The hypothesis advanced in this article requires further validation. Undoubtedly it will require modification as our knowledge of biochemical control increases. Nevertheless, it should prove useful in focusing attention on the apparent similarity in the response of a large number of specific cell types to particular stimuli. Emphasis has been placed on a few common and apparently key elements in these responses. It is recognized that other factors are undoubtedly involved. Specifically, the changes in membrane potentials indicate the likelihood of widespread changes in the properties of the cell membrane, for example, changes in Na(+) and K(+) transport and distribution. These aspects of cellular responses may eventually prove to be of equal or greater importance than those common aspects of the system already identified.

Cell Communication, Calcium Ion, and Cyclic Adenosine Monophosphate

Cyclic AMP (adenosine 3',5'-monophosphate or cyclic adenylate) has now been established as a second messenger mediating many of the effects of a variety of hormones. Several of the metabolic effects mediated by cyclic AMP are discussed, and it is suggested that certain other ("functional" or "mechanical") hormonal effects may be similarly mediated. In particular, the evidence presented supports the hypothesis that the positive inotropic response to the catecholamines is mediated by cyclic AMP. Although knowledge of the biological role of cyclic AMP has not been widely applied clinically, sufficient knowledge has now accumulated to make research in this area desirable.

Some Aspects of the Biological Role of Adenosine 3',5'-monophosphate (Cyclic AMP)

Beta-Adrenergic relaxation in bovine coronary arteries is enhanced by inhibition of eicosonoid metabolism and inhibited by its stimulation. We investigated the interaction between eicosonoid metabolism and beta-adrenergic mechanisms by studying the effect of perturbations of eicosonoid metabolism on vascular adenosine 3',5'-monophosphate (cAMP) content and the cAMP-dependent relaxation of isometric force and activation of glycogen phosphorylase. KCl (35 mM) elicited a contraction, activated phosphorylase, and slightly decreased cAMP content. Isoproterenol (10(-7) M) relaxed the KCl contraction, further increased phosphorylase activity, and increased cAMP. Neither indomethacin (5 X 10(-6) M) nor arachidonic acid (3 X 10(-5) M) affected the KCl contraction, but arachidonic acid increased both cAMP and phosphorylase activity and indomethacin decreased cAMP. Indomethacin potentiated the relaxation induced by isoproterenol but inhibited the activation of phosphorylase and had no effect on the isoproterenol-induced increase in cAMP. Arachidonic acid, on the other hand, inhibited the isoproterenol-induced relaxation but potentiated both the increases of phosphorylase activity and cAMP. Thus neither relaxation nor phosphorylase activity was related in a straightforward manner to the total cAMP content. A direct relation between cAMP, relaxation, and phosphorylase can be reconciled with the antiparallel effects of alterations of eicosonoid metabolism observed in this study by a proposed model in which the effects of cAMP are assumed to be functionally compartmentalized.

Eicosonoid metabolism and beta-adrenergic mechanisms in coronary arterial smooth muscle: potential compartmentation of cAMP

The biologic changes occurring in severely ischemic myocytes in vivo as the affected cells pass through the phase of reversible to the phase of lethal or irreversible injury are reviewed with special emphasis on the effect of ischemia on the production and utilization of highenergy phosphate, the destruction of the adenine nucleotide pool, and the appearance of signs of damage to the plasma membrane of the sarcolemma. Evidence is presented that indicates that the events occurring in severe ischemia in vivo are essentially identical to those found in total ischemia in vitro except that the biologic changes of ischemia develop more slowly in total ischemia in vitro than in severe ischemia in vivo. The slower time course of injury, together with the uniformity of injury provided by total ischemia in vitro, may allow for more precise identification of potential lethal cellular events in ischemic injury. The production of highenergy phosphates (HEP) from anaerobic glycolysis have been estimated in both in vivo and in vitro ischemia by the measurement of lactate accumulation, and total HEP utilization has been estimated from the depletion of stores of preformed HEP. The results show that between 80% and 90% of the HEP utilized by ischemic dog left ventricle is produced by anaerobic glycolysis. The onset of irreversibility is associated with marked depletion of the HEP and adenine nucleotide pools of the tissue and the cessation of energy production via glycolysis. The cessation of anaerobic glycolysis may be caused by the low sarcoplasmic, adenosine triphosphate (ATP) concentration of the dying myocyte. In addition to the foregoing changes, irreversibly injured tissue exhibits both ultrastructural and functional evidence of disruption of the plasmalemma of the sarcolemma. The possible relationships, causal and otherwise, between severe HEP depletion and membrane damage are discussed. Both HEP depletion (ATP < 3-8% of control) and membrane damage are considered to be objective signs of the presence of irreversible myocardial ischemic injury. However, at the present time, there is no proof that these changes are causally related either to each other or to cell death in severe in vivo ischemia.

