INTRACELLULAR [CA2-ENERGY OF THE NA+(] AND VO(2) AFTER MANIPULATION OF THE FREE)CA2+-EXCHANGER IN ISOLATED RAT VENTRICULAR MYOCYTES/

We have investigated whether the Na+/Ca2+-exchanger has a functional r egulatory role in the control of oxidative metabolism in suspensions o f isolated rat ventricular myocytes. Therefore we simultaneously measu red intracellular [Ca2+] ([Ca2+](i)) with Indo-1 and respiratory rate (VO2) after abrupt manipulation of the free-energy of the Na+/Ca2+-exc hanger (Delta G(exch)). The average fraction of viable myocytes was ab out 90% (82% rod-shaped plus 8% viable round cells). Delta G(exch) was manipulated either by an abrupt decrease,of [Na+](0) (in combination with an increase of [K+](0) or [Ca2+](0)) or by changing membrane pote ntial and/or intracellular cation activities with the use of gramicidi n or veratridine. A change of extracellular cation composition caused a transient increase of [Ca2+](i) and VO2, with peak values after 30 t o 40 s and a new steady state near control values after 180 to 240 s. Peak values of the transients were associated with the magnitude of th e thermodynamic disturbance. Inhibition of sodium-pump activity with o uabain greatly enhanced peak values and reduced the rate of return to a new steady state. Reversal of the initial disturbance of Delta G(exc h) by restoring [Na+](0) or reduction of [Ca2+](0) during the time cou rse of the transients greatly accelerated return to a new steady-state . An increase of sarcolemmal sodium permeability with the Na-channel l igand veratridine or manipulation of [Na+](i) and [K+](i) with the Na/K+-exchanger gramicidin caused monophasic increase of both [Ca2+](i) and VO2. The relationship between VO2 and [Ca2+](i) was the same, irre spective of the nature of the intervention (either extracellular or in tracellular manipulation of Delta G(exch)). We conclude that cytoplasm ic [Ca2+] (thermodynamically controlled by the Na+/Ca2+-exchanger) is a major regulator of the respiratory rate in (quiescent) myocytes. (C) 1995 Academic Press Limited