Concentration response curves for the positive chronotropic effects of glucagon (A) and isoproterenol (B), in the absence (▲) and in the presence of ruthenium red (10 μmol/L, ■), or ZD 7288 (0.5 μmol/L, ●) in the spontaneously beating rat right atria.

<p>Results are expressed as increase in basal rate (beats min^-1). Ruthenium red and ZD 7288 reduced basal rate by 23 ± 3 beats min^-1 (n = 7) and 93 ± 12 beats min^-1 (n = 11), respectively. When the preparation was treated with either ruthenium red or ZD 7288, 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.