The cardiovascular effects of intravertebral angiotensin II before and after treatment with clonidine.

1. In anaesthetized mongrel dogs it was shown that intravertebral infusions of angiotensin II (2-4 ng/kg per min) increased mean arterial pressure by causing an increase in cardiac output, while infusions of 10 ng/kg per min increased mean arterial pressure through an effect on peripheral resistance. After intravenous clonidine, intravertebral angiotensin no longer had any stimulatory effect on cardiac output, but arterial pressure still increased to the same extent. 2. It is concluded that intravertebral angiotensin can increase arterial pressure by increasing either cardiac output or peripheral resistance. The effects of intravertebral angiotensin on cardiac output can be reduced by concomitant stimulation of baroreflex pathways but its effects on peripheral resistance are not so readily antagonized.