α₁‐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.

Balloon Injury Alters Α‐adrenoceptor Expression Across Rat Carotid Artery Wall

“…The reasons for this disagreement are not evident, but may relate to minimal wall distension injury following simple endothelial denudation. Consistent with this, another study of balloon denudation of rabbit aorta endothelium reported that, 2 weeks later, when a small intimal thickening was present, maximal contraction to phenylephrine and potassium chloride was unchanged and potency to phenylephrine was reduced threefold 29 . It is interesting that unlike α‐AR downregulation by balloon injury, angiotensin receptors, which also mediate trophic activity on vascular SMC, are upregulated by balloon injury 30,31 …”

“…Consistent with this, another study of balloon denudation of rabbit aorta endothelium reported that, 2 weeks later, when a small intimal thickening was present, maximal contraction to phenylephrine and potassium chloride was unchanged and potency to phenylephrine was reduced threefold. 29 It is interesting that unlike a-AR downregulation by balloon injury, angiotensin receptors, which also mediate trophic activity on vascular SMC, are upregulated by balloon injury. 30,31 To begin examining possible mechanisms responsible for a-AR downregulation, we examined rat aorta SMC in early passage cell culture that express a 1B -and a 1D -AR (but not a 1A -or a 2D -AR) for an effect of FGF2 and TGF-b1 on expression.…”