Figure 3. Effects of isoproterenol (ISO) (n = 6), triiodothyronine (T sub 3) (n = 6), and thyroxine (T4) (n = 6, 3, 5, and 4, for bolus doses ranging from the lowest to the highest, respectively) on heart rate in isolated rat hearts. Data are presented as means plus/minus SEM. Differences in baseline heart rates in the T4studies are a consequence of the differing numbers of hearts in which these effects were observed. None of the drugs (at any study dose) produced a significant alteration in heart rate.