Clinical and Experimental Pharmacology and Physiology zyxwvu (1992) 19, 843-846 zyxw THE EFFECTS OF THYROXINE TREATMENT ON MUSCLE CONTRACTIONS OF THE CAT AND THEIR CYCLIC AMP LEVEL SLOW- AND FAST-CONTRACTING SKELETAL Zainuddin Merican,* S. Sukumaran,* Vijaya Lechimi Raj,* Mohd Hamim Rajikin' and B. A. K. Khalid* Departments of *Pharmacology, zyxwv 7 Physiology and $Medicine, Universiti Kebangsaan Malaysia, Jalan zyxwvut Raja Muda Abdul Aziz, Kuala Lurnpur, Malaysia (Received 29 August 1991; revision received 2 September 1992) SUMMARY 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 P2-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. zyxwvu 4. It is suggested that the muscle weakness and tremor observed in thyrotoxicosis and during administration of P2-adrenoceptor agonists are mediated by different mechanisms. Key words: contractions, cyclic AMP, extensor digitorum longus, soleus, thyroxine. INTRODUCTION The effects of thyrotoxicosis on contractions of slow- contracting mammalian skeletal muscles (Gold er zyxwv af. 1970; Fitts zyxwvuts et al. 1980; Merican zyxwvuts & Morat 1984) appear to be similar to the effects of catecholamines (CA) on this muscle type (Bowman & Zaimis 1958; Nott & Raper 1972; Apperley el al. 1979). CA also exert similar contractile effects on human muscles (Marsden & Meadows 1970). The tension depressant effect of CA on slow-contracting muscles is thought to be responsible for the tremor and muscle weakness experienced after administration of CA such as adrena- line (Bowman & Zaimis 1958). On fast-contracting mammalian skeletal muscles, thyrotoxicosis as well as CA appear to exert minimal effects. Thus neither thyrotoxicosis (Nicol & Bruce 1981) nor 'physiological' doses of CA (Bowman & Zaimis 1958; Bowman & Nott 1969) have significant effects on contractions of fast-contracting mammalian skeletal muscles. These few studies suggest that the effects of thyrotoxicosis on skeletal muscle contractions are largely similar to those of CA. The effects of CA on slow- and fast- contracting muscles are mediated through Bz- adrenoceptor stimulation (Bowman & Nott 1969), and cyclic AMP is believed to be the mediator of Correspondence: Dr Zainuddin Merican, Department of Pharmacology, Medical Faculty, Universiti Kebangsaan Malaysia, Jalan Raja Muda Abdul Aziz, 50300 Kuala Lumpur, Malaysia.