The effects of calcium on water transport.

Some critical level of calcium appears to be necessary for maximum expression of the hydroosmotic effect of vasopressin: both maneuvers which elevate cell calcium or activate calcium-dependent enzymes and maneuvers which diminish cell calcium or inhibit its effect diminish vasopressin-stimulated water flow. It would thus appear that cell calcium might well be maintained at some optimum level for mediating vasopressin-stimulated water flow across epithelia. At the same time, there is considerable evidence supporting vasopressin-elicited alterations in cell calcium and phospholipid metabolism; however, the linkage between these alterations and any eventual effects on water permeability has not yet been established with confidence.