Decreased endothelium-dependent vascular relaxation following subtotal coronary artery occlusion in dogs

Inhibition of arterial thrombus formation by ApoA1 Milano

The mutant form of human apoA1, known as apoA1 Milano, is formed as a result of arginine 173 to cysteine substitution and inhibits experimental atherosclerosis in cholesterol-fed animals. This study was designed to determine if apoA1 Milano would modify arterial thrombogenesis. Sprague Dawley rats were intravenously administered the carrier alone (n=8) or apoA1 Milano (20 mg. kg-1. d-1 for 4 to 10 days, n=17). The abdominal cavity was opened, and the abdominal aorta was isolated. Whatman paper impregnated with 35% FeCl3 was wrapped around the surface of the aorta, and aortic flow was recorded continuously. In carrier-treated rats, an occlusive platelet-fibrin-rich thrombus was formed in 21.2+/-4.1 (mean+/-SD) minutes. Treatment of rats with apoA1 Milano markedly delayed time to thrombus formation (38.8+/-11.9 versus 21.2+/-4.1 minutes, P<0. 01), inhibited platelet aggregation (25+/-7% versus 50+/-11%, P<0. 01), and reduced weight of the thrombus (18.5+/-1.8 versus 23.7+/-2. 3 mg/cm, P<0.01). Total cholesterol and HDL levels remained similar in both groups of rats, but plasma apoA1 Milano levels were elevated in apoA1 Milano-treated rats. In in vitro studies, incubation of platelets with apoA1 Milano reduced ADP-induced platelet aggregation by about 50%, but apoA1 Milano had no direct effect on vasoreactivity. This study provides further evidence for critical role of platelets in thrombosis. Use of apoA1 Milano offers a novel approach to inhibit arterial thrombosis.

Increased angiotensin II type 1 receptor expression in hypercholesterolemic atherosclerosis in rabbits

Angiotensin II (Ang II) promotes vascular smooth muscle growth and may be involved in the initiation and progression of atherosclerosis. To examine whether Ang II receptor expression in vascular tissues is altered in atherosclerosis, male New Zealand White rabbits were fed a high-cholesterol diet (1% cholesterol + 4% coconut oil mixed with regular chow; hypercholesterolemic group, n=12) or regular chow (control group, n=8) for 10 weeks. At the end of this period, the serum cholesterol level in the rabbits fed the high-cholesterol diet was higher than that in the control group (3616 +/- 144 versus 30 +/- 1 mg/dL, P<0.001). There was no atherosclerosis in the aortas of the control group, whereas 51 +/- 6% of the aorta was covered with atherosclerosis in the hypercholesterolemic group. Total Ang II receptor expression in the atherosclerotic aortic tissues was increased 5-fold in the hypercholesterolemic rabbits (292 +/- 28 versus 51 +/- 32 fmol/mg tissue, mean +/- SE, P<0.001), and the increased Ang II receptor expression was entirely due to enhanced Ang II type 1 (AT1) receptor expression (289 +/- 38 versus 38 +/- 18 fmol/mg, P<0.001), as Ang II type 2 receptor expression was unaltered (7 +/- 5 versus 3 +/- 2 fmol/mg, P=NS). AT1 receptors were localized primarily in the media and to some extent in the intima of the atherosclerotic aorta, as determined by immunohistochemistry with specific monoclonal and polyclonal AT1 receptor antibodies. Increased synthesis of AT1 receptor mRNA in atherosclerotic tissues was confirmed by reverse transcription-polymerase chain reaction. To evaluate the functional significance of increased AT1 receptor expression, the constrictor response of aortic rings to Ang II was examined and found to be markedly enhanced in atherosclerotic aortic rings (P<0.01 versus control aortic rings). The endothelium-dependent relaxation of aortic rings from hypercholesterolemic rabbits was markedly attenuated (P<0.001). This study shows that hypercholesterolemia in rabbits results in atherosclerosis, loss of endothelium-dependent relaxation, and increased Ang II receptor (entirely AT1 receptor) expression in aortic tissues, which may result in altered vasoreactivity.

Increased superoxide anion generation and altered vasoreactivity in rabbits on low-potassium diet

Potassium reduces blood pressure in populations at high risk of developing hypertension, which suggests that potassium depletion may increase vascular resistance. This study was designed to examine the effect of potassium depletion on the L-arginine-nitric oxide pathway in arterial tissues. New Zealand White rabbits were fed either a control diet, containing a normal amount of potassium, or a low-potassium diet for 1-3 wk. As expected, the low-potassium diet resulted in reduced serum and urinary potassium levels. Carotid arteries were excised, and their contractile and relaxant responses were determined in vitro. Carotid arterial ring contractile response to norepinephrine was enhanced, and relaxation in response to the endothelium-dependent vasodilators acetylcholine and calcium ionophore A-23187 was attenuated, in rabbits fed low-potassium diet (all P < 0.01 compared with responses in rabbits fed control diet). The vasomotor responses were similarly altered in rabbits fed low-potassium diet for 1 or 3 wk. Both the enhanced contraction and attenuated relaxation were abolished by treatment of arterial rings with superoxide dismutase but not by treatment with L-arginine or indomethacin. Carotid artery rings from rabbits fed the low-potassium diet showed approximately 100% greater superoxide anion formation than those from rabbits fed control diet (P < 0.01), whereas plasma and urinary nitrite levels were similar in both groups of rabbits. These observations indicate that low-potassium diet enhances the sensitivity of the carotid artery to vasoconstrictor stimuli and reduces the sensitivity to endothelium-dependent stimuli. Attenuation of endothelium-dependent relaxation appears to be secondary to increased free radical generation, which may degrade nitric oxide. Altered vasoreactivity may underlie the genesis of hypertension in populations consuming diets low in potassium.

Endothelium, coronary vasodilation, and organic nitrates

Recent investigations have suggested that the vascular endothelium is an active participant in the regulation of arterial tone and blood flow. In a state of health, the endothelium contributes to hemodynamic equilibrium; however, it rapidly becomes dysfunctional in hypercholesterolemia and diabetes mellitus or with exposure to the stress of hypertension or long-term smoking. Among the deficits observed during endothelial dysfunction is a reduction in the synthesis and release or an excessive degradation of EDRF. This potent vasorelaxant is derived from the amino acid L-arginine and has been characterized as NO or a closely related substance. EDRF relaxes vascular smooth muscle by activating guanylate cyclase. A deficiency in the activity of EDRF may be the mechanism of diminished coronary vasodilation in patients with ischemic heart disease. Organic nitrates, which are metabolized to NO or S-nitrosothiol at the cellular level, are often used in the management of myocardial ischemia; they also induce vasodilation by activating guanylate cyclase. The similarities between organic nitrates and endogenous EDRF and their interactions are discussed in this review.