Hagen Po

Local effects of nitric oxide supplementation and suppression in the development of intimal hyperplasia in experimental vein grafts

OBJECTIVES The universal response of vein grafts after insertion into the arterial circulation is the development of intimal hyperplasia; smooth muscle cell proliferation and connective tissue deposition, which may be modulated in part by dysfunctional endothelial nitric oxide (NO) metabolism. This study examines the effects of single dose, local application by pluronic gel of a NO donor, S-nitroso-N-acetylpenicillamine (SNAP) and an NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME) on the formation of intimal hyperplasia. MATERIALS Forty New Zealand white rabbits underwent jugular vein interposition grafting of the common carotid artery. DESIGN Ten animals were controls, 10 animals had the outer surface of the vein graft coated with 30% pluronic gel (2.5 ml), and 10 each were immersed for 15 min prior to insertion in Ringer lactate containing 10(-3) M of SNAP or L-NAME and then had their vein grafts coated with 2.5 ml of gel containing either SNAP (10(-3) M) or L-NAME (10(-3) M), which allows for sustained delivery for up to 6 h. On the 28th post operative day, the animals were sacrificed and vein grafts were harvested for morphology by electron microscopy (SEM and TEM) and dimensional analysis by videomorphometry. RESULTS All vein grafts developed intimal hyperplasia. On SEM the vein grafts had a confluent layer of endothelial cells with multiple layers of smooth muscle cells representing intimal hyperplasia in TEM. There were no demonstrable morphological differences between the four groups. Local treatment with SNAP produced a significant 36% decrease in mean intimal thickness (72 +/- 4 microns vs. 45 +/- 4 microns; mean +/- S.E.M.; p < 0.01) without a change in medial thickness compared to gel-only treated groups (58 +/- 6 microns vs. 61 +/- 7 microns; p = ns). Inhibition of NO synthase by L-NAME had no effect on the development of intimal hyperplasia (72 +/- 4 microns vs. 79 +/- 10 microns; p = ns); medial thickness was also unchanged. CONCLUSION These data confirm the protective effect of NO in vascular injury and suggest that NO synthase activity is either absent or reduced to such a level that further inhibition in this short time course is not relevant to the pathophysiology of vein graft intimal hyperplasia.

Characterisation of angiotensin II receptor mediated responses and inhibition of intimal hyperplasia in experimental vein grafts by the specific angiotensin II receptor inhibitor, L158,809

OBJECTIVES This study characterises pharmacologically the angiotensin II receptor in experimental vein grafts and examines the effect of the angiotensin II receptor (type 1) antagonist (L158,809) on the formation of vein graft intimal hyperplasia in vivo, as well as the in vitro physiological response to angiotensin II of vein grafts after chronic oral L158,809 treatment. MATERIALS Thirty New Zealand White rabbits had a right carotid interposition bypass graft using the external jugular vein and were killed on the 28th postoperative day. DESIGN To characterise the angiotensin II receptors, concentration response curves to angiotensin II were obtained in vitro in the presence or absence of L158,809. To determine the effect of L158,809 on the development of intimal hyperplasia, 10 animals received chronic oral therapy with L158,809 (10 mg/kg/day; begun 5 days before surgery and continued until harvest) and 10 animals received vehicle only as controls. These grafts were harvested either for histology (n = 6 per group) or for in vitro isometric tension studies to angiotensin II. RESULTS The monophasic contractile response to angiotensin II in the untreated vein grafts could be inhibited in a concentration dependent manner by L158,809 with first order kinetics. Chronic oral treatment with L158,809 produced a 48% decrease in intimal thickness from 82 +/- 1 micron (mean +/- S.E.M.) in the controls to 43 +/- 7 microns in the treated vein grafts (p = 0.002). There was also a significant decrease (45%) in the medial thickness between the control (76 +/- 6 microns) and L158,809 treated (42 +/- 6 microns) vein grafts (p = 0.007). The responses to angiotensin II were abolished in the vein grafts by chronic L158,809 therapy. CONCLUSIONS This study suggests that vein graft angiotensin II responses are mediated through a type 1 receptor and that chronic inhibition with L158,809, significantly reduces intimal hyperplasia and medial hypertrophy in experimental vein grafts and concomitantly abolishes the in vitro responses to angiotensin II. Therefore, angiotensin II acting through AT1 receptors mediates a significant part of the intimal hyperplastic response in vein grafts.

Reprinted Article “Pathophysiology of Vein Graft Failure: A Review”☆

Vein bypass grafting is an integral component of cardiovascular surgical practice for both arterial and venous diseases. However, many of these grafts will eventually fail due to either intrinsic or extrinsic causes. This review examines the current understanding and knowledge of venous histology, vein graft pathology and the associated endothelial and smooth muscle cell physiology and pharmacology. In addition, the status of research on the therapeutic control of vein graft intimal hyperplasia and accelerated atherosclerosis is assessed.

The morphology of venovenous bypass graft endothelium.

Venovenous bypass grafts are commonly used in the repair of vascular trauma to large- and small-caliber veins. This study examines the morphology of the venous wall in an experimental model of the venovenous bypass graft. The morphology of the venous endothelium from unmanipulated jugular veins and from jugular veins implanted as a venovenous bypass graft in the external jugular venous system for 10 min, 6 h, and 1, 3, 5, 7 and 28 days was examined. Veins and venovenous grafts were pressure fixed in situ at 80 mmHg and were examined by scanning and transmission electron microscopy. The endothelial cell lining remained confluent and intact over the 28-day period with evidence of endothelial cell contraction (spindle-shaped cells) for the first 72 h. Pinocytotic activity in endothelial cells and underlying smooth muscle cells was observed throughout the study, strongly indicating physiologically active cells. There was some accumulation of blood cells, predominantly polymorphonuclear leukocytes on the endothelial surface. Polymorphonuclear leukocytes were observed to infiltrate into the subendothelium through endothelial cell junctions within 6 h but by day 3, none was noted in the subendothelial space. There was no major disruption of the graft wall at any time point. By day 28, there was evidence of intimal thickening in the venovenous bypass grafts but no well-demarcated intimal hyperplasia. This study shows that there is no significant endothelial injury in the venovenous bypass grafts and that the endothelial cells remain physiologically active. Short-term failure of venovenous bypass grafts, therefore, appears not be due to significant endothelial cell damage in the graft.