[(3)H]Glycogen hydrolysis elicited by adenosine in rabbit retina: involvement of A(2)-receptors.
Adenosine promotes a concentration-dependent hydrolysis of [(3)H]glycogen newly synthesised from [(3)H]glucose by rabbit retinal slices. The EC(50) is 15 ?M. Theophylline and 8-phenyltheophylline antagonise the glycogenolysis induced by adenosine. The rank order of efficacy of adenosine agonists is chloroadenosine (CLA) and adenosine > d - phenylisopropyl - adenosine ( d - PIA ) > l - PIA > N- ethyl - carboxamidoadenosine ( NECA ) > cyclohexyladenosine (CHA), suggesting that adenosine promotes glycogenolysis via receptors of the A(2) type. This is supported by the finding that the rank order of effectiveness of adenosine agonists for stimulating C-AMP production is similar to that found for glycogenolysis though NECA is more efficacious than d-PIA and l-PIA. In addition, the weak stereospecificity observed for the glycogenolytic action d-PIA and l-PIA provides further support. It is suggested that some A(2)-receptors exist on Müller cells of the retina and that adenosine has a modulatory role in the retina, coupling neuronal activity and energy metabolism in the Müller cells.
Osborne
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Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford OX2 6AW, England.