1. Changes in plasma renin activity, plasma noradrenaline and adrenaline, and blood pressure were evaluated after mild haemorrhage (6 ml/kg) and subsequent intravenous captopril in conscious, restrained rabbits. Two protocols were followed. In the first, control animals (n = 8) were compared with rabbits receiving indomethacin (n = 8) in their drinking water for 5 days and intravenously during the haemorrhage and captopril study. In the second protocol, animals receiving propranolol in the drinking water and intravenously (n = 7) were compared with a group (n = 7) receiving propranolol plus indomethacin.

2. Urinary prostaglandin E excretion was decreased significantly with the oral administration of indomethacin in both protocols. Plasma renin activity levels were not significantly different at baseline, but the animals receiving indomethacin had significantly (P < 005) lower values after haemorrhage in both studies. The large increases in plasma renin activity after captopril were not influenced by indomethacin in either study.

3. The blood pressure response to captopril was significantly blunted in the indomethacin group in the first experiment.

4. Plasma noradrenaline and adrenaline levels were not affected by indomethacin treatment in either protocol.

5. It is concluded that the plasma renin activity response to haemorrhage in conscious rabbits is mediated in part by a cyclo-oxygenase-dependent component separable from the β-adrenoceptor. The renin response to converting enzyme inhibition is not influenced by cyclo-oxygenase inhibition. A component of the blood pressure response to captopril may involve a cyclo-oxygenase product.

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