Rostrocaudal pattern of fiber-type changes in an overloaded rat ankle extensor

Abstract

Our aim was to quantify the overload-induced hypertrophy and conversion of fiber types (type II to I) occurring in the medial head of the gastrocnemius muscle (MG). Overload of MG was induced by a bilateral tenotomy/retraction of synergists, followed by 12-18 wk of regular treadmill locomotion (2 h of walking/running per day on 3 of 4 days). We counted all type I fibers and determined type I and II mean fiber areas in eight equidistant sections taken along the length of control and overloaded MG. Increase in muscle weights (31%), as well as in total muscle cross-sectional areas (37%) and fiber areas (type I, 57%; type II, 34%), attested to a significant hypertrophic response in overloaded MG. An increase in type I fiber composition of MG from 7.0 to 11.5% occurred as a result of overload, with the greatest and only statistically significant changes (approximately 70-100%) being found in sections taken from the most rostral 45% of the muscle length. Results of analysis of sections taken from the largest muscle girth showed that it significantly underestimated the extent of fiber conversion that occurred throughout the muscle as a whole. These data obtained on the MG, which possesses a compartmentalization of fiber types, support the notion that all fiber types respond to this model with a similar degree of hypertrophy. Also, they emphasize the complex nature of the adaptive changes that occur in these types of muscles as a result of overload.

Three‐dimensional morphometric mapping of rat muscle fibers

“…Rat TA muscle contains predominantly fast muscle fibers (95%) . Our findings agree with previous studies that type I fibers often tend to accumulate toward deep muscle regions and that rat slow oxidative muscle fibers in fast hind limb muscles of the lower part exhibit a higher density in the proximal part, while there is a decrease in density toward the distal part of the muscle …”

Adaptations of motoneuron properties to chronic compensatory muscle overload

“…The compensatory overload of the MG muscle was induced by tenotomy of the lateral gastrocnemius, soleus, and plantaris muscles (Gardiner et al 1991;Kryściak et al In press). The operation was performed under general anesthesia (60 mg/kg ip pentobarbital sodium).…”

“…This indicates that the response to an increased level of chronic activation of MNs is relatively quick and stable. membrane properties; rat MUSCLE OVERLOAD CAN BE ACHIEVED by workload training (Fry 2004;McDonagh and Davies 1984), ablation or tenotomy of synergists (Gardiner et al 1991;Noble and Pettigrew 1989;Olha et al 1988), peripheral nerve damage, or denervation of synergistic muscles (Degens et al 1995). Overload induces measurable adaptive effects in the whole muscle, its motor units (MUs), and individual muscle fibers.…”

Modulation of MHC isoforms in functionally overloaded and exercised rat plantaris fibers

“…Thirty cross sections, 20 µm thick, were placed in precooled (220°C) microcentrifuge tubes and stored at 270°C in preparation for myofibrillar protein isolation. Because of differences in the fiber type distribution along the rostralcaudal axis of the highly compartmentalized rat plantaris (Roy and Edgerton, unpublished observations) and the reported differences in the adaptations to FO along the proximaldistal axis of a muscle (11), fibers from a deep region (close to the bone) in the midportion of the muscle were consistently chosen for analysis and comparison.…”