Effect of Ionic Strength on Length-dependent Ca2+Activation in Skinned Cardiac Muscle

S. H. Smith and F. Fuchs. Effect of Ionic Strength on Length-dependent Ca2+Activation in Skinned Cardiac Muscle. Journal of Molecular and Cellular Cardiology (1999) 31, 2115–2125. The length-dependence of myofilament Ca2+sensitivity is considered to be an important component of the steep force–length relationship in cardiac muscle (Frank–Starling relation). Recent studies suggest that Ca2+sensitivity is a function of the number of strong-binding cross-bridge interactions formed at a given sarcomere length. However, the length-dependent step in the thin filament activation process is still unknown. This study was designed to test the hypothesis that sarcomere length influences the transition of the thin filament from the unattached (blocked) state to the weakly bound (closed) state. This hypothesis was tested by determining the length-dependence of Ca2+sensitivity as a function of ionic strength in skinned bovine ventricular muscle. Previous studies have shown that reduction in ionic strength below a critical level, in the absence of Ca2+, shifts the thin filament to the closed state. In this study normal Ca2+regulation was maintained at low ionic strength but the length-dependence of Ca2+sensitivity and the length-dependence of Ca2+binding were eliminated. These results are consistent with the hypothesis that the transition from the blocked to the closed state is a function of filament geometry as well as Ca2+and ionic strength.