Cyclic AMP MICHAEL L. STEER, M.D.

Cyclic AMP is believed to be the intracellular agent which mediates the action of many hormones on their target cell. The mechanisms by which this nucleotide controls glycogen metabolism in liver and skeletal muscle seem to be firmly established. Data relevant to this area of research are selectively reviewed. In addition, the evidence is reviewed for and against a role for cyclic AMP in the regulation of a variety of other cellular functions including: cardiac contractility, smooth muscle relaxation, platelet aggregation, salivary gland amylase secretion, pancreatic exocrine secretion, and gastric acid secretion.

M ANY HORMONES affect their target cells by binding to surface membrane receptors and activating the enzyme adenyl cyclase which catalyzes the formation of cyclic AMP from ATP. The characteristics of adenyl cyclase were previously reviewed96 and derangements of the adenyl cyclase system which have been shown to result in disease states were discussed. The physiological role of cyclic AMP as the biochemical mediator of the effects of hormones on several tissues will be discussed in the present review. Cyclic AMP has been shown to be an important regulator of cellular function in a wide variety of tissues in humans and many other animals. To avoid an encyclopedic presentation, however, only selected cell types affected by cyclic AMP will be discussed here. Although cAMP has frequently been found to have more than one effect in a particular tissue, this review will focus on those effects which appear to be of greatest importance to the surgeon. Skeletal Muscle and Liver: Glycogen Metabolism It is appropriate that a discussion of the role of cyclic AMP in cell function begin by dealing with glycogen Submitted for publication December 5, 1975. Reprint requests: Michael L. Steer, M.D., Beth Israel Hospital, 330 Brookline Avenue, Boston, Massachusetts 02215.

From the Department of Surgery, Beth Israel Hospital and Harvard Medical School, Boston, Massachusetts

metabolism in liver and skeletal muscle because it was in this area of research that cyclic AMP was first discovered by Sutherland and co-workers in 1957.100 Since those early observations, extensive investigations focusing on the role of cyclic AMP in the regulation of glycogen metabolism have been reported, and a detailed molecular description of this process is now available. Unfortunately, our understanding of the biochemical steps involved in the regulation of cellular function by cyclic AMP in other cell types is much less advanced. It has been assumed, however, that the steps by which cyclic AMP affects muscle contraction, exocrine secretion, platelet aggregation, and the other cellular processes which will be discussed below are quite similar to the steps by which cyclic AMP controls glycogen metabolism. This appears to involve, initially, activation of a cyclic AMP-dependent protein kinase by cyclic AMP and, subsequently, the phosphorylation of specific cellular proteins by the activated protein kinase. In skeletal muscle and liver, the result is an increased rate of glycogen breakdown and a decreased rate of glycogen synthesis. Protein Kinases. Protein kinases are a group of enzymes which catalyze the phosphorylation of other proteins.46 The activity of some of these protein kinases is dependent upon the presence of cyclic AMP. ATP is used as the phosphate source.46 These cyclic AMPdependent protein kinases are composed of two subunits: a regulatory subunit and a catalytic subunit (Fig. 1). When both subunits are joined, the protein kinase is inactive as an enzyme. Cyclic AMP, when present, binds to the regulatory subunit thereby causing the two subunits to dissociate and liberating the free catalytic component which is now an active enzyme46 (Fig. 1). In liver

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FIG. 1. Activation of cyclic

AMP-dependent protein kinase. The protein kinase

CATALYTIC SUBUNIT

REGULATORY

SUBUNIT

+ CYCLIC AMP

N

ULA

SUBUNIT

RLY CYCLIC AMP

+

CATALYTIC SUBUNIT

consists of two subunits. Binding of cyclic AMP to the subunit regulatory causes the two subunits to

I_____l_____I___ dissociate leaving the free catalytic subunit which is ACTIVATED now an active enzyme. The PROrEIN activated protein kinase KINASE uses ATP as a phosphate

NON-ACrIVATED PROTEIN

KINASE

source and phosphorylates other cellular enzymes.

and skeletal muscle, this activated cyclic AMP-dependent protein kinase interacts with the enzyme systems controlling glycogen metabolism. The mechanism by which this results in a decreased rate of glycogen synthesis and an increased rate of glycogen degradation will now be discussed. Glycogen Synthesis. Glycogen is a polymer of glucose subunits. The immediate precursor of glycogen inside the cell is uridine-diphosphate-glucose (UDP-glucose). The synthesis of glycogen from UDP-glucose is catalyzed by the enzyme glycogen synthetase according to the following scheme: UDP-glucose + glycogen(, glucose groups) UDP

synthetase, however, may exist in two forms: an activated form (Synthetase I) and a non-activated form (Synthetase D).72 Cyclic AMP inhibits glycogen synthesis because the activated cyclic AMP-dependent protein kinase catalyzes the conversion of the activated Synthetase I into the non-activated Synthetase D95 according to the following scheme:

Synthetase I

+ ATP -*

Synthetase D + ADP.

