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

Abstract:

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.