Differential subunit composition of the G protein–activated inward-rectifier potassium channel during cardiac development

Parasympathetic slowing of the heart rate is predominantly mediated by acetylcholine-dependent activation of the G protein–gated potassium (K(+)) channel (I(K,ACh)). This channel is composed of 2 inward-rectifier K(+) (Kir) channel subunits, Kir3.1 and Kir3.4, that display distinct functional properties. Here we show that subunit composition of I(K,ACh) changes during embryonic development. At early stages, I(K,ACh) is primarily formed by Kir3.1, while in late embryonic and adult cells, Kir3.4 is the predominant subunit. This change in subunit composition results in reduced rectification of I(K,ACh), allowing for marked K(+) currents over the whole physiological voltage range. As a consequence, I(K,ACh) is able to generate the membrane hyperpolarization that underlies the strong negative chronotropy occurring in late- but not early-stage atrial cardiomyocytes upon application of muscarinic agonists. Both strong negative chronotropy and membrane hyperpolarization can be induced in early-stage cardiomyocytes by viral overexpression of the mildly rectifying Kir3.4 subunit. Thus, a switch in subunit composition is used to adopt I(K,ACh) to its functional role in adult cardiomyocytes.