In heart, ACh binds to the m2 receptor, leading to the dissociation of a G protein into G_α and G_βγ. Binding of G_βγ to the muscarinic K⁺ channel (a heteromultimer of Kir3.1 and Kir3.4) results in channel activation by an unknown mechanism. Although Sadja et al. (2001) have suggested that channel activation results from an opening of the channel’s inner vestibule, data from our laboratory suggest a different mechanism. It is likely that the mutation Kir3.4-E145Q alters the channel’s selectivity filter, because it abolishes K⁺ selectivity (Dibb et al. 2001) (furthermore, comparison with the crystal structure of the KcsA K⁺ channel suggests that Kir3.4-E145 is located immediately behind the selectivity filter). The mutation Kir3.4-E145Q also reduces Ba²⁺ block of the channel (Lancaster et al. 2000). These results suggest that Ba²⁺ blocks the channel by binding within the selectivity filter. Previously we have shown that Ba²⁺ is an open channel blocker, i.e. Ba²⁺ only blocks in the presence of ACh (Zang et al. 1995). This suggests that Ba²⁺ has access to its binding site (selectivity filter) only in the presence of ACh. This in turn suggests that the selectivity filter is only open in the presence of ACh. If ACh activation of the channel involves the selectivity filter, the change in the selectivity filter by mutation Kir3.4-E145Q may also affect channel activation. Kir3.1, Kir3.4 and the dopamine D₂ receptor (another G protein-coupled receptor) were expressed in oocytes from humanely killed Xenopus toads. Figure 1 shows current-voltage relationships in the absence and presence of dopamine. With the wild-type channel (Fig. 1A), there was current in the absence of dopamine and application of dopamine resulted in a substantial increase in current.With the mutant channel (Kir3.1 and Kir3.4-E145Q) there was again current in the absence of dopamine, but application of dopamine did not result in an increase – this suggests that the mutant channel is constitutively active. This is consistent with the hypothesis that agonist activation of the muscarinic K⁺ channel is the result of the opening of the selectivity filter of the channel.

Figure 1. Current-voltage relationships for wild-type (A) and mutant (B) channels in the absence and presence of 10 µM dopamine. Currents measured at end of 750 ms pulses to potentials shown (holding potential, 0 mV; 90 mM extracellular K+). Means ± S.E.M. (n = 6-7).

Dibb, K.M., Leach, R., Findlay, J.B.C. & Boyett, M.R. (2001). Biophys. J. 80, 624a.

Lancaster, M.K., Dibb, K.M., Quinn, C., Leach, R., Lee, J.-K., Findlay, J.B.C. & Boyett, M.R. (2000). J. Biol. Chem. 275, 35831-35839.

Sadja, R., Smadja, K., Alagem, N. & Reuveny, E. (2001). Neuron 29, 669-680.

Zang, W.-J., Yu, X.-J. & Boyett, M.R. (1995). Pflƒgers Arch. 430, 348-357.