R. C. Webb

Potassium-induced relaxation as an indicator of Na+-K+ ATPase activity in vascular smooth muscle.

Helical strips of rat tail artery were observed to relax in response to potassium after contraction induced by 10(-7) g/ml norepinephrine in potassium-free solution. After several minutes of relaxation, the strips showed an abrupt redevelopment of tension. The amplitude of the potassium-induced relaxation was employed as an index of the activity of the electrogenic sodium-potassium pump and hence of the Na+-K+ ATPase. This assumption seemed justified because the observed amplitude of potassium-induced relaxation paralleled known effects of the following variables on Na+-K+ ATPase: (1) intracellular sodium concentration; (2) ouabain administration; (3) magnesium; (4) temperature, and (5) potassium concentration. The relaxation that occurred in response to potassium is suggested to be due to an enhanced Na+-K+ ATPase resulting in increased electrogenic transport of sodium and potassium and, consequently, hyperpolarization. We propose that potassium-induced relaxation of rat tail artery may be used as a functional indicator of Na+-K+ ATPase activity in vascular smooth muscle.

Does nitric oxide regulate smooth muscle cell proliferation? A critical appraisal

Smooth muscle proliferation is involved in the pathogenesis of atherosclerosis, restenosis after angioplasty and vein graft failure due to neointimal hyperplasia. Nitric oxide (NO) inhibits smooth muscle cell growth in vitro and experimental neointimal hyperplasia in vivo, suggesting a role for NO as a regulator of smooth muscle cell proliferation. NO is also involved in the control of numerous other vascular functions including platelet and inflammatory cell adhesion, vascular reactivity and endothelial permeability. This review critically examines the experimental and clinical evidence that supports a role for NO as a modulator of smooth muscle cell proliferation, with an emphasis on the multiple mechanisms by which NO acts on vascular lesions.

Gap junctional communication and vascular smooth muscle reactivity: Use of tetraethylammonium chloride

Oscillatory contractions in uterine smooth muscle are mechanistically related to gap junction complex formation. We have tested the hypothesis that agonist-induced oscillations in vascular smooth muscle are also mediated by gap junctions and that gap junctions are important for vascular smooth muscle cell communication. Total RNA from cultured Wistar-Kyoto rat (WKY) mesenteric arterial cells hybridized strongly with a cDNA probe for the message for connexin43, a monomer of the gap junction. In these same cells, the quaternary ion tetraethylammonium (TEA) (10 mM) increased Lucifer yellow dye transfer between contiguous cells, a measure of cell-to-cell communication via gap junctions, approximately 35% above basal levels. Heptanol, an established inhibitor of gap junction communication, completely blocked both basal- and TEA-stimulated dye transfer between neighboring cells. In other experiments, helical strips of superior mesenteric and tail arteries from WKY rats were mounted in tissue baths for measurement of isometric contractile force. TEA (10(-3)-10(-1) M) induced oscillatory contractions (1-5 cycle/min) in both mesenteric and tail arteries. Removal of endothelium did not affect the pattern of TEA-stimulated oscillations. Oscillations to TEA were blocked in a concentration-dependent manner in both arteries by heptanol (10(-7)-10(-3) M). Heptanol (10(-3) M) also significantly reduced (40%) acetylcholine-induced relaxation in the mesenteric artery (contracted with phenylephrine).(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II-induced relaxation of vascular smooth muscle

