Normally cycling and latch bridges in venous smooth muscle

We have studied the mechanical properties of the crossbridges of canine saphenous venous smooth muscle during activation by electrical stimulation. A length-tension relationship study showed that at optimal length (1o) the maximum isometric tension (Po) developed was 87.9 mN/mm2. Resting tension at 1o was only 6% Po, which indicates a very compliant muscle. Active tension was observed at muscle lengths greater than 0.3 1o. The maximum shortening capacity was 0.65 1o. Force-velocity and series elastic characteristics were determined by the method of quick release to a set of load clamps applied during the course of an isometric contraction. The data were fitted by the hyperbolic Hill equation. Estimated maximum shortening velocity at zero load (Vo) an index of the crossbridge cycling rate) was 0.26 1o/s which was attained early in the contraction (5 s after the stimulation). A 11.9% decline in Vo was observed at the plateau of the contraction which occurred at 15 s. The stress (sigma)-strain (epsilon) curve for the series elastic component could be approximated by the equation sigma = B[exp(A epsilon)-1], where A = 76.51 (1/1o), B = 2.89 mN/mm2 at 5 s and A = 61.49 (1/1o), B = 1.85 mN/mm2 at 15 s. Like other smooth muscles, in general, saphenous vein develops considerable isometric tension and has a high shortening capacity but a low shortening velocity. The reduction in shortening velocity that occurs after 5 s in an isometric contraction is probably due to the development of so-called latch bridges.