Fig. 4

Concentration‒response curves for the positive chronotropic effects of glucagon in the absence (●) and presence of the nonselective phosphodiesterase inhibitor IBMX (30 μM, ■) in spontaneously beating rat right atria. The results are expressed as the increase in the basal rate (beats min⁻¹). IBMX increased the beating rate by 33 ± 12 beats min⁻¹ (n = 5). The beating rate in the presence of IBMX was used as the basal rate for the second concentration‒response curve of glucagon. Each point represents the mean value ± SEM (vertical bars) of 5 experiments. No significant differences were found when comparing any concentrations of glucagon in the absence or presence of IBMX (paired Student’s t test)