FAILURE OF TYRAMINE TO RELEASE NEURONAL ATP AS A COTRANSMITTER OF NORADRENALINE IN THE GUINEA-PIG VAS-DEFERENS

Contractions, release of noradrenaline and release of ATP elicited by the indirectly acting sympathomimetic amine tyramine and responses eli cited by exogenous noradrenaline were studied in the isolated vas defe rens of the guinea pig. Release of noradrenaline was assessed as overf low of tritium after preincubation with [H-3]-noradrenaline. ATP was m easured by means of the luciferin-luciferase technique. In tissues pre treated with pargyline 1 mM, tyramine 300 mu M, when added to the supe rfusion medium for 2 min, elicited contraction and an overflow of trit ium (mainly [H-3]-noradrenaline) and ATP. Contraction and ATP overflow responses were prevented and tritium overflow was greatly reduced by desipramine 10 mu M. Prazosin 0.3 CIM abolished contractions and evoke d ATP overflow without changing tritium overflow. Blockade of postjunc tional P-2,-purinoceptors by suramin 300 mu M caused a marked decrease of tyramine-evoked contractions and a slight reduction of tritium ove rflow whereas evoked ATP overflow was markedly increased. The effect o n contraction was not shared by two other P,(2)-purinoceptor antagonis ts, namely pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS ) 32 mu M and diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) 32 mu M: PPADS increased contractions about fourfold, whilst DIDS had no effect at all. When the vas deferens was superfused for 24 min with me dium containing tyramine 300 CIM, evoked contractions and tritium over flow continued throughout whereas ATP overflow faded rapidly to basal values. In the presence of prazosin 0.3 mu M, tyramine 300 mu M again failed to elicit contractions as well as an overflow of ATP. Applicati on of noradrenaline 10 mu M instead of tyramine also resulted in prolo nged contraction and an overflow of ATP that declined rapidly. It is c oncluded that all ATP released by tyramine is non-neuronal in origin, secondary to the activation of postjunctional alpha(1)-adrenoceptors b y released noradrenaline. The non-neural ATP does not seem to play a f unctional role in smooth muscle contraction and derives from a postjun ctional source which is subject to a rapid depletion upon sustained al pha(1)-adrenoceptor activation.