Carbon-dioxide-induced exocytotic insertion of H+ pumps in turtle-bladder luminal membrane: role of cell pH and calcium

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Abstract
The contents of endocytic vesicles and other intracellular organ-elles (such as Golgi and microsomes) are acidified by an elec-trogenic proton-translocating ATPase that is remarkably similar to that found in urinary epithelia1–10. We recently found that the number of H+ ATPases in the apical plasma membrane of these epithelia is regulated by exocytotic insertion of endocytic vesicles whose membranes contain this H+ pump2,10. Carbon dioxide, a major stimulus for urinary acidification, causes rapid fusion of these vesicles with the luminal membrane, thereby inserting these pumps there and increasing the rate of net transepithelial H+ secretion; CO2 also inhibits endocytic retrieval of the pumps from the luminal membrane11. Such reciprocal regulation of endocytosis and exocytosis by a physiological modulator makes this system particularly attractive for studying the cellular events regulating membrane fusion. Here we present evidence that CO2 induces exocytosis by a cascade of events, the first step of which is cytoplas-mic acidification. Cell acidification then increases calcium activity, which causes the fusion event.