Cytoplasmic sodium, calcium and free energy change of the Na+/Ca2+-exchanger in rat ventricular myocytes

The relationship between changing driving force of the Na+/Ca2+-exchanger (Delta G(exch)) and associated cytosolic calcium fluxes was studied in rat ventricular myocytes. Delta G(exch) was abruptly reversed by the reduction of extracellular sodium ([Na+](o)) with or without sustained depolarization by the elevation of potassium (K+](o)). Cytosolic sodium ([Na+](i)) and calcium ([Ca2+](i)) were measured with SBFI and indo-1 respectively and the time course of recovery of Delta G(exch) was calculated. Following abrupt reversal of Delta G(exch) from + 4.1 to - 9.2 kJ/mol [Na+](i) exponentially decreased from 9.6-2.5 mmol/l (t(1/2) about 30 s) and [Ca2+](i) transiently increased to a peak value after about 30 s. Negative values of Delta G(exch) were associated with an increase and positive values with a decrease of [Ca2+](i). Equilibrium (Delta G(exch)=0) was reached after about 30 s coinciding with the time to peak [Ca2+](i). After 180 s Delta G(exch) reached a new steady state at + 3.5 kJ/mol. Inhibition of SR with ryanodine or thapsigargin reduced the amplitude of the [Ca2+](i) transient and shifted its peak to 80 s, but did not affect the time course of [Na+](i) changes. In the presence of ryanodine or thapsigargin the time required for Delta G(exch) to recover to equilibrium was also shifted to 80 s. When we changed the Delta G(exch) to the same extent by the reduction of [Na+](o) in combination with a sustained depolarization, [Na+](i) decreased less and the amplitude of [Ca2+](i) transient was much enhanced. This increase of [Ca2+](i) was completely abolished by verapamil. Delta G(exch) only recovered to a little above equilibrium (+1 kJ/mol). Inhibition of the Na+/K+-ATPase with ouabain entirely prevented the decrease of [Na+](i) and caused a much larger increase of [Ca2+](i), which remained elevated; Delta G(exch) recovered to equilibrium and never returned to positive values. The rate of change of total cytosolic calcium was related to Delta G(exch), despite the fact that the calcium Bur associated with the exchanger itself contributed only about 10%; SR related nux contributed by about 90% to the rate of change of total cytosolic calcium. In summary, reduction of [Na+](o) causes reversal of the Na+/Ca2+-exchanger and its driving force Delta G(exch), a transient increase of [Ca2+](i) and a decrease of [Na+](i). The influx of calcium associated with reversed Delta G(exch) triggers the release of calcium from SR. Both the decrease of [Na+](i) and the increase of [Ca2+](i) contribute to the recovery of Delta G(exch) to equilibrium. The time at which Delta G(exch) reaches equilibrium always coincides with the time to peak of [Ca2+](i) transient. Activation of the Na+/K+-ATPase is required to reduce [Na+](i) and recover Delta G(exch) to positive Values in order to reduce [Ca2+](i). We conclude that Delta G(exch) is a major regulator of cytosolic calcium by interaction with SR. (C) 1998 Academic Press.