Potassium Channels and the Repolarization of Cardiac Cells“ L. J. DEFELICE, W. N. GOOLSBY, AND M. MAZZANTI zyxw Anatomy and Cell Biology zyx Emory University School of Medicine Atlanta, Georgia 30322 INTRODUCTION zyxw Though all currents are involved in each phase of the action potential, K currents in particular help repolarize heart cells, allowing them to beat again. Naming potassium as the repolarizing ion, however, is convention rather than a strict definition of its role. Though all currents interact in the free-running membrane, it is permissible to consider the contribution of each to the overall potential. The balance of the inward and outward currents causes some cells to beat spontaneously, as in the SA node, and requires others to be stimulated, as in the ventricle. Knowing the magnitude of a particular current at every moment allows a precise statement of how that current helps shape the action potential. This article shows how to calculate the contribution of each action current to the action potential, and it demonstrates how to measure the action currents directly. How does a particular class of channels contribute to the action potential? The traditional answer depends on a model of the voltage-dependent, time-variant con- ductance of the ion. The model of the current is usually derived from step-protocol, voltage-clamp experiments.’+ Driving a particular model with an action potential is one way to deduce the action current associated with an individual current. This paradigm, which Hodgkin and Huxley’ introduced around 1950 to study K and Na currents in squid axons, is the principal means of studying neurons, heart cells, and other excitable tissues. The patch-clamp technique Neher and Sakmann6 introduced in 1976 allows single-electrode, whole-cell voltage-clamp of individual cells.’ The protocols used in the patch-clamp technique most often retain the basic Hodgkin- Huxley approach. One way to look at the established method of finding action currents is to recognize that with traditional instruments it is impossible to measure current (voltage clamp) and voltage (current clamp) at the same time and place. Current meters and volt meters are too dissimilar to connect to the same membrane. Thus, we do not generally measure action potentials and action currents at the same time, and we are as a rule forced to go through the modeling procedure to answer the original question. Bren- ‘This work was supported by the National Institutes of Health HL-27385. zyx 174