Abstract
Glycerinated rabbit psoas fibres have been modified with paramagnetic probes (IASL and MSL) which react selectively with the reactive sulphydryl on the myosin head. The extent of SH-1 modification was monitored by extracting myosin and measuring its ATPase activity in the presence of EDTA and of Ca2+. The isometric tension, stiffness, maximum velocity of contraction (slack test), and the force-velocity relation was measured as a function of the degree of SH-1 modification. Reaction of up to 50% of SH-1, i.e. 50% reduction in the K+-EDTA ATPase activity of extracted myosin, produced little change (in vitro where SH-1 modification alters several rates in the interaction of myosin with ATP and decreases the actin-activated ATPase activity of myosin subfragments.