Evidence for an A2-subtype adenosine receptor on pancreatic glucagon secreting cells

The effects of a 5'-substituted analogue of adenosine, 5'-N-ethylcarboxamidoadenosine (NECA) have been studied on glucagon secretion in vitro, using the isolated pancreas of the rat perfused in the presence of glucose (2.8 mM). NECA provoked a peak of glucagon secretion, the kinetics of which were comparable to those previously obtained with adenosine. The effect was concentration-dependent and appeared at nanomolar concentrations. The EC50 was approximately 4 X 10(-8) M. A comparison of relative potency between adenosine and NECA showed that NECA was about 800 fold more potent than adenosine in inducing glucagon secretion. Theophylline (50 microM) considerably decreased the peak of glucagon secretion induced by 1.65 microM NECA and totally suppressed the effect of 16.5 nM NECA. These results indicate the involvement of an adenosine receptor. These and other previous results (low stereoselectivity of N6-phenylisopropyladenosine) provide evidence for an adenosine receptor of the A2-subtype being involved in glucagon secretion.