Force-velocity characteristics and active tension in relation to content and orientation of smooth muscle cells in aortas from normotensive and spontaneous hypertensive rats.

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Abstract
Segments of abdominal aorta from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats (20-25 weeks) were compared with respect to force production and dynamic mechanical properties. The preparations were mounted in vitro for determination of optimal length (lo) for active force, and then maximally stimulated (high-K+ solution, 10 mm Ca2+, 10(-5) m noradrenaline), and fixed for electron microscopy. Muscle cellular volume per mm vessel wall was significantly (p less than 0.01) higher in SHR (0.14 +/- 0.009 mm3, n = 7) compared to WKY (0.11 +/- 0.004 mm3, n = 7). Unchanged cell length and unaltered cross-sectional area (17 micrometers2) of nucleus containing cell profiles in SHR suggest an increased number of cells in the media. No difference was found in maximum force per unit cell area between SHR (271 +/- 31, n = 7) and WKY (305 +/- 49 mN/mm2, n = 7). Cell orientation was almost circular in both groups, showing that force was measured in parallel to the cell long axis. Aortic segments were mounted in an apparatus for quick-release experiments. They were maximally stimulated and force steps were imposed at peak of contractions. The series elastic component, characterized by the initial elastic recoils at 0.75 lo, has similar stiffness values in SHR and WKY. Velocities were measured 100 msec after release. The results were fitted to Hill's equation and maximum shortening velocity (Vmax) computed. No difference in Vmax was found at 0.75 lo (WKY: 0.048 +/- 0.005; SHR: 0.042 +/- 0.006 lo/s, n = 6 for both). At 0.85 lo, the data were corrected for passive tension (40% to total). Vmax at 0.85 lo was 0.071 +/- 0.009 lo/s (n = 5) for WKY, and 0.069 +/- 0.007 lo/s (n = 5) for SHR. Similar Vmax and force per cell cross-sectional area suggest similar characteristics of actomyosin interaction in SHR and WKY aorta.