COMPARATIVE ASPECTS OF IONIC CONDUCTANCES IN NERVE AND HEART CELL MEMBRANES

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This project is concerned with a comparative analysis of ionic current channels in nerve and heart cell membranes and the relationship of these channels to electrical activity, with a particular emphasis on potassium ion channels in both preparations and the effects of various ionic blockers on these channels. During the past year the primary experimental preparations which have been used are squid giant axons, chick embryonic heart cells, and mongrel dog hearts. The mechanisms by which ionic blockers and other agents alter potassium ion currents in the squid and chick heart cell preparations have been investigated with the voltage clamp technique. This work has focused recently on the derivatives of triethylammonium ions. A major finding has been the discovery of a relationship between the size of the ionic blocker and the mechanism of blockade. Specifically, the smaller sized members of this sequence, such as methyltriethylammonium and tetraethylammonium block channels without altering channel gating, whereas blockage by larger sized ions, such as n-pentyltriethylammonium and n-nonyltriethylammonium is accompanied by an alteration of gating. The relationship between potassium channel blockade and the mechanism of anti-fibrillatory drugs has been further investigated with the open-chested dog heart preparation using quaternary derivatives of lidocaine. In particular, QX314 and QX572 produce a significant increase in ventricular fibrillation threshold, which is well correlated with the blockade of potassium current in the squid and chick heart cell preparations which these agents produce. The relationship between ionic currents and spontaneous electrical activity in the heart has been further investigated with our ionic current model of embryonic chick atrial cells. Addition of 10-8M tetrodotoxin alters the shape of the action potential without altering beat rate. An analysis of our measurements of sodium ion current, I-Na, from single cells using the suction pipette technique before and after application of TTX, together with our computer model, illustrates the role of I-Na in spontaneous activity.