Glycogen Breakdown: In addition to inhibiting glycogen synthesis, cyclic AMP also stimulates glycogen breakdown. This process, however, is more complex and involves a cascade of enzymatic steps (Fig. 2). Briefly, the activated cyclic AMP-dependent protein kinase con+ glycogen(n + 1 glucose groups)* verts a non-activated enzyme (phosphorylase kinase) This reaction is dependent on the presence of an activated into its activated form.104 The activated phosphorylase form of the enzyme glycogen synthetase. Glycogen kinase itself converts another non-activated enzyme (glycogen phosphorylase b) into its activated form (glycogen phosphorylase a)." Glycogen phosphorylase a then catacyclic AMP lyzes the breakdown of glycogen into glucose-i-phosphate37 according to the following scheme: Non- ~l~A Ptin Kinase

glycogen(n glucose groups) + phosphate -glycogen{n - 1 glucose groups) + glucose-I

Activoted

Protein Kinose

Non-octivoted Phosphorylose Kinose

Acti0ted

Phosphorylase Kinase

Phosphorylose b (non-activoted)

Phosphorylase (activated)

Glycogen (n Glucose groups)

o

Glycogen + Glucose- - P (n-l Glucose groups)

FIG. 2. Stimulation of glycogen breakdown by cyclic AMP. The activated protein kinase precipitates a cascade of enzymatic steps resulting in liberation of glucose-l-phosphate from glycogen. These steps (1,11,11) each involve phosphorylation and use cellular ATP as the phosphate source leaving ADP as one of the products.

-

P.

The liberated glucose-I - P is now available to serve as an intracellular energy source or to be released into the extracellular space as glucose for use by other tissues. Similarities in Other Tissues. Cyclic AMP-dependent protein kinases from many cell types have now been studied and, in general, they are quite similar.46 These kinases usually exist in an inactive form which becomes activated upon binding of cyclic AMP to the regulatory subunit. The activated catalytic subunit of the protein kinase then regulates specific metabolic pathways by catalyzing the phosphorylation of the enzymes which control these metabolic pathways. In some cases, the enzyme which is phosphorylated by the activated protein kinase is activated, as when glycogen phosphorylase kinase is activated by the protein kinase. In other situations, the phosphorylated enzyme is inhibited as when activated enzyme glycogen synthetase I is converted

