Systolic quick releases of nonexcised blood-perfused canine papillary muscle

Systolic stiffness was studied in five coronary perfused twitching papillary muscles of canine right ventricles. The muscles beat at regular sinus rhythms at 37 degrees C. They were shortened quickly at about 10 muscle lengths/s in the middle of isometric contractions. Systolic force decreased exponentially with the shortening in the force-length diagram. Relating Lagrangian stress (sigma) and strain change (delta epsilon) relative to isometric length calculated from the force-length relationship curve during quick release, I found a linear relationship between ln sigma and delta epsilon. This indicates that the systolic stres-strain relationship of the canine papillary muscle can be approximated by an exponential curve, sigma = sigma m exp(k . delta epsilon), where sigma m = isometric stress at the onset of quick release. The mean +/- SE of the stiffness constant k was calculated to be 55 +/- 8 (dimensionless). These k values are greater than those of excised cat papillary muscles at 37 degrees C reported in literature. This difference in k values may be ascribed to the difference in the mechanical property of the uncut pinned end of the present preparation as opposed to the cut clamped end of the conventional preparation, although the canine and feline papillary muscles may simply have the different k values.