Observations on the Mechanism of Decreased Tubular Reabsorption of Sodium and Water during Saline Loading.
چکیده
It has been demonstrated by several recent studies that the regulation of sodium excretion involves factors other than the total filtered load of sodium and the renal tubular effects of mineralo-corticoids. Contrary to earlier suggestions that an increase in the filtered load of sodium may be sufficient to account for the natriuresis of saline loading in the dog, de Wardener, Mills, Chapman, and Hayter (1) reported that the administration of saline to dogs receiving an exogenous mineralo-corticoid results in an increased excretion of sodium that is not necessarily accompanied by an increased glomerular filtration rate. On the basis of this and other considerations, these authors suggested that natriuresis resulting from saline loading may be due largely to some unidentified circulating factor that affects tubular reabsorption. Similar observations that saline loading may result in natriuresis without a spontaneous increase in the filtered load of sodium have been made in this laboratory (2). Levinsky and Lalone (3), Blythe and Welt (4), and Rector, Van Giesen, Kiil, and Seldin (5) have demonstrated that hemo-dynamically induced reductions in the filtered load of sodium do not abolish the natriuresis of iso-tonic (3, 5) or hypertonic (4) saline loading. Thus, several recent investigations are consistent with the view that the excretion of sodium may be regulated in part by yet unidentified factors that alter net tubular reabsorption. It was the purpose of the present studies to determine if specific changes in urinary composition may be associated with saline loading, under conditions of controlled urine flow. When the diuretic response to saline loading was prevented in one kidney by ureteral or aortic constriction, a clear effect of the saline load was evident, de. spite no change in the rate of urine flow. In hy-dropenic animals receiving vasopressin, saline loading with a constant urine flow was associated with decreased urinary osmolality, and urinary electrolyte concentrations (when initially low) were increased. These changes occurred as glo-merular filtration rate decreased. The present results are consistent with the concept that saline loading in the dog results in decreased tubular re-absorption of electrolyte through a mechanism that also affects the urinary concentrating capacity. Methods Mongrel dogs, ranging in weight from 12.5 to 15.5 kg, received no food for 24 hours and no water for 48 hours before study. Twelve to 16 hours before experiment, the animals received by intramuscular injection 5 U of vasopressin1 and 10 mg of desoxycorticosterone acetate (DOCA).2 In …