Responses of Femoral Resistance Vessels to Angiotensin in Vitro

The effect of angiotensin II and angiotensin I on isolated rat resistance vessels (inner diameter ca. 200 micron) was investigated. Angiotensin II caused a contraction (ED50 = 0.58 +/- 0.17 X 10(-8) M) of rat femoral and cerebral arteries and to a lesser extent of mesenteric and renal arteries. However, all vessels showed strong tachyphylaxis on repeated stimulation with angiotensin II. Tachyphylaxis was avoided by inducing submaximal tone in the vessels with either K, noradrenaline or serotonin. The response to angiotensin II was inhibited by saralasin but not by captopril. Angiotensin I also caused contraction of the femoral arteries (ED50 = 2.68 +/- 0.32 X 10(-8) M). These responses were inhibited by captopril and saralasin. Functional removal of the endothelium had little effect on the contractile responses to either angiotensin I or II. These results indicate that there are functional receptors to angiotensin II in the resistance vessels of the rat and that, in the presence of tone (a more physiological condition), the vessels contract to angiotensin II without tachyphylaxis. In addition, angiotensin II may be formed from angiotensin I by the angiotensin converting enzyme which may be situated in the vessel wall as well as in the endothelium.