Activation of P1‐ and P2Y‐purinoceptors by ADP‐ribose in the guinea‐pig taenia coli, but not of P2X‐purinoceptors in the vas deferens

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Abstract
The activity of adenosine 5′‐diphosphoribose (ADP‐ribose), a ribosylated purine nucleotide, was investigated on the carbachol‐contracted taenia coli, a tissue possessing P1‐ (A2) and P2Y‐purinoceptors and on the guinea‐pig vas deferens which possesses P2X‐purinoceptors. In the vas deferens, where ATP (1 μm‐1 mm) produced concentration‐dependent contractions, ADP‐ribose was without effect at concentrations up to 1 mm. In the taenia coli, ADP‐ribose (0.1 μm‐1 mm) produced concentration‐dependent relaxations with a potency similar to that of adenosine, but less than that of ATP. The pD2 values for ADP‐ribose, adenosine and ATP were 4.5 ± 0.07 (27), 4.4 ± 0.10 (9) and 5.5 ± 0.14 (21), respectively. The time‐course of the relaxations elicited by ADP‐ribose was found to be significantly longer than that for ATP and significantly shorter than that for adenosine. The P1‐purinoceptor antagonist, 8‐phenyltheophylline (5 μm), produced parallel rightward shifts in the concentration‐response curves of the relaxations of the taenia coli elicited by ADP‐ribose and adenosine but not ATP. Dipyridamole (0.3 μm), a purine nucleoside uptake inhibitor, potentiated the responses to adenosine and ADP‐ribose in the taenia coli. These potentiations were sensitive to 8‐phenyltheophylline (5 μm).6 Reactive blue 2, a P2Y‐purinoceptor antagonist, antagonized the inhibitory responses of ADP‐ribose and ATP in the taenia coli, without significantly altering the inhibitory responses of either adenosine or noradrenaline.7 In the presence of the potassium channel blocker, apamin (0.3 μm), the inhibitory responses of ADP‐ribose were severely attenuated, and the inhibitory responses of ATP in the taenia coli were converted to transient contractions. Further addition of 8‐PT blocked the residual responses of ADP‐ribose.8 The P2‐purinoceptor antagonist, suramin (500 μm), antagonized responses to ATP and ADP‐ribose, but not adenosine. Further addition of 8‐PT antagonized the residual responses to ADP‐ribose, but not to ATP.9 It is concluded that ADP‐ribose has a mixed pharmacological profile, evoking both P1 (A2)‐purinoceptor‐mediated responses and P2Y‐purinoceptor‐mediated responses, while being inert at P2X‐purinoceptors. It is suggested that ADP‐ribose may provide a useful starting point for the generation of structural analogues which have specific activity at the P2Y‐purinoceptor.