Relaxation gating of the acetylcholine-activated inward rectifier K+ current is mediated by intrinsic voltage sensitivity of the muscarinic receptor.

Normal heart rate variability is critically dependent upon the G-protein-coupled, acetylcholine (ACh)-activated inward rectifier K+ current, I(KACh). A unique feature of I(KACh) is the so-called ‘relaxation' gating property that contributes to increased current at hyperpolarized membrane potentials. I(KACh) relaxation refers to a slow decrease or increase in current magnitude with depolarization or hyperpolarization, respectively. The molecular mechanism underlying this perplexing gating behaviour remains unclear. Here, we consider a novel explanation for I(KACh) relaxation based upon the recent finding that G-protein-coupled receptors (GPCRs) are intrinsically voltage sensitive and that the muscarinic agonists acetylcholine (ACh) and pilocarpine (Pilo) manifest opposite voltage-dependent I(KACh) modulation. We show that Pilo activation of I(KACh) displays relaxation characteristics opposite to that of ACh. We explain the opposite effects of ACh and Pilo using Markov models of I(KACh) that incorporate ligand-specific, voltage-dependent parameters. Based on experimental and computational findings, we propose a novel molecular mechanism to describe the enigmatic relaxation gating process: I(KACh) relaxation represents a voltage-dependent change in agonist affinity as a consequence of a voltage-dependent conformational change in the muscarinic receptor.