Aerobic Activity in the Axis of Growing Myofibers in the Porcine Biceps Femoris

Twenty-four pigs between 9 and 29 wk of age were slaughtered at intervals of 4 wk. A microscope photometer was used to measure the staining intensity of histochemical types of myofibers. Biceps femoris myofibers were categorized as either strong-ATPase or weak-ATPase. The growth in cross sectional area of strong-ATPase myofibers was greater than that of weak-ATPase myofibers (allometric growth ratio, k = 1.36, P less than .005). Among strong-ATPase myofibers of animals at each different age, myofibers with a large diameter tended to have weaker succinate dehydrogenase (SDH) activity in their central axis than myofibers with a small diameter. Hence, correlations of cross sectional area with axial SDH staining (transmittance) ranged from r = .27 at 9 wk to r = .43 at 29 wk (all correlations significant, P less than .005). When the means for strong-ATPase myofibers of pigs at different ages were compared, SDH staining in the central axis of strong-ATPase myofibers tended to decrease as myofibers grew larger in cross sectional area. On this basis, myofiber cross sectional area was correlated with SDH transmittance, r = .85 (P less than .025). Among weak-ATPase myofibers of animals at each different age, large diameter myofibers sometimes tended to have weaker axial SDH activity than small myofibers, but no significant decrease in axial SDH activity was detected as myofibers grew larger.