Transmembrane Na+ and Ca2+ electrochemical gradients in cardiac muscle and their relationship to force development

Abstract

Na+- and Ca2+-sensitive microelectrodes were used to measure intracellular Na+ and Ca2+ activities (aiae and ace) of sheep ventricular muscle and Purkinje strands to study the interrelationship between Na+ and Ca2+ electrochemical gradients (ΔµNa and ΔµCa) under various conditions. In ventricular muscle, aiNa was 6.4 ± 1.2 mM and acai was 87 ± 20 nM ([Ca2+] = 272 nM). A graded decrease of external Na+ activity (aNa0) resulted in decrease of aNai, and increase of aiNa. There was increase of twitch tension in low-aiNa solutions, and occasional increase of resting tension in 40% aiNa. Increase of external Ca2+ (aiNa) resulted in increase of aiNa and decrease of aiNa. Decrease of ace resulted in decrease of ace and increase of aNa. The apparent resting Na-Ca energy ratio (ΔµCa and ΔµNa) was between 2.43 and 2.63. When the membrane potential (Vm) was depolarized by 50 mM K+ in ventricular muscle, Vm depolarized by 50 mV, aNa decreased, and ac e increased, with the development of a contracture. The apparent energy coupling ratio did not change with depolarization. 5 × 10-6 M ouabain induced a large increase in aNa and aca, accompanied by an increase in twitch and resting tension. Under the conditions we have studied, ΔµNa and ΔµCa appeared to be coupled and n was nearly constant at 2.5, as would be expected if the Na-Ca exchange system was able to set the steady level of aiCa. Tension threshold was about 230 nM aiCa. The magnitude of twitch tension was directly related to aiCa. © 1982, Rockefeller University Press., All rights reserved.