The effects of intracellular Na on contraction and intracellular pH in mammalian cardiac muscle.
Intracellular Na and pH were measured with recessed-tip ion-selective microelectrodes in voltage-clamped sheep cardiac Purkinje fibers. Intracellular Na activity (aiNa) was elevated by inhibiting the Na/K pump. This produced an increase of twitch tension that had a steep dependence on the increase of aiNa. These effects of aiNa on twitch tension are probably mediated by an Na-Ca exchange. An increase of aiNa also produced a component of tonic tension that appears to be produced directly by the Na-Ca exchange. The dependence of tonic tension and aiNa on membrane potential suggests that this exchange process may be voltage-sensitive. The increase of aiNa 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.
Vaughan-Jones
R D
RD
Eisner
D A
DA
Lederer
W J
WJ
D000818:Animals; D002118:Calcium / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000378:metabolism; D008564:Membrane Potentials; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D011690:Purkinje Fibers / Q000502:physiology; D012756:Sheep; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology
D007473:Ion Channels; D012964:Sodium; D002118:Calcium