Autoregulation in vasa recta of the rat kidney

Vasa recta blood flow autoregulation was studied by measuring flow velocity in individual vessels on the papilla surface with a video adaptation of the dual-slit erythrocyte velocity method. Vessel diameter did not vary with arterial pressure in the range of 60-150 mmHg, allowing the calculation of the ratio of flows in a single vessel at two pressures from the ratio of velocities. Flow velocity in single vasa recta increased with arterial pressure to 75 mmHg, remained constant in the range of 75-125 mmHg, and increased with higher pressures. In a second series of animals, whole kidney blood flow auto-regulated above 90 mmHg. Vasa recta and whole kidney flow patterns were not changed by extracellular fluid volume expansion. Volume expansion caused a greater increase in ascending than in descending vasa recta flow, reflecting the volume load from enhanced collecting duct reabsorption in diuresis. In a final series, Na excretion varied with arterial pressure in the range of 90-130 mmHg. Because vasa recta velocity remains constant within this range, pressure diuresis cannot be caused by the lack of autoregulation of vasa recta blood flow, at least to 130 mmHg.