Reversal of current through calcium channels in dialysed single heart cells

Abstract

Calcium channels contribute to important cellular functions such as impulse conduction, rhythmic activity, muscle contraction, and secretion¹ and are a major target for modulation by neurohormones and drugs^2,3. Despite their importance, Ca channels are much less well understood than Na and K channels because of several problems reviewed by Hagiwara and Byerly¹. First, few preparations allow good control of electrical potential and ionic composition on both sides of the membrane, yet display strong Ca currents. Second, current carried by Ca channels is often obscured by ion movements through other membrane pathways. Third, outward current flow through Ca channels, carried by Ca²⁺ or any other ion, has been difficult to demonstrate. This is unfortunate because measurements of the potential at which current reverses (E_rev) have been crucial in understanding ion permeation and selectivity in other ionic channels (see ref. 4). In molluscan neurones, attempts at recording outward current through the Ca channel have not succeeded, largely because of overlap by nonspecific outward current^5–7. In multicellular cardiac muscle preparations, strong depolarizations produce a decaying outward current that Renter and Scholz attributed to K efflux through the Ca channel⁸. Their interpretation is controversial, however, since the evidence leaves open other explanations for the outward current—for example, Na efflux via Na–Ca exchange⁹, or K efflux through K-selective channels. To overcome these problems, we studied Ca channels in single isolated heart muscle cells^10–12 using a suction pipette method¹³. We were able to record robust Ca currents with minimal interference from other time-dependent currents while controlling potential and ion composition on both sides of the membrane. Here we present experimental evidence for a genuine reversal of ionic current through Ca channels due to outward movement of K⁺ ions, in support of the hypothesis of Reuter and Scholz.