Figure 1. Exercise training induces sinus bradycardia and remodelling of the sinus node.

a, Increase in $\stackrel{.}{V}{O}_{2,max}$ in the rat following the 12 week training period. Normalised $\stackrel{.}{V}{O}_{2,max}$ of sedentary and trained rats before and after the 12 week training period shown (n=11/group). In all bar graphs: black bars, data from sedentary animals; hatched bars, data from trained animals. b, Significant correlation between heart rate in vitro and normalised $\stackrel{.}{V}{O}_{2,max}$ in sedentary and trained rats (measured after 12 week training period). Each point corresponds to a different animal. Data fit with a straight line by linear regression (n=5/9; R² and P values shown). c, Representative ECG traces obtained from sedentary and trained (unrestrained and conscious) rats and mice demonstrating longer R-R intervals in trained animals. d and e, Mean (+SEM) R-R intervals (d) and corresponding heart rates (e) measured in vivo (rats, n=5/9; mice, n=6/8) and in vitro (from isolated sinus node preparations; rats, n=5/9; mice, n=6/7) in sedentary and trained animals. f, Mean (+SEM) heart rate measured in vivo in conscious mice at baseline (n=9/11) and after complete autonomic block with propranolol and atropine (n=7/14). g, Mean (+SEM) sympathetic tone and vagal tone in conscious sedentary and trained mice (n=6/6). h, Expression of transcripts in the sinus node (grey bars) and atrial muscle (open bars) of trained rats as a percentage of that of sedentary rats. The vertical line corresponds to 100%, i.e. no change. Values below 100% correspond to a decrease on training and above 100% an increase. Student’s t test used to test differences between data from trained and sedentary animals. Normal distribution of data was tested using the Shapiro-Wilk W test and equal variance was tested using the F test. When the null hypothesis of normality and/or equal variance was rejected, the nonparametric Mann Whitney U test was used. *P<0.05.