[Potential-independent Na+-stimulated transport of Ca2+ into a smooth muscle sarcolemma fraction].

In the K+-valinomycin system Na+ (140 mM) induces Ca2+ release from cow myometrium sarcolemmal vesicles at the membrane potential value of -61.5 mV. The Na+-stimulated component of Ca2+ release from polarized vesicles does not obey the kinetic regularities of a simple diffusion reaction but is characterized by saturation with the transport substrate. The value of this component is identical at -61.5 and at 0 mV, i.e., approximately 2 nmol Ca2+/min/mg protein). Alteration of the potential sign (-43 divided by 18.5 mV) does not affect the Na+-stimulated release of Ca2+. In the absence of Na+ and Ca2+ gradients, no transmembrane transport of Ca2+ utilizing the energy of electric field occurs. In the absence of initial gradients of transport substrates, the temperature changes (from 23 degrees C to 37 degrees C) have no influence on the intracellular level of Ca2+ in sarcolemmal vesicles at changeable values of the membrane potential. The data obtained suggest that the myometrium tissue contains a system of non-electrogenic ion antiport which possesses a low affinity for Ca2+ and protects myocytes against the damaging action of high (up to greater than or equal to 10(-5) M) Ca2+ concentration, i.e., from the impairment of intracellular homeostasis of Ca2+.