Divalent cation-activated ecto-ATPase activity of rat glomerular mesangial cells

Evidence is presented of a Mg2+ and Ca(2+)-activated ATPase at the surface of cultured rat glomerular mesangial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. The apparent Km values were 0.46 and 0.32 mM for Mg(2+)-ATPase and Ca(2+)-ATPase, respectively. Inhibition studies have revealed that this enzyme is different from the (Na+ + K+)-ATPase and from the divalent cation activated mitochondrial ATPases. Culture in the serum-free medium markedly decreased ecto-ATPase activity. Con A treatment reduced at 50 micrograms/ml by 15% the Ca(2+)-ATPase activity. Endocytosis of serum-treated zymosan (STZ) did not significantly affect ecto-ATPase activity. In contrast, endocytosis of STZ by macrophages was accompanied by a biphasic response, an increase in Mg(2+)-ATPase and Ca(2+)-ATPase activities after ingestion of smaller amounts of STZ and a marked decrease after loading doses of STZ.