The effect of P2 receptor antagonists and ATPase inhibition on sympatheticpurinergic neurotransmission in the guinea-pig isolated vas deferens

1 Intracellular microelectrodes were used to record the transmembrane poten tial and excitatory junction potentials (e.j.p.s) produced by sympathetic n erve stimulation (1 Hz) in smooth muscle cells of the guinea-pig isolated v as deferens. 2 The symmetrical 3'-urea of 8-(benzamido)naphthalene-1,3,5-trisulphonic ac id (NF023) produced a concentration-dependent inhibition of e.j.p. magnitud e (IC50 = 4.8 x 10(-6) M), but had no effect on the resting membrane potent ial of the smooth muscle cells. 3 Pyridoxal-5-phosphate (P-5-P) also depressed e.j.p. magnitude in a concen tration-dependent manner, but was less potent than NF023 (IC50 = 2.2 x 10(- 5) M). At 10(-4) M and above P-5-P significantly depolarized the smooth mus cle cells. 4 The nucleoside triphosphatase inhibitor 6-N,N-diethyl-D-beta,gamma-dibrom omethyleneATP (ARL 67156) (5 x 10(-5) M) significantly increased e.j.p. amp litude. ARL 67156 (10(-4) M) further increased e.j.p. amplitude such that t hey often reached threshold for initiation of action potentials, causing mu scle contraction and expulsion of the recording electrode. 5 After reduction of e.j.p.s by NF023 or P-5-P (both 10(-5) M), subsequent co-addition of ARL 67156 (10(-4) M) significantly increased their magnitude . 6 The overflow of endogenous ATP evoked by field stimulation of sympathetic nerves (8 Hz, 1 min) was measured by HPLC and flurometric detection. ARL 6 7156 (10(-4) M) enhanced ATP overflow by almost 700% compared to control. 7 We conclude that for electrophysiological studies NF023 is preferable to other P2X receptor antagonists such as pyridoxalphosphate-6-azophenyl-2',4' -disulphonic acid (PPADS), suramin or P-5-P. Furthermore, breakdown of endo genous ATP by nucleoside triphosphatases is an important modulator of purin ergic neurotransmission in the guinea-pig vas deferens.