A divergent selection experiment was conducted for 8-wk BW in chickens. At 3, 6, 9,
and 12 wk of age, samples of pectoralis profundus (PP) and biceps femoris (BF) muscles
from fast-growing and slow-growing lines were used to estimate the enzyme activities
and muscle fiber diameter. Microphotometric measurements made in situ of succinate
dehydrogenase (SDH, EC 1.3.99.1) and glycerol-3-phosphate dehydrogenase (GPDH, EC
1.1.99.5) were completed on serial sections of PP and BF muscles from male chickens,
in order to examine the ratio of SDH:GPDH activity in single fibers. On the basis
of the SDH:GPDH activity ratios, muscle fibers were divided using cluster analysis
into 3 populations of different fiber types (O = oxidative, OG = oxidative-glycolytic,
and G = glycolytic). Cockerels of the SGL attained an 8.1-fold increase and those
of the FGL a 6.8-fold increase in BW at 12 wk compared with that at 3 wk of age. The
O, OG, and G type fibers of the BF muscles of the SGL had significantly (P ≤ 0.001)
lower SDH:GPDH activity ratios than those of the FGL. A step decrease in the SDH:GPDH
activity of O, OG, and G fibers in the PP of both lines occurred, and this differed
significantly between SGL and FGL (P ≤ 0.001). Age and line effects influenced the
diameter of the 3 fiber types in the BF muscle only. In contrast to this response,
all 3 fiber types of the PP muscles reached similar diameters in both lines during
the growth process from wk 3 to 12. From the results of this study, we concluded that
the activities of metabolic enzymes in skeletal muscle fibers are under the influence
of muscle type, age, and selection pressure. Microphotometry is a suitable method
for the evaluation of enzyme activity measured in a single muscle fiber. The method
enables precise estimation of enzyme activities, especially in muscles composed of
populations of different metabolic fiber types.