Control of rat renal vascular resistance at reduced perfusion pressure.

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Abstract
Determination of the factors influencing renal vascular resistance (RVR) at very low perfusion pressures (RPP) is of interest, since prolonged periods of reduced perfusion can lead to acute renal failure. The effects of inhibition of the renin-angiotensin [A] system on renal blood flow (RBF) were measured by electromagnetic flow probe, over a subautoregulatory pressure range of 90-25 mm Hg. These studies were carried out in rats on low N diets (LS), in which AII is increased and high Na diets (HS) in which AII is suppressed. The effect of altering the kallikrein-kinin system was studied since previous studies indicated an interaction between this system and AII in control of RVR at normal RPP. Variations in Na intake did not alter the regression of RBF on RPP over the entire low pressure range. RVR in LS rats increased from 15.8 .+-. 0.4 between 65-90 mm Hg to 21.2 .+-. 1.2 mm Hg/ml per min at pressures < 45 mm Hg (P < 0.05). In HS rats RVR rose from 13.8 .+-. 0.5 at RPP between 65-90 mm Hg to 18.6 .+-. 0.9 mm Hg/ml per min at RPP < 45 mm Hg (P < 0.001). Saralasin, an AII antagonist, increased RBF and reduced RVR over the entire low pressure range in LS rats, but had no effect on RVR in HS rats. Captopril, which inhibits the formation of AII as well as the degradation kinins, increases RBF in LS and HS rats. Aprotinin, an inhibitor of kallikrein, reduced RBF and increased RVR over the entire low pressure range in LS rats. Aprotinin had no effect on RVR in HS rats, but pretreatment with this agent blocked the vasodilatory effect of captopril. Infusion of kinins raised RBF in HS rats but had no effect on RBF of LS rats. RVR apparently is not fixed or minimal in the subautoregulatory pressure range. AII acts as a vasoconstrictor throughout this low pressure range in LS rats, but its effect is countered by the vasodilator influence of kinins. Neither hormone substantially influenced RVR in HS rats. Parallel changes in the activity of these 2 hormones maintains a relatively constant RVR despite changes in dietary Na intake.

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