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K+ fluctuations in the extracellular spaces of cardiac muscle. Evidence from the voltage clamp and extracellular K+ - selective microelectrodes.

UNDERSTANDING cellular cardiac electrophysiology is an important first step in developing the ability to predict and alter pathological conditions in the whole heart. Over the past 30 years, many cardiac electrophysiologists have attempted to analyze the membrane properties of cardiac muscle in a manner similar to the classic analysis of the squid axon (Hodgkin and Huxley, 1952). The upstroke (Weidmann, 1955; Dudel and Rudel, 1970; Colatsky and Tsien, 1979), the plateau (Beeler and Reuter, 1970; Noble and Tsien, 1969), and the diastolic depolarization (Noble and Tsien, 1968) have been interpreted by assuming a number of parallel ionic channels with timeand voltage-dependent conductances. Neurophysiologists have enjoyed significant experimental advantages over the cardiac electrophysiologist in pursuing the initial observations of Hodgkin and Huxley. First, they were able to control the ionic concentrations on either side of the membrane more effectively because of their ability to perfuse the squid axon internally (Baker et al., 1962). Second, selective blockers of the two timeand voltage-dependent channels (tetrodotoxin for the Na channel and tetraethylammonium for the K channel) were available. Third, the squid axon can be voltage clamped by inserting an axial wire