Lethal myocardial ischemic injury.

Compartments of cyclic AMP and protein kinase in mammalian cardiomyocytes.

The effect of epinephrine on the biochemical sequence of events leading to the activation of glycogen phosphorylase was studied in the isolated, perfused rat heart. In arrested preparations, perfused with a medium either devoid of Ca++ or containing a depolarizing concentration of K+, the epinephrine-induced activation of phosphorylase was markedly diminished. In the absence of Ca++, however, the epinephrine-induced rise in cyclic adenosine 39,59-monophosphate (cyclic AMP) concentration and the activation of phosphorylase kinase were not greatly altered. This suggested that the effect of omission of Ca++ occurred at the phosphorylase b to a conversion step. The effect of epinephrine on phosphorylase activation was blocked simultaneously with the cessation of contraction following perfusion with a Ca++-deficient medium. The store of Ca++ required for the maintenance of contractile activity appeared to be similar to that necessary for the conversion of phosphorylase b to a. Elevation of Ca++ in the perfusion medium resulted in activation of phosphorylase but not of phosphorylase kinase; nor was there an increase in cyclic AMP concentration. This observation also suggested that Ca++ is required for the catalytic activity of phosphorylase kinase. In the heart depolarized with K+, only a small and transient increase in cyclic AMP concentration occurred in response to epinephrine. Activation of phosphorylase kinase was considerably less than that observed in the beating heart. This suggested that an excess of K+ interfered with the action of epinephrine at a site at or before adenyl cyclase. Thus the activation of phosphorylase in cardiac muscle can be the consequence of one of the following events: (a) the activation of phosphorylase kinase as a result of a rise in cyclic AMP concentration; (b) an increase in the catalytic activity of phosphorylase kinase as a result of an increase in the concentration of Ca++ available to the enzyme; or (c) a combination of these two mechanisms.

The role of potassium and calcium ions in the effect of epinephrine on cardiac cyclic adenosine 3',5'-monophosphate, phosphorylase kinase, and phosphorylase.

The purpose of this investigation was to contrast the effect of glucagon and that of epinephrine on the concentration of cyclic adenosine 3',5'-monophosphate (cyclic AMP), the activity of phosphorylase a and the contractile amplitude of isolated perfused rat hearts. The two drugs were about equally effective except that the maximal augmentation of contractility by epinephrine (5 x 10-9 moles) was twice that produced by an equivalent dose of glucagon with a fourfold greater increase in cyclic AMP concentration. Combination of large doses of the two drugs caused increases in the cyclic nucleotide considerably greater than those required for maximal phosphorylase activation or associated with a maximal inotropic response. The effects of glucagon also developed more slowly than those of epinephrine. An increase in cyclic AMP was not detectable until after phosphorylase a and contractile amplitude had increased.

The beta-receptor-blocking agents dichloroisoproterenol and pronethalol did not block the biochemical responses to glucagon in doses which abolished the epinephrine-induced increases in cyclic AMP and phosphorylase a . These results, along with those obtained by other investigators, indicate that glucagon can elicit the same biochemical responses in intact heart as have been obtained with epinephrine, but by action at a different receptor site.

Effect of Glucagon on Cyclic 3',5'-AMP, Phosphorylase Activity and Contractility of Heart Muscle of the Rat

The effects of cocaine on the contractile force and phosphorylase responses to catecholamines and on the myocardial uptake of these agents has been studied in the dog heart in situ . The effects of norepinephrine on the heart were potentiated by cocaine to a greater extent than were those of epinephrine, while those of isoproterenol were not potentiated at all. Cocaine decreased the amount of H3-norepinephrine and H3-epinephnne found in the heart at the peak of the contractile force response less than 30 seconds after their injection, but had no effect on the amount of H3-isoproterenol in the heart. It was shown by the concurrent administration of K42 that the changes induced by cocaine in myocardial uptake of catecholamines were not due to alterations in blood flow to the heart. The results support the concept that the ability of cocaine to potentiate the actions of a catecholamine in a tissue depends on the extent to which the amine is inactivated in that tissue by an uptake mechanism which is sensitive to inhibition by cocaine.

