Sodium-calcium exchange in regulation of cardiac contraction

The origin and possible regulatory mechanism of tonic tension (ICa-independent component of active contractile activity) were investigated in frog antrial muscle under voltage-clamp conditions. Replacement of NaCl by LiCl resulted in a fast decrease in tonic tension; a similar fast decrease of this contractile component was induced by Ca-free solution. When low Na Ringer's solution was applied, tonic tension increased transiently and then decreased to a steady amplitude; at return to normal Ringer's, a further, substantial decrease in tonic tension occurred before the original level was reached. Similar behavior of tonic tension was observed when both [Na] o and [Ca]o were lowered, but the ratio [Ca]o/[Na]o2 remained constant; the transient changes were prevented by using low Ca and Na solutions and keeping the ratio of [Ca]o/[Na]o5 constant. The significance of Na-Ca exchange in regulating tonic tension and the possibility that this exchange may be electrogenic are discussed.