Stop flow studies of renal concentrating mechanism

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Abstract
Stop flow patterns of sodium, urea, and osmolality were obtained in dogs producing hypotonic urine following water loading, water loading plus urea, and water loading plus urea plus Pitressin. "Distal" and "proximal" peaks in osmolality, separated by a region where osmolality fell to or below free flow values, were regularly observed during water diuresis and water plus urea. The region of minimal osmolal concentration coincided with the distal region of minimal sodium concentration. In more proximal samples sodium concentration rose sharply above free flow levels to a maximum that coincided with and was adequate to account for the proximal osmolality peak. The proximal rise in osmolality, accompanied by a sharp rise in sodium concentration in the absence of a concomitant proximal rise in creatinine suggests re-entry of sodium into some proximal region of the nephron, probably the descending portion of Henle's loop. Urea showed a distal concentration rise but no proximal rise. Following Pitressin administration, the two osmolality peaks and the proximal sodium peak are obliterated and replaced by a single distal osmolality peak.