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

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Abstract
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. 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=4.8×10−6 M), but had no effect on the resting membrane potential of the smooth muscle cells. Pyridoxal‐5‐phosphate (P‐5‐P) also depressed e.j.p. magnitude in a concentration‐dependent manner, but was less potent than NF023 (IC50=2.2×10−5 M). At 10−4 M and above P‐5‐P significantly depolarized the smooth muscle cells. The nucleoside triphosphatase inhibitor 6‐N,N‐diethyl‐D‐β,γ‐dibromomethyleneATP (ARL 67156) (5×10−5 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. 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. 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. 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. British Journal of Pharmacology (2000) 129, 1089–1094; doi:10.1038/sj.bjp.0703163