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to the non-activated glycogen synthetase D by the activa- evidence would suggest that the effects of cyclic AMP ted cyclic AMP-dependent protein kinase. Thus the vary- and calcium are interrelated. Further investigations will ing effects of cyclic AMP on different tissues appear probably show that yet other regulatory agents are imto be related to the presence of differing cyclic AMP- portant and it is likely that a better understanding of dependent protein kinases and to the fact that different the mechanisms regulating cardiac contractility will enzymes in these cells are phosphorylated by the acti- emerge. vated cyclic AMP-dependent protein kinase. In many instances, however, the exact enzyme which acts as a Smooth Muscle: Relaxation substrate for protein kinase catalyzed phosphorylation has not been identified. A search for these substrate The primary effect of cyclic AMP in smooth muscle enzymes is being pursued in many laboratories. appears to be relaxation. Smooth muscle in various tissues has been found to contain hormone sensitive adenyl and the hormones which stimulate this adenyl cyclase Cardiac Muscle: Contractility cyclase cause muscle relaxation. Addition of exogenous The positive inotropic effect of catecholamines, cyclic AMP or its dibutyryl derivative leads to relaxahistamine, and glucagon has been extensively investigated tion. This response also follows administration of phosand considerable data have accumulated suggesting that phodiesterase inhibitors such as theophylline which inthis effect is mediated by cyclic AMP. These hormones crease cyclic AMP levels (for recent reviews, see referhave been shown to activate myocardial adenyl cy- ences 14, 86). Some of the findings implicating cyclic AMP clase42"8 -59 and cause a rise in myocardial cyclic AMP in smooth muscle relaxation include the following: 1) levels.70 In addition, administration of either exogenous Tracheal and bronchial smooth muscle has been shown cyclic AMP or its dibutyryl derivative increases cardiac to relax following 8-adrenergic stimulation or following contractility.1 92 Other agents, such as phosphodiesterase administration of dibutyryl cyclic AMP.58'61 2) The histainhibitors, increase myocardial cyclic AMP levels, and mine induced contraction of bronchial smooth muscle also induce a positive inotropic effect.18,66 The myo- is antagonized by dibutyryl cyclic AMP73 and this antagcardium also contains cyclic AMP-dependent protein onistic effect of the cyclic AMP analog is potentiated kinases which, when activated by cyclic AMP, are cap- by theophylline. 3) Intestinal and colonic smooth able of phosphorylating other proteins.19-" Cardiac gly- muscle7'8 relaxes after ,3-adrenergic stimulation or adcogen phosphorylase may be activated by a cyclic ministration of dibutyryl cyclic AMP and /8-adrenergic AMP-dependent mechanism which is similar to that stimulation sufficient to cause relaxation is associated already described for the liver and skeletal muscle with an increase in muscle cyclic AMP levels. 4) Reenzyme. Another mechanism by which cyclic AMP may laxation of the sphincter of Oddi follows adenyl cyclase alter cardiac contractility, which involves the contractile stimulation by cholecystokinin or catecholamines and reelements themselves. has been proposed. The activated laxation of the sphincter of Oddi can be elicited by protein kinase has been shown to be capable of phos- addition of dibutyryl cyclic AMP.4 (Interestingly, phorylating troponin,13'74 an important element in the con- cholecystokinin causes contraction of the gallbladder tractile apparatus. It has also been found that activated but this may be related to a fall in cyclic AMP protein kinases can phosphorylate various components levels since exogenous dibutyryl cyclic AMP or theoof the cell or sarcoplasmic membranes and thereby phylline inhibit contraction and/or cause relaxation.2'3 alter intracellular calcium distribution and promote con- 5) Uterine smooth muscle relaxes following ,8-adrenergic tato 29.41.102,103 stimulation and this is associated with increased uterine This evidence, however, does not prove that cyclic cyclic AMP levels.2571 Uterine relaxation can also be AMP regulates cardiac contractility. In fact, it has been produced by administration of dibutyryl cyclic AMP and shown experimentally that contractility can increase this effect is potentiated by theophylline.25'71 6) In some without a change in cyclic AMP levels.90 Myocardial gly- cases vascular smooth muscle relaxation follows catecogen phosphorylase also can be activated without a cholamine stimulation. This effect on vascular smooth change in cyclic AMP levels" and troponin can be muscle is associated with a rise in cyclic AMP levels phosphorylated by cyclic AMP independent mech- suggesting that the process of vascular relaxation is anisms.97 Similarly, calcium changes may be induced by cyclic AMP mediated. Other pharmacologic agents which methods which bypass adenyl cyclase and cyclic AMP.0 cause vasodilatation, such as diazoxide, hydralazine, Thus, the physiological role of cyclic AMP in regulating and nitroglycerine, also elevate cyclic AMP levels.6'14 myocardial contractility remains to be fully elucidated The evidence presented above strongly suggests that but it is currently under intensive study. The importance cyclic AMP mediates smooth muscle relaxation. The of calcium has already been mentioned. The current molecular basis for this process is not completely

