Characteristics of Na⁺‐Ca²⁺ exchange in frog skeletal muscle

Abstract

1. Fluxes studies were carried out to investigate the Na⁺-dependent outward movement of Ca²⁺ in intact frog sartorius muscle from <i>Leptodactylus ocellatus</i>, 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⁺-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.