Radial mass transfer of cross-bridges in a tetanized ferret heart muscle.

An X-ray diffraction experiment on ferret heart muscle was made to examine the relationship between tension and mass transfer from the thick to the thin filament associated with the interaction of cross-bridges with actin. A ferret papillary muscle was electrically stimulated for 8 s in the presence of 5 microM ryanodine to give a tetanus. At different extracellular Ca2+ concentrations (2-20 mM), a linear relationship was found between the tension and the mass transfer. It was concluded that a change in tension caused by altering the extracellular Ca2+ concentration is due to a change in the number of myosin heads bound to actin. This is in contrast to the results obtained on skinned preparations, which showed a markedly nonlinear relationship between the number of heads in the vicinity of the thin filaments and force. It was found that the tension per myosin head is similar in twitch and tetanus of cardiac muscle. Also, a similar fraction of myosin heads is recruited in tetanus of cardiac and skeletal muscles.