| Abstract: | 1 The effects of adenosine, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloroadenosine, 2-azidoadenosine, and their L-enantiomers were examined on driven left atria, trachea and transmurally stimulated ileum of the guinea-pig. 2 In each tissue the order of potency of the D-enantiomers for producing inhibitory effects was NECA greater than 2-chloroadenosine greater than 2-azidoadenosine greater than adenosine. 3 The log concentration-response curve of each agonist was shifted to the right in the presence of the P1-purinoceptor antagonist, theophylline. 4 Dipyridamole, which blocks adenosine uptake, potentiated the effects of adenosine but not those of the D-enantiomers of adenosine analogues. 5 The greater potency of the adenosine analogues therefore, is at least partly due to their resistance to tissue uptake and subsequent enzymatic destruction. 6 The L-enantiomers of adenosine and its analogues did not produce inhibitory responses in the driven left atria or transmurally stimulated ileum. At high concentrations relaxations of the tracheal muscle were obtained, with the potency series L-NECA greater than 2-chloro-L-adenosine greater than 2-azido-L-adenosine greater than L-adenosine. 7 It is concluded that the postsynaptic P1-purinoceptors in the guinea-pig atria and trachea and the presynaptic P1-purinoceptors on cholinergic nerve terminals in guinea-pig ileum are stereospecific for the D-enantiomers of adenosine and its analogues. |
