Inhibition by ethanol of contractions of rat vas deferens: no evidence for selective blockade of P_2X-purinoceptors

Summary

Some ligand-gated ion channels are important sites of action of ethanol. The aim of the study was to find out whether the P_2X-purinoceptors mediating contraction of the rat isolated vas deferens also are selectively sensitive to ethanol. Contractions were elicited by ATP (1 mmol/1), α,ß-methylene ATP (0.3 μmol/1), noradrenaline (3 μmol/1), high K⁺ (20 mmol/1) or electrical (neural) stimulation by pairs of pulses 3 s apart. In electrical stimulation experiments, purinergic and adrenergic response components were isolated by prazosin and suramin, respectively. Concentration-effect curves were determined for ethanol and, for comparison, nifedipine. Tritium outflow from tissues preincubated with ³H-noradrenaline was also examined.

Ethanol at relatively low concentrations reduced contractions elicited by high K⁺ (IC₃₀ 145 mmol/1), ATP (IC₃₀ 211 mmol/1) and α,ß-methylene ATP(IC₃₀ 215 mmol/1) as well the purinergic component of neurogenic twitches (IC₃₀ 110-126 mmol/1; a significant effect at 10–32 mmol/1) and the adrenergic component of twitch 2 of the twitch pairs (IC₃₀ 63 mmol/1). These contractions also were very sensitive to nifedipine. Higher concentrations of ethanol were needed to reduce contractions elicited by noradrenaline (IC₃₀ 365 mmol/1) and the adrenergic component of twitch 1 of the twitch pairs (IC₃₀ 382 mmol/1), contractions that also were less sensitive to nifedipine. Ethanol 1 mol/l abolished all contractions. In contrast, concentration-effect curves for the inhibition by nifedipine of contractions evoked by ATP, α,ß-methylene ATP and noradrenaline (rapid phase) levelled off at 60–70% inhibition. The contractions that remained when these agonists were administered in the presence of nifedipine 10 μmol/l were depressed by ethanol (IC₃₀ 242–387 mmol/1). Ethanol 320 mmol/1 did not change the electrically evoked overflow of tritium from vasa deferentia preincubated with ³H-noradrenaline.

It is concluded that the P_2X-purinoceptors of rat vas deferens smooth muscle, although ligand-gated ion channels, are not selectively sensitive to ethanol. The reduction of contractions can be explained by, first, an inhibition of L-type voltage-dependent Ca²⁺ channels for which relatively low concentrations of ethanol are needed, and second, a “non-specific” depressant effect at an unknown site or at unknown sites which requires relatively high concentrations.