Influence of hydration on renal function and medullary sodium during vasopressin infusion

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Abstract
Mild and severe degrees of hydration were produced in anesthetized dogs by constant infusion of dextrose in water at different rates. When water diuresis ensued, and after adequate control periods, vasopressin was infused for 2 1/2 hr. The magnitudes of urine flow and Na excretion varied directly and osmolal urine-to-plasma ratio varied inversely with the degree of hydration. During mild hydration, glomerular filtration rate and osmolal clearance remained practically unchanged, whereas during severe hydration they increased. In another group of animals, similarly prepared, kidneys were removed at appropriate times and tissue was obtained from various areas for Na analysis. During uninterrupted water diuresis the medullary gradient for Na was practically dissipated. Vasopressin promoted the repletion of medullary Na during mild hydration, but only slightly ameliorated the disturbance during severe hydration. These data demonstrate that during vasopressin infusion the renal concentrating response and Na excretion were significantly modified by the degree of hydration, and evidence is presented for several possible causes.