Relaxation Is Impaired in the Diaphragm Muscle of the Cardiomyopathic Syrian Hamster

Relaxation was examined in diaphragm muscle strips of cardiomyopathic Syrian hamsters and control hamsters. Isotonic lengthening velocity and isometric tension decay were analyzed over the load continuum in response to twitch. For each load level (P), we measured the maximum extent of shortening (deltaL), the peak lengthening velocity (VL), and the peak rate of tension decline (-dP/dtmax). The kinetics of sarcomere length (SL) were simultaneously measured by laser diffraction. In an attempt to account for the influence of shortening and/or load on relaxation, we calculated the slopes of the VL - deltaL and -dP/dtmax - P relationships. In both groups, there was a direct relationship between (1) VL and deltaL and (2) -dP/dtmax and P. In myopathic hamsters, we observed a decrease in the slope of the VL - deltaL relationship (p < 0.05), a decrease in VL at any common value of both muscle and sarcomere extent of shortening, and an increase in the duration of overall lengthening. Isometric tension decay was significantly prolonged in myopathic muscle strips, while the -dP/dtmax - P relationship was not significantly different than in controls. At low-to-medium loads, SLs at the onset of tension decline were greater in myopathy. These data indicate that relaxation of the diaphragm was both slowed and prolonged in myopathic Syrian hamsters.