Donnan potentials from the A- and I-bands of glycerinated and chemically skinned muscles, relaxed and in rigor.

Using a combination of microelectrode measurements and high-power microscopy we have demonstrated that different Donnan potentials can be recorded from the A- and I-bands of glycerinated and chemically skinned muscles in rigor, so that the A-band fixed charge concentration exceeds the I-band fixed charge concentration in the rigor condition. In relaxation the two potentials, and therefore the two charge concentrations, are equal in the two bands. X-ray data are presented for relaxed and rigor rat semitendinosus muscle, chemically skinned, and actin and myosin filament charges are calculated under a variety of conditions. Our conclusions are that (a) the fixed (protein) charge is different in the A- and I-bands of striated muscle in the rigor state; (b) the fixed charges are equal in the A- and I-bands of relaxed muscle; (c) the largest charge change between relaxation and rigor is on the thick filament. This occurs whether or not the myosin heads are cross-linked to the thin filaments. (d) Possibly an event on the myosin molecule, the binding of ATP (or certain other ligands) causes a disseminated change that modifies the ion-binding capacity of the myosin rods, or part of them.