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understood, but it is currently believed that cyclic AMP causes smooth muscle relaxation by decreasing intracellular calcium levels. Agents, such as acetyl choline, which cause smooth muscle contraction, appear to do so by increasing intracellular calcium levels (for a recent review, see 16). It would seem that intracellular calcium levels determine smooth muscle tone. Cyclic AMP decreases calcium levels by either increasing the rate of calcium efflux, decreasing the rate of influx, or promoting sequestration of calcium through binding of calcium to the sarcoplasmic reticulum.5'9'16 Platelets: Aggregation The complex process by which a platelet plug is formed at the site of vascular injury begins when platelets come in contact with exposed subintimal connective tissue. Platelets adhere to the exposed collagen fibers and this initiates a process known as the "release reaction" in which the platelets discharge intracellular granules.77 These granules contain several agents including adenosine diphosphate (ADP), serotonin, and calcium which cause other platelets to undergo the release reaction and aggregate.77 Thrombin, which may be generated locally as a result of the intrinsic coagulation system can also induce the platelet release reaction and promote platelet aggregation.77 Many agents have now been identified which can either initiate or inhibit the release reaction. In general, compounds which promote the release reaction (a-adrenergic catecholamines, collagen, ADP, thrombin, and prostaglandin E2) also cause platelet cyclic AMP levels to fall while agents which inhibit the release reaction (prostaglandin E1, theophylline, caffeine, papaverine, and dibutyryl cyclic AMP) elevate platelet cyclic AMP levels. 43,54,5578'80'81'107'108 This has suggested that the governing factor in regulating platelet aggregation may be platelet cyclic AMP. It should be emphasized, however, that this is only a hypothesis since several reports suggest that cyclic AMP may not alone determine aggregation. For example, if cyclic AMP levels are initially raised (by prostaglandin E1, for example), aggregation can still be induced although the aggregating agent has not reduced cyclic AMP levels below the basal value.62 Thus, although considerable circumstantial evidence suggests that cyclic AMtP regulates platelet aggregation, further experimentation is needed to confirm this hypothesis. It has been suggested that calcium also plays an important role in regulating platelet aggregation and that cyclic AMP might act by altering platelet calcium levels.30'57 For example, cyclic AMP might activate platelet cyclic AMP-dependent protein kinases.56'82 Once activated, these protein kinases could phosphorylate the proteins which regulate the activity of the membranebound calcium pump. A decrease in the activity of this

Ann. Surg. * July 1976

would result in a net increase in calcium influx. The additional calcium could then activate contractile proteins within the platelet and initiate the release reaction (79). At present, however, this must be considered only as a working hypothesis. Platelet prostaglandins also play an important role in regulating platelet aggregation. The ability of prostaglandin E1 to inhibit aggregation, apparently by increasing platelet cyclic AMP levels, has already been mentioned. Another prostaglandin, PGD2, has a similar effect.60'4 These and other prostaglandins are formed from arachidonic acid by the enzyme prostaglandin synethetase in the plasma membrane of platelets. During the process of prostaglandin synthesis, intermediate compounds (called endoperoxide precursors) accumulate and these compump

pounds

can cause platelet aggregation.34'35'93106 Aspirin and indomethacin inhibit prostaglandin synthesis and thus prevent the accumulation of the endoperoxide precursors. This explains the ability of these drugs to prevent platelet aggregation. At the present time, however, the relationship between platelet cyclic AMP and the endoperoxide precursors of prostaglandins is not clear.

Salivary Glands: Amylase Secretion

Salivary glands such as the parotid gland respond to both adrenergic and cholinergic stimulation. Both cholinergic and a-adrenergic stimulation result in fluid and electrolyte secretion83 while,8-adrenergic stimulation causes amylase secretion.15 It has become increasingly apparent that the ,-adrenergic stimulation of amylase secretion is mediated by cyclic AMP. p-adrenergic stimulation is associated with adenyl cyclase stimulation and elevations in tissue cyclic AMP levels.51'84 Moreover, addition of exogenous cyclic AMP in the form of its dibutyryl analog or elevation of cyclic AMP by inhibition of phosphodiesterase also leads to amylase secretion.12 These findings have led to the conclusion that amylase secretion is mediated by cyclic AMP. This concept is supported by the finding that the parotid gland contains a cyclic AMP-dependent protein kinase,89 but the mechanism by which activation of this protein kinase leads to amylase secretion is not known. It has been suggested that calcium plays an important role in coupling cyclic AMP increases with amylase secretion.67'88 It is possible that the final step involves cyclic AMP stimulated calcium binding to contractile proteins such as the microtubules and microfilaments and that this, in turn, leads to the discharge of amylase containing secretory granules at the cell surface.20 Pancreas: Exocrine Secretion The exocrine pancreas is a mixed gland consisting of at least two components. The ductal cells secrete

