Relationship between plasma renin activity and hypotensive effect of captopril in dogs

The ability of captopril (SQ-14225), an angiotensin I-converting enzyme inhibitor, to reduce blood pressure in dogs anesthetized with either pentobarbital sodium or a combination of morphine plus chloralose was studied over a sixty minute period. In addition, since both of these anesthetic agents cause an increase in the plasma renin activity of these animals, the drug was used to further investigate whether the hypotensive action of this drug is dependent on the level of plasma renin activity. Captopril produced significant but similar decreases in blood pressure of both the anesthetized groups, even though the blood pressure of the morphine-chloralose dogs was significantly lower than that of the pentobarbital animals. Further, the hypotensive action of captopril was not related to the level of plasma renin activity, since the morphine-chloralose dogs had a significantly higher plasma renin activity. It is concluded that the magnitude of blood pressure reduction following the administration of captopril is not related to the level of blood pressure and the pre-drug plasma renin activity and hitherto unexplained mechanisms in addition to the converting enzyme inhibition may be involved in the hypotensive action of this drug.