P2 purinoceptor antagonist properties of pyridoxal-5-phosphate.
The antagonist properties of pyridoxal-5-phosphate, a synthesis precursor of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, were investigated on P2 purinoceptor-mediated responses of the rat isolated vagus nerve and vas deferens. In addition, the effect 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 P2x 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 [3H]alpha,beta-meATP to homogenised membranes of the rat vas deferens with a pKi 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 pKi 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 P2 purinoceptors in the vagus nerve and vas deferens of the rat and that the phosphate moiety is required for activity.(ABSTRACT TRUNCATED AT 250 WORDS)
Trezise
D J
DJ
Glaxo Institute of Applied Pharmacology, Department of Pharmacology, University of Cambridge, UK.
Bell
N J
NJ
Khakh
B S
BS
Michel
A D
AD
Humphrey
P A
PA
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D058919:Purinergic P2 Receptor Antagonists; D011732:Pyridoxal Phosphate / Q000494:pharmacology*; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D014630:Vagus Nerve / Q000502:physiology; D014649:Vas Deferens / Q000187:drug effects
D058919:Purinergic P2 Receptor Antagonists; D011732:Pyridoxal Phosphate; D000255:Adenosine Triphosphate; C002630:alpha,beta-methyleneadenosine 5'-triphosphate