Changes in urine concentration during prolonged administration of vasopressin and water

Vasopressin and water were administered to dogs for periods of up to a month. The initial increase in urine osmolality was maintained for only 3–12 days. Thereafter, urine concentration fell to less than half the maximum value and remained at these lower levels indefinitely. The decrease was associated with expansion and dilution of body fluids. It could not be correlated with the rises in filtration rate which occur, nor with the decreased plasma osmolality and sodium per se, nor was it associated with increased solute excretion. A low salt diet alone or with desoxycosterone acetate (DCA) decreased the maximum urine concentration achieved after vasopressin. Adrenalectomy abolished this effect of the low salt diet.