Abstract
The β-adrenergic action of catecholamines is known to exhibit remarkable influence on the excitation-contraction (E-C) coupling of the myocardium to alter the contractility of heart muscle. Cyclic AMP (cAMP) serves as a second messenger of such a catecholamine action, activating cAMP-dependent protein kinase. Protein kinase catalyzes phosphorylation of at least three kinds of important proteins in the myocardial cells, phospholamban of sarcoplasmic reticulum (SR), a protein of sarcolemma, and myofibrillar protein troponin I. It is intriguing to note that all of these phosphorylation reactions are associated with Ca related events within the cell, thus serving to link the interplay between cAMP and Ca2+. Among these, phosphorylation of phospholamban and its functional consequences are extensively defined, in that phospholamban presumably serves to modulate Ca pump ATPase of SR by augmenting the key elementary steps of ATPase (1). The cAMP-mediated cascade of intracellular reactions, leading to increased turnover of Ca fluxes, may largely contribute to alter the contraction-relaxation process of the myocardium (2).
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