Age-related Changes in the Concentration of Hydroxypyridinium Crosslinks in Functionally Different Skeletal Muscles

High-performance liquid chromatography methods were developed to measure the concentration of hydroxypyridinium crosslinks in the intramuscular collagen and tendinous parts of functionally different skeletal muscles at different ages. A significant increase in pyridinoline concentration took place during maturation reaching 0.32 + 0.07 (moUmol collagen) in soleus (slow plantar flexor) and 0.28 ± 0.07 in plantaris (fast "mixed" plantar flexor) at the age of 4 months. In medial and lateral gastrocnemius (fast "mixed" plantar flexors) the pyridinoline concentrations (mol/mol collagen) reached 0.24 + 0.06 and 0.19 + 0.04, respectively, similar to those in both the extensor digitorum longus (fast "mixed" dorsi flexor) and rectus femoris (fast "mixed" knee extensor) muscles, but higher than in the fast "mixed" dorsi flexor muscle, anterior tibialis (0.11 ± 0.05 moUmol). By comparison, pyridinoline concentrations of 0.33 moll mol collagen (± 0.10) was measured from longissimus dorsi, a slow-twitch back posture muscle. After maturation the most significant increase in pyridinoline concentration was measured in soleus and gastrocnemius muscles. No differences in the crosslinking between different parts of muscle belly were noticed at any time-point. However, significantly fewer pyridinoline crosslinks were found in tendinous parts of soleus, extensor digitorum longus and anterior tibialis than in intramuscular collagen. The concentration of pyridinoline crosslinks tended to be highest in slow-twitch postural muscles, soleus and longissimus dorsi, and generally higher in plantar flexors which are exposed to higher stretch than dorsal flexors. The reasons for the unexpectedly low concentrations of pyridinoline crosslinks in the tendinous parts of muscles remain to be clarified.