The nature and origin of spontaneous noise in G protein-gated ion channels.
Arrival of agonist is generally thought to initiate the signal transduction process in G protein-receptor coupled systems. However, the muscarinic atrial K+ (K+[ACh]) channel opens spontaneously in the absence of applied agonist, giving a noisy appearance to the current records. We investigated the nature and origin of the noise by measuring single channel currents in cell-attached or excised, inside-out membrane patches. Guanosine triphosphate (GTP) produced identical single channel currents in a concentration- and Mg(2+)-dependent manner in the presence or absence of carbachol, but the requirements for GTP were greater in the absence of agonist. Hence the agonist-independent currents appeared to be produced by an endogenous G protein, Gk. This prediction was confirmed when an affinity-purified, sequence-specific Gi-3 alpha antibody or pertussis toxin (PTX) blocked the agonist-independent currents. Candidate endogenous agonists were ruled out by the lack of effect of their corresponding antagonists. Thus agonist-independent currents had the same nature as agonist-dependent K+[ACh] currents and seemed to originate in the same way. We have developed a hypothesis in which agonist-free, empty receptors prime Gk with GTP and Gk activates atrial K+ [ACh] channels producing basal currents or noise. Agonist-independent activation by G proteins of effectors including ion channels appears to be a common occurrence.
Okabe
K
K
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030.
Yatani
A
A
Brown
A M
AM
D000324:Adrenocorticotropic Hormone / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002217:Carbachol / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins / Q000502:physiology*; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*
D015221:Potassium Channels; D006160:Guanosine Triphosphate; D002217:Carbachol; D000324:Adrenocorticotropic Hormone; D019204:GTP-Binding Proteins; D008274:Magnesium; D011188:Potassium