Alpha 1-Adrenoceptors on rabbit aortic smooth muscle cells in culture and in experimental intimal thickening.

Abstract

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 cell culture which had undergone serial passaging and changes in phenotype. 2. [3H]-prazosin binding to SMC from control aorta, vessels 2 weeks after endothelial denudation and sub-cultured SMC (passage 3-6) was specific (displaceable with 10 mumol/L phentolamine), and of high affinity to a single class of sites (KD range: 71-114 pmol/L). The maximum binding density (Bmax) of alpha 1-adrenoceptors on SMC from the neointima (11,105 +/- 771 sites/cell) was not significantly different to that of control medial SMC (14,014 +/- 2472 sites/cell). However, SMC cultured to passage 6, showed a 2-fold increase in Bmax (30,227 +/- 4349 sites/cell). 3. The production of inositol phosphates (IP1, IP2 and IP3) by SMC following 10 mumol/L phenylephrine was assayed. Both freshly-dispersed aortic SMC and sub-cultured SMC were stimulated to produce increased inositol phosphates by the addition of phenylephrine which was completely inhibited by pre-incubation with 10 mumol/L phentolamine, suggesting that the stimulation was via alpha 1-adrenoceptors. 4. Maximal contractile responses of isolated thoracic and abdominal aortic rings to KCl (100 mmol/L), 5-HT and phenylephrine were unchanged two weeks after endothelial denudation. However, phenylephrine was significantly less potent (2.7-fold) in both areas of the aorta, while the potency of 5-HT was significantly enhanced (2.7-fold) after endothelial denudation only in the abdominal aorta. 5. The decreased sensitivity of the rabbit aorta to alpha 1-adrenoceptor agonists following endothelial 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 cytodifferentiation of irreversible synthetic state SMC which are similar to those in atherosclerotic lesions.