Intravenous anaesthetics inhibit nicotinic acetylcholine receptor-mediated currents and Ca2+ transients in rat intracardiac ganglion neurons
The effects of intravenous (i.v.) anaesthetics on nicotinic acetylcholine receptor (nAChR)-induced transients in intracellular free Ca 2+ concentration ([Ca 2+] i) and membrane currents were investigated in neonatal rat intracardiac neurons. In fura-2-loaded neurons, nAChR activation evoked a transient increase in [Ca 2+] i, which was inhibited reversibly and selectively by clinically relevant concentrations of thiopental. The half-maximal concentration for thiopental inhibition of nAChR-induced [Ca 2+] i transients was 28 μM, close to the estimated clinical EC 50 (clinically relevant (half-maximal) effective concentration) of thiopental. In fura-2-loaded neurons, voltage clamped at -60 mV to eliminate any contribution of voltage-gated Ca 2+ channels, thiopental (25 μM) simultaneously inhibited nAChR-induced increases in [Ca 2+] i and peak current amplitudes. Thiopental inhibited nAChR-induced peak current amplitudes in dialysed whole-cell recordings by ∼ 40% at -120, -80 and -40 mV holding potential, indicating that the inhibition is voltage independent. The barbiturate, pentobarbital and the dissociative anaesthetic, ketamine, used at clinical EC 50 were also shown to inhibit nAChR-induced increases in [Ca 2+], by ∼40%. Thiopental (25 μM) did not inhibit caffeine-, muscarine- or ATP-evoked increases in [Ca 2+] i, indicating that inhibition of Ca 2+ release from internal stores via either ryanodine receptor or inositol-1,4,5-trisphosphate receptor channels is unlikely. Depolarization-activated Ca 2+ channel currents were unaffected in the presence of thiopental (25 μM), pentobarbital (50 μM) and ketamine (10 μM). In conclusion, i.v. anaesthetics inhibit nAChR-induced currents and [Ca 2+] i transients in intracardiac neurons by binding to nAChRs and thereby may contribute to changes in heart rate and cardiac output under clinical conditions.