The length-tension characteristics of small coronary arteries vary with transmural origin

There are transmural differences in the structure of arteries across the left ventricular wall. For example, for arteries of the same size, wall thickness is greater in arteries of the epicardium than those from the endocardium. This observation suggests that there could be differences in their passive and active length-tension relationships, as different amounts of connective tissue or smooth muscle would be expected to alter these characteristics. We tested this hypothesis by studying similarly sized porcine coronary arteries from opposite transmural locations. Endocardial arteries had a diameter of 389 ± 33 µm (n = 8), while epicardial arteries measured 388 ± 50 µm (n = 6). A wire myograph was used to study the mechanical properties of these arteries under isometric conditions in KrebsHenseleit buffer at 37 oC. Arteries were cut into rings with an axial length of 2 mm. Rings were repetitively stimulated to contract at increasing lengths with the addition of high extracellular K + (80 mM). Coronary arteries developed active tension to a plateau level over approximately 3-5 min and K + -induced contractions readily washed out. Arteries from the epicardium were stiffer, as the passive-length tension curve of these vessels was elevated over arteries from the endocardium. Passive tensions at optimal length were 3.2 ± 0.4 vs. 5.6 ± 1.5 mN/mm (p < 0.05). The active tension developed in response to K + depolarization was greater in epicardial arteries. Active tensions at optimal length were 3.4 ± 1.1 vs. 2.4 ± 0.3 mN/mm (p < 0.05). Our results represent the first comparison of transmural differences in coronary arteries under isometric tension. Our findings support the hypothesis that differences exist in the passive and active length-tension relationships of epicardial and endocardial arteries that correlate with wall thickness.