Are angiotensin receptors in vascular smooth muscles a homogeneous population?

The effects of angiotensin II (AII) and angiotensin III (AIII) on mean arterial pressure (MAP) and mean circulatory filling pressure (MCFP), an index of total body venous tone, in the presence and absence of [Sar1,Ile8]AII in conscious rats were examined. The infusion of AII caused dose-dependent increases in MAP and MCFP. The dose-response curves of MAP and MCFP for AII were displaced to the right in the presence of various doses of [Sar1,Ile8]AII. The pA2 values obtained for AII in the presence of the antagonist were 9.2 and 8.4 for the arterioles and veins respectively. The infusion of AIII also caused dose-dependent increases in MAP and MCFP. In the presence of the antagonist the AIII dose-response curves for MAP and MCFP were not displaced to right. The same maximum MAP was obtained for both AII and AIII but the maximum MCFP obtained following the infusion of AIII was smaller than that for AII. It is concluded that AII may act on different sub-classes of angiotensin receptors in arterioles and veins. AIII caused vasoconstriction in arterioles by acting on a sub-class of angiotensin receptors different from the ones activated by AII. AIII may act as a partial agonist on the same types of receptors as AIII in the venous bed.