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 Mg2+-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, G(k). This prediction was confirmed when an affinity-purified, sequence-specific G(i)-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 G(k) 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.