Classification of Antiarrhythmic Actions

Abstract

In cardiac cells there are four main sources of EMF, as noted in the preceding chapter, engendered by concentration differences across the cell membrane of Na, Ca, K and Cl ions, which would be in equilibrium at intracellular potentials of approximately +56, +120, -94 and -40mV respectively. In order that current from these sources may be used for physiological functions, ion- selective pathways through the membrane can be opened and closed. The function of sodium current is to depolarize atrial and ventricular muscle cells and Purkinje cells, and of calcium current to depolarize nodal cells. Slow depolarization in the central SA node cells permits faster depolarization in the surrounding transitional cells, when they reach threshold, to overtake them and synchronize firing of the node as a whole, from which a ring of excitation spreads outwards in all directions. Slow depolarization of the AV node provides the delay of conduction required while blood is transferred from atrium to ventricle. Calcium current is also involved in excitation-contraction coupling, and can be increased by the opening of an additional set of pathways under the control of adrenergic stimulation. The main function of potassium current is repolarization, but the function of chloride current is uncertain. High Cl permeability in P cells (pale sinoatrial node (SAN) cells) may provide a sink for depolarizing neighbours in the pacemaking process. Chloride-bicarbonate exchange may be concerned in the control of intracellular pH (Vaughan-Jones 1979).