Mechanisms of Adrenergic and Cholinergic Regulation of Myocardial Contractility. In: Sperelakis N, editor. Physiology and Pathophysiology of the Heart. Boston: Springer US; 1989. pp. 423-52. [DOI: 10.1007/978-1-4613-0873-7_20]

Muscarinic receptor agonists and antagonists: effects on cardiovascular function

Muscarinic receptor activation plays an essential role in parasympathetic regulation of cardiovascular function. The primary effect of parasympathetic stimulation is to decrease cardiac output by inhibiting heart rate. However, pharmacologically, muscarinic agonists are actually capable of producing both inhibitory and stimulatory effects on the heart as well as vasculature. This reflects the fact that muscarinic receptors are expressed throughout the cardiovascular system, even though they are not always involved in mediating parasympathetic responses. In the heart, in addition to regulating heart rate by altering the electrical activity of the sinoatrial node, activation of M₂ receptors can affect conduction of electrical impulses through the atrioventricular node. These same receptors can also regulate the electrical and mechanical activity of the atria and ventricles. In the vasculature, activation of M₃ and M₅ receptors in epithelial cells can cause vasorelaxation, while activation of M₁ or M₃ receptors in vascular smooth muscle cells can cause vasoconstriction in the absence of endothelium. This review focuses on our current understanding of the signaling mechanisms involved in mediating these responses.

Intracellular localization and functional effects of P21-activated kinase-1 (Pak1) in cardiac myocytes

We investigated intracellular localization and substrate specificity of P21-activated kinase-1 (Pak1) in rat cardiac myocytes. Pak1 is a serine/threonine protein kinase that is activated by Rac1/Cdc42 and important in signaling of stress responses. Yet the localization and in vivo function of Pak1 in heart cells is poorly understood. Studies reported here indicate that Pak1 physically interacts with protein phosphatase 2a and localizes to the Z-disk, cell membrane, intercalated disc, and nuclear membrane of adult rat heart myocytes. We compared levels of phosphorylation of cardiac troponin I (cTnI) in control myocytes with phosphorylation of cTnI and myosin binding protein C (C-protein) in myocytes with increased Pak1 activity. The increase in activity was induced by infection of myocytes with a recombinant adenovirus (AdPak1) containing cDNA for a constitutively active Pak1. Control cells were infected with a virus (AdLacZ) containing LacZ. Basal levels of phosphorylation of cTnI and C-protein were relatively high in the myocytes infected with AdLacZ. However, phosphorylation of cTnI and C-protein in cells expressing constitutively active Pak1 was significantly reduced compared with those expressing LacZ. Measurement of Ca2+ tension relations in single myocytes demonstrated that this reduction in phosphorylation of cTnI and C-protein was associated with the predicted increase in sensitivity to Ca2+. Our data provide evidence for a novel pathway of phosphatase regulation in cardiac myocytes.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

Inhibition of type 1 protein phosphatase activity by activation of beta-adrenoceptors in ventricular myocardium

The regulation of protein phosphatase (PP) activity by cardiac beta-adrenergic receptor stimulation with isoproterenol (ISO) was studied in four groups of guinea pigs consisting of seven animals each. Group 1 received the vehicle solution only intraperitoneally; group 2, 6 microg/kg of ISO; group 3, 60 microg/kg of ISO; and group 4, 600 microg/kg of ISO. Total PP activity (consisting of both type 1 and type 2A PP), activity of each PP subtype, the cAMP-dependent protein kinase activity ratio (-cAMP/+cAMP), the phosphorylation of PP inhibitor 1, and the phosphorylation of phospholamban were measured in ventricular tissue. PP activity was also studied in ventricular cardiomyocytes isolated from guinea pigs treated with and without 1 microM ISO or 1 microM ISO plus 10 microM propranolol, an antagonist of the beta-adrenoceptor. PP activity decreased significantly in membrane vesicles, but not in cytosolic fractions, of guinea pigs treated with 60 and 600 microg/kg of ISO compared with untreated animals. The PKA activity ratio, PLB phosphorylation, and PP inhibitor 1 phosphorylation increased in ventricles of guinea pigs treated with 60 and 600 microg/kg of ISO compared with vehicle-treated animals. The decrease in overall PP activity was due primarily to a reduction in type 1 but not type 2A PP activity. In isolated ventricular cardiomyocytes, PP activity was decreased significantly after treatment with 1 microM ISO, and this inhibition was reversed by treatment with 10 microM propranolol. The membrane vesicles of group 1 animals did not release any catalytic subunit of type 1 PP upon phosphorylation by exogenous PKA. These results indicate that activation of cardiac beta-adrenoceptors inhibits type 1 PP activity via phosphorylation of PP inhibitor 1 in the ventricles. This effect is associated with the well-known effect of ISO on increases in the PKA activity ratio and PLB phosphorylation. Inhibition of type 1 PP activity could be one possible mechanism, in addition to activation of adenylate cyclase, by which ISO mediates enhanced contractility of the heart.

Mechanisms for vagal modulation of ventricular repolarization and of coronary occlusion-induced lethal arrhythmias in cats

Our goal was to better understand the mechanisms underlying muscarinic receptor actions on the ventricle in vivo. Therefore, we studied the effects of vagal stimulation on ventricular repolarization and of vagal tone on lethal arrhythmias induced by 30 minutes of left anterior descending coronary artery ligation in anesthetized cats. Experimental groups included normal control cats subjected only to coronary ligation and cats pretreated with atropine, pertussis toxin (PTX), or propranolol. All cats received bilateral cervical vagal stimulation (Vstim) at 1, 3, and 5 Hz for 1 minute at 10-minute intervals. Before coronary ligation, Vstim slowed sinus rate, prolonged the PR interval, and lowered blood pressure. Most important from the point of view of electrophysiological function was a vagally induced acceleration of ventricular repolarization in paced and unpaced hearts, which could be explained by the effects of acetylcholine (ie, shortening the subepicardial muscle action potentials). The effect on repolarization was blocked by atropine or PTX but not by propranolol. The extent of sinus slowing and acceleration of repolarization was directly related to the level of functional PTX-sensitive G protein (P < .05). Coronary occlusion was performed during atrial pacing such that the heart rate in all groups was equal. The incidence of ventricular fibrillation (VF) was 10% in the control group and 50% and 54% in atropine and PTX groups, respectively (P < .05). During atrial pacing before coronary occlusion, a vagal index was calculated as percent QTc shortening during Vstim. When the vagal index was 13% to 26%, the incidence of VF during occlusion was zero. When the vagal index was 0% to 12%, VF was 52% (P < .01). Conclusions are as follows: (1) Vstim accelerates ventricular repolarization in cats via a pathway that incorporates a PTX-sensitive G protein and involves an altered gradient between epicardium and endocardium. (2) Removal of vagal tone during ischemia favors VF, as predicted by a vagal index.

The Sicilian gambit. A new approach to the classification of antiarrhythmic drugs based on their actions on arrhythmogenic mechanisms. Task Force of the Working Group on Arrhythmias of the European Society of Cardiology

The Queen's Gambit is an opening move in chess that provides a variety of aggressive options to the player electing it. This report represents a similar gambit (the Sicilian Gambit) on the part of a group of basic and clinical investigators who met in Taormina, Sicily to consider the classification of antiarrhythmic drugs. Paramount to their considerations were 1) dissatisfaction with the options offered by existing classification systems for inspiring and directing research, development, and therapy, 2) the disarray in the field of antiarrhythmic drug development and testing in this post-Cardiac Arrhythmia Suppression Trial (CAST) era, and 3) the desire to provide an operational framework for consideration of antiarrhythmic drugs that will both encourage advancement and have the plasticity to grow as a result of the advances that occur. The multifaceted approach suggested is, like the title of the article, a gambit. It is an opening rather than a compendium and is intended to challenge thought and investigation rather than to resolve issues. The article incorporates first, a discussion of the shortcomings of the present system for drug classification; second, a review of the molecular targets on which drugs act (including channels and receptors); third, a consideration of the mechanisms responsible for arrhythmias, including the identification of "vulnerable parameter" that might be most accessible to drug effect; and finally, clinical considerations with respect to antiarrhythmic drugs. Information relating to the various levels of information is correlated across categories (i.e., clinical arrhythmias, cellular mechanisms, and molecular targets), and a "spread sheet" approach to antiarrhythmic action is presented that considers each drug as a unit, with similarities to and dissimilarities from other drugs being highlighted. A complete reference list for this work would require as many pages as the text itself. For this reason, referencing is selective and incomplete. It is designed, in fact, to provide sufficient background information to give the interested reader a starting frame of reference rather than to recognize the complete body of literature that is the basis for this article.