The Effects of Thyroxine Treatment on Slow‐ and Fast‐contracting Skeletal Muscle Contractions of the Cat and Their Cyclic Amp Level

Abstract

1. The effects of thyroxine treatment on soleus and extensor digitorum longus (EDL) muscle contractions and their cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels were examined in anaesthetized cats. 2. Thyroxine treatment decreased the tension of incomplete tetanic contractions of the soleus as well as the EDL muscles. The effect on tension of these muscles was not associated with an increase in the cyclic AMP level of the muscle as is the case with a beta 2-adrenoceptor agonist effect. 3. The results do not support the involvement of cyclic AMP in the tension depressant effect of thyroxine on contractions of skeletal muscle. 4. It is suggested that the muscle weakness and tremor observed in thyrotoxicosis and during administration of beta 2-adrenoceptor agonists are mediated by different mechanisms.

Effect of Altered Thyroid State on the in situ Mechanical Properties of Adult Cat Soleus

“…The lower dose resulted in a significant decrease in Pt, TPT and HRT, but not Po, muscle weight or body weight [Merican and Morat, 1984]. The higher dose had similar effects on the twitch properties, but also resulted in a decrease in Po without affecting muscle or body weights [Merican et al, 1992;Zaiton et al, 1993]. No previous data were found on the effects of a chronic decrease in thyroid hormone on the mechanical properties of cat skeletal muscles.…”

“…The dosage of thyroxine administered to the hyperthyroid cats in the present study was F50 Ìg/kg body weight/ day compared to F4 and 60 Ìg/kg body weight/day for the low and high doses administered by Merican and coworkers [Merican and Morat, 1984;Merican et al, 1992;Zaiton et al, 1993], based on a mean body weight of 2 kg and treatment 2 and 3 times/week, respectively. Thus, the dose used in the present study was similar to the higher dose used in the previous studies.…”

“…The most consistent effects across studies and species, including humans, are on the isometric twitch TPT and HRT and the frequency-tension response [Takamori et al, 1971;Wiles et al, 1979;Fitts et al, 1980;Nicol and Maybee, 1982;Nwoye et al, 1982;Caiozzo et al, 1991Caiozzo et al, , 1992Merican et al, 1992;Larsson et al, 1994;Li and Larsson, 1997]. Both of these measures are highly dependent on the dynamic calcium properties of the sarcoplasmic reticulum.…”

“…The limited existing literature is controversial. A hyperthyroid state has been reported to consistently decrease the isometric twitch tension and contraction time [Merican and Morat, 1984;Merican et al, 1992;Zaiton et al, 1993], but to either decrease [Zaiton et al, 1993] or maintain the maximum tetanic tension (Po) at control levels [Merican and Morat, 1984]. To our knowledge, the effects of elevated thyroid hormone levels on Vmax and of depressed thyroid hormone levels on any mechanical property of the cat soleus muscle have not been determined.…”

“…Merican and coworkers [Merican and Morat, 1984;Merican et al, 1992;Zaiton et al, 1993] have studied the effects of 10 weeks of thyroxine treatment on the in situ isometric properties of adult male cats. The effects of two doses of thyroxine administered intraperitoneally for 10 weeks were studied; 25 Ìg/kg every 3rd day and 250 Ìg/kg 3 times/week.…”