ISOMETRIC RELAXATION OF ISOLATED DIAPHRAGM MUSCLE - INFLUENCE OF LOAD, LENGTH, TIME, AND STIMULATION

Determinants of the isometric relaxation rate were investigated in iso lated rat diaphragm (n = 30). We tested the hypothesis that these dete rminants could include loading conditions, namely preload and afterloa d; abrupt changes in load during the contraction phase; stimulation co nditions; and time. Two relaxation sequences were studied. When isomet ric relaxation occurred at initial muscle length (isotonic-isometric s equence), an increase in total load (P) accelerated the negative peak rate of tension decline (-dP/dt(max)). Variations in initial length, s timulation, and onset of relaxation did not modify the -dP/dt(max) vs. afterload relationship. When isometric relaxation was analyzed after -dP/dt(max) for a given afterload level the instantaneous rate of tens ion decline (-dP/dt) was a unique function of instantaneous tension, r egardless of previous loading conditions, stimulation mode, or time. W hen the isometric relaxation occurred at end-shortening muscle length (isometric-isotonic sequence), the -dP/dt(max) vs. P relationship was flat. The rate of tension decay, as attested by either -dP/dt(max) or instantaneous -dP/dt vs. instantaneous tension phase plane, differed m arkedly depending on stimulation conditions. Thus the regulation of is ometric relaxation rate differed according to the relaxation sequence. In muscle isometrically relaxing at initial muscle length, peak isome tric relaxation rate was mainly determined by afterload. Conversely, i n muscle isometrically relaxing at end-shortening length, isometric re laxation rate was highly dependent on the level of activation and was independent of preload and afterload.