Age effects on cytochrome oxidase activities during denervation and recovery of three muscle fiber types

The rat nasolabialis muscle is comprised of a mosaic of red, white, and intermediate muscle fiber types. Using computerized microdensitometry, cytochrome oxidase (COX) activity was quantitatively analyzed in each fiber type throughout the period of denervation and recovery in young adult (3‐month) and middle‐aged (15‐month) male Sprague‐Dawley rats. In animals of both age groups, the nasolabialis muscle on one side of the head was denervated by crushing the facial nerve. At specific days post crush (dpc) ranging from 2 days–2 months, animals were sacrificed and thick sections of normal and denervated muscles were incubated to demonstrate the activity of COX, a mitochondrial enzyme, which differentiates between the three fiber types. Enzyme activities in individual fibers were microdensitometrically analyzed using a digitizing image analyzer. Although a denervation‐induced decrease followed by eventual recovery occurred in all fibers of each type, age‐related differences were evident. For all types, younger fibers consistently showed decreased COX activity sooner than their older counterparts, and older fibers of all types consistently showed a greater decreased COX activity than the younger fibers. Denervation‐induced de‐differentiation of muscle fibers led to a more homogeneous population of fiber types in both age groups. Following recovery of function, the magnitude of the fiber enzyme activity change differed according to fiber type and to age, and was consistently smaller in older animals. The normal mosaic pattern of fiber type distribution and normal COX levels were restored 2 months after nerve lesion in both age groups.