Effect of calcium and magnesium on transport processes by the turtle bladder

The role of the changes in intracellular concentration of Ca and Mg in Na and H+ transport by the turtle bladder in vitro was studied with the use of the ionophore A23187. This ionophore increases the permeability of living and artificial membranes to divalent cations. In the presence of a minimal concentration of Ca (0.2 mM), which is necessary for the membranes to remain stable, the ionophore failed to alter the rate of Na transport or H+ secretion in the presence of 2.5 mM Mg. These data demonstrate that changes in intracellular Mg do not play an important role in Na or H+ transport by the turtle bladder. In the presence of 1.8 mM Ca the ionophore caused a significant decrease in Na transport as compared to control hemibladders treated with dimethylsulfoxide, the vehicle used to dissolve the ionophore. In the presence of a minimal concentration of Ca (0.2 mM) the ionophore failed to alter the rate of Na transport. These observations demonstrate that the decrease in Na transport in the presence of Ca was the result of changes in intracellular Ca concentration rather than a non-specific effect of the ionophore. The data further demonostrate that calcium plays an important role in Na transport by the turtle bladder but that Mg does not seem to be important in the regulation of Na or H+ transport by this membrane.