Fig. 4. The concentration–response curves for forskolin (A ) and norepinephrine (B ) in the absence (open circle; n = 8 for forskolin and n = 6 for norepinephrine) and presence of 2 mm CsCl (closed circle; n = 6 for forskolin and n = 6 for norepinephrine) in the spontaneously beating guinea pig right atrium. Points are mean ± SD. The increase in the beating rate (beats/min [bpm]) produced maximally by forskolin or norepinephrine was taken as 100%. When the preparation was treated with CsCl, the beating rate in the presence of CsCl was taken as the basal beating rate. *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 beating rate. C = control; T = treatment.