Abstract
Changing external sodium concentration results in only transient changes in myocardial tension development. The contractile response to altered [Na] o consists of a brief transient with T 1 2 similar to interstitial space exchange (1.1 min), followed by a return to tention levels obtained prior to changing [Na] o. The initial fast component of the response is dependent on the ratio [Ca] o [Na] o but the steady-state tension development is independent of external Na. This contractile response can be explained by a Na-Ca exchange system where changes in [Na] o result in (1) external competition with Ca for binding sites on the carrier thus affecting Ca influx and tension development, (2) changes in internal Na concentration consequent to changed [Na] o modify Ca o-Na i exchange, compensating for the external effect. In this fashion, steady-state Ca influx (and tension) is maintained despite variations in external [Na].
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