ALTERATIONS IN SINGLE TYPE I SKELETAL MUSCLE FIBER CONTRACTILE FUNCTION WITH INACTIVITY AND THERAPEUTIC EXERCISE IN AGED ANIMALS

Abstract

The benefits of therapeutic interventions during periods of inactivity are unknown in the older population. Therefore, the purpose of this study was to determine the contractile function of single muscle fibers from the soleus and gastrocnemius muscles of 36-month old Fisher 344/BN rats following two weeks of inactivity and two weeks of inactivity plus intermittent treadmill walking. It was hypothesized that the contractile function of single fibers would adapt to inactivity and that treadmill walking for 15 minutes, 4X/day, would be an effective exercise. In order to test this hypothesis there were three experimental groups (8 animals per group): cage control, two weeks of hindlimb unweighting to mimic inactivity, and two weeks of hindlimb unweighting plus intermittent treadmill walking. Contractile function was determined in 180 single permeabilized fiber preparations. Peak force, peak specific force (Po), maximal shortening velocity (Vo, tested by the slack test and Vmax, extrapolated from the force-velocity relationship) and peak power (calculated from the force-velocity relationship) were determined in myosin heavy chain type I fibers (determined by SDS PAGE). In type I fibers from the soleus, inactivity-induced a decline in fiber peak force (31%), Po (10%), and peak power, whereas Vo and Vmax increased. In type I fibers from the gastrocnemius, there was a decline in fiber peak force (50%), Po (24%) and peak power with inactivity. In contrast, Vo and Vmax of type I fibers from the gastrocnemius did not change with inactivity. Attenuation of the inactivity-induced decline in single fiber contractile function was greater in the type I fibers from the gastrocnemius muscle compared to the soleus muscle with treadmill walking. Although the contractile function of single type I fibers from aged animals adapt to altered activity patterns, the extent of change with inactivity and intermittent treadmill walking is dependent on the specific muscle and the contractile property examined. Underlying mechanisms responsible for the contractile property-specific adaptation and the benefits of treadmill walking are unknown. Supported by the AHA-Minnesota Affiliate and Foundation for Physical Therapy.