THE EFFECTS OF BETA-ADRENOCEPTOR ACTIVATION ON CONTRACTION IN ISOLATED FAST-TWITCH AND SLOW-TWITCH SKELETAL-MUSCLE FIBERS OF THE RAT

1 The aim of the experiments was to examined the effects of beta-adrenoceptor activation on twitch and tetanic contractions in fast- and slow-twitch mammalian skeletal muscle fibres. Isometric force was recorded from bundles of intact fibres isolated from the normal and denervated slow-twitch soleus and normal fast-twitch sternomastoid muscles of the rat. 2 Terbutaline (10 muM), a beta2-adrenoceptor agonist, induced an average 15% potentiation of peak twitch and peak tetanic force in normal soleus fibres and abbreviated twitch and tetanic relaxation. In white- and red-sternomastoid fibres, 10 muM terbutaline potentiated peak twitch force by about 7% and slowed twitch relaxation. 3 The potentiation of twitches and tetani by terbutaline was quantitatively similar in normal and denervated soleus fibres. However, in contrast to the normal soleus, terbutaline slowed twitch relaxation and had no effect on tetanic relaxation in denervated soleus fibres. 4 Adrenaline (10 muM) increased peak tetanic force by about 7% in both normal and denervated soleus fibres. 5 Exposure to (+/-)-propranolol (0.1 muM), a general beta-adrenoceptor blocker, completely abolished the tetanus potentiation by terbutaline. 6 Dibutyryl-cyclic AMP (2 mm) mimicked the effects of 10 muM terbutaline on peak tetanic force and tetanic relaxation in normal and denervated soleus fibres. Dibutyryl-cyclic AMP also potentiated peak twitch force in denervated soleus fibres but only after a brief period of twitch depression: the twitch depression might be due to butyrate. 7 The results suggest that the increase in peak twitch and tetanic force and abbreviation of tetanic relaxation induced by terbutaline depend on the activation of beta-adrenoceptors and a consequent increase in the myoplasmic cyclic AMP concentration.