The effect of age on the nucleic acid content of slow- and fast-twitch muscle in normal and dystrophic mice and their litter mates

Abstract

RNA, DNA, and NCP content were measured in fast- and slow-twitch skeletal muscle of normal and dystrophic mice (HDM) and their littermates at ages 4 through 29 weeks. In normal and litter mate mice RNA and DNA content were far greater in the soleus than in the gastrocnemius while the RNA/DNA ratio and NCP content were greater in the gastrocnemius. In dystrophic mice, however, the differences between nucleic acid content of the 2 muscles were far less, apparently due to a proportionately higher content in the dystrophic gastrocnemius. Due to a proportionately lower ratio in the gastrocnemius, dystrophic RNA/DNA ratios for the 2 muscles were essentially the same. Age had a marked effect on the nucleic acid content of both muscles in all 3 mice types but to varying degrees. In the soleus, RNA and DNA content rapidly decreased until 9 to 10 weeks of age followed by a gradual decline. Soleus RNA/DNA ratios showed little change with age except in the HDM mice in which there was a significant overall decline. In the gastrocnemius, RNA content followed the same pattern but with a smaller decline in the younger ages. Age had no affect on DNA content in the normal gastrocnemius, but there was a significant decline in the HDM gastrocnemius. RNA/DNA gastrocnemius ratios showed marked fluctuations in both normal and dystrophic mice but did not appear to be affected by age. Depending upon age and muscle type, dystrophic RNA content was significantly higher in both muscle types from 5 to 9 weeks of age, and then decreased below normal levels. DNA content of both dystrophic soleus and gastrocnemius was also elevated in the young mice. In the soleus, DNA also decreased below normal values during middle age but then increased again in the middle and older age mice. The RNA/DNA ratio of the dystrophic gastrocnemius was decreased in most age groups. The ratio for the soleus, however, had 3 phases; a reduced ratio in the young mice, a higher ratio in the middle-aged mice, and a reduced ratio in the oldest mice.