Interestingly, the single-channel conductances for channels expressed in heterologous expression systems (38-40 pS) are fairly uniform compared to those observed in neurons, which reveal a variety of brain region-specific differences (see Table 2). In native tissues, the subunit composition is not known but single-cell RT-PCR data have recently provided information about possible types of heterotetramers (Table 2). Differences in conductance between native and heterologously expressed GIRK channels suggest that the subunit composition of native channels may not be symmetric (e.g. two GIRK1, one GIRK2 and one GIRK3) or that there are channels composed of currently unknown subunits. More detailed single channel and biochemical studies are needed to resolve these differences.
