Direct evidence for the presence of a different converting enzyme in the hamster cheek pouch

Kininase II (angiotensin I-converting enzyme) is generally accepted to be the enzyme responsible for the conversion of angiotensin I (A I) to angiotensin (A II). This study examined the response of the microvasculature of the hamster cheek pouch to the local application of A I, A II, and the renin substrate, tetradecapeptide (TDP). A I and TDP caused a localized vasoconstriction that was not blocked by converting enzyme inhibitors (CEI; BPF(5a) for A I and BPF(5a) and the nonapeptide inhibitor for TDP. However, both the A II antagonist [Sar¹, Ala⁸]angiotensin II and the antiserum to A II blocked completely the A I- and TDP-induced vasoconstriction. Sixty-eight percent of the applied A I was converted to A II in the presence of CEI as well as in its absence. It is concluded that the vasculature of the hamster cheek pouch converts significant amounts of A I to A II by a route that does not involve kininase II.