We studied the effects of interruption of the renin-angiotensin system (RAS) in rats that were volume depicted by water deprivation for 48 hours (AWD) with/without furosemide (AWD + F), a condition known to activate RAS. Following baseline micropuncture, AWD rats (N = 6) were treated with a specific angiotensin II type 1 receptor antagonist (AIIRA; 4 mg/kg body wt bolus i.v. and then continuous infusion) and glomerular hemodynamics compared to those obtained during angiotensin I converting enzyme inhibitor treatment (ACEI; 24 mg/kg bolus i.v. and then continuous infusion). Systemic blood pressure decreased equally following AIIRA and ACEI. Single nephron glomerular filtration rate (SNGFR) increased from baseline following AIIRA (24 nl/min vs. 30, P < 0.025). While a decrease in efferent arteriolar resistance (R(E)) reduced glomerular capillary pressure (P(GC); 67 mm Hg vs. 60, P < 0.05), this change in R(E) together with decrease in afferent arteriolar resistance (R(A)), enhanced glomerular plasma flow rate (Q(A); 80 nl/min vs. 111). Antagonizing angiotensin Il receptor increased Q(A) which, together with the tendency to increase glomerular capillary ultrafiltration coefficient, K(f), served to improve glomerular filtration. By contrast, although inhibition of the angiotensin I converting enzyme caused greater vasodilatation, no increase in SNGFR occurred. The lack of response in filtration after ACEI was due to a further fall in P(GC) to 52 mm Hg (P < 0.01 vs. AIIRA), reflecting profound reduction in R(E). Since ACEI but not AIIRA potentiates bradykinin activity we examined effects of a specific bradykinin antagonist (Hoe). In a separate group of AWD+F rats (N = 6), ACEI again caused significant renal vasodilatation without affecting filtration rate. The ACEI-induced decrease in resistances was substantially antagonized by inhibition of bradykinin activity. Thus, Hoe significantly increased R(E), effectively raising P(GC) by 9 mm Hg (P < 0.05 vs. ACEI) and thus secondarily effecting increased filtration (SNGFR 22 nl/min vs. 28, P < 0.05, ACEI vs. Hoe). In contrast to these results with ACEI, treatment with AIIRA significantly increased GFR, and addition of bradykinin antagonist (N = 4) did not cause further increase in filtration. Thus, in conditions where high P(GC) is crucial in maintaining GFR, activation of bradykinin by ACEI can compromise GFR secondary to bradykinin's selective vasodilatory action on the efferent arteriole.
