Influence of age, growth, and sex on cardiac myocyte size and number in rats

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Abstract
The effects of altered neonatal nutrition on cardiac myocyte size and number was examined in 21-day-old and 3-month-old rats. Nutritional differences in growth rate were produced in newborns by adjusting litter size to four (fast-growing), eight (normally growing), or 16 (slow-growing) pups per litter. Isolated myocytes were prepared from animals in each group to evaluate changes in cell size and number. Heart weight (mg ± S.D.), at 21 days of age, was 398 ± 51 for “fast-growing” rats, 329 ± 43 for “normally growing” rats, and 228 ± 24 for “slow-growing” rats. Body weights showed a comparable decline with reduced nutrition. In adults, treatment-related differences in body and heart weight were present in males but not females. “Slow-growing” rats had 21% fewer myocytes than “fast-growing” rats at 21 days of age, a change that persisted in adults. Values for myocyte number from “normally growing” rats were intermediate between those of “fast and slow-growing” rats at both 21 days and 3 months of age. In each heart region of weanling rats, myocyte length and volume were smallest in 16 per litter rats. Cellular dimensions increased progressively with better nutrition. In adults, differences in myocyte size as a function of altered neonatal nutrition were not statistically significant. Sex-related differences in myocyte size or number were not found in males and females from any group at the time of weaning. In adults, it was also noted that males and females have a similar number of cardiac myocytes, but the larger heart weight found in males was primarily due to increased myocyte size. Myocyte size and number were also examined in 2-year-old female rats. There were no differences in cardiac myocyte size or number between 2-year-old females and 8-month-old weight-matched females.

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