Carbon-dioxide-induced exocytotic insertion of H+ pumps in turtle-bladder luminal membrane: role of cell pH and calcium.
The contents of endocytic vesicles and other intracellular organelles (such as Golgi and microsomes) are acidified by an electrogenic proton-translocating ATPase that is remarkably similar to that found in urinary epithelia. 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+ pump. 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 membrane. 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 cytoplasmic acidification. Cell acidification then increases calcium activity, which causes the fusion event.
Cannon
C
C
van Adelsberg
J
J
Kelly
S
S
Al-Awqati
Q
Q
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000502:physiology*; D002245:Carbon Dioxide / Q000502:physiology*; D003078:Colchicine / Q000494:pharmacology; D005089:Exocytosis; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D008561:Membrane Fusion; D006180:Proton-Translocating ATPases / Q000378:metabolism*; D014426:Turtles; D001743:Urinary Bladder / Q000378:metabolism
D002245:Carbon Dioxide; D006180:Proton-Translocating ATPases; D003078:Colchicine; D002118:Calcium