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Open state destabilization by ATP occupancy is mechanism speeding burst exit underlying KATP channel inhibition by ATP.

Li L, Geng X, Drain P - J. Gen. Physiol. (2002)

Bottom Line: We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes.In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations.The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, 3500 Terrace Street, Pittsburgh, PA 15261, USA.

ABSTRACT
The ATP-sensitive potassium (K(ATP)) channel is named after its characteristic inhibition by intracellular ATP. The inhibition is a centerpiece of how the K(ATP) channel sets electrical signaling to the energy state of the cell. In the beta cell of the endocrine pancreas, for example, ATP inhibition results from high blood glucose levels and turns on electrical activity leading to insulin release. The underlying gating mechanism (ATP inhibition gating) includes ATP stabilization of closed states, but the action of ATP on the open state of the channel is disputed. The original models of ATP inhibition gating proposed that ATP directly binds the open state, whereas recent models indicate a prerequisite transition from the open to a closed state before ATP binds and inhibits activity. We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes. In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations. The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

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“Ligand-insensitive bursts” in the presence of 5 mM MgATP. Typically (92/97 patches) in the absence of creatine phosphate/kinase to scavenge any MgADP generated in the patch, tens and hundreds of one or two opening bursts would be accompanied by one to a few long bursts, as above. Note the occasional long bursts each with 10–100 openings with long mean durations, compared with the more frequent short bursts each with 1–2 openings with short mean durations. Thus, one long burst can have approximately the same number of openings as in 50 or more short bursts.
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Figure 6: “Ligand-insensitive bursts” in the presence of 5 mM MgATP. Typically (92/97 patches) in the absence of creatine phosphate/kinase to scavenge any MgADP generated in the patch, tens and hundreds of one or two opening bursts would be accompanied by one to a few long bursts, as above. Note the occasional long bursts each with 10–100 openings with long mean durations, compared with the more frequent short bursts each with 1–2 openings with short mean durations. Thus, one long burst can have approximately the same number of openings as in 50 or more short bursts.

Mentions: We also studied the response of open times of the KATP channel in conditions where creatine phosphate and kinase were not added. Physiologically, similar conditions may occur where creatine kinase is downregulated or absent. Fig. 6 shows that without creatine phosphate and kinase that KATP channel activity is increased with the appearance of long duration bursts, even though 0.6 mM ATP is present. The long bursts, evidently refractory to the high ATP, are similar to the “ligand-insensitive” gating previously reported for the cardiac channel KATP channel in the presence of 2 mM MgUDP (Alekseev et al. 1998). They have demonstrated that MgUDP can constrain single cardiac KATP channels to gating transitions within the active intraburst, with infrequent transitions to the interburst. When MgUDP is bound, presumably at NBD2 of SUR2A, the cardiac channel exhibits extraordinarily long burst durations, as if it were insensitive to the tri-phosphate nucleoside ligands. Although evidently still sensitive to diphosphate nucleoside ligands, the term ligand insensitivity thus was used only with respect to inhibition of the KATP channel by triphosphate nucleoside ligands. We confirmed that the long duration bursts of our pancreatic KATP channel were due to MgADP generated by the patch in high MgATP as follows.


Open state destabilization by ATP occupancy is mechanism speeding burst exit underlying KATP channel inhibition by ATP.

Li L, Geng X, Drain P - J. Gen. Physiol. (2002)

“Ligand-insensitive bursts” in the presence of 5 mM MgATP. Typically (92/97 patches) in the absence of creatine phosphate/kinase to scavenge any MgADP generated in the patch, tens and hundreds of one or two opening bursts would be accompanied by one to a few long bursts, as above. Note the occasional long bursts each with 10–100 openings with long mean durations, compared with the more frequent short bursts each with 1–2 openings with short mean durations. Thus, one long burst can have approximately the same number of openings as in 50 or more short bursts.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233857&req=5

Figure 6: “Ligand-insensitive bursts” in the presence of 5 mM MgATP. Typically (92/97 patches) in the absence of creatine phosphate/kinase to scavenge any MgADP generated in the patch, tens and hundreds of one or two opening bursts would be accompanied by one to a few long bursts, as above. Note the occasional long bursts each with 10–100 openings with long mean durations, compared with the more frequent short bursts each with 1–2 openings with short mean durations. Thus, one long burst can have approximately the same number of openings as in 50 or more short bursts.
Mentions: We also studied the response of open times of the KATP channel in conditions where creatine phosphate and kinase were not added. Physiologically, similar conditions may occur where creatine kinase is downregulated or absent. Fig. 6 shows that without creatine phosphate and kinase that KATP channel activity is increased with the appearance of long duration bursts, even though 0.6 mM ATP is present. The long bursts, evidently refractory to the high ATP, are similar to the “ligand-insensitive” gating previously reported for the cardiac channel KATP channel in the presence of 2 mM MgUDP (Alekseev et al. 1998). They have demonstrated that MgUDP can constrain single cardiac KATP channels to gating transitions within the active intraburst, with infrequent transitions to the interburst. When MgUDP is bound, presumably at NBD2 of SUR2A, the cardiac channel exhibits extraordinarily long burst durations, as if it were insensitive to the tri-phosphate nucleoside ligands. Although evidently still sensitive to diphosphate nucleoside ligands, the term ligand insensitivity thus was used only with respect to inhibition of the KATP channel by triphosphate nucleoside ligands. We confirmed that the long duration bursts of our pancreatic KATP channel were due to MgADP generated by the patch in high MgATP as follows.

Bottom Line: We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes.In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations.The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, 3500 Terrace Street, Pittsburgh, PA 15261, USA.

ABSTRACT
The ATP-sensitive potassium (K(ATP)) channel is named after its characteristic inhibition by intracellular ATP. The inhibition is a centerpiece of how the K(ATP) channel sets electrical signaling to the energy state of the cell. In the beta cell of the endocrine pancreas, for example, ATP inhibition results from high blood glucose levels and turns on electrical activity leading to insulin release. The underlying gating mechanism (ATP inhibition gating) includes ATP stabilization of closed states, but the action of ATP on the open state of the channel is disputed. The original models of ATP inhibition gating proposed that ATP directly binds the open state, whereas recent models indicate a prerequisite transition from the open to a closed state before ATP binds and inhibits activity. We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes. In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations. The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

Show MeSH
Related in: MedlinePlus