Differentiation of Ca2+ pumps linked to plasma membrane and endoplasmic reticulum in the microsomal fraction from intestinal smooth muscle

ATP promotes ⁴⁵Ca uptake by the microsomal fraction from the longitudinal smooth muscle of guinea-pig ileum and this uptake is stimulated by oxalate. As the microsomal fraction is made up of various subcellular entities, we examined the localization of the Ca²⁺-transport activity by density gradient centrifugation, taking advantage of the selective effect of digitonin (at low concentration) on the density of plasmalemmal elements. When the ⁴⁵Ca-uptake activity was measured in the absence of oxalate, its behavior in subfractionation experiments closely paralleled that of the plasmalemmal marker 5′-nucleotidase. In contrast, the additional Ca²⁺-transport activity elicited by oxalate behaved like NADH-cytochrome $\text{c}$ reductase, a putative endoplasmic reticulum marker. The endoplasmic reticulum vesicles constituted only a small part of the membranes in the microsomal fraction, which explains that their Ca²⁺-storage capacity was not detectable in the absence of Ca²⁺-trapping agent. Low digitonin concentrations selectively increased the Ca²⁺ permeability of the plasmalemmal vesicles. The two Ca²⁺-transport activities were further differentiated by their distinct sensitivities to K⁺, vanadate and calmodulin. In this respect, the oxalte-insensitive and oxalate-stimulated Ca²⁺-transport systems resembled, respectively, the sarcolemmal and sarcoplasmic reticulum Ca²⁺ pumps in cardiac and skeletal muscle, in accordance with the subcellular locations established by density gradient centrifugation.