THE EFFECT OF HYPERTHYROIDISM ON MULLERS MUSCLE CONTRACTILITY IN RATS

The objective of this study was to determine if hyperthyroidism affect s the responses of Muller's muscle to alpha-(1) adrenoceptor agonists and consequently, if these responses might explain thyroid eyelid retr action. Sprague-Dawley adult rats (n = 37) were divided into control a nd treated groups and given either placebo or intraperitoneal triiodot hyronine (250 mu g/kg/d) for 1, 2, or 3 weeks. A suture was passed thr ough their upper eyelid and connected to a force transducer that measu red Muller's muscle contractions. Responses to phenylephrine (0.015-0. 61 mmol) were compared with respect to peak amplitude and 50% duration of action. Mean maximum force values [+/-1 standard error of the mean (SEM)] in response to phenylephrine were 1.254 +/- 0.071 gr for contr ols and 0.963 +/- 0.062 gr for thyroid-treated subjects (p = 0.005). M ean 50% duration of response values (+/-1 SEM) were 9.143 +/- 1.108 mi n for controls and 5.763 +/- 0.973 min for thyroid-treated subjects (p = 0.014). Hyperthyroid rats had a significantly lower Muller's muscle response amplitude than control rats; however, duration of response w as not significantly different between the groups. We believe that hyp erthyroidism caused intrinsic changes in Muller's muscle that resulted in eyelid retraction. Based on hypotheses discussed in this article, we expect that further studies will localize these changes to the thyr oid hormone receptor on Muller's muscle or calcium-triggered intracell ular second messengers. Clinical significance would then be the abilit y to treat hyperthyroid eyelid retraction with drugs. This study provi des the first evidence of functional impairment of Muller's muscle due to hyperthyroidism in an animal model.