Cytoplasmic Sodium, Calcium and Free Energy Change of the Na+/Ca2+-Exchanger in Rat Ventricular Myocytes

• Published in: Journal of molecular and cellular cardiology 1998-11, Vol.30 (11), p.2437-2447
• Main Authors: Baartscheer, A., Schumacher, C.A., Fiolet, J.W.T.
• Format: Article
• Language: English
• Subjects: Animals; Calcium - metabolism; Cytoplasm - metabolism; Energy Metabolism; Free energy; Heart Ventricles - metabolism; Intracellular calcium; Intracellular sodium; Male; Myocytes; Na+/Ca2+-exchanger; Rats; Rats, Wistar; Sodium - metabolism; Sodium-Calcium Exchanger - metabolism
• ISSN: 0022-2828; 1095-8584
• DOI: 10.1006/jmcc.1998.0803

The relationship between changing driving force of the Na+/Ca2+-exchanger (ΔGexch) and associated cytosolic calcium fluxes was studied in rat ventricular myocytes. ΔGexchwas 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 ΔGexchwas calculated. Following abrupt reversal of ΔGexchfrom +4.1 to −9.2 kJ/mol [Na+]iexponentially decreased from 9.6–2.5 mmol/l (t1/2about 30 s) and [Ca2+]itransiently increased to a peak value after about 30 s. Negative values of ΔGexchwere associated with an increase and positive values with a decrease of [Ca2+]i. Equilibrium (ΔGexch=0) was reached after about 30 s coinciding with the time to peak [Ca2+]i. After 180 s ΔGexchreached a new steady state at +3.5 kJ/mol. Inhibition of SR with ryanodine or thapsigargin reduced the amplitude of the [Ca2+]itransient and shifted its peak to 80 s, but did not affect the time course of [Na+]Ichanges. In the presence of ryanodine or thapsigargin the time required for ΔGexchto recover to equilibrium was also shifted to 80 s. When we changed the ΔGexchto the same extent by the reduction of [Na+]oin combination with a sustained depolarization, [Na+]idecreased less and the amplitude of [Ca2+]itransient was much enhanced. This increase of [Ca2+]iwas completely abolished by verapamil. ΔGexchonly recovered to a little above equilibrium (+1 kJ/mol). Inhibition of the Na+/K+-ATPase with ouabain entirely prevented the decrease of [Na+]iand caused a much larger increase of [Ca2+]i, which remained elevated; ΔGexchrecovered to equilibrium and never returned to positive values. The rate of change of total cytosolic calcium was related to ΔGexch, despite the fact that the calcium flux associated with the exchanger itself contributed only about 10%; SR related flux contributed by about 90% to the rate of change of total cytosolic calcium. In summary, reduction of [Na+]ocauses reversal of the Na+/Ca2+-exchanger and its driving force ΔGexch, a transient increase of [Ca2+]iand a decrease of [Na+]i. The influx of calcium associated with reversed ΔGexchtriggers the release of calcium from SR. Both the decrease of [Na+]iand the increase of [Ca2+]icontribute to the recovery of ΔGexchto equilibrium. The time at which ΔGexchreaches equilibrium always coincides with the time to peak of [Ca2+]i