Medullary collecting-duct function in antidiuretic and in salt- or water-diuretic rats.

SONNENBERG, H. Medullary collecting-duct function in antidiuretic and in saltor water-diuretic rats. Am. J. Physiol. 226(3): 501-506. 1974.-Microcatheterization of the medullary collecting duct of adult rats was used to study the contribution of water, sodium, and potassium transport in this nephron segment to urinary excretion. In antidiuresis medullary reabsorption of fluid and sodium was 50/;, and 3y0 of filtered load, respectively. Animals chronically ingesting saline, followed by infusion at a rate comparable to prior voluntary intake, showed an increase in the absolute amount of water and Na delivered to the medullary duct system from the cortex of the experimental kidney. The absence of any further reabsorption of either substance resulted in diuresis and natriuresis. In rats on high water intake with subsequent infusion, an increased amount of water, but not sodium, was delivered to the medullary collecting duct. Essentially complete inhibition of further water reabsorption, associated with Na reabsorption as great as, or greater than, that during antidiuresis, resulted in excretion of a large volume of hypotonic urine. No net transport of potassium was found in the collecting duct either in antidiuresis or saline diuresis, -while in water diuresis net reabsorption was detectable. It is concluded that the medullary collecting duct is a major site for the regulation of urinary excretion of both salt and water.