Effect Of Hyperthyroidism On The Orbicularis Oculi Muscle In Rabbits

Purpose:To determine the effect of hyperthyroidism on the myofiber number,myosin heavy-chain isoform composition and the inflammatory cell infiltrate,GAG level within the palpebral orbicularis oculi muscle in rabbits.Methods:Ten rabbits were assigned randomly into two groups.Five rabbits were made hyperthyroid by injection of 3,5,3'-triiodothyroinine(T3)intraperitoneally every other day for 1 month(0.2mg/kg.wt).Five rabbits were used as control animals.The weight,serum FT3,FT4, TSH,TRAb were detected in this period.After 1 month the rabbits were euthanized,and the eyelids were excised and sectioned.The sections were HE,Alcian blue stained to determine the inflammatory cell infiltrate,GAG level.Then were immunostained to determine the presence of fast,slow and neonatal myosin heavy-chain isoforms.To determine alterations in myofiber number,differential counts of myofiber number and the cross-sectional areas of the muscle fibers were performed with the use of computerized morphometry.Results:The hyperthyroid rabbits showed signs of hyperthyroidism such as decreased body weight,increased serum FT3 etc.The orbicularis oculi muscle in the palpebral portion of the eyelids from hyperthyroid rabbits had significantly fewer myofibers compared with control eyelids.The remaining fibers showed continued expression of fast,slow and neonatal myosin, but upregulated coexpression of slow myosin isoform.There are no significant deviation of inflammatory cell infiltrate,GAG aggregation in the orbicularis oculi muscle of hyperthroid animals and control animals.Conclusion:Hyperthyroidism led to reduced orbicularis oculi muscle in the rabbit model and an alteration in the myosin heavy-chain isoform composition.This finding may help explain the clinical finding of eyelid retraction,upper eyelid drop delay,nictitate decrease in patients with thyroid associated ophthalmopathy.