Fig. 1

Cardiomyocyte cell cycle activity is increased in the exercised heart ¹⁵N-thymidine was administered continuously for 8 weeks to young adult mice (2 months old) undergoing voluntary wheel running vs. sedentary activity. a Mass ¹⁴N image (top left, bottom left) shows histological details such as sarcomeres (large red arrows), mass ³¹P image (center, top and bottom) shows nucleus and chromatin condensation, while the hue-saturation-intensity image (mosaic, top, and bottom right) demonstrates nuclear ¹⁵N labeling of a cardiomyocyte (yellow asterisk), non-labeled cardiomyocytes (large white arrows) can also be found in that section, as well as, two labeled non-cardiomyocytes (bottom right, small white arrows) and one non-labeled non-cardiomyocyte (top right, small white arrow). The scale ranges from blue, where the ratio is equivalent to natural ratio (0.37%, expressed as 0% above natural ratio (enrichment over natural ratio)), to red, where the ratio is 150% above natural ratio. ¹⁵N-thymidine has labeled the nucleus while the cytoplasm is at the natural abundance level. Scale bar = 10 μm. b Comparison of the percentage of ¹⁵N-labeled cardiomyocyte nuclei in exercised to sedentary young adult hearts. Exercise increases cardiomyocyte cell cycle activity (sedentary:exercise = 1.24:3.62%; >1000 cardiomyocytes from four mice per group were counted, ***p = 0.0003, Fisher’s exact test). c Contingency table showing the absolute numbers and percentage calculations of ¹⁵N-positive and ¹⁵N-negative cardiomyocytes