Altered catecholamine receptor affinity in rabbit aortic intimal hyperplasia

Chronic extrinsic denervation of the small bowel: effect on adrenergic and cholinergic contractile mechanisms in canine ileal circular muscle

BACKGROUND

Small bowel transplantation necessitates chronic extrinsic denervation and is often followed by enteric dysfunction. Our aim was to study canine ileal contractile activity after extrinsic denervation.

METHODS

In vitro dose responses to cholinergic and adrenergic agonists were evaluated in canine ileal muscle strips in control subjects and after jejunoileal extrinsic denervation (EX DEN) at 0, 2, and 8 weeks after operative preparation.

RESULTS

Spontaneous activity and the increased activity after tetrodotoxin (enteric nerve blockade) did not differ between groups. Response to acetylcholine and bethanechol did not differ at any time in the control or EX DEN group. In contrast, the EX DEN group demonstrated a procontractile hypersensitivity to norepinephrine at 2 and 8 weeks that was not seen in the control group. This adrenergic hypersensitivity in the EX DEN group was insensitive to intramural neural blockade with tetrodotoxin.

CONCLUSIONS

Extrinsic denervation does not affect basal contractile activity, augmented contractile activity to intramural neural blockade, nor response to cholinergic agonists. However, it induces a procontractile adrenergic hypersensitivity in canine ileal circular muscle mediated at the level of the smooth muscle and not at the enteric nervous system.

Neuronal adrenergic and muscular cholinergic contractile hypersensitivity in canine jejunum after extrinsic denervation

Extrinsic denervation may be responsible for motor dysfunction after small bowel transplantation. The aim of this study was to examine the role of extrinsic innervation of canine jejunum on contractile activity. An in vitro dose response of cholinergic and adrenergic agonists was evaluated in canine jejunal strips of circular muscle at 0, 2, and 8 weeks in a control group and after jejunoileal extrinsic denervation (EX DEN). Neurons in circular muscle were quantitated by means of immunohistochemical techniques. Adrenergic and cholinergic responses did not differ at any time in the control group. However, at 2 and 8 weeks, extrinsic denervation caused an increased sensitivity to the procontractile effects of the cholinergic agonist bethanechol at the level of the smooth muscle cells, and increased sensitivity to the inhibitory effects of the adrenergic agent norepinephrine mediated at the level of the enteric nervous system. Immunohistochemical analysis showed a reduction in all neurons and a complete lack of adrenergic fibers in the EX DEN group after 2 and 8 weeks. Extrinsic denervation induces enteric neuronal cholinergic and adrenergic smooth muscle hypersensitivity in canine jejunal circular muscle.

Neural regulation of intestinal smooth muscle growth in vitro

The loss of intrinsic neurons is an early event in inflammation of the rat intestine that precedes the growth of intestinal smooth muscle cells (ISMC). To study this relationship, we cocultured ISMC and myenteric plexus neurons from the rat small intestine and examined the effect of scorpion venom, a selective neurotoxin, on ISMC growth. By 5 days after neuronal ablation, ISMC number increased to 141+/-13% (n = 6) and the uptake of [(3)H]thymidine in response to mitogenic stimulation was nearly doubled. Atropine caused a dose-dependent increase in [(3)H]thymidine uptake in cocultures, suggesting the involvement of neural stimulation of cholinergic receptors in regulation of ISMC growth. In contrast, coculture of ISMC with sympathetic neurons increased [(3)H]thymidine uptake by 45-80%, which was sensitive to propranolol (30 microM) and was lost when the neurons were separated from ISMC by a permeable filter. Western blotting showed that coculture with myenteric neurons increased alpha-smooth muscle-specific actin nearly threefold to a level close to ISMC in vivo. Therefore, factors derived from enteric neurons maintain the phenotype of ISMC through suppression of the growth response, whereas catecholamines released by neurons extrinsic to the intestine may stimulate their growth. Thus inflammation-induced damage to intestinal innervation may initiate or modulate ISMC hyperplasia.

Balloon catheter injury and vein graft morphology and function

Endovascular interventions to salvage failing vein bypass grafts are often associated with suboptimal outcomes. This study examines the effect of experimental vein graft catheter injury on vein graft morphology and vasomotor function. Thirty New Zealand white rabbits underwent a right common carotid interposition vein bypass graft. Ten grafts were harvested at 14 days, 10 were harvested at 28 days, and 10 had a balloon catheter injury induced at 14 days (4 F Fogarty catheter, 0.6 to 0.75 ml water inflation, 3 passes) and these 10 grafts were harvested after an additional 14 days. Morphologic and morphometric determinations (n = 5) or in vitro contractile studies (n = 5) were performed on segments of the vein grafts. Intimal thickness, without any intervention, increased by 84% from 14 to 28 days (p < 0.01), whereas catheter injury at 14 days induced a twofold increase (p < 0.001) in the formation of intimal hyperplasia by 28 days. Scanning electron microscopy demonstrated near-complete endothelial denudation after balloon catheter injury. In the 14- and 28-day control vein grafts, and in the balloon-injured vein grafts, the vascular surfaces had confluent endothelial linings. However, the ultrastructural features of the endothelial cells were group specific. Transmission electron microscopy of the same specimens confirmed this. There were no significant differences in contractility between the 28-day control and the catheter-injured vein grafts. This study demonstrates that balloon catheter injury doubles the rate at which intimal hyperplasia develops in vein grafts without significantly altering the physiologic phenotype of the smooth muscle cells as defined by their vasomotor function.

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 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.

Pathobiology of intimal hyperplasia

In the current vascular interventional environment, high restenosis rates have increased awareness of the significance of intimal hyperplasia, a chronic structural lesion that develops after vessel wall injury, and which can lead to luminal stenosis and occlusion. Intimal hyperplasia may be defined as the abnormal migration and proliferation of vascular smooth muscle cells with associated deposition of extracellular connective tissue matrix. The pathology of intimal hyperplasia is reviewed with particular attention to its physiology, pharmacology, cell biology and molecular biology.

Long-term reduction of intimal hyperplasia by the selective alpha-1 adrenergic antagonist doxazosin

Studies have shown that alpha 1-adrenergic blockade reduces intimal hyperplasia in the rabbit aorta. In this study a selective alpha 1-adrenergic antagonist, doxazosin, has been used to examine whether this effect is persistent and dose dependent. Forty-eight New Zealand White rabbits underwent endothelial denudation of the abdominal aorta using a Fogarty balloon catheter. Test rabbits were given low-dose (2 mg) or high-dose (8 mg) doxazosin daily and all animals killed at either 1 or 12 weeks after the procedure. The aortas were harvested after fixation in situ with 4 per cent glutaraldehyde and neointimal hyperplasia was measured, using an x-y digitizer, as the percentage reduction in luminal cross-sectional area. At 1 week after surgery, rabbits receiving the low dose had a median area reduction of 7.7 per cent and those receiving the high dose a reduction of 8.2 per cent; both had significantly less intimal hyperplasia than control rabbits, which had a median area reduction of 14.8 per cent (P < 0.01). However, at 12 weeks, when compared with the 32.6 per cent reduction in the control group, only those rabbits receiving high-dose doxazosin had significantly less intimal hyperplasia, with a reduction of 5.5 per cent (P < 0.001). It is concluded that selective alpha 1-adrenergic blockade significantly reduces neointimal hyperplasia, that this effect is dose dependent, and that it persists for at least 3 months.