Nitric oxide impairs Ca2+ activation and slows cross-bridge cycling kinetics in skeletal muscle

The effects of the nitric oxide (NO) donor spermine NONOate (Sp-NO, 1.0 mM) on cross-bridge recruitment and cross-bridge cycling kinetics were studied in permeabilized rabbit psoas muscle fibers. Fibers were activated at vari ous Ca2+ concentrations (pCa, negative logarithm of Ca2+ concentration), an d the pCa at which force was maximal (pCa 4.0) and similar to 50% of maxima l (pCa(50) 5.6) were determined. Fiber stiffness was determined using 1-kHz sinusoidal length perturbations, and the fraction of cross bridges in the force-generating state was estimated by the ratio of stiffness during maxim al (pCa 4.0) and submaximal (pCa 5.6) Ca2+ activation to stiffness during r igor (at pCa 4.0). Cross-bridge cycling kinetics were evaluated by measurin g the rate constant for force redevelopment after quick release (by 15% of optimal fiber length, L-o) and restretch of the fiber to L-o. Exposing fibe rs to Sp-NO for 10 min reduced force and the fraction of cross bridges in t he force-generating state at maximal and submaximal (pCa(50)) Ca2+ activati on. However, the effects of Sp-NO were more pronounced during submaximal Ca 2+ activation. Sp-NO also reduced the rate constant for force redevelopment but only during submaximal Ca2+ activation. We conclude that Sp-NO reduces Ca2+ sensitivity by decreasing the number of cross bridges in the strongly bound state and also impairs cross-bridge cycling kinetics during submaxim al activation.