title: Background conductance attributable to spontaneous opening of muscarinic K+ channels in rabbit sino-atrial node cells.
abstract: Single myocytes were dissociated from the rabbit sino-atrial node, and the membrane background conductance produced by spontaneous opening of the muscarinic K+ channels was investigated by recording whole-cell and single channel currents in both normal K+ (5.4 mM) and high-K+ (145 mM) external solutions. Increasing external K+ concentration ([K+]o) from 5.4 to 145 mM induced a large inward shift of the whole-cell current accompanied by considerable current fluctuations at -50 mV. The high-K(+)-induced current was both K+ selective and voltage dependent, which was examined by varying [K+]o. This current was almost completely suppressed by 1-5 mM Ba2+ or 2-10 mM Cs+ and it was partly blocked by 10 microM atropine. In high-K+ (145 mM) solution, 20 nM acetylcholine (ACh) further increased the K+ conductance as well as the current noise. The power density spectrum of the noise was fitted with a sum of two Lorentzian functions. The corner frequencies of both the slow (approximately 5 Hz) and fast (approximately 120 Hz) components were comparable between the noise before and during the ACh application. Internal dialysis with a non-hydrolysable derivative of ATP, 5'-adenylylimido-diphosphate (AMP-PNP) or Mg(2+)-free solution markedly decreased both the amplitude and fluctuations of the high-K(+)-induced current. The relation between the variance of the current fluctuations and the mean current amplitude was linear in every experiment using dialysis of AMP-PNP or Mg(2+)-free internal solution, or using superfusion of ACh. The slopes of these relations gave comparable single channel current amplitudes of -0.7 pA at -50 mV. These results indicate that the spontaneous opening of the muscarinic K+ channels is largely responsible for the high-K(+)-induced current. In the high-K+ solution, the variance-mean relation at -50 mV showed that the muscarinic K+ channel provides an inward current of 3.12 +/- 2.13 pA pF-1 (n = 23), which was about 60% of the total inward background current. In the normal K+ solution, the variance-mean relation at -50 mV indicated that an outward current of 6.0 +/- 2.0 pA (0.33 +/- 0.28 pA pF-1, n = 8) was provided by the K+ channel. The single channel current amplitude was estimated to be 0.06 +/- 0.02 pA (n = 9). Cell-attached recordings in the absence of ACh demonstrated sporadic and brief openings of channels identical to the ACh-induced channels. The power density spectra of the single channel currents exhibited kinetic properties comparable with those of the whole-cell currents.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Ito
authors.forename: K
authors.initials: K
authors.lastname: Ono
authors.forename: A
authors.initials: A
authors.lastname: Noma
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000266:Adenylyl Imidodiphosphate / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002478:Cells, Cultured; D003470:Culture Media; D019204:GTP-Binding Proteins / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels / Q000378:metabolism*; D011817:Rabbits; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012849:Sinoatrial Node / Q000166:cytology / Q000378:metabolism*
chemical_list: D003470:Culture Media; D015221:Potassium Channels; D011976:Receptors, Muscarinic; D006153:Guanosine Diphosphate; D000266:Adenylyl Imidodiphosphate; D001285:Atropine; D006160:Guanosine Triphosphate; D019204:GTP-Binding Proteins; D000109:Acetylcholine; D011188:Potassium