Angiotensin II surface receptor coupling to inositol trisphosphate formation in vascular smooth muscle cells

In some systems there are spare receptors for hormone action, i.e. only a fraction of the total number of surface receptors need be occupied by agonist to elicit maximum cellular responses. The purpose of this study was to determine the relationship between angiotensin II (AII) surface receptor number and AII-induced inositol trisphosphate (IP3) formation in rat-cultured vascular smooth muscle cells. To accomplish this purpose, it was necessary to develop a method to modulate AII surface receptor number without activating phospholipase C. Incubation with the putative AII receptor antagonist Sar1,Leu8-AII (SL) caused reductions in AII surface receptor number by redistribution of receptors to the cell interior. However, in contrast to AII, SL did not elicit IP3 responses. By varying the conditions of incubation with SL, graded (32-60%) reductions in AII surface receptor number were achieved. In association with reductions in surface receptors there were comparable reductions in AII-stimulated IP3 formation. The correlation between receptor number and stimulated IP3 formation was highly linear (r = 0.99, p less than 0.01). To determine if incubation with AII also caused reductions in stimulated IP3 formation in proportion to the degree of receptor loss, AII surface receptor number was decreased by incubation with AII. Despite decreases in AII receptor number comparable to those achieved with SL, incubation with AII resulted in 2-fold greater loss of AII-stimulated IP3 formation than did incubation with SL. We conclude that in vascular smooth muscle cells 1) the AII receptor antagonist SL stimulates AII receptor trafficking without eliciting IP3 formation, 2) there are no spare AII receptors for phospholipase C-mediated IP3 formation, and 3) AII desensitization of IP3 formation is mediated by reductions in surface receptors as well as by post-receptor mechanisms.