Voltage Clamp Analysis of Embryonic Heart Cell Aggregates

I N T R O D U C T I O N T h e voltage c lamp technique has been used by n u m e r o u s investigators to describe the voltageand t i me -dependen t conductances in cardiac tissue. T h e main conclusion f rom these analyses is that the ionic cur ren ts that under l ie the action potent ial in the hear t are m o r e n u m e r o u s and complex than in the classic squid axon (for reviews, see Trau twe in , 1973; McAllister et al., 1975; N o m a and Ir isawa, 1976; Beeler and Reuter , 1977; T rau twe in and McDonald, 1978). A major p rob lem in the quanti tat ive in te rpre ta t ion o f voltage c lamp data in hear t tissue arises out o f the multicellular na tu re of the p repara t ion . H e a r t muscle is com pos ed of individual cells connec ted by junc t ions of low but vary ing resistance, into fibers o f complex geome t ry (DeHaan and Fozzard, 1975; Kensler et al., 1977). Substantial voltage gradients may exist within such fibers, and currents crossing the m e m b r a n e o f one cell may d i f fe r f r o m those f lowing in a distant cell. T h e p rob l em of electrical inhomogene i ty of cardiac tissue and the limitations it places on voltage c lamp analyses have been d o c u m e n t e d and discussed at length. I t is general ly ag reed that, because o f the multicellular na ture o f hear t muscle, it has not been possible to control ideally the m e m b r a n e potential o f any p repa ra t ion . Thus , there r ema in substantial uncertaint ies J. GEN. PHYSIOL. 9 The Rockefeller University Press 9 0022-1295[79/02/0175-2451.00 175 Volume 73 February 1979 175-198 on Jne 3, 2017 D ow nladed fom Published February 1, 1979