Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus.

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Abstract
Regulation of the Na permeability of the luminal membrane is the major mechanism by which the net rate of Na transport across tight epithelia is varied. The permeability of the luminal membrane might be regulated by changes in intracellular Na or Ca activities. To test this directly, a fraction of the plasma membrane was isolated from the toad urinary bladder, which contains a fast, amiloride-sensitive Na flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of Na efflux from these vesicles in the time range, the isotope exchange permeability of these vesicles is very sensitive to Ca. Ca reduces the Na permeability, and the half-maximal inhibitory concentration is 0.5 .mu.M, well within the range of Ca activity found in cells. The permeability of the luminal membrane vesicles is little affected by the ambient Na concentration. Cell Ca may be an important regulator of transepithelial Na transport by its effect on luminal Na permeability. The effect of cell Na on permeability may be mediated by Ca rather by Na itself.