Isolated, perfused rabbit ear artery: a model for studying segmental vasoconstriction and dilatation

"Sausage-string" appearance of arteries and arterioles can be caused by an instability of the blood vessel wall

Vascular damage induced by acute hypertension is preceded by a peculiar pattern where blood vessels show alternating regions of constrictions and dilations ("sausages on a string"). The pattern occurs in the smaller blood vessels, and it plays a central role in causing the vascular damage. A related vascular pattern has been observed in larger vessels from several organs during angiography. In the larger vessels the occurrence of the pattern does not appear to be related to acute hypertension. A unifying feature between the phenomenon in large and small vessels seems to be an increase in vascular wall tension. Despite much research, the mechanisms underlying the sausage pattern have remained unknown. Here we present an anisotropic model of the vessel wall and show that the sausage pattern can arise because of an instability of the vessel wall. The model reproduces many of the key features observed experimentally. Most importantly, it suggests that the "sausaging" phenomenon is neither caused by a mechanical failure of the vessel wall due to a high blood pressure nor is it due to standing pressure waves caused by the beating of the heart. Rather, it is the expression of a general instability phenomenon. Experimental data suggest that the structural changes induced by the instability may cause secondary damage to the wall of small arteries and arterioles in the form of endothelial hyperpermeability followed by local fibrinoid necrosis of the vascular wall.

Hypertensive arteriolar necrosis revisited

Arteriolar necrosis is the histopathological hallmark of malignant hypertension. In the period from 1940 to 1980, intensive studies of the pathogenesis of hypertensive fibrinoid necrosis were carried out. From studies of mesenteric vessels in animals with experimental hypertension it was discovered that arteriolar necrosis is preceded by an abnormal vascular reaction pattern consisting of alternating constriction and localized dilatations. The development of the abnormal reaction pattern is followed by endothelial hyperpermeability resulting in transsudation of plasma and macromolecules into the wall of the arteriole. The hyperpermeability, and, in turn, arteriolar necrosis, is exclusively found in the dilated segments of the vessel. The abnormal vascular reaction pattern can be induced experimentally by various techniques such as infusion of angiotensin II or stimulation of perivascular nerves, and it can be demonstrated in all target organs. Since 1980 a remarkable falling-off in the research on the pathogenesis of hypertensive arteriolar necrosis has been noted. Recent studies in this area utilizing current knowledge and major advances in microvascular research methods are very few in number. As of today, the chain of pathogenetic processes leading eventually to the all-important lesion of arteriolar fibrinoid necrosis remains poorly understood.

Antihypertensive drugs and endothelial cell function

Arterial hypertension is associated with increased capillary permeability, a possible contributor to the vascular remodeling process which could be involved in certain pathological conditions arising from elevated blood pressure. This study evaluated the effects of various antihypertensive drugs on capillary permeability in the normal rat, using Evan's blue dye (EB) as a marker of albumin extravasation. The results reveal that acute injection of certain diuretics (furosemide, indapamide, hydrochlorothiazide) increase while others (amiloride, cicletanine) decrease capillary permeability via stimulation of the cyclooxygenase pathway. 10 day gavage with indapamide, amiloride and cicletanine, as well as angiotensin-converting enzyme (ACE) inhibitor perindopril and calcium channel blockers nifedipine and verapamil decreases capillary permeability, whereas furosemide, hydrochlorothiazide, ACE inhibitor captopril and calcium channel blocker clentiazem do not modify or increase EB extravasation. Hence, selected antihypertensive agents reduce capillary permeability and could therefore have a supplemental protective vascular effect, in addition to their lowering arterial pressure.