The effect of shortening on crossbridge dynamics in cardiac muscle

Abstract

Landesberg and Sideman have proposed a model for cross-bridge cycling where the rate constant for crossbridge weakening, G, is a function of the sarcomere shortening velocity such that G = Go + G1 • V , where Go is the rate constant in the isometric state and G\ is the mechanical feedback. The effect of my osin heavy chain isoform on G\ is unknown. Methods: Rats were made to express either pure a.-MHC, by treatment with thyroid hormone (T3), or pure j3-MHC, by treatment with PTU. Force-velocity curves were obtained from cardiac trabeculae (Ca2+ = 1.5 mM, 25°C). To determine G\, ~FN the instantaneous force difference between a shortening and isometric contraction was measured.

Results: The sarcomere shortening velocity ofT3 rats were twice that of PTU treated rats at all levels of relative load (VMAX= 15.99 Ilm/S - T3, 8.95 Ilm!s - PTU). There was no difference in the curvature of the force-velocity relationship between the two groups. G\ as measured from slow (:S 4llm/s) releases was 4.7611lm (STD DEV = 1.43) for T3rats and 4.88 Illm (STD DEV = 1.29) for PTU rats.