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⁺ (kCa,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 kCa,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 kCa,o. This effect of Ni²⁺ was associated with a concomitant caffeine-stimulated Ni²⁺ influx. In the absence of caffeine Ni²⁺ did not affect kCa,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⁺-Ca²⁺ 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 1Ni²⁺-1Ca²⁺ exchange.