Fig 6. (A) Concentration response curves for the positive chronotropic effects of glucagon in the absence (▲) and in the presence of the non selective PDE inhibitor IBMX (3 μmol/L, ●), the selective PDE3 inhibitor cilostamide (0.3 μmol/L, ■) or the selective PDE4 inhibitor rolipram (1 μmol/L, ▼) in the spontaneously beating rat right atria. (B) Concentration response curves for the positive chronotropic effects of isoproterenol in the absence (▲) and in the presence of the non selective PDE inhibitor IBMX (3 μmol/L, ●) in the spontaneously beating rat right atria.

Results are expressed as increase in basal rate (beats min^-1). Both IBMX and rolipram increased beating rate by 26 ± 3 beats min^-1 (n = 8) and 36 ± 6 beats min^-1 (n = 5) respectively. Cilostamide did not changed basal atrial rate. When the preparation was treated with either IBMX or rolipram the beating rate in the presence of each of these agents was taken as the basal beating rate. Each point represents the mean value ± s.e.m (vertical bars) of 4–6 experiments.