Effect of sodium on transmembrane calcium movement in the cat ileal longitudinal muscle

Abstract

To get a better insight into the existence and the role of a Na−Ca exchange mechanisms in smooth muscle, the effect of Na substitution with sucrose on tension development, cellular Ca uptake and45Ca efflux was investigated using isolated cat ileal longtitudinal muscle strips. Experimental results were summarized as follows; 1) Exposure of the cat ileal longitudinal muscle to Na-free solution induced a contraction, and the magnitude of the contraction increased after incubation of the muscle strips with ouabain (2×10−5 M) for 1 hr. 2) Cellular Ca uptake in Na-free solution increasedin Na content of the Na-loading media, and a linear relationship existed between tissue Na content and cellular Ca uptake for 10 min. 3) After tissues were equilibrated in PSS containing45Ca for 2hr, cellular Ca uptake decreased with raising the external Na concentration. 4) Removal of medium Na or inhibition of the Na−K pump decreased the rate of45Ca efflux. These results strongly suggested that Na substitution increases cellular Ca uptake and decreases the rate of45Ca efflux via a Na−Ca exchange mechanism.