulation curve to higher concentrations of stimulator as compared with the adenylate cyclase activation curve. This accounts for much of the data in the literature of hormonal effects on phosphodiesterase activity. It shows that the characteristics of cyclic nucleotide phosphodiesterases are as important as those of adenylate cyclase in determining the response of the system.     

Variations in cyclic AMP level and specific activities of adenylate cyclase and cyclic AMP phosphodiesterase during the cell cycle of an Actinomycete (author's transl)

The variations in the concentrations of intra- and extracellular cyclic AMP and in he specific activities of adenylate cyclase (EC 4.6.1.1) and cyclic AMP phosphodiesterase (EC 3.1.4.17) have been monitored in synchronized cultures of Nocardia restricta, a prokaryote belonging to the group of Actinomycetes. At the beginning of the cell cycle, during a first period of RNA and protein synthesis, there is an increasing synthesis of adenylate cyclase which can be suppressed in the presence of chloramphenicol or rifampicin. Simultaneously, the specific activity of cyclic AMP phosphodiesterase decreases and the concentrations of intra- and extracellular cyclic AMP rise. After the end of DNA replication, during a second period of RNA and protein synthesis, the specific activity of cyclic AMP phosphodiesterase increases; during the same time, the specific activity of adenylate cyclase and the level of intracellular cyclic AMP drop. It appears that the overall metabolism of cyclic AMP is coordinated so that the cyclic AMP level will be high at the beginning of DNA replication and will fall thereafter. The results are discussed in comparison with known data about the variations of cyclic AMP during the cell cycle of mammalian cells in cultures.     

Effect of high level section of the spinal cord on the cyclic AMP system in rat liver.

The effect of high level section of the spinal cord upon the hepatic cyclic AMP system was investigated in the rat. We report that transection of the spinal cord dramatically decreases the basal level of cyclic AMP from 0.88 nmol/g liver to 0.36 nmol/g at 1 h and to 0.20 nmol/g at 4 h. This was not due to increased activity of cyclic AMP phosphodiesterase or to decreased activity of basal adenylate cyclase. The sensitivity of adenylate cyclase to its usual effectors in vitro was not impaired. It is proposed that the lowering of liver cyclic AMP below its basal level after high level section of the spinal cord is due to decreased levels of hepatic catecholamines and/or plasma glucagon.