Whole Kidney Response to Reduced Arterial Pressure during Converting Enzyme Inhibition in the Rat

Abstract

Autoregulatory efficiency of renal blood flow (RBF) and glomerular filtration rate (GFR) was evaluated in male Sprague-Dawley rats during interference with the renin-angiotensin system by a converting enzyme inhibitor (CEI), captopril (3 mg · h^-1 · kg^-1 BW). RBF and GFR were approximately 25 (p < 0.01) and 20% (p < 0.02) higher, respectively, in rats infused with CEI than in control rats at spontaneous renal arterial pressure (RAP). A reduction of RAP to 100 mm Hg (within the autoregulatory range) resulted in effective autoregulation of GFR and RBF in control rats. In rats given CEI, however, the autoregulation of GFR was markedly impaired. GFR decreased by 35% (p < 0.001), while RBF remained relatively unchanged. This caused the filtration fraction to decrease from 0.33 ± 0.01 to 0.29 ± 0.01 (p < 0.001). RAP had a consistent effect on the urine flow rate, even though both GFR and RBF were well autoregulated in control rats. No significant decrease in electrolyte excretion was detected within the autoregulatory range in control rats, but during converting enzyme blockade this excretion decreased progressively as RAP was reduced, and the decrease correlated well to the reduction in GFR. In summary, these results suggest that the renin-angiotensin system plays an important intrarenal role in the autoregulation of GFR, probably through an efferent arteriolar mechanism. Furthermore, it is demonstrated that the contra-lateral kidney efficiently compensates in urinary electrolyte excretion for an acute unilateral reduction of RAP, both under control conditions when the perfusion pressure to the contra-lateral kidney increased and during blockade of the renin-angiotensin system when the increase in perfusion pressure was inhibited.

Renal Autoregulation in Health and Disease

“…ANG II also did not affect renal medullary blood flow autoregulation (288). However, ANG II was more important in steadystate autoregulation of the GFR than the RBF (488,550,774,1261), especially at lower RPP, most likely by maintaining efferent arteriolar tone. However, a few studies have reported that the RAS was essential for effective steadystate autoregulation of RBF and GFR (764,1479).…”

The effect of pressor doses of Angiotensin II on autoregulation and intrarenal distribution of glomerular filtration rate in the rat

“…This apparent contradiction may, however, be an effect of a time-dependency of the mechanisms responsible for GFR regulation. Sjoquist & Goransson (1984) observed that SNGFR was efficiently autoregulated immediately after a reduction in RAP to 70 mmHg. A gradual decrease in SNGFR was seen when RAP remained lowered for 5 min or more.…”

“…The preparatory procedures have been described in detail earlier (Sjoquist & Goransson 1984). but will be summarized here.…”

“…SNGFR was obtained by dividing the filtered volume by the number of glomeruli in the sample. The amount of intratubularly trapped tracer substance was calculated by subtracting the amount of tracer within the vascular and interstitial compartments from the total amount in each slice (Sjoquist & Goransson 1984).…”

“…Alterations in renal perfusion pressure are yet another situation calling for adjustment of intrarenal mechanisms governing electrolyte excretion rates. Sjoquist & Goransson (1984) found that when t h e kidney was submitted to a renal arterial pressure (RAP) of 70 mmHg f o r 5 min or longer a fractional redistribution of SNGFR towards deeper nephrons t o o k place. A concomitant increase in central pressure (CAP) was observed, a phenomenon which is known t o be due to activation of t h e renin-angiotensin system.…”