Rectification of muscarinic K+ current by magnesium ion in guinea pig atrial cells.
Rectifying properties of the acetylcholine (ACh)-sensitive K+ channels were studied using a patch-clamp method in single atrial cells prepared enzymatically from adult guinea pig hearts. In the presence of micromolar concentration of nonhydrolyzable guanosine 5'-triphosphate (GTP) analogue 5'-guanylylimidodiphosphate (GppNHp) and the absence of Mg2+ at the inner surface of patch membrane [( Mg2+]i), the channel activity recovered in inside-out patch condition. The single channel conductance became ohmic between -80 and +80 mV (symmetrical 150 mM K+ solutions). The rapid relaxation of outward single channel currents was disclosed on a depolarization. [Mg2+]i blocked the outward current through the channel dose- and voltage-dependently and also induced a dose-dependent increase in the channel activation. The apparent paradoxical role of [Mg2+]i is important for the cholinergic control in the heart; voltage-dependent Mg block ensures a low K+ conductance of cell membrane at the plateau of action potentials during the exposure to ACh, thereby slowing the heart rate without unfavorable shortening of the action potentials.
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D004553:Electric Conductivity; D006168:Guinea Pigs; D006325:Heart Atria; D007473:Ion Channels / Q000502:physiology*; D007477:Ions; D008274:Magnesium / Q000494:pharmacology*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism* / Q000502:physiology
D007473:Ion Channels; D007477:Ions; D008274:Magnesium; D000109:Acetylcholine; D011188:Potassium