Regulation of spontaneous opening of muscarinic K+ channels in rabbit atrium

J Physiol. 1991 Feb;433:589-613. doi: 10.1113/jphysiol.1991.sp018445.

Abstract

1. Intracellular mechanism(s) for controlling the opening of muscarinic K+ channels in the absence of an applied muscarinic agonist were studied in rabbit atrium by applying the patch clamp technique to isolated single myocytes. 2. In the cell-attached patch configuration, currents due to the activity of both the muscarinic K+ channel and the inward rectifying K+ channel were recorded. However, while the inward rectifying K+ channel currents were observed in only ten patches of 211 examined, spontaneous opening (i.e. in the absence of a muscarinic agonist) of the muscarinic K+ channel currents was observed in all patches examined in these atrial cells. 3. The single-channel currents due to spontaneous opening of muscarinic K+ channels were identified on the basis of their very similar conductance and gating properties to the unitary events which have been recorded when 0.5 microM-acetylcholine is included in the pipette and 10 microM-GTP is present in the internal side of the patch membrane. 4. Although the spontaneous opening of the muscarinic K+ channels disappeared soon after excision of the patch membrane, this type of channel activity reappeared following application of ATP and MgCl2 to the internal side of the torn-off patch, as expected from previous publications. 5. The K+ channel activity induced by the ATP and Mg2+ (measured as the product of the number of channels, N, times the probability of opening, Po) was strongly dependent upon concentration of free Mg2+; it was half-maximal at 2.2 x 10(-4) M [Mg2+]i. However, after the muscarinic K+ channels had been activated by 100 microM-guanosine 5'-O-3-thiotriphosphate (GTP gamma S) together with ATP and Mg2+, an increase in the Mg2+ concentration from 5.5 x 10(-5) to 2 x 10(-3) M failed to enhance this channel activity. 6. Pertussis toxin, which is known to uncouple muscarinic receptors from associated G-proteins (G(i) or G(o)), failed to inhibit the ATP- and Mg(2+)-induced activation of this K+ channel in the absence agonists. 7. In experiments made to test whether the Mg(2+)-ATP requirement results from an obligatory phosphorylation reaction, ATP was replaced with adenylyl-imidodiphosphate (AMP-PNP), an analogue of ATP which is resistant to hydrolysis. This K+ channel activity was not present when ATP was replaced with AMP-PNP.(ABSTRACT TRUNCATED AT 400 WORDS)

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphate / analogs & derivatives
  • Adenosine Triphosphate / metabolism
  • Adenosine Triphosphate / pharmacology
  • Animals
  • Electrophysiology
  • Guanosine 5'-O-(3-Thiotriphosphate) / pharmacology
  • Heart Atria / metabolism
  • In Vitro Techniques
  • Magnesium / metabolism
  • Magnesium / pharmacology
  • Myocardium / metabolism*
  • Nucleotides / pharmacology
  • Pertussis Toxin
  • Phosphorylation
  • Potassium Channels / drug effects
  • Potassium Channels / metabolism*
  • Protein Kinase C / metabolism
  • Rabbits
  • Receptors, Muscarinic / drug effects
  • Receptors, Muscarinic / metabolism
  • Virulence Factors, Bordetella / pharmacology

Substances

  • Nucleotides
  • Potassium Channels
  • Receptors, Muscarinic
  • Virulence Factors, Bordetella
  • Guanosine 5'-O-(3-Thiotriphosphate)
  • Adenosine Triphosphate
  • Pertussis Toxin
  • Protein Kinase C
  • Magnesium