G protein-activated (GIRK) current in rat ventricular myocytes is masked by constitutive inward rectifier current (IK1)

Inwardly-rectifying K+ channel subunits are not homogenously expressed in different cardiac tissues. In ventricular myocytes (VM) the background current-voltage relation is dominated by I-K1, carried by channels composed of Kir2.x subunits, which is less important in atrial myocytes (AM). On the other hand in AM a large G protein gated current carried by Kir3.1/3.4 complexes can be activated by stimulation of muscarinic M-2 receptors (I-K(ACh)), which is assumed to be marginal in VM. Recent evidence suggests that total current carried by cardiac inward-rectifiers (I-K(ATP), I-K(ACh), I-K1) in both, AM and VM is limited, due to K+ accumulation/ depletion. This lead us to hypothesize that in conventional whole cell recordings I-K(ACh) in VM is underestimated as a consequence of constitutive I-K1. In that case a reduction in density of I-K1 should be paralleled by an increase in density of I-K(ACh). Three different experimental strategies have been used to test for this hypothesis: (i) Adenovirus-driven expression of a dominant-negative mutant of Kir2.1, one of the subunits supposed to form I-K1 channels, in VM caused a reduction in I-K1-density by about 80 %. In those cells I-K(ACh) was increased about 4 fold. ( ii) A comparable increase in I-K(ACh) was observed upon reduction of I-K1 caused by adenovirus-mediated RNA interference.(iii) Ba2+ in a concentration of 2 mu M blocks I-K1 in VM by about 60 % without affecting atrial I-K(ACh). The reduction in I-K1 by 2 mu M Ba2+ is paralleled by a reversible increase in I-K(ACh) by about 100%. These data demonstrate that the increase in K+ conductance underlying ventricular I-K(ACh) is largely underestimated, suggesting that it might be of greater physiological relevance than previously thought. copyright (c) 2008 S. Karger AG, Basel.