A conserved inhibitory and differential stimulatory action of nucleotides on KIR6.0/SUR complexes is essential for excitation-metabolism coupling by KATP channels

The mechanism by which ubiquitous adenine nucleotide-gated K(IR)6.0(4)/SUR4 channels link membrane excitability with cellular metabolism is controversial. Is a decreased sensitivity to inhibitory ATP required, or is the Mg-ADP/ATP-dependent stimulatory action of the ATPase, sulfonylurea receptor (SUR), on K-IR sufficient to elicit a physiologically significant open channel probability? To evaluate the roles of nucleotide inhibition versus stimulation, we compared K(IR)6.1-based K-NDP channels with K(IR)6.2-based K-ATP channels and all possible K(IR)6.1/6.2 hybrids. Although K-NDP channels are thought to be poorly sensitive to inhibitory ATP and to require Mg-nucleotide diphosphates for activity, we demonstrate that, like K-ATP, and hybrid channels, they are inhibited with an IC50(ATP) 100-fold lower than [ATP](i). K(IR)6.1 is, however, more efficiently stimulated by SUR than K(IR)6.2, thus providing a mechanism for differential nucleotide regulation, in addition to the known differential interactions of Mg-nucleotides with SUR isoforms. The on-cell and spontaneous activities of K-NDP, K-ATP, and hybrid channels identified in native cells, are different; thus, their similar IC50(ATP) values argue the regulatory "beta" SUR subunits play a preeminent role in coupling excitation to metabolism and pose questions about the physiologic significance of models, which assume the ATP insensitivity of open K(IR)s.