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

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