Effects of adrenaline infusion on cAMP and glycogen phosphorylase in fast‐twitch and slow‐twitch rat muscles

CHASIOTIS, D. 1985. Effects of adrenaline infusion on cAMP and glycogen phosphorylase in fast‐twitch and slow‐twitch rat muscles. 1985 Acta Physiol Scand 125 , 537–540. Received 15 February 1985, accepted 28 May 1985. ISSN 0001–6772. Department of Clinical Chemistry II, Karolinska Institutet, Huddinge University Hospital, Huddinge, Sweden. This study investigated the effect of adrenaline infusion on the cAMP content, glycogen phosphorylase activity and the rate of glycogen breakdown in rat extensor digitorum longus (EDL) and soleus muscles. Adrenaline was constantly infused in a dose of 0.15 μg kg ‐1 body wt min ‐1 . The cAMP content increased approximately 2.8‐fold in both muscles after 2 min of infusion. Phosphorylase a 4‐ b activity was six times higher in fast‐twitch muscle (EDL) than in slow‐twitch (soleus) and remained unchanged during the infusion. Phosphorylase a activity increased by 8.4‐fold in EDL and 2.4‐fold in soleus muscles during the infusion period. Glycogen content decreased in EDL muscle by 10% whereas no change was observed in soleus. It is concluded that β‐adrenergic stimulation by adrenaline results in a similar cAMP increase in both muscles. The low rate of glycogen breakdown in EDL and the unchanged content of glycogen in soleus muscle suggest that cAMP mediated transformation of phosphorylase b to a in itself is not adequate for a rapid glycogenosis in muscle phosphorylase.