Effects of secretin on peritubular capillary physical factors and proximal fluid reabsorption in the rat

The effects of secretin vasodilation on peritubular capillary Starling forces and absolute proximal reabsorption were examined in the rat. Secretin was infused at 75 mU/kg per min into the aorta above the left renal artery. Efferent plasma flow increased the left renal artery. Efferent plasma flow increased from 125 ± 28 to 230 ± 40 nl/min with secretin infusion. Single nephron filtration rate (44 ± 6 vs. 44 ± 7 nl/min) and absolute proximal reabsorption (21 ± 5 vs. 21 ± 4 nl/min) were not significantly changed. Peritubular capillary and interstitial hydrostatic pressures increased with secretin infusions (from 9 ± 0.4 to 15 ± 0.7 mmHg and from 3 ± 0.2 to 4 ± 0.2 mmHg, respectively). Both peritubular capillary and interstitial oncotic pressures decreased (from 25 ± 2 to 20 ± 2 mmHg and from 10 ± 1 to 4 ± 1 mmHg, respectively) during secretin infusion. The net reabsorption pressure for peritubular capillary uptake significantly decreased from 9 ± 2 to 5 ± 2 mmHg and the coefficient of reabsorption increased from 3 ± 1 to 6 ± 2 nl/min per mmHg. We conclude that although secretin causes a vasodilation and decreases net reabsorption pressure, absolute proximal reabsorption is unchanged. Peritubular capillary uptake is maintained, and since net reabsorption pressure is decreased, the coefficient of reabsorption is increased.