ADENOSINE EXERTS A GLYCOGEN-SPARING ACTION IN CONTRACTING RAT SKELETAL-MUSCLE

The role of adenosine in regulating glycogen breakdown during electric ally induced muscle contractions was investigated in isolated rat hind quarters perfused with a standard medium either lacking or containing 100 mu U/ml insulin and/or 1.67 nM isoprenaline. Nonselective A(1)/A(2 )-adenosine receptor antagonism via caffeine enhanced (P < 0.05) glyco gen breakdown in contracting fast-oxidative (FO) fibers by 40%, provid ed they were exposed to both insulin and isoprenaline. Combined A(1)/A (2)-receptor antagonism by 8-cyclopentyl-1,3-dipropylxanthine (CPDPX) plus 3,7-dimethyl-1-proparglyxanthine (DMPX) fully reproduced (P < 0.0 5) this stimulatory effect. Furthermore, CPDPX plus BMPX also enhanced (P < 0.05) glycogenolysis during contractions in soleus but not in wh ite gastrocnemius muscle. In contrast, CPDPX or DMPX alone did not aff ect glycogenolysis in either fiber type. Muscle adenosine 3',5'-cyclic monophosphate concentration during contractions was increased (P < 0. 05) by CPDPX plus DMPX in both fiber types, whereas glycogen synthase fractional activity was depressed (P < 0.05). Phosphorylase activity w as not changed by CPDPX plus DMPX. It is concluded that adenosine exer ts a glycogen-sparing action in oxidative skeletal muscle exposed to b oth insulin and beta-adrenergic stimulation during contraction, presum ably via stimulation of glycogen synthase activity.