Co-transmitter mediated facilitation by sympathetic nerve stimulation of evoked acetylcholine release from the rabbit perfused atria preparation

Rabbit atria were isolated with the extrinsic right sympathetic and vagus nerves attached and perfused with Tyrode solution. Acetylcholine overflow was determined after labelling of the transmitter stores with ¹⁴C]choline and fractionation of the radioactivity on cation exchange columns. Sympathetic nerve stimulation (SNS, 2 Hz, 3 min) carried out together with vagus nerve stimulation (VNS, 2 Hz, 3 min), but each SNS pulse preceding a vagal one by 19 ms, caused a facilitation of acetylcholine overflow of about 60% versus independent controls in the absence of SNS. Antagonists of putative neurotransmitters were tested to find out the prejunctional mediator involved in the facilitation.The facilitation was not significantly reduced by prazosin, rauwolscine, idazoxan, or propranolol, excluding mediation by - or \-adrenoceptors. However, guanethidine abolished evoked noradrenaline release and facilitation, suggesting that it is due to a compound co-released with noradrenaline from postganglionic noradrenergic nerves. Pretreatment of rabbits with reserpine which reduced noradrenaline content of atria and SNS evoked overflow by 94% did not affect the facilitation of acetylcholine release which, due to the cardiostimulatory action of SNS being absent, resulted in enhanced depression of atrial force. We conclude that the facilitation is due to release of a reserpine-resistant co-transmitter from sympathetic nerves.Possible mediation of the facilitation by ATP through P_2X- or P_2Y-purinoceptors was excluded by ineffectiveness of , \-methylene ATP-preperfusion, of suramin and cibacron blue, respectively. However, the selective A₂ adenosine receptor antagonist CP 66,713 reduced the facilitation by 25% whereas DPCPX (A₁-selective) had no effect. Of the non-subtype-selective antagonists only 8-phenyltheophylline but not XAC decreased the facilitation by 40%. Mediation of the facilitation by tachykinin-, angiotensin-, opioid, AMPA/kainate-, M₁ muscarinic, 5-hydroxytryptamine 5-HT_1–4-receptors, or by nitric oxide, was excluded by administration of respective antagonists or inhibitors. Thus, whilst adenosine seems to be responsible for about one-fourth of the effect of sympathetic nerve stimulation, the major part of the facilitation of acetylcholine release remains unexplained.Under conditions of a low rate of evoked acetylcholine overflow prazosin enhanced the facilitatory action of SNS, suggesting an ₁-adrenoceptor mediated prejunctional inhibition of acetylcholine release.