Influence of ionic strength on the actomyosin reaction steps in contracting skeletal muscle fibers

Muscle contraction occurs as the result of actin-myosin interaction, which is mediated by the intermolecular forces exerted at the actin-myosin interface. To obtain information about the nature of these intermolecular forces, we tested the sensitivity of various contractile parameters of skinned skeletal muscle fibers to ionic strength (IS) at 3-5 degrees C; IS variation is a useful technique for distinguishing between ionic and nonionic (primarily hydrophobic) types of intermolecular forces. The most striking effect of elevated IS was the strong suppression of isometric tension. However, none of the measured parameters suggested a corresponding decrease in the number of force-generating myosin heads on actin. The rate of actin-myosin association seemed to be only modestly IS-sensitive. The following force-generating isomerization was apparently IS-insensitive. The dissociation of the force-generating actomyosin complex was decelerated by elevated IS, contrary to the expectation from the suppressed isometric tension, These results led us to conclude that an IS-sensitive step, responsible for the large suppression of tension, occurs after force-generating isomerization but before dissociation. The present study suggests that the actomyosin interaction is generally nonionic in nature, but there are at least two ionic processes, one at the beginning and the other close to the end of the actomyosin interaction.