Cs+ block of the cardiac muscarinic K+ channel, GIRK1/GIRK4, is not dependent on the aspartate residue at position 173

Cs+ block of GIRK1/GIRK4 expressed in Xenopus oocytes has been investigated . It has been reported that a negatively charged aspartate residue at posit ion 172 in IRK1 is responsible for Cs+ block of the channel. IRK1, a homote tramer, has four aspartate residues at this: position. GIRK1/GIRK4 is a het erotetramer and has two aspartate residues at the equivalent position (GIRK 1-D173) and, consequently, it should be less sensitive to Cs+. Cs+ caused v oltage-dependent block of GIRK1/GIRK4 current (measured with the two-microe lectrode voltage-clamp technique). The apparent fraction of the electrical field through which Cs+ moves in order to reach its site of block (delta co ngruent to1.66) is comparable to that in IRK1, suggesting that Cs+ binds to a similar site in the two channels. GIRK1/GIRK4 was less sensitive than IR K1 to Cs+ - the K-d was 3.0-8.5 times greater and at potentials more negati ve than congruent to -130 mV there was voltage-dependent relief of block of GIRK1/GIRK4 (not the case with IRK1). However, the mutations GIRK1-D173A a nd GIRK1-D173Q increased the sensitivity of the channel to Cs+, while addin g a negatively charged aspartate residue to GIRK4 at the equivalent positio n (GIRK4-N179D) decreased Cs+ sensitivity. GIRK1-D173 cannot be the site of Cs+ block of GIRK1/GIRK4.