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

1 Intracellular microelectrodes were used to record the transmembrane potential and excitatory junction potentials (e.j.p.s) produced by sympathetic nerve stimulation (1 Hz) in smooth muscle cells of the guinea-pig isolated vas deferens. 2 The symmetrical 3'-urea of 8-(benzamido)naphthalene-1,3,5-trisulphonic acid (NF023) produced a concentration-dependent inhibition of e.j.p. magnitude (IC50 = 4.8 x 10(-6) M), but had no effect on the resting membrane potential of the smooth muscle cells. 3 Pyridoxal-5-phosphate (P-5-P) also depressed e.j.p. magnitude in a concentration-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 muscle cells. 4 The nucleoside triphosphatase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156) (5 x 10(-5) M) significantly increased e.j.p. amplitude. ARL 67156 (10(-4) M) further increased e.j.p. amplitude such that they often reached threshold for initiation of action potentials, causing muscle 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 67156 (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 endogenous ATP by nucleoside triphosphatases is an important modulator of purinergic neurotransmission in the guinea-pig vas deferens.