The mechanism of action of adrenaline on cardiac contractility in rat papillary muscles containing V1 and V3 isomyosins was analyzed during barium-activated contractures at 25 degrees C by frequency domain analysis using pseudo-random binary noise-modulated perturbations. The analysis characterizes a frequency (fmin) at which dynamic stiffness of a muscle is a minimum, a parameter that reflects the rate of cycling of crossbridges. We have previously shown that fmin for V1- and V3-containing papillary muscles were 2.1 +/- 0.2 Hz (mean +/- SD) (n = 10) and 1.1 +/- 0.2 Hz (n = 8), respectively, and that these values were independent of the level of activation. The present study's goal was to determine whether the inotropic action of adrenaline was associated with an increased rate of crossbridge cycling. The results show that a saturating dose of adrenaline increased fmin in V1 hearts by 49 +/- 2% (n = 11). The action on V3 hearts was significantly less; the increase in fmin was 26 +/- 2% (n = 6). The increase in fmin for V1 hearts was shown to be sensitive to the beta-blocking agent propranolol. These results suggest that adrenaline significantly increases the rate of crossbridge cycling by a beta-receptor-mediated mechanism. We conclude that the increased contractility of the heart in the presence of adrenaline arises not only from more complete activation of the contractile proteins but also from the increased rate at which each crossbridge can transduce energy.

Experiments were done on four-week-old rats, containing biochemically verified V1 only, and thyroidectomized adult rats, treated with propylthiouracil, verified to contain V3 only. Contracture tension was induced in isolated papillary muscles either by high potassium solution or 0.5 mmol l-1 Ba2+. Small amplitude length perturbations with peak-to-peak value not exceeding 0.15% L0 were applied to the activated muscle. Both the applied length perturbations and the corresponding resulting force changes were analysed by computer for dynamic stiffness and phase values. In order to reduce data acquisition time, pseudo-random binary noise length changes, rather than the conventional sinusoidal length changes, were used. The plot of the dynamic stiffness against frequency displays a minimum, akin to a resonance phenomenon. The frequency, fmin, at which this resonance occurs, reflects crossbridge kinetics. It was found that the fmin values for the two types of papillary muscles differed by a factor of two. Experiments were also done on chemically skinned muscles containing V1 or V3 isomyosin activated by different concentrations of either barium or calcium ions. It was found that fmin values of skinned fibres were higher than those obtained from intact fibres. However, for each type of muscle the fmin was independent of the activator used as well as the level of activation. The ratio of fmin for V3 to that for V1 remained the same as for intact preparations. We conclude that the difference in mechanical parameters did not arise a possible difference in excitation-contraction coupling mechanism, but rather is a difference in the dynamic properties of the two types of crossbridges.

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