Effects of ovariectomy on rat genioglossal muscle contractile properties and fiber-type distribution.

OBJECTIVE

To test the hypothesis that ovariectomy has no effects on contractile, histochemical, or biochemical properties of the rat genioglossus (GG).

MATERIALS AND METHODS

Eight-week-old female Sprague-Dawley rats were randomly assigned into three groups: normal group (Normal), sham-operated group (Sham), and ovariectomized group (OVX). Four weeks later, genioglossal electromyography activity (EMGgg) and contractile properties were measured, including relative integrated EMG (iEMG), maximal twitch tension, 70%-decay time, and fatigue index (FI). Then rats were sacrificed and paired GG were removed for further analysis. Adenosine-triphosphatase (ATPase) staining was performed to determine the percent fiber-type distribution and to identify cross-sectional area (CSA) of muscle fibers. Myosin heavy chain (MHC) phenotypes were determined by gel electrophoresis.

RESULTS

Ovariectomy reduced EMG activity and contractile properties of the GG. Following ovariectomy, the CSA of type IIA and the proportion of MHCIIA decreased significantly. The MHC isoform composition of GG transferred from relative slow-twitch to fast-twitch isoform, following the order MHCIIB --> MHCIIX --> MHCIIA. Sham operation had no effect on any of the parameters.

CONCLUSIONS

The hypothesis is rejected. The contractile properties of the GG are sensitive to ovariectomy. These changes were, at least in part, associated with changes in the amount and type of contractile protein expressed.