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fluid rich in electrolytes whereas the acinar cell secretion is rich in protein composed primarily of digestive enzymes. In many ways these two components behave as separate organs and the mechanisms governing their secretion differ considerably. They will, therefore, be discussed separately. Electrolyte secretion. Secretin, released from the duodenum, appears to act directly on the ductal cells to stimulate fluid and electrolyte secretion.21 Several factors suggest that the secretin effect is mediated by cyclic AMP. It can be reproduced by dibutyryl cyclic AMP23 and potentiated by the phosphodiesterase inhibitor theophylline.23 Also, secretin induced stimulation of electrolyte secretion is associated with a rise in the pancreatic content of cyclic AMP and this rise precedes the secretory response.22 In addition, adenyl cyclase in isolated pancreatic cell membranes can be stimulated by concentrations of secretin similar to those which stimulate electrolyte secretion by the intact gland.75 The preceding evidence would strongly suggest that secretin stimulates electrolyte secretion by a cyclic AMP-dependent pathway. Although this concept is accepted generally, data in conflict with this hypothesis have been presented. For example, in one study theophylline was found to decrease electrolyte secretion.33 Thus, it would appear that further experimentation is needed before cyclic AMP can be positively identified as the mediator of secretin stimulated electrolyte secretion. Protein Secretion. The pancreatic acinar cells synthesize many digestive enzymes which are stored intracellularly in the form of zymogen granules. These granules are discharged into the ductal collecting system after stimulation by pancreozymin or cholinergic agents.21 The evidence would suggest that cholinergic stimulation alters membrane potential resulting in changes in calcium flux and distribution which eventually lead to protein secretion.85'87 A role for cyclic AMP in this cholinergically stimulated process has not been established. A great deal of experimental effort, however, has been devoted to establishing whether or not the response to pancreozymin is mediated by cyclic AMP but a clear answer to this question is not presently available. If the pancreozymin effect is mediated by cyclic AMP, one should observe the following effects (101): 1) concentrations of pancreozymin which stimulate protein secretion should be capable of stimulating pancreatic adenyl cyclase; 2) pancreozymin stimulation of the pancreas should result in a rise in pancreatic cyclic AMP and this rise should precede protein secretion; 3) exogenous cyclic AMP or its dibutyryl derivative should induce protein secretion and their effect should be potentiated by phosphodiesterase inhibitors. Unfortunately, available data relative to this issue are conflicting. Although some observers have found that exogenous cyclic AMP or dibutyryl cyclic

111

AMP and theophylline stimulate protein secretion,4569 other investigators have failed to observe this response.23'105 In one study pancreozymin has been found to be incapable of stimulating adenyl cyclase,47 although other reports show that this hormone, at high concentrations, can stimulate adenyl cyclase.75 Recently, however, an explanation for this latter finding has been proposed. Kempen et al.39 have shown that, in the presence of certain phospholipids, pancreozymin at low concentrations stimulates adenyl cyclase. In addition to establishing that the pancreozymin effect could be mediated by cyclic AMP, these workers have demonstrated that certain technical problems encountered in handling pancreatic tissue may be the explanation for the previously conflicting data reported in this field.39 Studies of pancreatic function are difficult because the gland is composed of several cell types. Thus, identification of the cell type responding to a specific stimulus is difficult and interpretation of data may be confused. Recently, Gardner and co-workers, studied the effects of these various stimuli on preparations of isolated pancreatic acinar cells.31 They have shown that cholinergic stimulation results in a decrease in membrane-bound calcium and that this effect triggers protein secretion. Pancreozymin stimulation also leads to a release of membrane-bound calcium. They have found that cholinergic agents and pancreozymin cause an increase in calcium efflux, an effect which leads to activation of guanylate cyclase and formation of cyclic GMP. Thus, while the preliminary data suggested that cyclic AMP mediates acinar cell secretion of protein, the more recent data suggest that calcium and cyclic GMP may be more important. Here too, further studies are necessary before a precise picture of the mediation of pancreatic protein secretion can be drawn.

Stomach: Acid Secretion Gastric secretion of acid is a complex process which occurs in response to gastrin, histamine, or cholinergic stimulation. The question of whether or not cyclic AMP plays a role in gastric acid secretion is, at present, quite controversial.40 Most of the data suggesting that cyclic AMP mediates this process have been obtained from studies of rodents or amphibians. Unfortunately, the results of studies dealing with human and canine gastric secretion have been less conclusive. Therefore, although the data from rodents and amphibians are obviously not directly applicable to human secretion, I will concentrate on them in this review since they are complete enough to allow at least tentative conclusions to be drawn. The present understanding of the role of cyclic AMP in human and canine gastric secretion will be briefly discussed. Cholinergic Mechanisms. The mechanism whereby

