[Mechanical determinants of isotonic relaxation of the isolated diaphragmatic muscle]

The aim of this study was to define the mechanical determinants of isotonic relaxation of isolated diaphragm muscle in the rat over a load range (n = 30). We tested several hypotheses to determine the effect of i) load conditions (preload, post-load), ii) sudden changes in load during contraction, iii) length of the muscle at peak shortening, iv) maximal amplitude of shortening (delta L) and v) stimulation conditions on peak rate of isotonic reelongation (+dL/dmax). At tetanus at 30 Hz, +dL/dmax was linearly correlated to delta L peak shortening and total load. Variations in preload, peak shortening or postload did not modify the +dL/dmax vs delta L relationship but such variations did affected the relationships +dL/dmax vs total load or +dL/dmax vs peak shortening. For a given value of L, +dL/dmax was weaker for twitch than for tetanus. In conclusion, four findings show that over a wide lad range the maximal amplitude of shortening the main mechanical determinant of the rate of isotonic reelongation of the isolated diaphragm muscle, independently of the length of the muscle at peak shortening, the initial length of the muscle and independently of the load during reelongation.