Plasma membranes were isolated from smooth muscles of rat mesenteric veins. The plasma membrane fraction is relatively pure according to its morphological and enzymatic characteristics. The membrane distribution, enzymatic activities, as well as calcium accumulation by the plasma membrane fraction from venous smooth muscle were compared to those from arterial smooth muscle. The isolated venous smooth muscle plasma membranes formed primarily closed vesicles which were capable of accumulating Ca2+ in the presence of ATP suggesting active transport of Ca2+ across the membrane vesicles. Evidence obtained from several approaches by studying the effect of A23187, phosphate ions and hypotonic shock on the Ca2+ accumulation in the presence of ATP revealed that there is an active transport of Ca2+ across isolated vascular smooth muscle membrane vesicles in addition to binding of Ca2+. However, venous smooth muscle plasma membrane fraction appears to be different from arterial smooth muscle plasma membrane fraction in its lower activity of alkaline phosphatase, greater Ca2+ binding and lower Ca2+ transport. These and previous studies show that the plasma membrane of vascular muscles may play an important role in the steady state regulation of cellular calcium concentration during excitation-contraction coupling, especially in small arteries and veins.

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