Terminal papillary collecting duct reabsorption of water, sodium, and potassium in Psammomys obesus

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Abstract
Micropuncture studies were performed to assess the contribution of the terminal collecting duct to reabsorption of water, Na and K in the desert rodent, P. obesus. Tubule fluid was collected at base and tip of the exposed papilla before (period 1) and after (period 2) administration of furosemide. In period 1, a significant rise occurred in tubule fluid-to-plasma (TF/P) ratios of inulin, osmolality and Na and K from base to tip; 2.3 .+-. 0.8% of filtered water and 5.1 .+-. 1.7% of filtered Na were reabsorbed by the collecting duct. In period 2, water, Na and K delivery to the collecting duct was significantly increased. Although TF/P ratios for inulin, osmolality and Na and K were lower at base and tip in period 2 than corresponding values in period 1, all ratios increased between base and tip. Approximately 3 times as much water (7.4 .+-. 1.8%) and twice as much Na (10.1 .+-. 3.0%) were reabsorbed by the exposed collecting duct in period 2. These results reveal a remarkable reabsorptive capacity and suggest that the terminal collecting duct plays an important role in regulation of water and Na excretion in P. obesus.

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