The effects of angiotensin II (AII) on contractile tension were studied in vascular smooth muscle from dogs, pigs and rabbits. Helically cut strips of renal veins were mounted in organ chambers and isometric contractions were recorded. Contraction of the venous strips was induced by application of 10(-8) g/ml norepinephrine (NE). Subsequent addition of 5 X 10(-8) g/ml AII caused a triphasic response: (1) there was an initial contraction (subsequent contractions were tachyphylactic in all species); (2) the contraction was followed by a relaxation below the contraction induced by NE (subsequent relaxation responses were tachyphylactic in dog and pig veins), and (3) there was a return from the relaxation to the level of the NE-induced contraction. The duration of the entire response was approximately 5 min. The magnitude of the relaxation varied inversely with the level of the NE contraction when the contractile state was altered by changing the NE concentration. Conditions which inhibit the sodium pump (potassium-free solution and ouabain) and beta-adrenergic blockade with propranolol had no effect on the AII-induced relaxation. The relaxation was temperature sensitive. Inhibitors of prostaglandin synthesis (indomethacin and aspirin) and saralasin attenuated the relaxation in response to AII. Prostaglandins E1 and E2 and arachidonic acid caused relaxation of renal vein strips contracted with NE; the relaxant effect of arachidonic acid was blocked by indomethacin. These results suggest that: (1) All stimulates the synthesis of prostaglandins in isolated venous smooth muscle, and (2) endogenous prostaglandins modulate the response of venous smooth muscle to AII.

Prostanoids contribute to endothelium-dependent coronary vasodilation in guinea pigs.

This study characterizes the contribution of prostanoids to endothelium-dependent responses in two vascular regions of the guinea pig. We compared the mechanisms of relaxation responses to acetylcholine and adenosine triphosphate (ATP) in the coronary vasculature and in the abdominal aorta of the guinea pig. Endothelium-dependent responses were examined in an isolated, potassium-arrested guinea pig heart utilizing a modified Langendorff preparation. Coronary vessels were constricted with prostaglandin F2 alpha and dilated with acetylcholine (10(-9)-10(-6) mol) or ATP (10(-10)-10(-7) mol) before and after exposure to indomethacin (14 microM, n = 6) or ibuprofen (150 microM, n = 5). Helically cut strips of abdominal aorta (n = 6) were suspended in isolated tissue baths for measurement of isometric force. Relaxation to acetylcholine (5.5 x 10(-7) M) and ATP (10(-5) M) was quantified in strips contracted with norepinephrine before and after exposure to indomethacin (14 microM). In addition, the endothelium was damaged by exposing vessels to free radicals generated by electrolysis of the buffer (4 Hz, 9 V, 1 ms, 5 min). Following electrolysis of the buffer, relaxation responses to acetylcholine and ATP were significantly attenuated in both preparations. In the perfused heart, endothelium-dependent dilatation to acetylcholine, but not ATP were significantly inhibited in the presence of indomethacin or ibuprofen. In contrast, acetylcholine- and ATP-induced relaxation responses in the aorta were not altered by indomethacin. We conclude that prostaglandins contribute to acetylcholine-induced dilatation in the coronary bed but not in the abdominal aorta of the guinea pig. Furthermore, in the coronary bed, different endothelial factors mediate relaxation to acetylcholine and ATP.

Neuraminidase and contractile responses to norepinephrine in rat tail artery

Sialic acids are negatively charged groups in the carbohydrate side chains of glycolipids and glycoproteins which line the external membrane surface. The goal of this study was to characterize the effect of neuraminidase, which selectively cleaves sialic acids, on contractile activity in vascular smooth muscle. Helically cut strips of rat tail artery were mounted in an organ chamber and isometric contractions were recorded. Following treatment with neuraminidase (0.2 U/ml, 1 h), contractile responses to norepinephrine were significantly greater than control responses. Phasic contractions to norepinephrine in calcium-free medium were not altered by neuraminidase, whereas following calcium depletion with EGTA, contractile responses to added calcium were greater in enzyme-treated strips than in control when activated with norepinephrine. The augmentation of norepinephrine-induced contractions following neuraminidase treatment was reversed by incubation of the arterial strips with N-acetylneuraminic acid (10(-4) M). Neuraminidase had no effect on contractile responses to potassium chloride, angiotensin II, and caffeine. Biochemical assay indicated that approximately 63% of the total sialic acid residues were removed from the arterial strips during incubation with the enzyme. It is concluded that a component for the control of the transmembrane calcium movement in response to norepinephrine is dependent on the presence of sialic acid residues.