Effect of the goitrogen methimazole on skeletal muscle growth of chick embryos

Abstract

The effect of methimazole (MMI) treatment on the body growth of chick embryos, with particular reference to skeletal muscle growth, was ascertained during the second half of embryogenesis. MMI-induced hypothyroidism is easily achieved and the optimum effective dose appears to be 2.5 mg/100 g body wt/day. When this dose is administered from 10 through 19 days of incubation, the following parameters at 20 days of incubation are reduced below control levels by the percentages indicated in parentheses: body wt (38%), shank-toe length (20%a), gastrocnemius muscle weight (50%), gastrocnemius total actin (75%), gastrocnemius total crude fibrillar protein (60%), sartorius muscle weight (48%), sartorius total actin (65%), and sartorius total crude fibrillar protein (50%). Muscle DNA levels were not appreciably altered by the MMI treatment. Most of the effect of hypothyroidism on body weight and muscle growth becomes apparent between 17 and 20 days of incubation when there are normally large increases in these parameters. The effects of MMI were counteracted, in part, by thyroxine replacement from 16 through 19 days of incubation. The effect of MMI administration from 10 through 19 days of incubation was compared to the effectiveness when administered from 10 through 14 or 15 through 19 days. At 20 days of incubation, body weight and gastrocnemius muscle weight and actin content were affected nearly the same following either 5-day treatment as following 10-day MMI treatment. Apparently, early hypothyroidism effects a change in body and gastrocnemius muscle growth which persists after withdrawal of MMI. Also, this early influence of thyroid deprivation does not seem to be required for a nearly full effect of MMI-induced hypothyroidism on these parameters from 15 to 20 days of incubation.