Cocaine potentiation of the cardiac inotropic and phosphorylase responses to catecholamines as related to the uptake of h3-catecholamines

Several hormones known to affect tissue glycogen concentration have been studied in the open-chested rat in relation to their effects on the enzymes of cardiac glycogen metabolism. Epinephrine, given by rapid intravenous injection, increased phosphorylase a activity and glucose-6-P concentration in both heart and skeletal muscle. In contrast, glycogen synthetase I activity was increased in heart and decreased in skeletal muscle. Continuous infusion of epinephrine (1 µg/kg min-1) caused marked glycogenolysis in skeletal muscle, but no change in heart glycogen concentration. In these experiments a transient increase was seen in cardiac glycogen synthetase I activity, while the percentage synthetase I decreased in skeletal muscle. Infusion of 2.5 µg/kg min-1 of epinephrine decreased cardiac glycogen and produced a biphasic change in synthetase I activity, consisting of first an increase, then a decrease in heart percentage of glycogen synthetase I. All these effects of epinephrine were prevented by prior administration of the beta adrenergic blocking agent pronethalol. Glucagon caused marked cardiac glycogenolysis, an increase in the percentage of phosphorylase a, but no change in the percentage of synthetase I. These effects were also antagonized by pronethalol pretreatment. Insulin caused an increase in the percentage of synthetase I in heart without affecting phosphorylase activity. The increase in synthetase I activity was unaffected by the administration of pronethalol. Thus, in heart as in skeletal muscle, hormones affect the critical steps in glycogen synthesis and degradation, perhaps in part through a common pathway sensitive to beta adrenergic blockade. However, other factors such as physiological changes in cardiac muscle may be involved in the magnitude and direction of the response.

Hormonal effects on glycogen metabolism in the rat heart in situ.

Two types of receptors have been described as reacting with sympathomimetic drugs and adrenergic blocking agents a t effector sites for physiological responses. The classification of metabolic responses to adrenergic stimuli, however, follows a clear pattern to only a limited extent (TABLE 1). The transformation of cardiac glycogen phosphorylase b to the a form of the enzyme is specifically mediated by a beta receptor. The order of potency of catecholamines in causing this response is the same as their effectiveness as physiological agonists in the heart2 (isoproterenol > epinephrine = norepinephrine), and the effect of these drugs is specifically inhibited by beta adrenergic blockade.\"2\"\"' The augmentation of glycogenolysis in skeletal muscle can also be said to involve a beta receptor with respect to both the relative potencies of the catecholamines and the effectiveness of beta but not of alpha antagonists in blocking the glycogenolytic re~ponse .~\" However, in contrast t o heart, the methoxamine type of antagorists are effective in skeletal Methoxamine and related sympathomimetic agents do not readily fit into the category of beta adrenergic blocking drugs because, while they block certain metabolic responses to catecholamines, they do not interfere with the contractile responses and ~ e a k l y , ' ' ~ if a t all,\" with the biochemical responses of the heart to adrenergic stimulation. I t is not possible t o classify the receptor which mediates glycogenolysis in the liver as either alpha or beta. Epinephrine and isoproterenol are equieffective but more potent than norepinephrine in the intact dog4 and in uitro. However, both alpha and beta blocking agents interfere with these effects. Ergot alkaloids abolish the glycogenolytic response to adrenergic stimula-

Steven E. Mayer

Papers

The role of potassium and calcium ions in the effect of epinephrine on cardiac cyclic adenosine 3',5'-monophosphate, phosphorylase kinase, and phosphorylase.

Effect of Glucagon on Cyclic 3',5'-AMP, Phosphorylase Activity and Contractility of Heart Muscle of the Rat

Cocaine potentiation of the cardiac inotropic and phosphorylase responses to catecholamines as related to the uptake of h3-catecholamines

Hormonal effects on glycogen metabolism in the rat heart in situ.

Adrenergic mechanisms in cardiac glycogen metabolism.