The Effects of Intracellular Na on Contraction and Intracellular pH in Mammalian Cardiac Muscle
Intracelluar Na and pH were measured with recessed-tip ion-selective microelectrodes in voltage-clamped sheep cardiac Purkinje fibers. Intracellular Na activity a Na i was elevated by inhibiting the Na/K pump. This produced an increase of twitch tension that had a steep dependence on the increase of a Na i . These effects of a Na i on twitch tension are probably mediated by an Na-Ca exchange. An increase of a Na i also produced a component of tonic tension that appears to be produced directly by the Na-Ca exchange. The dependence of tonic tension and a Na i on membrane potential suggests that this exchange process may be voltage-sensitive. The increase of a Na i is associated with an intracellular acidification that appears to be secondary to an increase of [Ca2+]i produced by Na-Ca exchange. Therefore, as well as affecting [Ca2+]i, Na-Ca exchange can under some circumstances influence pHi indirectly, and this complicates the interpretation of changes in tension,. since protons and Ca ions have opposite effects on contractile force.
KeywordsMembrane Potential Myocardial Contraction Tonic Contracture Squid Axon Cardiac Purkinje Fibre
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