Activation of the A1 adenosine receptor increases insulin-stimulated glucose transport in isolated rat soleus muscle

The A, adenosine receptor (AIAR) has been suggested to participate in insulin- and contraction-stimulated glucose transport in skeletal muscle, but the qualitative and quantitative nature of the effect are controversial. We sought to determine if AIAR is expressed in rat soleus muscle and then characterize its role in glucose transport in this muscle. AIAR mRNA and protein expression were determined by RT-PCR and Western blotting, respectively. To examine the role of adenosine in 3-O-methylglucose transport, isolated muscles were exposed to adenosine deaminase and cl,p-methylene adenosine diphosphate to remove endogenous adenosine and were left unstimulated (basal) or stimulated with insulin. To assess the functional participation of AIAR in 3-O-methylglucose transport, muscles were incubated with A(1)-selective agonist and (or) antagonist in the absence of endogenous adenosine and with or without insulin. AIAR mRNA was expressed in soleus muscle and A(1)AR was present at the plasma membrane. Removal of endogenous adenosine reduced glucose transport in response to 100 mu U/mL insulin (similar to 50%). The A(1)-selective agonist, N-6-cyclopentyladenosine, increased submaximal (100 mu U/mL) insulin-stimulated glucose transport in a dose-dependent manner (0.001-1.0 mu mol/L). This stimulatory effect was inhibited by the A(1)-selective receptor antagonist 1,3-dipi-opyl-8-cyclopentylxinthine in a concentration-dependent manner (0.001-1.0 mu mol/L). However, neither activation nor inhibition of AIAR altered basal or maximal (10 mU/rnL) insulin-stimulated glucose transport. Our results suggest that adenosine contributes similar to 50% to insulin-stimulated muscle glucose transport by activating the AIAR. This effect is limited to increasing insulin sensitivity, but not to either basal or maximal insulin-stimulated glucose uptake in rat soleus muscle.