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-adren oceptor activation on twitch and tetanic contractions in fast- and slo w-twitch mammalian skeletal muscle fibres. Isometric force was recorde d from bundles of intact fibres isolated from the normal and denervate d slow-twitch soleus and normal fast-twitch sternomastoid muscles of t he rat. 2 Terbutaline (10 muM), a beta2-adrenoceptor agonist, induced an average 15% potentiation of peak twitch and peak tetanic force in n ormal soleus fibres and abbreviated twitch and tetanic relaxation. In white- and red-sternomastoid fibres, 10 muM terbutaline potentiated pe ak twitch force by about 7% and slowed twitch relaxation. 3 The potent iation of twitches and tetani by terbutaline was quantitatively simila r 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 mu M) increased peak tetanic force by about 7% in both normal and denerva ted soleus fibres. 5 Exposure to (+/-)-propranolol (0.1 muM), a genera l beta-adrenoceptor blocker, completely abolished the tetanus potentia tion by terbutaline. 6 Dibutyryl-cyclic AMP (2 mm) mimicked the effect s of 10 muM terbutaline on peak tetanic force and tetanic relaxation i n normal and denervated soleus fibres. Dibutyryl-cyclic AMP also poten tiated 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 an d tetanic force and abbreviation of tetanic relaxation induced by terb utaline depend on the activation of beta-adrenoceptors and a consequen t increase in the myoplasmic cyclic AMP concentration.