Abstract
1. 1. 22Na efflux from rat ileal smooth muscle possessed a slow, presumably intracellular, compartment of only 10% total activity. 2. 2. Na-free and K-free media reduced 22Na efflux as did 2 × 10 −4M ouabain, but a significant ouabain-insensitive efflux fraction remained. 3. 3. Quabain inhibited 45Ca efflux from the ileum indicating that some calcium extrusion depended upon an ouabain-sensitive sodium carrier mechanism. 4. 4. 24Na and 45Ca double labelling showed that Na-free media stimulated 24Na and 45Ca efflux if calcium was present in the buffer. 5. 5. In Ca-free media, Na-free conditions inhibited both Na and Ca efflux, indicating the presence of a sodium-dependent calcium efflux. 6. 6. Mg-free media stimulated 45Ca efflux in the presence of external calcium, but in Ca-free conditions, Mg-free media inhibited 45Ca efflux, indicating the presence of a Mg-dependent Ca efflux. 7. 7. It was concluded that calcium regulation in this muscle depended upon counterexchanges with Na and Mg as well as cytoplasmic buffering.
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