Enzymatic alterations in single type IIB skeletal muscle fibers with inactivity and exercise in 12- and 30-month-old rats.

The purpose of the present study was to examine the effects of aging, inactivity and weight-bearing exercise on fast-twitch single Type IIB skeletal muscle fibers from the superficial region of the lateral head of the gastrocnemius (Type IIB fibers). Specifically, this study compared the biochemical properties of Type IIB fibers after 7 days of hindlimb unweighting (HU), 7 days of HU with intermittent weight-bearing (HU-Ex), and cage control (C) from adult and aged Fischer 344 Brown Norway F1 Hybrid rats (12- and 30-month old). Biochemical measurements included total lactate dehydrogenase (LDH) and beta-hydroxy-acyl-coenzyme A dehydrogenase activities (BHAD), expressed in nmoles/microg/hr dry weight. Fiber-typing for myosin heavy chain isoform was determined by SDS-PAGE. With age, LDH activity in Type IIB fibers decreased from 52.0 +/- 3.4 nmoles/microg/hr (12-month old) to 39.5 +/- 2.9 nmoles/microg/hr (30-month old). Following HU, LDH activity of single Type IIB fibers increased by 22% (52.0 +/- 3.4 to 66.4 +/- 3.2 nmoles/microg/hr) in the 12-month-old animals, whereas no difference was observed with HU in the Type IIB fibers of the 30-month-old animals. Following HU-Ex, LDH activity of Type IIB fibers in the 12-month-old animals was not significantly different from that of Type IIB fibers from HU animals, whereas a significant increase was observed (38.1 +/- 2.9 to 51.8 +/- 3.1 nmoles/microg/hr) in Type IIB fibers of 30-month-old animals, for HU and HU-Ex, respectively. Analysis of variance revealed an interaction between age and condition, indicating that Type IIB fibers from adult and aged animals have a different biochemical response to inactivity. The enzyme activities for BHAD were not different between the experimental conditions. The results demonstrate that the total LDH enzyme activities of the Type IIB fibers decrease with age, suggesting an age-related shift in the biochemical profile. Further, single skeletal muscle fiber adaptation is age-dependent.