The work loop technique was used to examine the effects of adrenaline on the mechanics of cardiac muscle contraction in vitro. The length for maximum active force (Lmax) and net work production (Lopt) for rat papillary muscles was determined under control conditions (without adrenaline). The concentration of adrenaline producing the maximum inotropic effect was determined. This concentration was used in the remainder of the experiments. Sinusoidal strain cycles about Lopt were performed over a physiologically relevant range of cycle frequencies (4-11 Hz). Maximum work and the frequency for maximum work increased from 1.91 J kg-1 at 3 Hz in controls to 2.97 J kg-1 at 6 Hz with adrenaline. Similarly, maximum power output and the frequency for maximum power output (fopt) increased from 8.62 W kg-1 at 6 Hz in controls to 19.95 W kg-1 at 8 Hz with adrenaline. We suggest that the power-frequency relationship, derived using the work loop technique, represents a useful index with which to assess the effects of pharmacological interventions on cardiac muscle contractility.

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