Interaction of Myosin with Thin Filaments during Contraction and Relaxation: Effect of Ionic Strength

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Abstract
The influence of ionic strength on the isometric tension, stiffness, shortening velocity and ATPase activity of glycerol-treated rabbit psoas muscle fiber in the presence and the absence of Ca 2+ has been studied. When the ionic strength of an activating solution (containing Mg 2+ -ATP and Ca 2+ ) was decreased by varying the KCl concentration from 120 to 5 mM at 20°C, the isometric tension and stiffness increased by 30% and 50%, respectively. The ATPase activity increased 3-fold, while the shortening velocity decreased to one-fourth. At 6°C, similar results were obtained. These results suggest that at low ionic strengths ATP is hydrolyzed predominantly without dissociation of myosin cross-bridges from F-actin. In the absence of Ca 2+ , with decreasing KCl concentration the isometric tension and stiffness developed remarkably at 20°C. However, the ATPase activity and shortening velocity were very low. At low ionic strength, even in the absence of Ca 2+ myosin heads are bound to thin filaments. The development of the tension and stiffness were greatly reduced at 6°C or at physiological ionic strength.

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