In vivo pulmonary metabolism of bradykinin, angiotensin I and 5-hydroxtryptamine in the rat.

Involvement of the pulmonary vasculature of the rat in the in vivo metabolism of bradykinin, angiotensin I and 5-hydroxytryptamine was studied by monitoring the systemic blood pressure response of the test animal during intravenous or intraarterial administration of these substances. Observation of essentially the same blood pressure response regardless of the route of injection indicated that neither agiotensins I or II nor 5-hydroxytryptamine was metabolized by the pulmonary vasculature, while bradykinin was inactivated to the extent of 95+% on one passage through the lung. In the rat the major portion of bradykinin inactivation and angiotensin conversion apparently occur at different places in the vascular tree and therefore could not be carried out by the same enzyme. Differential inhibition by 2-mercaptoethanol and the bradykinin potentiating peptide, Pyr-Lys-Trp-Ala-Pro, support this postulate. One rat was found which had no pulmonary kininase but could convert angiotensin I to angiotensin II. In addition, infusion of large doses of known inhibitors of lung converting enzyme had no effect on either angiotensin I conversion or bradykinin inactivation in this system. These results support the idea that the pulmonary vasculature is not a major site for conversion of angiotensin I in the rat.