Influence of loading patterns on peak length-tension relation and on relaxation in cardiac muscle

To analyze the influence of loading patterns on cardiac pump performance and cardiac relaxation [corrected], the effects of preload on peak length-tension relation [corrected] and of systolic load clamps on peak length-tension relation and on relaxation were analyzed in isolated cat papillary muscles. Preload reduction and early loading clamps induced a shift to the left of the peak length-tension relation, that is, a smaller muscle length for the same tension at peak shortening. Unloading clamps induced a shift to the right of the peak length-tension relation, that is, a larger muscle length for the same tension at peak shortening. The effects of load clamps on relaxation depended on when they were applied during isotonic shortening. Changes induced by load clamps could not be summarized in terms of enhanced or delayed relaxation, illustrating that shortening duration, isometric tension decline and isotonic lengthening have different determinants. In conclusion, not only peak or mean systolic pressure but also the entire loading pattern has to be taken into account whenever pressure-volume data or relaxation variables are interpreted.