A novel plasmalemmal magnesium-independent, high-affinity calcium-adenosine triphosphatase in smooth muscles

In this communication, we report the presence of a smooth muscle membrane ATPase activity which is activated by physiological concentrations of intracellular Ca²⁺ [K(m) for Ca²⁺ = 0.3 - 0.6 μM), has no requirement for Mg²⁺, parallels the plasma membrane marker enzyme distribution and is widely present in several smooth muscle preparations. This work is the result of our initial attempt to identify the Mg²⁺-dependent, Ca²⁺-ATPase as the Ca-pump in isolated smooth muscle plasma membranes. Further characterization of this plasmalemmal, Mg²⁺-independent, high affinity Ca²⁺-ATPase in smooth muscle would be necessary in order to determine the relationship between this Ca²⁺-ATPase activity and the Ca-pump ATPase activity reported in smooth muscle by other investigators. However, the common occurrence of this Ca²⁺-ATPase in different types of smooth muscle with a similarly high affinity of Ca²⁺ suggests that it may have a definite physiological significance in the energy-requiring process involving cytoplasmic Ca²⁺.