Isolated secretion granules from parotid glands of chronically stimulated rats possess an alkaline internal pH and inward-directed H+ pump activity.

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Abstract
Secretion granules have been isolated from the parotid glands of rats that have been chronically stimulated with the beta-adrenergic agonist, isoproterenol. These granules are of interest because they package a quantitatively different set of secretory proteins in comparison with granules from the normal gland. Polypeptides enriched in proline, glycine, and glutamine, which are known to have pI's greater than 10, replace alpha-amylase (pI's = 6.8) as the principal content species. The internal pH of granules from the treated rats ranges from 7.8 in a potassium sulfate medium to 6.9 in a choline chloride medium. The increased pH over that of normal parotid granules (approximately 6.8) appears to reflect the change in composition of the secretory content. Whereas normal mature parotid granules have practically negligible levels of H+ pumping ATPase activity (Arvan, P., G. Rudnick, and J. D. Castle, 1985, J. Biol. Chem., 260, 14945-14952) the isolated granules from isoproterenol-treated rats undergo a time-dependent internal acidification (approximately 0.2 pH unit) that requires the presence of ATP and is abolished by an H+ ionophore. Additionally, an inside-positive granule transmembrane potential develops after ATP addition that depends upon ATP hydrolysis. Two independent methods have been used that exclude the possibility that contaminating organelles are the source of the H+-ATPase activity. Together these data provide clear evidence for the presence of an H+ pump in the membranes of parotid granules from chronically stimulated rats. However, despite the presence of H+-pump activity, fluorescence microscopy with the weak base, acridine orange, reveals that the intragranular pH in live cells is greater than that of the cytoplasm.