Hepatic stellate cells express Ca2+ pump-ATPase and Ca2+-Mg2+-ATPase in plasma membrane of caveolae

Intracytoplasmic free calcium ions (Ca2+) are maintained at a very low concentration in mammalian tissue by the extrusion of Ca2+ across a steep extracellular Ca2+ gradient, mainly through the activity of plasma membrane Ca2+ pump-ATPase. The present study aimed to identify, by electron cytochemical and electron immunogold methods, the ultrastructural localizations of two types of plasma membrane Ca2+-ATPase; Ca2+-Mg2+-ATPase and Ca2+ pump-ATPase, in hepatic stellate cells. Liver tissues and isolated hepatic stellate cells (HSCs) were studied. The ultrastructural localization of Ca2+-Mg2+-ATPase activity was examined by the electron cytochemical method of Ando. The localization of Ca2+ pump-ATPase was identified by immunofluorescence. The ultrastructural localization of Ca2+ pump-ATPase was identified by the electron immunogold method. The cytochemical reaction products of Ca2+-Mg2+-ATPase activity were localized on the outer (cavity) side of the plasma membrane of caveolae. Immunofluorescence of Ca2+ pump-ATPase was seen as small dots along the cell edge in HSCs. Immunogold particles indicating the presence of Ca2+ pump-ATPase were identified on the inner (cytoplasmic) side of the plasma membrane of caveolae. We localized Ca2+ pump-ATPase on the inner side of the plasma membrane caveolae and Ca2+-Mg2+-ATPase on the outer leaflet of the caveolar plasma membrane in stellate cells, suggesting that Ca2+ pump-ATPase may play a key role in the Ca2+ reflux.