The passive mechanical properties of blood vessel mainly stem from the interaction of collagen and elastin fibers, but vessel constriction is attributed to smooth muscle cell (SMC) contraction. Although the passive properties of coronary arteries have been well characterized, the active biaxial stress-strain relationship is not known. Here, we carry out biaxial (inflation and axial extension) mechanical tests in right coronary arteries that provide the active coronary stress-strain relationship in circumferential and axial directions. Based on the measurements, a biaxial active strain energy function is proposed to quantify the constitutive stress-strain relationship in the physiological range of loading. The strain energy is expressed as a Gauss error function in the physiological pressure range. In K ϩ -induced vasoconstriction, the mean Ϯ SE values of outer diameters at transmural pressure of 80 mmHg were 3.41 Ϯ 0.17 and 3.28 Ϯ 0.24 mm at axial stretch ratios of 1.3 and 1.5, respectively, which were significantly smaller than those in Ca 2ϩ -free-induced vasodilated state (i.e., 4.01 Ϯ 0.16 and 3.75 Ϯ 0.20 mm, respectively). The mean Ϯ SE values of the inner and outer diameters in no-load state and the opening angles in zero-stress state were 1.69 Ϯ 0.04 mm and 2.25 Ϯ 0.08 mm and 126 Ϯ 22°, respectively. The active stresses have a maximal value at the passive pressure of 80 -100 mmHg and at the active pressure of 140 -160 mmHg. Moreover, a mechanical analysis shows a significant reduction of mean stress and strain (averaged through the vessel wall). These findings have important implications for understanding SMC mechanics.contraction; constitutive equation; stress-strain relation; vessel mechanics VASOACTIVITY OF LARGE EPICARDIAL coronary arteries is affected by cardiovascular diseases such as diabetes (14, 15), hypertension (14, 20), atherosclerosis (11), vasospasm (10, 32), and aneurysm (26), which are major risk factors for angina pectoris or myocardial infarction in patients. The constitutive passive and active stress-strain relationships can characterize the vasoactivity and are fundamental for understanding the mechanical behaviors of vascular smooth muscle cell (SMC) in health and disease (5). The strain energy function has been widely used to characterize the passive mechanical properties of blood vessels (5,8). For large epicardial coronary arteries, extensive mechanical measurements and analysis were carried out in the passive state (18,19,23,30,33,34).The active mechanical properties of coronary arteries are much known. To date, there are only uniaxial active constitutive length-tension relationships in the circumferential direction of coronary arteries (1, 2, 24, 31). Although some multi-axial active models have been proposed, those have been of theoretical forms not rooted in experimental measurements (25, 36). Clearly, there is a need for multi-axial active mechanical measurements and experimentally determined multi-dimensional active strain energy functions for coronary arteries.The objective ...

GS. Two-layer model of coronary artery vasoactivity. J Appl Physiol 114: 1451-1459, 2013. First published March 7, 2013 doi:10.1152/japplphysiol.01237.2012.-Since vascular tone is regulated by smooth muscle cells in the media layer, a multilayer mechanical model is required for blood vessels. Here, we performed biaxial mechanical tests in the intima-media layer of right coronary artery to determine the passive and active properties in conjunction with the passive properties of adventitia for a full vessel wall model. A two-layer (intima-media and adventitia) model was developed to determine the transmural stress and stretch across the vessel wall. The mean Ϯ SE values of the outer diameters of intima-media layers at transmural pressure of 60 mmHg in active state were 3.17 Ϯ 0.16 and 3.07 Ϯ 0.18 mm at axial stretch ratio of 1.2 and 1.3, respectively, which were significantly smaller than those in passive state (i.e., 3.62 Ϯ 0.19 and 3.49 Ϯ 0.22 mm, respectively, P Ͻ 0.05). The inner and outer diameters in no-load state of intima-media layers were 1.17 Ϯ 0.09 and 2.08 Ϯ 0.09 mm, respectively. The opening angles in zero-stress state had values of 159 Ϯ 21°for intima-media layers and 98 Ϯ 15°for adventitia layers, which suggests a residual strain between the two layers. There were slightly decreased active circumferential stresses (Ͻ10%), but significantly decreased active axial stresses (Ͼ25%) in the intima-media layer compared with those in the intact vessel. This suggests that the adventitia layer affects vascular contraction. The two-layer analysis showed that the intima-media layer bears the majority of circumferential tensions, in contrast to the adventitia layer, while contraction results in decreased stress and stretch in both layers.contraction; constitutive stress-strain relation; vessel mechanics THE EPICARDIAL CORONARY ARTERY wall is composed of intima, media, and adventitia layers, while smooth muscle cells (SMC) mainly reside in the media layer. Since dysfunction in vasoactivity is present in coronary artery disease (15, 30), the assessment of active and passive properties of coronary arteries provides essential diagnostics in subjects at risk (1,4,9,22,24). In passive state, our laboratory has previously treated the vessel wall as a two-layered structure, consisting of intimamedia layer (endothelial cells and SMC, as well as elastin and collagen) and adventitia layer (collagen, fibroblasts, and elastin) and carried out extensive experiments for determination of passive mechanical properties of the two layers (17,20,27). More recently, our laboratory determined the biaxial (circumferential and axial) active properties of intact right coronary arteries (RCA) under K ϩ -induced SMC contraction (10). The mechanical properties of the isolated intima-media layer are not available to determine the active and passive contributions of the various layers of the vessel wall.The objective of the study is to determine the biaxial (circumferential and axial) active properties in the intimamedia layer and to comput...