The nature and origin of spontaneous noise in G protein-gated ion channels.

Abstract

A B S TRACT 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, insideout membrane patches. Guanosine triphosphate (GTP) produced identical single channel currents in a concentration-and Mg~+-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 afffinity-purified, sequence-specific G~-3a antibody or pertussis toxin (PTX) blocked the agonistindependent 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 G k 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.

Agonist‐independent effects of muscarinic antagonists on Ca2+ and K+ currents in frog and rat cardiac cells.

“…As will be outlined in the discussion below, our findings are best interpreted by the two following assumptions: (1) an agonist-independent/mAChR-dependent mechanism mediates a spontaneous activation of Gi, as has been recently suggested for GK by Ito, Sugimoto, Kobayashi, Takahashi &Kurachi (1991) andOKabe et al (1991). As a result, cardiac adenylyl cyclase is under a tonic Gi-mediated inhibition which holds back its basal and/or hormonally stimulated activity; (2) in addition to displacing muscarinic agonists from their binding site on the mAChR, atropine and 'M2-selective' antagonists exhibit intrinsic negative activity on the cardiac mAChR which leads to a reduced interaction between the receptor and the corresponding G proteins, Gi and GK.…”

“…Also it remains unclear to what extent G proteins sustain a spontaneous agonist-independent activation under in vivo conditions. If not negligible, this basal G protein activation is likely to affect adenylyl cyclase and IK(ACh) Interestingly, it has been shown that single mAChR-activated K+ channels can open spontaneously even in the absence of mAChR stimulation (Soejima & Noma, 1984;Kaibara, Nakajima, Irisawa & Giles, 1991;Okabe et al 1991;Ito et al 1991) and that this spontaneous activity can be totally suppressed by pertussis toxin pretreatment (but see Kaibara et al 1991).…”

Estrogen directly acts on osteoclasts via inhibition of inward rectifier K + channels

“…Potassium channels regulated by GTP-binding proteins, which exhibit inward rectification, have been identified in a number of cells including osteoclasts (Okabe et al 1991;Arkett et al 1994). Furthermore, estrogen has been shown to block K + conductance through a second-messenger system involving cAMP in hypothalamic neurons (Minami et al 1990).…”

Membrane-delimited activation of muscarinic K current by an albumin-associated factor in guinea-pig atrial myocytes

“…3. Spontaneous, i. e. agonist-independent opening activity, which is an intrinsic property of this channel species, caused by agonist-independent G-protein and channel interaction [11,21], has been recorded starting about 5 s after formation of the seal. The figure depicts a sequence of traces, each corresponding to 6 s, interrupted by periods of 12 s, i. e. the entire recording covers a period of time of about 8 min.…”