Abstract

Recently we observed that the FDL muscle of the F344 rat, which is comprised primarily of type IIb muscle, does not change in size, fiber type, or physiological characteristics during senescence. This muscle was utilized to determine whether a predominantly fast-twitch glycolytic muscle would respond to tonic electrical stimulation (ES) with the same degree of fiber-type transformation in aging and young rats. The extent of transformation was quantified in the FDL muscle after ES (10 Hz, 8 h/day) imposed on the tibial nerve for periods of 0-90 days in young adult (YG), middle-aged (MA), and senescent (SN) male F344 rats. Although ES induced a IIb-to-IIa fiber-type shift in all groups, in SN rats the shift was significantly less pronounced at the intermediate time points and remained incomplete after 90 days, compared with YG and MA rats. ES resulted in a reduction in tetanic tension (Po), which in the YG and MA rats was due to a reduction in muscle cross-sectional area. In the SN rats the reduced Po was due to a combined loss of cross-sectional area and specific tension (Po, N/square-cm). Contraction and half-relaxation times were largely unaffected by ES, and maximal velocity of unloaded shortening declined throughout ES in all groups. Citrate synthase activity and fatigue index increased in all groups after ES; however, the rate of change in SN rats was slower than in YG rats. Results of this study indicate some loss of plasticity as a result of aging in response to ES, particularly in greater atrophy in SN rats. Verf.-Referat