RELEASE OF CALCIUM FROM GUINEA-PIG CARDIAC SARCOPLASMIC-RETICULUM INDUCED BY SODIUM-CALCIUM EXCHANGE

Objective: Depolarisation-induced Na+ influx through tetrodotoxin sensitive Na+ channels causes a rapid increase in intracellular Ca2+ concentration ([Ca2+](i)). The Na+ current (I-Na) induced [Ca2+](i) transients: (a) occur after blocking sarcolemmal Ca2+ channels with nisoldipine or D-600, (b) are inhibited by ryanodine, and (c) are dependent upon extracellular Ca2+. Thus the I-Na induced [Ca2+](i) transients arise from sarcoplasmic reticular Ca2+ release triggered by Ca2+ entering the myocyte, following a transient rise in intracellular Na+ ([Na+](i)), via a pathway distinct from sarcolemmal Ca2+ channels. Reverse mode Na+-Ca2+ exchange could provide such a pathway for Ca2+ entry. The aim of this study was to ascertain directly whether Na+-Ca2+ exchange mediates the I-Na induced release of Ca2+ from sarcoplasmic reticulum. Methods: Whole cell voltage and current clamped guinea pig ventricular myocytes dialysed with indo-1 were used; Ca2+ transients elicited upon activation of I-Na before and after inhibiting the exchanger were measured. Results: Following conditioning protocols to load Ca2+ stores, activation of I-Na during a test pulse to -50 mV from a holding potential of -80 mV elicited [Ca2+](i) transients in caesium loaded myocytes superfused with solutions containing 2.5 mM Ca2+ and 5 mu M nisoldipine. When extracellular Na+ was replaced with equimolar lithium, which carries current through Na+ channels but does not readily substitute for Na+ on the Na+-Ca2+ exchanger, or when Ni2+ (5 mM) or dichlorobenzamil (10 mu M), which block the exchanger, were added to superfusion solutions, activation of I,, failed to elicit [Ca2+](i) transients. Lithium and Ni2+ also inhibited nisoldipine insensitive [Ca2+](i) transients elicited by action potentials, indicating that I-Na and Na+-Ca2+ exchange may play a role in excitation-contraction coupling under physiological conditions. Conclusions: Activation of I-Na appears to promote Ca2+ entry into cardiac cells by stimulation of reverse mode Na+-Ca2+ exchange, triggering Ca2+ release from the sarcoplasmic reticulum.