Inhibitory and Facilitatory Effects of Purines on Transmitter Release from Sympathetic Nerves1

The effects of several punnoceptor agonists and antagonists on ingly, the nucleotide and nucleoside agonists facilitated, rather norepinephnne overflow were examined in the electrically field than inhibited, the electrical field stimulation-evoked release of stimulated vas deferens and saphenous artery ofthe rabbit. Both norepinephrine from the in vitro perfused saphenous artery. This the adenine nucleotides AlP and 3,-y-methylene ATP and the facilitation was antagonized by 8-(p-sulfophenyl)-theophylline but adenine nucleosides adenosine and 2-chloroadenosine reduced not by aj9-methylene ATP. These results indicate that there may the electrical field stimulation-evoked release of norepinephnne be facilitatory prejunctional purinoceptors that exhibit structure-from the in vitro vas deferens. These responses are antagonized activity relationships similar to, but perhaps not identical to, those by both 8-(p-sulfophenyl)-theophylline and aj9-methylene ATP, exhibited by inhibitory prejunctional purinoceptors. indicating that this effect is mediated by P3-receptors. Interest-At a number of adrenergic neuroeffector junctions, ATP is a receptors for adenosine or of the P2 receptors for ATP. Instead cotransmitter with NE (see reviews by Burnstock, 1990; West- we refer to these receptors as P3 (Shinozuka et al., 1988). Several fail et at., 1991). An extension of the idea that ATP is a characteristics of these prejunctional purinoceptors have cotransmitter with NE is the possibility that the nucleotide emerged. For example, adenosine and ATP are roughly equi-may also modulate transmitter release via prejunctional pun- potent, the responses to both nucleosides and nucleotides ar