Epinephrine, hypoxia, and the relation of relaxation to peak tension in cardiac muscle

The relationship of the maximal and of the average rate of relaxation to the peak tension developed during isometric contractions of cat papillary muscles at 30° was studied. The frequency force relationship (0.05-0.5/sec) showed that the relationship of the 2 measures of relaxation to the peak tension was linear. This relationship was not affected by changes in muscle length, nor by epinephrine (5 x 10^-8 g/ml) and quinidine (2 x 10^-5 g/ml), i.e. the rates of relaxation were those to be expected from the peak tensions. Gross deviations from the relationship in the direction of slower relaxation were observed during recovery from 30-60 sec of stimulation at 2/sec. These were reduced by decreases and potentiated by increases in [Ca]. Recovery from 10 min of hypoxia caused deviations similar to those resulting from drive. Hypoxia itself caused more rapid relaxation than was to be expected at the tensions observed. It appears to be useful to refer rates of relaxation after isometric contractions to the peak tension. When this is done, the previously reported effect of epinephrine to speed relaxation appears to be only secondary to its inotropic effect whereas recovery from hypoxia or from rapid stimulation appears to have a primary effect on relaxation.