EFFECTS OF A-23187 AND VERAPAMIL ON THE ACTIVE-TRANSPORT ENZYMES IN TURTLE BLADDER EPITHELIAL-CELLS

These experiments were designed to examine the effects of A-23187 (5 x 10(-4) M) and verapamil (100 mu M) on membrane transport, Ca-45 fluxe s, and adenosinetriphosphatase (ATPase) activities in turtle bladder. In the intact membrane, the calcium ionophore decreased proton secreti on and sodium transport [short-circuit current (SCC)I to approximately the same degree (by similar to 55% at 30 min). During the same period of time, verapamil decreased SCC (by similar to 58%), but proton secr etion was unaffected. The turtle bladder membrane is composed predomin ately of two cell types: I)the mitochondrial-rich cells (MR cells) tho ught to be involved in proton (and bicarbonate) secretion containing s ignificant H+-ATPase and Ca2+-ATPase and 2) the granular cells (G cell s), postulated important in sodium reabsorption, having abundant Na+-K +-ATPase. That Na+-K+-ATPase activity was unchanged by either a calciu m ionophore or a calcium channel blocker suggests that the decrease in SCC noted in the intact membrane is not directly mediated by changes in the sodium ''pump.'' The decrease of H+-ATPase in MR cells, which r esulted after the A-23187, suggests that it probably exerts a direct a ction on the proton pump, which decreases hydrogen ion secretion. The increase in ATP-dependent Ca-45 transport seen after the ionophore (or the decrease in ATP-independent Ca-45 transport after verapamil) most likely reflects increased (or decreased) Ca2+ availability within the cytosol, and the high (or low) cell calcium could decrease the SCC. T hese results thus suggest that cytosolic Ca2+ reciprocally sets, by di fferent mechanisms, the rate of proton secretion in MR cells and the s odium reabsorption in G cells.