J Biol Chem, Vol. 273, Issue 44, 28868-28872, October 30, 1998

Receptor-independent Activation of Atrial Muscarinic Potassium Channels in the Absence of Nucleotides

From the Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia 22906-0011

The removal of nucleotides from the solution bathing the inner face of excised patches of frog atrial membranes was found to activate muscarinic K⁺ channels in the absence of agonists. Channel activation was also observed in Mg²⁺-free solutions and blocked by low (0.1-10 µM) concentrations of GDP or GTP. After full activation was achieved, channel openings were abolished by the application of GDP-bound G_i2 but were not affected by exogenous G dimers, suggesting that effector activation is a consequence of the liberation of subunits from endogenous G proteins. The process of channel activation in the absence of nucleotides seems to be receptor-independent, because it is not influenced by muscarinic receptor agonists and antagonists or by treatment with uncoupling agents such as pertussis toxin or N-ethyl maleimide. Taken together, these results suggest that the loss of GDP from the G protein nucleotide binding site promotes its uncoupling from receptors and destabilizes the G_(empty) heterotrimer. Therefore, the nucleotide-free form of G proteins has some of the characteristics of the GTP-bound, activated form.