Fig. 6. The concentration–response curves for the positive inotropic effects of norepinephrine in the absence (open circle; n = 4) and presence of 100 μm amrinone (closed circle; n = 5) in the electrically driven guinea pig left atrium. Points are mean ± SD. The increase in the contractile force produced maximally by norepinephrine was taken as 100%. When the preparation was treated with amrinone, the contractile force in the presence of amrinone was taken as the basal contractile force. *Significantly different from corresponding control (P < 0.05) by analysis of variance followed by Bonferroni t test. (Inset ) Concentration–response curves depicted on the basis of changes in absolute values of the contractile force. C = control; T = treatment.