The calcium uptake in smooth muscle microsomal vesicles is reduced by centrifugation

A membrane fraction was isolated from the smooth muscle of the pig stomach by density gradient centrifugation. It was observed that the ATP-dependent Ca uptake in this fraction was diminished if the microsomes were pelleted by differential centrifugation. The decrease of the oxalate-independent Ca uptake was relatively small, but the oxalate-stimulated Ca uptake was reduced dramatically. Evidence is presented which indicates that the selective decrease of the oxalate-stimulated Ca uptake is mainly caused by mechanical damage of the vesicles. Since the oxalate-stimulated Ca uptake can be largely preserved by avoiding pelleting during the membrane fractionation, this observation may be very useful for the further study of Ca transport in subcellular fractions of smooth muscle.