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 microM) on membrane transport, 45Ca fluxes, and adenosine-triphosphatase (ATPase) activities in turtle bladder. In the intact membrane, the calcium ionophore decreased proton secretion and sodium transport [short-circuit current (SCC)] to approximately the same degree (by approximately 55% at 30 min). During the same period of time, verapamil decreased SCC (by approximately 58%), but proton secretion was unaffected. The turtle bladder membrane is composed predominantly of two cell types: 1) the mitochondrial-rich cells (MR cells) thought to be involved in proton (and bicarbonate) secretion containing significant H(+)-ATPase and Ca(2+)-ATPase and 2) the granular cells (G cells), postulated important in sodium reabsorption, having abundant Na(+)-K(+)-ATPase. That Na(+)-K(+)-ATPase activity was unchanged by either a calcium 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 resulted after the A-23187, suggests that it probably exerts a direct action on the proton pump, which decreases hydrogen ion secretion. The increase in ATP-dependent 45Ca transport seen after the ionophore (or the decrease in ATP-independent 45Ca 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. These results thus suggest that cytosolic Ca2+ reciprocally sets, by different mechanisms, the rate of proton secretion in MR cells and the sodium reabsorption in G cells.
Sabatini
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Department of Physiology, Texas Tech University Health Sciences Center, Lubbock 79430, USA.
D000818:Animals; D001693:Biological Transport, Active; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004553:Electric Conductivity; D004848:Epithelium / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D006180:Proton-Translocating ATPases / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D014426:Turtles; D001743:Urinary Bladder / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology*
D000001:Calcimycin; D012964:Sodium; D014700:Verapamil; D006180:Proton-Translocating ATPases; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium