Dual contractile effects of ATP released by field stimulation revealed by effects of alpha,beta-methylene ATP and suramin in rat tail artery.

1. The field stimulation-induced release of endogenous ATP and noradrenaline (NA) and contractile response in rat isolated tail artery were examined. The release of ATP was studied by extracellular electrophysiological recording and that of NA by a novel voltammetrical technique. The effects of the P2-purinceptor antagonist, suramin, on these parameters were compared with those of alpha,beta-methylene ATP, a P2X-purinoceptor desensitizing agent. 2. Neither alpha,beta-methylene ATP (10 microM) nor suramin (100-500 microM) had significant effects on the extracellularly recorded nerve terminal action potential but both abolished the ATP-induced excitatory junction current caused by stimulation at 0.1 Hz. Neither agent affected significantly the voltammetrically measured release of NA induced by 10 or 100 pulses at 20 Hz. 3. Combined blockade of both postjunctional alpha 1- and alpha 2-adrenoceptors by prazosin and yohimbine (both 0.1 microM) profoundly depressed the contractile response to 10 pulses at 20 Hz. The small and fast residual contraction in the presence of these agents was abolished by alpha,beta-methylene ATP (10 microM) and inhibited by suramin in a concentration-dependent manner (10-500 microM; IC50 75 microM) and was hence probably caused by ATP or a related nucleotide. 4. When added first, alpha,beta-methylene ATP (10 microM) or suramin (100-500 microM) delayed the onset and enhanced the amplitude of the neurogenic contraction. This enhanced response was abolished by further addition of prazosin and yohimbine (both 0.1 microM). 5. The K+ channel blocker, tetraethylammonium (10 mM), dramatically enhanced the contractile response to 100 pulses at 1 Hz and caused it to become diphasic. Addition of alpha,beta-methylene ATP (10 microM)or suramin (100-500 microM) abolished the large initial twitch component of this contraction and depressed the tonic phase.6. Like alpha,beta-methylene ATP, suramin (500 microM) had no effect on the contraction caused by exogenous NA (1O nM-l10 microM) or KCI (60 mM); both agents almost abolished the contraction caused by ATP(100 microM).7. In conclusion, (i) the contractile response of rat tail artery to electrical field stimulation is mediated by both ATP and NA, and is thus an expression of ATP-NA co-transmission, (ii) the released ATP exerts two opposite effects via 'P2x-like' purinoceptors, triggering the initial rapid phase of the neurogenic contraction and restricting the NA-mediated component of the contraction; and (iii) the source and possible physiological role of the ATP which causes the inhibitory effect are unknown at present.