Non-linear correlation between nerve terminal parameters and muscle fibre diameter of exercised mice during aging

ABSTRACT

The effect of exercise on morphological age changes in endplate parameters and their relation to muscle fibre diameter was studied in male [C57B1/6J] mice. Animals [N = 60; half of them served as controls] aged 12, 18 and 24 months at sacrifice, were treadmill exercised for two months for 1 h/day at velocities up to 30 m/min. The area [A], perimeter [P], nerve longitudinal extent length [E] and fibre diameter [FD] from camera lucida drawings of extensor digitorum longus zinc iodide osmium-stained nerve terminals were measured. Non-linear regression and multivariate analysis were used to study the inter-relationships among these parameters under aging and exercise conditions. At 12 months E was strongly correlated with FD, but the correlation badly deteriorated with aging, and exercise did not improve it. At all ages A was not correlated with FD during aging alone, but with exercise the correlation improved, especially at 18 months [r2 = 0.73]. The ratio E/FD was not correlated with the other endplate parameters except in exercised animals, in whom E/FD showed a high correlation with A, especially at age 12 months [r2 = 0.72]. FD showed a high correlation with P/A at 12 months of age, but exercise decreased this correlation at all ages. Multivariate analysis showed no significant relationship between FD and nerve terminal parameters at any age in control animals, but in exercised animals a close relationship appeared, given by the equation FD = 18.75 + 0.025A - 0.08E + 0.01P with P < 0.01 and r2 = 0.34. The discovery of a non-linear relationship between FD and nerve terminal parameters makes possible a further analysis of the effects of exercise on morphological age changes at the neuromuscular junction