Interfilament spacing, Ca2+ sensitivity, and Ca2+ binding in skinned bovine cardiac muscle

Abstract

The length-dependence of myofilament Ca²⁺ sensitivity in cardiac muscle appears to be a function of length-dependent variation in the lateral separation of actin and myosin filaments. The goal of this study was to determine how force, Ca²⁺ sensitivity, and Ca²⁺ binding to troponin C are correlated in skinned bovine ventricular muscle bundles set at sarcomere length 1.9 μm and subjected to varying degrees of osmotic compression with Dextran T-500. With 5, 10, and 15% Dextran T-500 the muscle diameter was reduced by 13, 21, and 25%, respectively. Addition of 5% Dextran T-500 caused increases in developed force, Ca²⁺ sensitivity, and in the affinity of Ca²⁺ for the regulatory binding site on troponin C. All of these parameters were reversed back toward control levels with 10% Dextran T-500. With 15% Dextran T-500 all parameters were decreased to below control levels. These data indicate that (1) there is an optimal filament separation at which both Ca²⁺ sensitivity and Ca²⁺ binding are maximized, and (2) Ca²⁺ –troponin C affinity is linked to changes in Ca²⁺ sensitivity rather than to changes in interfilament spacing.