Mechanical behavior of vascular smooth muscle in cylindrical segments of arteries in vitro

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Abstract
Vascular smooth muscle mechanics have been studied in vitro in cylindrical segments of dog carotid artery, human internal mammary artery, and human saphenous vein. Such cylindrical preparations maintain normal vessel geometry and also permit correlation of mechanical phenomena with transmural pressure. These studies show that the vascular muscle in cylindrical arteries develops a maximum active stress of 1.1 X 10(5) N/m2 for the whole wall, or 2.2-3.7 X 10(5) N/m2 for the volume of the wall occupied by vascular muscle. These values are similar to those reported for strip studies of vascular muscle and various preparations of skeletal muscle, but are two to five times that reported for cardiac papillary muscle preparations. In cylindrical preparations of arteries, maximum isometric active stress occurs at 150 mm Hg, whereas that in veins occurs at less than 15 mm Hg. Quick release experiments of cylindrical segments of vessels avoid the compliance of inactive tissue trapped beneath ligatures in strip studies. Quick release experiments in cylindrical segments of dog carotid artery reveal that at maximum isometric stress, the series elastic component (SEC) is extended 8-11%. Experiments employing temperature variations and degradative enzymes show that the SEC is located largely in elastin, with a lesser portion located in the contractile apparatus. At short- and long-muscle lengths, the active muscle develops decreased active stress and that developed at long lengths persists at all muscle lengths, even after shortening. This has been termed "attenuation" and appears to contribute to the static length-stress and pressure-diameter hysteresis exhibited by vessels. Excitation of vascular muscle in vessel segments held at constant pressure discloses that isobaric contraction decreases artery diameter a maximum of approximately 25%. This occurs at a dimension corresponding to approximately 100 mm Hg in the relaxed vessel. Isometrically and isobarically contracted vessels tend to fall along the same pressure-diameter coordinates, indicating equivalence of both modes of contraction. Distention of contracted vessels indicates that active vascular muscle markedly resists distention up to 150-250 mm Hg; at higher pressures the contracted vessel exhibits decreased stiffness as the contracted muscle yields. The vascular muscle, therefore, has a biphasic effect on circumferential elastic modulus relative to that of the relaxed vessel.(ABSTRACT TRUNCATED AT 400 WORDS)