Fig. 5. Effects of protamine (10 μg/ml) on the positive inotropic and lusitropic effects of dibutyryl 3′,5′-cAMP (0.5 mm) V^max= maximum unloaded shortening velocity; AF = active isometric force; R1 = ratio of maximum shortening velocity to maximum lengthening velocity; R2 = ratio of the peak of the positive force derivative to the peak of the negative force derivative. Data are mean percentage of baseline ± SD; n = 8 in each group. *P < 0.05 versus  baseline. P  values refer to between-groups differences. NS = not significant.