P-2 PURINOCEPTOR ANTAGONIST PROPERTIES OF PYRIDOXAL-5-PHOSPHATE

The antagonist properties of pyridoxal-5-phosphate, a synthesis precursor of pyridoxalphosphate-6-azophenyl-2',4'-dis acid, were investigated on P-2 purinoceptor-mediated responses of the rat isolated vagus nerve and vas deferens. In addition, the efect of this agent was studied on high affinity tritiated alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) binding to rat vas deferens membranes, thought to represent binding to functional P-2X purinoceptors. In the rat vagus nerve, pyridoxal-5-phosphate (10(-5)-10(-4) M) produced concentration-related antagonism of depolarisation responses induced by alpha,beta-meATP, measured using an extracellular recording technique. In contrast, depolarisation responses to 5-hydroxytryptamine (5-HT) were unaffected by pyridoxal-5-phosphate. In the rat vas deferens, pyridoxal-5-phosphate (10(-5)-10(-4) M) antagonised contractile responses produced by alpha,beta-meATP while contractions to phenylephrine were unaffected. However, responses of the vagus nerve and the vas deferens to alpha,beta-meATP were not antagonised by pyridoxal hydrochloride (10(-4) M). Pyridoxal-5-phosphate competed for high affinity binding of [H-3]cr,beta-meATP to homogenised membranes of the rat vas deferens with a pK(i) estimate of 4.91 +/- 0.12 and a Hill slope of 0.80 +/- 0.03. Pyridoxal hydrochloride only competed for binding at concentrations in excess of 10(-4) M, yielding a pK(i) estimate of 3.21 +/- 0.04 and a Hill slope of 1.82 +/- 0.12. These findings indicate that pyridoxal-5-phosphate acts as a specific antagonist of P-2 purinoceptors in the vagus nerve and vas deferens of the rat and that the phosphate moiety is required for activity. From the radioligand binding studies it seems likely that pyridoxal-5-phosphate binds to the same receptor as alpha,beta-meATP.