Prostaglandins and the renal responses to haemorrhage, angiotensin II and methoxamine in conscious rabbits

Effects of indomethacin on responses of regional kidney perfusion to vasoactive agents in rabbits

1. To determine whether differential release of products of arachidonic acid metabolism, via the cyclo-oxygenase pathway, underlies the diversity of responses of regional kidney perfusion to vasoactive agents, we tested the effects of intravenous indomethacin on responses to renal arterial bolus doses of vasoactive agents in pentobarbitone-anaesthetized rabbits. 2. Total renal blood flow (RBF) and regional kidney perfusion were determined by transit time ultrasound flowmetry and laser-Doppler flowmetry, respectively. 3. Responses of regional kidney blood flow to vasoactive agents were diverse: noradrenaline reduced cortical but not medullary perfusion, [Phe 2,Ile 3,Orn 8]-vasopressin reduced medullary perfusion more than cortical perfusion, endothelin-1 and angiotensin II increased medullary perfusion in the face of reduced cortical perfusion, while acetylcholine, bradykinin and the nitric oxide donor methylamine hexamethylene methylamine (MAHMA) NONOate all increased both cortical and medullary perfusion. 4. Indomethacin administration was followed by reductions in total RBF (17 +/- 6%), cortical perfusion (13 +/- 5%) and medullary perfusion (40 +/- 8%). Angiotensin II- and endothelin-1-induced increases in medullary perfusion were abolished by indomethacin, but indomethacin had no significant effects on responses of regional kidney perfusion to acetylcholine, bradykinin, MAHMA NONOate, noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin. 5. Our results suggest that vasodilator cyclo-oxygenase products contribute to the maintenance of resting renal vascular tone, particularly in vascular elements controlling medullary perfusion. Cyclo-oxygenase products also appear to mediate endothelin-1- and angiotensin II-induced increases in medullary perfusion. However, regionally specific engagement of cyclo-oxygenase-dependent arachidonic acid metabolism does not appear to contribute to the differential effects of noradrenaline and [Phe 2,Ile 3,Orn 8]-vasopressin on cortical and medullary perfusion.

Mediators of the hypotensive response to increased renal perfusion in rabbits

We have previously shown that increasing the renal perfusion pressure by using an extracorporeal circuit in anesthetized rabbits resulted in a progressive fall in systemic arterial pressure. Prior ablation of the renal medulla with 2-bromoethylamine abolished the hypotensive response. In the present study, we investigated whether vasodilator prostanoids or platelet activating factor (PAF), both known to be produced in the renal medulla, were responsible for the hypotensive response to increased renal perfusion pressure. Anesthetized animals were treated with indomethacin (5 mg/kg + 0.5 mg/kg per hour), the PAF antagonist WEB 2086 (0.5 mg/kg + 0.5 mg/kg per hour), enalaprilat (2 mg/kg + 10 micrograms/kg per hour), or all three agents. In response to acute elevation of renal artery pressure to 170 mm Hg, systemic mean arterial pressure fell at 0.76 +/- 0.17, 0.59 +/- 0.08, and 0.76 +/- 0.17 mm Hg/min in the indomethacin, WEB 2086, and enalapril groups, respectively. These responses were not significantly different from the rate of 1.00 +/- 0.21 mm Hg/min in a control group that received vehicle infusion alone. Renal blood flow and the diuretic and natriuretic responses were also similar in all groups. Thus, increased renal perfusion pressure resulted in a progressive fall in systemic arterial pressure that was not mediated by PAF, prostaglandins, or suppression of renin release and angiotensin II production.

Indomethacin does not potentiate renovascular constriction during hypercapnia in unanesthetized rabbits

The role of prostanoids in regulation of the renal circulation during hypercapnia was examined in unanesthetized rabbits. Renal blood flow (RBF) was determined with 15 micron radioactive microspheres during normocapnia (PaCO2 congruent to 30 mmHg) and hypercapnia (PaCO2 congruent to 60 mmHg), before and after intravenous administration of indomethacin (10 mg/kg) or vehicle (n = 6 for each group). Arterial blood pressure was not different among the 4 conditions in each group. RBF was 438 +/- 61 and 326 +/- 69 (P less than 0.05) ml/min per 100 g during normocapnia and hypercapnia, respectively, before indomethacin, and following administration of indomethacin, RBF was 426 +/- 59 ml/min per 100 g during normocapnia and 295 +/- 60 ml/min per 100 g during hypercapnia (P less than 0.05). In the vehicle group, RBF was 409 +/- 74 and 226 +/- 45 (P less than 0.05) ml/min per 100 g during normocapnia and hypercapnia, respectively, before vehicle; and following administration of vehicle, RBF was 371 +/- 46 ml/min per 100 g during normocapnia and 219 +/- 50 (P less than 0.05) ml/min per 100 g during hypercapnia. RBF during normocapnia was not affected by administration of indomethacin or vehicle. The successive responses to hypercapnia were not different within the indomethacin and vehicle groups, and the second responses to hypercapnia were not different between the two groups. These findings suggest that prostanoids do not contribute significantly to regulation of the renal circulation during normocapnia and hypercapnia in unanesthetized rabbits.