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 v entricular myocytes. Delta G(exch) was abruptly reversed by the reduction o f extracellular sodium ([Na+](o)) with or without sustained depolarization by the elevation of potassium (K+](o)). Cytosolic sodium ([Na+](i)) and cal cium ([Ca2+](i)) were measured with SBFI and indo-1 respectively and the ti me course of recovery of Delta G(exch) was calculated. Following abrupt rev ersal of Delta G(exch) from + 4.1 to - 9.2 kJ/mol [Na+](i) exponentially de creased from 9.6-2.5 mmol/l (t(1/2) about 30 s) and [Ca2+](i) transiently i ncreased 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 [C a2+](i). Equilibrium (Delta G(exch)=0) was reached after about 30 s coincid ing with the time to peak [Ca2+](i). After 180 s Delta G(exch) reached a ne w steady state at + 3.5 kJ/mol. Inhibition of SR with ryanodine or thapsiga rgin 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 pre sence of ryanodine or thapsigargin the time required for Delta G(exch) to r ecover 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 [C a2+](i) transient was much enhanced. This increase of [Ca2+](i) was complet ely abolished by verapamil. Delta G(exch) only recovered to a little above equilibrium (+ 1 kJ/mol). Inhibition of the Na+/K+-ATPase with ouabain enti rely prevented the decrease of [Na+](i) and caused a much larger increase o f [Ca2+](i), which remained elevated; Delta G(exch) recovered to equilibriu m and never returned to positive values. The rate of change of total cytoso lic calcium was related to Delta G(exch), despite the fact that the calcium Bur associated with the exchanger itself contributed only about 10%; SR re lated 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 [Ca 2+](i) and a decrease of [Na+](i). The influx of calcium associated with re versed Delta G(exch) triggers the release of calcium from SR. Both the decr ease of [Na+](i) and the increase of [Ca2+](i) contribute to the recovery o f Delta G(exch) to equilibrium. The time at which Delta G(exch) reaches equ ilibrium always coincides with the time to peak of [Ca2+](i) transient. Act ivation of the Na+/K+-ATPase is required to reduce [Na+](i) and recover Del ta G(exch) to positive Values in order to reduce [Ca2+](i). We conclude tha t Delta G(exch) is a major regulator of cytosolic calcium by interaction wi th SR. (C) 1998 Academic Press.