The effect of a converting enzyme inhibitor on autoregulation and intrarenal distribution of glomerular filtration in the rat

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Abstract
The role of the renin‐angiotensin system in the autoregulation and distribution of the single nephron glomerular filtration rate (SNGFR) in anaesthetized, normotensive rats was investigated. SNGFR in outer cortical (OC) and inner cortical (IC) nephrons of the left kidney were measured with a modified Hanssen technique at three levels of renal arterial pressure (RAP): (1) at a spontaneous arterial pressure; (2) at a value within the autoregulatory limit, 100 mmHg; and (3) at the lower limit of the autoregulatory range, 70 mmHg. This was done in control rats and in rats given a continuous i.v. infusion of the converting enzyme inhibitor (CEI) captopril (3 mg · h‐1· kg‐1BW). In control rats there was complete autoregulation of SNGFR in both OC and IC nephrons when RAP was reduced to 100 mmHg. Further reduction to 70 mmHg elicited different responses among the cortical layers, associated with a decrease in SNGFR. A fractional redistribution of glomerular filtration rate towards IC nephrons was evident. Administration of CEI at spontaneous RAP increased SNGFR in IC nephrons compared with values in control rats, but did not notably alter SNGFR in OC nephrons. Reduction of RAP to 100 mmHg during CEI infusion caused SNGFR to decrease below control values in both OC and IC nephrons, and the autoregulation as found in control rats was impaired. When RAP was lowered to 70 mmHg during CEI administration there was a progressive decrease in SNGFR in all cortical layers, although absolute changes were much greater in IC nephrons than in OC nephrons. This resulted in a fractional redistribution of SNGFR towards OC nephrons, opposite to that seen in control rats when the pressure was reduced. These results show that the autoregulation and distribution of SNGFR are dependent on the renin‐angiotensin system and that IC nephrons are more affected by inhibition of converting enzyme than OC nephrons.