CHARACTERISTICS OF NA+ CA2(+) EXCHANGE IN FROG SKELETAL-MUSCLE

1. Fluxes studies were carried out to investigate the Na+-dependent outward movement of Ca2+ in intact frog sartorius muscle from Leptodactylus ocellatus, a preparation for which published data on the subject are sparse. 2. Under normal resting conditions the Na+-Ca2+ exchange was not readily detectable. 3. When muscles were exposed to 4 mM caffeine, the rate of fractional loss of Ca2+ (k(Ca,o)) increased by about 50%. Most of this increase exhibits characteristics typical of the Na+-Ca2+ antiport working in the forward mode found in other cells. 4. The increase in k(Ca,o) promoted by caffeine was decreased by: (a) 72% in the absence of external Na+ (Na-o(+)); (b) 73% in Na+-loaded muscles ([Na+](i) = 98 mM); (c) 70% when fibres were depolarized to -27 mV ([K+](o) = 50 mM); and (d) 80% in the presence of 5 mM amiloride. 5. Ni2+ (5 mM), an inhibitor of the Na+-Ca2+ exchanger current, unexpectedly increased the caffeine-promoted rise in k(Ca,o). This effect of Ni2+ was associated with a concomitant caffeine-stimulated Ni2+ influx. In the absence of caffeine Ni2+ did not affect k(Ca,o). 6. It was concluded that: (a) under resting conditions the sarcolemmal Ca2+ pump suffices to handle the cytosolic calcium concentration ([Ca2+](i)); (b) Na+-C2+ exchange activity becomes apparent when [Ca2+](i) is substantially increased by caffeine-induced Ca2+ release from the sarcoplasmic reticulum; and (c) the blocking effect of Ni2+ on the current generated by a Na+-Ca2+ exchange with a coupling ratio > 2 might actually represent a shift of the antiport mode toward an electroneutral 1Ni(2+)-1Ca(2+) exchange.