Ion permeation and conduction in a human recombinant 5‐HT3 receptor subunit (h5‐HT3A)

1 A human recombinant homo‐oligomeric 5‐HT3 receptor (h5‐HT3A) expressed in a human embryonic kidney cell line (HEK 293) was characterized using the whole‐cell recording configuration of the patch clamp technique. 2 5‐HT evoked transient inward currents (EC50= 3.4 μM; Hill coefficient = 1.8) that were blocked by the 5‐HT3 receptor antagonist ondansetron (IC50= 103 pM) and by the non‐selective agents metoclopramide (IC50= 69 nM), cocaine (IC50= 459 nM) and (+)‐tubocurarine (IC50= 2.8 μM). 3 5‐HT‐induced currents rectified inwardly and reversed in sign (E5‐HT) at a potential of −2.2 mV. N‐Methyl‐D‐glucamine was finitely permeant. Permeability ratios PNa/PCs and PNMDG/PCs were 0.90 and 0.083, respectively. 4 Permeability towards divalent cations was assessed from measurements of E5‐HT in media where Ca2+ and Mg2+ replaced Na+. PCa/PCs and PMg/PCs were calculated to be 1.00 and 0.61, respectively. 5 Single channel chord conductance (γ) estimated from fluctuation analysis of macroscopic currents increased with membrane hyperpolarization from 243 fS at −40 mV to 742 fS at −100 mV. 6 Reducing [Ca2+]o from 2 to 0.1 mM caused an increase in the whole‐cell current evoked by 5‐HT. A concomitant reduction in [Mg2+]o produced further potentiation. Fluctuation analysis indicates that a voltage‐independent augmentation of γ contributes to this phenomenon. 7 The data indicate that homo‐oligomeric receptors composed of h5‐HT3A subunits form inwardly rectifying cation‐selective ion channels of low conductance that are permeable to Ca2+ and Mg2+.