ALPHA(1)-ADRENOCEPTORS ON RABBIT AORTIC SMOOTH-MUSCLE CELLS IN CULTURE AND IN EXPERIMENTAL INTIMAL THICKENING

1. This study has defined alpha(1)-adrenoceptors and their reactivity in rabbit aorta, following removal of the endothelium and formation of a myointimal thickening, and also in smooth muscle cells (SMC) in cel l culture which had undergone serial passaging and changes in phenotyp e. 2. [H-3]-prazosin binding to SMC from control aorta, vessels 2 week s after endothelial denudation and sub-cultured SMC (passage 3-6) was specific (displaceable with 10 mu mol/L phentolamine), and of high aff inity to a single class of sites (K-D range: 71-114 pmol/L), The maxim um binding density (B-max) of alpha(1)-adrenoceptors on SMC from the n eointima (11105+/-771 sites/cell) was not significantly different to t hat of control medial SMC (14014+/-2472 sites/cell). However, SMC cult ured to passage 6, showed a 2-fold increase in B-max (30227+/-4349 sit es/cell). 3. The production of inositol phosphates (IP1, IP2 and IP3) by SMC following 10 mu mol/L phenylephrine was assayed, Both freshly-d ispersed aortic SMC and sub-cultured SMC were stimulated to produce in creased inositol phosphates by the addition of phenylephrine which was completely inhibited by pre-incubation with 10 mu mol/L phentolamine, suggesting that the stimulation was via alpha(1)-adrenoceptors. 4. Ma ximal contractile responses of isolated thoracic and abdominal aortic rings to KCl (100 mmol/L), 5-HT and phenylephrine were unchanged two w eeks after endothelial denudation. However, phenylephrine was signific antly less potent (2.7-fold) in both areas of the aorta, while the pot ency of 5-HT was significantly enhanced (2.7-fold) after endothelial d enudation only in the abdominal aorta. 5. The decreased sensitivity of the rabbit aorta to alpha(1)-adrenoceptor agonists following endothel ial denudation and the formation of a myointimal thickening is not due to changes in affinity or density of alpha(1)-adrenoceptors. However multiple passaging of SMC in culture leads to an increase in alpha(1)- adrenoceptor density, This change can be related to the altered cytodi fferentiation of irreversible synthetic state SMC which are similar to those in atherosclerotic lesions.