Effects of chloride replacement and chloride transport blockade on the tonic tension of frog atrial trabeculae.

The effects of a chloride-poor medium (methanesulfonate substituted) and a chloride transport inhibitor (SITS) on the outward delayed current and the tonic tension were studied on frog atrial trabeculae under voltage-clamp conditions. The outward delayed current decreased in low-chloride medium (10.5 mmol/l) or in the presence of SITS (2 mmol/l). The tonic tension increased in chloride-poor solution and decreased following SITS. The replacement of chloride by methanesulfonate enhanced the transient increase of tonic tension induced by low external sodium concentration while SITS reduced it. In the same conditions, the effect of the chloride-poor medium was abolished in the presence of SITS. The results showing an increase in Na-Ca exchange in low-chloride medium and a decrease by SITS are discussed assuming that changes in the inner negative charge density influenced the Na-Ca exchange mechanism; the influence of pHi variation are also considered.