Effect of captopril on blood pressure responses to enkephalins in chloralose-anaesthetized rats.

Captopril lowered blood pressure in chloralose-anaesthetized rats and enhanced the depressor responses to metenkephalin and D-Ala-metenkephalinamide to the same extent. Since metenkephalin is a much better substrate than D-Ala-metenkephalinamide for dipeptidylcarboxypeptidase, some mechanism other than inhibition of this enzyme appears to be responsible for the enhancement by captopril of the responses to opioids. When the blood pressure had been lowered by haemorrhage, naloxone was no more effective than saline control in restoring blood pressure. However, when captopril was given after haemorrhage, the blood pressure was further lowered, and then naloxone produced a significant restoration. Hydralazine given after haemorrhage also caused a further lowering of blood pressure, then naloxone produced a lesser restoration of blood pressure than in haemorrhage plus captopril experiments. The results suggest that release of endogenous opioids may contribute to the fall in blood pressure upon bleeding.