DIVALENT CATION-ACTIVATED ECTO-ATPASE ACTIVITY OF RAT GLOMERULAR MESANGIAL CELLS

Evidence is presented of a Mg2+ and Ca2+-activated ATPase at the surfa ce of cultured rat glomerular mesangial cells. Substrate specificity w as very low when different nucleoside-5'-triphosphates were examined. The apparent Km values were 0.46 and 0.32 mM for Mg2+-ATPase and Ca2+- ATPase, respectively. Inhibition studies have revealed that this enzym e is different from the (Na+ + K+)-ATPase and from the divalent cation activated mitochondrial ATPases. Culture in the serum-free medium mar kedly decreased ecto-ATPase activity. Con A treatment reduced at 50 mu g/ml by 15% the Ca2+-ATPase activity. Endocytosis of serum-treated zy mosan (STZ) did not significantly affect ecto-ATPase activity. In cont rast, endocytosis of STZ by macrophages was accompanied by a biphasic response, an increase in Mg2+-ATPase and Ca2+-ATPase activities after ingestion of smaller amounts of STZ and a marked decrease after loadin g doses of STZ.