Specific Pharmacology of Calcium in Myocardium, Cardiac Pacemakers, and Vascular Smooth Muscle

Abstract

The essential connection of the basic physiological processes of excitation and contraction with transmembrane movements of Na, K, and Ca ions probably origi­ nates from an early stage of cellular evolution. Despite innumerable modifications the fundamental processes that developed in the ocean have not undergone major changes during the course of development of higher forms of animal life. Thus in a wide variety of excitable cells the transmembrane exchange of the monovalent marine cations Na and K can be considered the substantial basis of bioelectric membrane activity, whereas Ca ions are required as mediators when, by this superfi­ cial process, intracellular reactions such as muscular contraction, glandular secre­ tion, or liberation of transmitter substances are initiated ( 1, 2). Ca ions can exert this messenger function either in a primitive way, by penetrating into the intracellu­ lar space across the depolarized cell membrane or, at a more advanced stage of evolution, by being released from intracellulariy located endoplasmic stores. As to contractile tissues, the development of large endoplasmic Ca pools is most obvious in skeletal muscle, whereas myocardial fibers and, particularly, smooth muscle cells are less specialized in this respect. The natural consequences are as follows: