Involvement of a Na-Ca exchange mechanism in contraction induced by low-Na solution in isolated guinea-pig aorta

The possibility of involvement of a Na-Ca exchange mechanism in the contractile responses induced by a reduction of external Na concentration ([Na]0) has been studied in isolated guinea-pig aorta. Low-Na (11.9 mM) solution (Li-substituted) produced a contraction in ouabain-treated muscles in the presence of phentolamine (10(-6) M). The magnitude of the contraction was dependent on the duration of the pretreatment with ouabain (2 x 10(-5) M). Ca-free solution, but not verapamil (10(-6) M), abolished the contraction induced by low-Na solution. The muscles were loaded with various amounts of Na by incubating the tissue with ouabain and varying [Na]0 (11.9--148.7 mM) in the absence of Ca. The magnitude of the contractions induced in these muscles by low-Na solution containing Ca (2.25 mM) was dependent on the cellular Na content. Loss of cellular Na into low-Na solution followed a single exponential time course and the rate coefficient of Na-loss in the presence of external Ca was about twice as great as in the absence of Ca. Cellular 45Ca uptake in low-Na solution was significantly greater in Na-loaded tissues (pretreated with ouabain for 3 h) than in normal tissues. The 45Ca uptake in low-Na solution was not inhibited by verapamil. These results suggest that the contraction induced by low-Na solution is caused by a Ca influx which is dependent on internal Na (a Na-Ca exchange mechanism).