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vagal stimulation or administration of cholinergic agents stimulates gastric acid secretion is incompletely understood. Cholinergic stimulation has been shown to release gastrin from antral cells.50 This response can also be elicited by addition of cyclic AMP50 suggesting that the cholinergic release of gastrin may be mediated by cyclic AMP. Cholinergic stimulation is also associated with rises in cyclic GMP,28 thus implicating this cyclic nucleotide as another possible mediator of the cholinergic effect. Gastrin. The site at which gastrin acts to promote acid secretion has not been clearly shown. Gastrin stimulation of gastric acid secretion can be blocked by histamine (H2) receptor blockers.3249 In fact, patients with gastrin secreting tumors have been successfully treated with H2 receptor blockers.68 This has suggested that gastrin liberates histamine and that histamine acts directly to stimulate acid secretion. Gastrin has been reported to increase histidine decarboxylase activity in gastric mucosa,10'"1 an effect that could result in increased production of histamine since this enzyme converts histidine to histamine. This relationship between gastrin and histamine does not appear to involve cyclic AMP although further studies are needed to clarify this issue. The evidence presented suggests that histamine and not gastrin is the direct stimulant of acid production.40 However, the evidence does not permit one to exclude the possibility that gastrin also directly stimulates acid production. Histamine. Histamine, but not gastrin, has been shown to be capable of directly stimulating adenyl cyclase and histamine is a potent stimulant for mucosal acid secretion.26,27,65,99 Cyclic AMP, dibutyryl cyclic AMP or theophylline also stimulate acid secretion.24 Thus, the data are consistent with the hypothesis that histamine directly stimulates acid secretion by a cyclic AMP-dependent mechanism while gastrin acts by stimulating histamine release and/or formation by a cyclic AMPindependent mechanism. Recent data suggest that histamine, through a cyclic AMP-dependent pathway, stimulates acid secretion by increasing mucosal carbonic anhydrase activity.f4 76 This activation of carbonic anhydrase involves a cyclic AMP-dependent protein kinase and, presumably, reflects phosphorylation of carbonic anhydrase. Canine and Human Gastric Secretion. It would seem unlikely that the basic mechanisms controlling acid secretion in man should differ greatly from those in rodents. Yet, most of the evidence suggests that human and canine acid secretion is not mediated by cyclic AMP. For example, canine gastric mucosal adenyl cyclase does not respond to histamine53 and administration of cyclic AMP does not result in increased acid secretion.53 Administration of gastrin or histamine does not alter mucosal cyclic AMP content although gastric acid secretion is

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increased.52 In addition, although administration of phosphodiesterase inhibitors can increase mucosal cyclic AMP levels, these agents do not stimulate acid secretion.52'53 This evidence suggests that human and canine gastric acid secretion is not cyclic AMP mediated. Recently, however, Bieck et al. have presented data which suggests that cyclic AMP does mediate gastric acid secretion, at least in the dog.17 In their studies, histamine and gastrin were found to increase mucosal cyclic AMP levels and the cyclic AMP content in gastric juice closely paralleled the acid content. Phosphodiesterase inhibitors were found to potentiate the effects of sub-maximal doses of histamine on secretion of acid and of cyclic AMP. Finally, some inhibitors of gastric acid secretion were noted to decrease the secretion of cyclic AMP into gastric juice. Hopefully, further experimentation will establish whether or not the canine and human gastric secretion of acid is mediated by cyclic AMP. It is possible, for example, that the experimental inconsistencies seen in gastric acid secretory studies result from the fact that the gastric mucosa contains a mixture of cell types. Thus, as with the pancreas, further insight would be gained from studies dealing with preparations of isolated acid secreting cells. This approach is being pursued in several laboratories and hopefully will elucidate, more precisely, the role, if any, that cyclic AMP plays in mediating the secretion of acid in the canine and human stomach. Conclusions In this review I have summarized the biochemical steps by which cyclic AMP regulates glycogen metabolism in liver and skeletal muscle. Although incompletely studied, it is likely that very similar steps enable cyclic AMP to mediate hormone-induced effects on other cells. Evidence has been presented to suggest that cyclic AMP mediates the hormonally stimulated processes of smooth muscle relaxation, platelet aggregation, salivary gland amylase secretion, pancreatic fluid and electrolyte secretion, and the inotropic effect of certain hormones on the heart. Although further documentation is required, the evidence would also suggest that pancretic enzyme secretion and, at least in some species, gastric secretion of acid is also mediated by cyclic AMP. Many other hormone-stimulated cell processes are believed to be mediated by cyclic AMP and it should be recognized that the data summarized in this review represent merely a few of the effects of this interesting nucleotide. References 1. Ahren, K., Hjalmarson, A. and Isaksson, 0.: Inotropic and

Metabolic Effects of Dibutyryl Cyclic Adenosine 3',5'-Monophosphate in the Perfused Rat Heart. Acta Physiol. Scand., 82:79, 1971.

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