Ion permeation properties of a cloned human 5-HT3 receptor transiently expressed in HEK 293 cells

Human 5-HT3 receptors expressed in HEK 293 cells were studied using patch-c lamp techniques. The permeability ratios of cations to Na+ were Li+, 1.16; K+, 1.04; Rb+, 1.11; Cs+, 1.11; NMDG(+), 0.04; Ca2+, 0.49, and Mg2+, 0.37. The permeability sequence of the alkali metal cations was Li+ > Rb+ = Cs+ > K+ > Na+. Increased external concentrations of Ca2+ or Mg2+ decreased 5-HT -induced currents at all potentials tested in a voltage-independent manner. The single-channel conductance of human 5-HT3 receptors measured by fluctu ation analysis of whole-cell currents was 790 +/- 100 fS. Differences in th e basic properties of 5-HT3 receptors between species may explain interspec ies differences in pharmacological properties.