Time-dependent heterogeneity of filtration rate in the autoreguiating rat kidney

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Abstract
Experiments were performed to study the regulation of the single-nephron glomerular filtration rate (SNGFR) in superficial and juxtamedullary nephrons, as the left kidney of Sprague Dawley rats was submitted to a reduced arterial pressure of 70 mm Hg by means of an aortic clamp. The SNGFR at different cortical levels was measured 0.5, 1, 5, 20 or 45 min after the reduction, in order to ascertain whether the effects of the regulatory mechanisms are modified with time. A Hanssen technique was used, which allows one determination of filtration rates per animal. At a renal arterial pressure (RAP) of 100 mm Hg (= control animals) the SNGFR amounted to 20 .+-. 1.2 and 23 .+-. 0.8 nl .cntdot. min-1 .cntdot.g-1 kidney wt in the outer and inner cortical (OC, IC) nephrons. When RAP was further reduced to 70 mm Hg, the autoregulation of SNGFR, determined after 0.5 min, was highly efficient for both OC and IC nephrons (19 .+-. 2.0, 23 .+-. 2.6). A prolonged reduction in RAP caused a gradual decline in SNGFR. The filtration rate measured after 5 min was 15 .+-. 1.4 for OC and 20 .+-. 1.8 for IC nephrons. The decline was most pronounced for OC nephrons, which led to a fractional redistibution in favor of IC nephrons. Thus, SNGFRIC/SNGFROC was 1.16 .+-. 0.065 when RAP was 100 mm Hg and 1.41 .+-. 0.126 after 5 min with an RAP of 70 mm Hg. Suprarenal aortic occlusion is a powerful stimulus for the release of renin. This was manifested as an increase in the arterial pressure proximal to the aortic clamp. The fractional redistibution to IC nephrons and the loss of autoregulation, when the renal hypotension was sustained, may be an expression of the intrarenal mechanisms attempting to restore RAP. The renin/angiotensin system is involved in these processes.