IMPACT OF CYCLOOXYGENASE BLOCKADE ON JUXTAMEDULLARY MICROVASCULAR RESPONSES TO ANGIOTENSIN-II IN RAT-KIDNEY

1. Experiments were designed to evaluate the hypothesis that cyclo-oxy genase products modulate the influence of angiotensin II (AII) on the renal juxtamedullary microvasculature of enalaprilat-treated rats. 2. The in vitro blood-perfused juxtamedullary nephron technique was utili zed to provide access to afferent arterioles, efferent arterioles and descending vasa recta located in the outer stripe of the outer medulla . 3. Baseline afferent arteriolar diameter was 20.8+/-1.9 mu m in kidn eys subjected to cyclo-oxygenase blockade (1 mu mol/L piroxicam), a va lue significantly lower than that observed in untreated kidneys (26.1/-1.0 mu m) Baseline diameters of efferent arterioles and outer medull ary descending vasa recta did not differ between untreated and piroxic am-treated groups. 4. Topical application of 1 nmol/L AII reduced bloo d now through outer medullary descending vasa recta by 22+/-6% in untr eated kidneys and by 24+/-7% in piroxicam-treated kidneys. 5. In untre ated kidneys, AII (0.01-100 nmol/L) produced concentration-dependent a fferent and efferent arteriolar constrictor responses of similar magni tudes. Neither afferent nor efferent arteriolar An responsiveness was significantly altered in piroxicam-treated kidneys, although afferent responses exceeded efferent responses at An concentrations greater tha n or equal to 10 nmol/L. 6. We conclude that endogenous cyclo-oxygenas e products exert a vasodilator influence on juxtamedullary afferent ar terioles under baseline conditions. Although cyclo-oxygenase inhibitio n had little effect on juxtamedullary microvascular responses to AII, the response to high AII concentrations may be modulated by cyclo-oxyg enase products in a manner which delicately alters the relative influe nce of the peptide on pre- vs postglomerular resistances.