Ion channels in cardiac cell membranes.

Abstract

The electrophysiology of cardiac muscle is notoriously complicated. In the past, some of the difficulties in interpreting voltage clamp data arose from the complex morphology of cardiac tissues and from inadequate tools for une­ quivocal separation of the various ionic current components (2, 13). However, new methods have been developed. Single cardiac cells can be voltage­ clamped and internally dialyzed (51, 54), and single ion channels can be identified and analyzed by the patch clamp method (34). First attempts to reconstitute cardiac sarcolemmal ion channels have been successful (24), Specific binding of radioligands to ion channels emerges as an additional . method for identifying the channels (31, 49). Some of the previous results obtained in intact cardiac tissue and some older concepts have been confirmed, and important new insight has been gained by the new techniques. Although the molecular analysis of ion channels in the heart is still in its early infancy, some initial developments in this field are discussed in this review, In some in­ stances new results from cardiac cells are compared with those from other cells, in order to indicate possible similarities between ion channels in various biological tissues. Because of space limitations, only a few of the many pertinent papers can be discussed. Important new developments in the field of junctional channels and tissue impedance (16, 17,55) are beyond the scope of this review. Table 1 gives an overview of the various currents and the respective identi­ fied channels observed in cardiac cell membranes. The variety of channels is puzzling, a feature that cardiac muscle shares with other excitable tissues, e.g. molluscan neurons (1).