Fig. 3. Effects of protamine (100 μg/ml) on the positive lusitropic effects of β-adrenoceptor stimulation induced by isoproterenol under low (A ) and high (B ) loads. 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.