Relaxation in atrial and ventricular myocardium: activation decay and different load sensitivity.

Isolated atrial and ventricular preparations from rat heart have been compared. In atrial specimens relaxation is faster than in papillary muscles both in isometric and isotonic conditions. In papillary muscles the tension decay occurs earlier in isotonic than isometric contractions and a stretch applied at or after the peak of isometric twitches promotes a faster relaxation: this load dependence of relaxation is less pronounced in atrial specimens. The decay of activation, evaluated from the decline of the muscle shortening ability, is faster in atrium than in ventricle. These findings suggest that the sensitivity of relaxation to the loading conditions might be determined by both the activation decay rate and the cross bridge kinetics.