THE MECHANISM OF SODIUM DIURESIS AFTER SALINE INFUSION IN THE DOG*

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Abstract
The mechanism of the Na diuresis after saline infusion was studied in dogs in which filtered Na could be reduced by means of a clamp around the aorta. Mineralocorticoids were administered in amounts sufficient to eliminate changes in endogenous secretion of aldosterone as a factor in these experiments. In 22 acute experiments, control periods were obtained, after which dogs were infused with 1,400 to 5,400 ml of isotonic saline or modified Ringer''s solution. Plasma Na was stable. When filtered Na after saline loading was reduced significantly (300 to 4,700 uEq per minute) below control values by aortic clamping, sodium excretion was 20 to 280 uEq per minute above control. It is concluded that neither increased filtered Na nor decreased aldosterone secretion can account for the increase in Na excretion that occurs after saline loading in the dog. Part of the increase must result from the action of another as yet undefined factor which decreases tubular reabsorption of Na. The magnitude of this effect apparently increases with progressive saline loading.