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

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Abstract
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+.