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Ca45, K42, and Na24 exchanges were compared with tension changes induced by epinephrine (1.5 x 10–5 m) and by lowering the extracellular sodium concentration (Na)0 in normal and glycerol-extracted isolated spiral strips of rabbit aortas. Contractions induced by epinephrine and lowering the (Na)0 to one-fifth of normal resulted in an increased Ca influx of 107 and 225%, respectively. Epinephrine had no effect on tension or Ca influx in glycerol-extracted preparations. Ca45 efflux was not affected. Epinephrine in normal Ringer's solution had no effect on Na24 net entry, but increased K efflux 486% and K influx 107% with the development of 1.467 g of tension. K efflux and influx increased 215 and 30%, respectively, when the aortas were placed in Ca-free Ringer's solution, whereas tension decreased 0.158 g. These data were interpreted to suggest that Ca, not Na or K, was directly involved in contraction in isolated rabbit aortic strips. Na appears to act through an antagonism with Ca.
Briggs et al. (1961) studied this question.