Role of polymorphonuclear leukocytes in collar-induced intimal thickening in the rabbit carotid artery.

In this study, the involvement of polymorphonuclear leukocytes (PMNs) in the development of intimal thickening was investigated. A fibromuscular intima was induced by placing a silicone collar around the rabbit carotid artery for 3 days or 2 weeks; the contralateral artery was sham operated. Rabbits received placebo treatments (groups 1 and 3), granulocyte-colony stimulating factor (group 2; G-CSF, 20 microg x kg(-1) x d(-1), delivered by subcutaneous osmotic pumps), or an anti-CD18 monoclonal antibody (group 4; 1.5 mg/kg i.v.). The G-CSF treatment raised the peripheral PMN count 5- to 12-fold but had no effect on intimal thickening on day 3, 12, or 14. A single injection of anti-CD18 prevented PMN extravasation 6 hours after collar implantation without influencing intimal hyperplasia on day 14. Repeated daily administration of anti-CD18 strongly bound to CD18 on peripheral PMNs and inhibited both PMN-dependent plasma extravasation in the skin and accumulation of CD14-immunoreactive leukocytes in the intima and media. However, anti-CD18 did not suppress early intimal thickening or accumulation of alpha-smooth muscle actin-immunoreactive cells by day 3. It thus appears that the PMN influx in the intima and media evoked by the perivascular collar is of little functional relevance to the subsequent smooth muscle cell migration and intimal thickening in this model.

Possible mechanisms of collar-induced intimal thickening.

The positioning of a soft silicone collar around the rabbit carotid artery induces intimal thickening. We investigated to which extent occlusion of the vasa vasorum, damage of the perivascular nerve network, and/or changes in blood flow velocity contribute to intimal thickening. To this end, collars with different bores (diameter of inlet and outlet) were positioned around the carotid artery of male rabbits for 14 days. In another experiment, 75% of the wall of fitting collars was removed (open collar). In the midcollar region, the cross-sectional area of the intima reached a maximum (72 +/- 14 mm2/1000) when the endings of the collar fitted the artery closely. Removal of the side wall of these fitting collars reduced intimal thickening by 90%. Examination of unoperated carotid arteries never showed penetration of the adventitia or the media by vasa vasorum. The perivascular neuronal network in the region surrounded by a closed or an open collar was almost completely lost as compared with the zones outside the collar. Both the closed and open collar slightly bent the artery and increased the peak systolic velocity, measured with pulsed color Doppler after 6 hours, to a similar extent as compared with the proximal zone outside the collar. After 2 weeks, the peak systolic velocity within both the closed and open collar was partly normalized and was statistically not different from the proximal zone outside the collar. In conclusion, the geometry of the collar influenced the extent of intimal thickening, whereby more intimal thickening was obtained with a collar whose endings fit the carotid artery, rather than with a loose collar. Moreover, a closed structure was essential. The results obtained with the open collar exclude occlusion of vasa vasorum, damage of the perivascular neuronal network, kinking of the artery, and changes in blood flow velocity as major factors in the collar-induced intimal thickening. Our findings are consistent with the possibility that intimal thickening is the consequence of the combination of both vascular injury and hindrance of transmural flow by the collar. The obstruction of transmural fluid transport may then lead to retention of toxic metabolites, and/or cytokines within the segment enclosed by the collar.

Dexamethasone influences intimal thickening and vascular reactivity in the rabbit collared carotid artery.

Intimal thickening predisposes to atherosclerosis and is often associated with alterations of the vascular reactivity of the artery. We investigated whether dexamethasone inhibited the intimal thickening and reactivity changes induced by a silicone collar placed around the left rabbit carotid artery for 2 weeks. The sham-operated, right artery served as control. Dexamethasone (1 mg/kg/day), given in the drinking water (n = 10) or by a subcutaneous minipump (n = 10), abolished intimal thickening compared to that of both placebo groups (n = 10). Both dexamethasone and the collar suppressed the isometric force development of isolated segments elicited by KCl in organ chamber experiments. The collar raised the sensitivity to serotonin (5-hydroxytryptamine, 5-HT) and the maximum force development (Emax) after normalization for the KCl responses. Dexamethasone exerted complex effects on 5-HT contractions in sham arteries: the curves often became biphasic, and sensitivity and Emax of the first phase were depressed by dexamethasone. In contrast, dexamethasone raised the hypersensitivity of collared arteries to 5-HT even further. Collar and dexamethasone did not influence endothelium-dependent relaxations elicited by acetylcholine or the calcium ionophore A-23187. It is concluded that dexamethasone interfered with neo-intima formation in the collar model, presumably by inhibition of smooth muscle cell migration and/or proliferation, without restoring contractile behaviour. Therefore, the collar-induced alterations in the reactivity of the smooth muscle cells in the media appear to be unrelated to the process of intimal thickening.