Dual effects of intracellular magnesium on muscarinic potassium channel current in single guinea-pig atrial cells

J Physiol. 1989 Jan;408:313-32. doi: 10.1113/jphysiol.1989.sp017461.

Abstract

1. The effects of internal Mg2+ ions on the muscarinic acetylcholine (ACh) receptor-mediated K+ currents were investigated in single atrial cells of guinea-pigs, using the whole-cell and inside-out modes of the patch-clamp technique. 2. During cell dialysis in the whole-cell-clamp condition, the depletion of internal Mg2+ increased outward muscarinic K+ currents but decreased inward currents, thereby reducing the inwardly rectifying property of the channels. 3. When inside-out patches were prepared, channel availability was abolished and was reactivated by internal application of guanosine 5'-triphosphate (GTP) or its non-hydrolysable analogue, 5'-guanylyl imidodiphosphate (GppNHp), in the presence of Mg2+. GppNHp led to a recovery of the channels also in the nominal absence of Mg2+ (0[Mg2+]i). 4. The activation of single-channel currents by intracellular GTP and Mg2+ was dose-dependent. Both concentration-response curves were fitted by saturation kinetics with Hill coefficients of 1, and the half-maximum doses were 24 +/- 8 microM for GTP and 67 +/- 14 microM for Mg2+. The effects of Mg2+ on activation of K+ currents were additive with those of GTP, suggesting the presence of two independent binding sites for GTP and Mg2+. 5. The single-channel conductance became virtually ohmic when measured at nominally zero [Mg2+]i while GppNHp was used to recover the channel activity. Micromolar [Mg2+]i reduced the unitary amplitude of single open-channel currents in a dose- and voltage-dependent manner, showing half-blocking doses of 293 microM at +40 mV and 115 microM at +60 mV. 6. Voltage-dependent kinetics of Mg2+ block were described using equations based on Eyring rate theory (Woodbury, 1971; Hille, 1984), where the coefficient for voltage dependence (delta) was 0.63. 7. Intracellular Mg2+, at a physiological concentration, has a dual action on the muscarinic K+ channel: first Mg2+ activates the channel in the presence of GTP through GTP-binding proteins (G proteins), and secondly it blocks outward currents through the channel, thereby causing the inwardly rectifying property.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcholine / pharmacology
  • Animals
  • Dose-Response Relationship, Drug
  • Electric Conductivity
  • Female
  • Guanosine Triphosphate / pharmacology
  • Guanylyl Imidodiphosphate / pharmacology
  • Guinea Pigs
  • Heart / physiology*
  • Heart Atria
  • Magnesium / physiology*
  • Male
  • Membrane Potentials / drug effects
  • Potassium Channels / drug effects
  • Potassium Channels / physiology*
  • Receptors, Muscarinic / physiology

Substances

  • Potassium Channels
  • Receptors, Muscarinic
  • Guanylyl Imidodiphosphate
  • Guanosine Triphosphate
  • Magnesium
  • Acetylcholine