The effects of β-adrenoceptor blockade on resting heart rate.

Figure 1: The effect of β-adrenoceptor antagonists on resting heart rate of healthy individuals over time. Placebo (vitamin B6, 10 mg), atenolol (50 mg) or pindolol (10 mg) were administered at 30 min. * p<0.05 drug vs placebo; n = 68-69 per treatment. Bpm, beats per minute.

In a previous trial, it was found that the decrease in heart rate observed with pindolol, was faster than that observed with atenolol. What may have caused the difference in effect between atenolol and pindolol at 15 min post-treatment?