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

We have investigated whether the Na+/Ca2+-exchanger has a functional regulatory role in the control of oxidative metabolism in suspensions of isolated rat ventricular myocytes. Therefore we simultaneously measured intracellular [Ca2+] ([Ca2+](i)) with Indo-1 and respiratory rate (VO2) after abrupt manipulation of the free-energy of the Na+/Ca2+-exchanger (Delta G(exch)). The average fraction of viable myocytes was about 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 potential and/or intracellular cation activities with the use of gramicidin or veratridine. A change of extracellular cation composition caused a transient increase of [Ca2+](i) and VO2, with peak values after 30 to 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 the thermodynamic disturbance. Inhibition of sodium-pump activity with ouabain greatly enhanced peak values and reduced the rate of return to a new steady state. Reversal of the initial disturbance of Delta G(exch) by restoring [Na+](0) or reduction of [Ca2+](0) during the time course of the transients greatly accelerated return to a new steady-state. An increase of sarcolemmal sodium permeability with the Na-channel ligand 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, irrespective of the nature of the intervention (either extracellular or intracellular manipulation of Delta G(exch)). We conclude that cytoplasmic [Ca2+] (thermodynamically controlled by the Na+/Ca2+-exchanger) is a major regulator of the respiratory rate in (quiescent) myocytes. (C) 1995 Academic Press Limited