External calcium sensitivity of low sodium contractures in the control and hypertrophied right ventricle of the ferret.

The existence of possible differences of calcium (Ca2+) fluxes through the sarcolemmal sodium-calcium (Na+/Ca2+) exchanger during hypertrophy has been tested by comparing the characteristics of the contracture--as an indicator of the intracellular Ca2+ concentration--induced by partial or total withdrawal of external sodium (Na+), in the absence of external potassium, in the right ventricular trabeculae of adult ferret hearts. Pressure-overload was induced by pulmonary artery clipping and led to an increase of the right ventricular weight of 60%. At an external Ca2+ concentration ([Ca2+]o) of 3 mM, the dependence of the contractures on extracellular sodium concentration ([Na+]o), the rate of tension development, the time course of spontaneous relaxation and the time course for the repriming of the contracture were unchanged by hypertrophy. However, the relationship between [Ca2+]o and contracture amplitude at various [Na+]o showed that the apparent affinity of the contracture for [Ca2+]o was decreased in hypertrophied preparations. Thus, in 0 mM [Na+]o, half-maximal contracture was induced at a [Ca2+]o of 0.012 +/- 0.016 mM and 0.171 +/- 0.021 mM in control (n = 11) and hypertrophy (n = 12) respectively (P less than 0.001). Although these data may be indicative of a decreased Ca2+ influx through the Na+/Ca2+ exchanger, it cannot be excluded that intracellular buffering mechanism may also be involved in this differential response to [Na+]o withdrawal.