ATP-dependent Ca-uptake by rat vas deferens smooth muscle microsomes: properties of oxalate stimulated and oxalate-independent Ca-uptake.
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The oxalate-stimulated and -independent components of the ATP-dependent azide insensitive Ca-uptake by rat vas deferens smooth muscle microsomes differ in the following properties: (a) 5, 5'-disulfonate stilbene (DIDS) and digitonin inhibited the oxalate-independent Ca-uptake more strongly than the oxalate-stimulated component; (b) phosphatidylserine inhibited the oxalate-stimulated Ca-uptake and had no significant effect on the oxalate-independent uptake and (c) digitonin treatment of microsomes changed the density distributions of the two modes differentially. The untreated microsomes gave two Ca-uptake peaks on sucrose density gradients: one at density 1.130 +/- 0.010 g/ml and the other at 1.214 +/- 0.005 g/ml. Digitonin treatment shifted the distribution of the lower density peak to slightly higher density for the oxalate-independent Ca-uptake but not for the oxalate-stimulated Ca-uptake. Distribution of phosphodiesterase I was also shifted to higher densities while distributions of the higher density Ca-uptake peak, NADPH: cytochrome c reductase and of cytochrome c oxidase were not significantly altered. Thus clearly the oxalate-stimulated and the oxalate-independent Ca-uptake in rat vas deferens smooth muscle microsomes show several differences but it is unknown whether the differences are due to the existence of two distinct Ca-pump proteins or due to different microenvironment of the same protein resulting from membrane heterogeneity.
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