Total and local renal blood flow and filtration in the rat during reduced renal arterial blood pressure.

Discrepancies concerning the lower pressure limit of renal blood flow (RBF) autoregulation, different autoregulatory adjustments in deep and superficial renal zones and dissociation of RBF and filtrate production stimulated the present study. Autoregulation of renal blood flow was investigated at reduced renal arterial blood pressure (RAP) in Sprague-Dawley rats using 4 different flow methods: uptake of 125I-iodoantipyrine (I-Ap) and 86Rb, local detection of hydrogen gas washout rate (H2) and, in the autoperfused kidney, electromagnetic flowmetry (FM). With I-Ap and 86Rb, RBF was maintained at a RAP as low as 80 mmHg, compared to contralateral RBF. However, with the other two methods where each kidney serves as its own control, a 15% RBF reduction was obtained at this RAP. This discrepancy (p less than 0.001) infers a contralateral renal vasoconstriction during ipsilateral renal hypotension and vasodilation. Arterial blood pressure increased during unilateral renal hypotension, suggesting that contralateral renal constriction was part of a general increase in total body vascular resistance. Following abrupt RAP reduction RBF was immediately readjusted (2-3 s) and maintained for up to 40 min. No significant change in intrarenal blood flow distribution was observed with I-Ap. Superficial and deep cortical single nephron glomerular filtration rates were equally reduced at lowered RAP as determined by the ferrocyanide technique. However, a dissociation between the autoregulation of RBF and glomerular filtration rate (GFR) in the direction of less well maintained GFR was observed. Renal arterial acetylcholine infusion increased RBF by about 40% and effectively abolished RBF autoregulation.