Normally Cycling and Latch Bridges in Venous Smooth Muscle

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Abstract
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 (1₀) the maximum isometric tension (P₀) developed was 87.9 mN/mm2. Resting tension at lo was only 6% P₀, which indicates a very compliant muscle. Active tension was observed at muscle lengths greater than 0.3 lo. The maximum shortening capacity was 0.65 lo. 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 (V₀) an index of the crossbridge cycling rate) was 0.26 lo/s which was attained early in the contraction (5 s after the stimulation). A 11.9% decline in V₀ was observed at the plateau of the contraction which occurred at 15 s. The stress (a)-strain (e) curve for the series elastic component could be approximated by the equation a = B[exp(Ae) – 1], where A = 76.51 (1/1₀), B = 2.89 mN/mm2 at 5 s and A = 61.49 (1/lo), 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.

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