Identification of a plasma membrane Ca2+-ATPase in epithelial cells and aposomes of the rat coagulating gland

BACKGROUND. Epithelial cells of the rat coagulating gland secrete a minor fraction of proteins by means of an alternative export mode named apocrine secretion. Thereby, proteins are released by means of membrane bounded blebs or "aposomes" arising from the apical plasma membrane. Little is known about the composition of the aposomal membrane and whether or not proteins located in the apical plasma membrane are integrated into the aposomes. METHODS. To show expression and localization of Ca2+-ATPase in rat coagulating gland, reverse transcriptase-polymerase chain reaction, Western blotting experiments, and Ca2+-ATPase activity assays, as well as immunofluorescence studies were performed. RESULTS. Ca2+-ATPase is located in the apical plasma membrane of the epithelial cells of the rat coagulating gland and is also included in the aposomes. Mg2+-dependent and Mg2+-independent Ca2+-ATPase activity was observed in coagulating gland primary epithelial cells and tissue. Gene expression of plasma membrane Ca2+-ATPase isoforms 1 and 4 was detected in cultured primary epithelial cells of the rat coagulating gland and coagulating gland tissue. CONCLUSIONS. We show for the first time that Ca2+-ATPase as an important, functionally active membrane protein of the apical plasma membrane is incorporated into the aposomal membranes and is released from the cells during apocrine secretion process.