Na+-K+ ATPase, Na-Ca exchange, and excitation-contraction coupling in smooth muscle

The evidence in favor of a direct role of active Na transport in the regulation of excitation-contraction coupling in vascular smooth muscle has been examined. The observations in vivo and those obtained in isolated tissues do not always lead to the same conclusions. The changes of the membrane potential obtained in vitro by slight reductions in, or increases of [K]o do not modify the resting potential of the cells sufficiently to make them contract. Applying K-free or Na-free medium on isolated tissues is a much more vigorous procedure than the limited changes of [K]o that can occur in vascular beds in situ. The Na-Ca exchange-mechanism does not seem to play a major role in those smooth-muscle cells that have been analyzed in detail, but even here the experimental procedures have neither given precise information about the composition of the intracellular compartment nor allowed sufficient control of the parameters studied. The comparison of membrane vesicles from smooth muscle and from cardiac muscle indicates that important differences exist in Na-Ca exchange and in activities of Na+-K+ ATPase and Ca2+-Mg2+ ATPase. These findings suggest a poor development of Na-Ca exchange in smooth muscle as compared to cardiac muscle. Finally, the changes in the Na metabolism of erythrocytes from hypertensives are mentioned, and the present difficulties of linking those changes to an increased reactivity of vascular smooth-muscle cells are briefly discussed.