Receptor-independent activation of atrial muscarinic potassium channels in the absence of nucleotides.

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 microM) concentrations of GDP or GTP. After full activation was achieved, channel openings were abolished by the application of GDP-bound Galphai2 but were not affected by exogenous Gbetagamma dimers, suggesting that effector activation is a consequence of the liberation of betagamma 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 Galpha(empty)betagamma heterotrimer. Therefore, the nucleotide-free form of G proteins has some of the characteristics of the GTP-bound, activated form.