Studies on canine gastric antrum smooth muscle: preparation and characterization of a plasma membrane enriched fraction

A method is described for preparation of large amounts of a plasma membrane (PM) enriched fraction from the smooth muscle of dog antrum. It consists of preparing microsomes, treating them with ATP + EGTA + Mg, centrifuging in 30% sucrose and then centrifuging the resulting supernatant in 15% sucrose to yield the plasma membrane enriched fraction P6. The subcellular fractions obtained at various steps during purification were characterized by: 5'-nucleotidase and phosphodiesterase I as plasma membrane markers; cytochrome c oxidase as an inner mitochondrial marker; NADPH-cytochrome c reductase as a putative endoplasmic reticulum marker; electron microscopy; polyacrylamide sodium dodecyl sulfate slab gel electrophoresis. The distribution of ATP-dependent and independent Ca uptake in presence and absence of azide and the effect of 5 mM oxalate or 25 mM phosphate on this uptake was also examined. The fraction P6 consists of mostly smooth surface vesicles 164.3 +/- 7.2 nm in diameter, has an exclusion volume of 9.7 microL/mg for [3H]inulin and 11.1 microL/mg for [3H]sucrose. P6 is maximally enriched in the ATP-dependent azide-insensitive Ca-uptake capacity and as compared with the postnuclear supernatant (S1) it shows a very small percent stimulation by oxalate and phosphate. The ATP-dependent Ca uptake by the P6 fraction occurs optimally at pH 7.0-7.4 and is much larger than the ATP-independent Ca uptake. At pH 7.1, the ATP-dependent Ca uptake occurs with a Km of 0.27 microM and a Hill coefficient greater than 2 for Ca2+. Half maximum binding of Ca2+ occurred at 300 microM Ca2+. Ca ionophores A23187 and ionomycin inhibited the ATP-dependent Ca uptake, and if added after the uptake, these caused a release of the accumulated Ca2+. From these and other data it is concluded that this PM preparation contains a Ca transport system which can lead to formation of greater than 1000-fold Ca2+ concentration gradient across the vesicle membrane in 1 min when extravesicular Ca2+ concentration is 0.3 microM. Thus this preparation is an extremely useful material for studying the mechanism of action of the Ca pump in smooth muscle plasma membrane.