Kir6.2 variant E23K increases ATP-sensitive K+ channel activity and is associated with impaired insulin release and enhanced insulin sensitivity in adults with normal glucose tolerance.
OBJECTIVE
The E23K variant in the Kir6.2 subunit of the ATP-sensitive K(+) channel (K(ATP) channel) is associated with increased risk of type 2 diabetes. The present study was undertaken to increase our understanding of the mechanisms responsible. To avoid confounding effects of hyperglycemia, insulin secretion and action were studied in subjects with the variant who had normal glucose tolerance.


METHODS
Nine subjects with the E23K genotype K/K and nine matched subjects with the E/E genotype underwent 5-h oral glucose tolerance tests (OGTTs), graded glucose infusion, and hyperinsulinemic-euglycemic clamp with stable-isotope-labeled tracer infusions to assess insulin secretion, action, and clearance. A total of 461 volunteers consecutively genotyped for the E23K variant also underwent OGTTs. Functional studies of the wild-type and E23K variant potassium channels were conducted.


RESULTS
Insulin secretory responses to oral and intravenous glucose were reduced by approximately 40% in glucose-tolerant subjects homozygous for E23K. Normal glucose tolerance with reduced insulin secretion suggests a change in insulin sensitivity. The hyperinsulinemic-euglycemic clamp revealed that hepatic insulin sensitivity is approximately 40% greater in subjects with the E23K variant, and these subjects demonstrate increased insulin sensitivity after oral glucose. The reconstituted E23K channels confirm reduced sensitivity to inhibitory ATP and increase in open probability, a direct molecular explanation for reduced insulin secretion.


CONCLUSIONS
The E23K variant leads to overactivity of the K(ATP) channel, resulting in reduced insulin secretion. Initially, insulin sensitivity is enhanced, thereby maintaining normal glucose tolerance. Presumably, over time, as insulin secretion falls further or insulin resistance develops, glucose levels rise resulting in type 2 diabetes.
Villareal
Dennis T
DT
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri, USA.
Koster
Joseph C
JC
Robertson
Heather
H
Akrouh
Alejandro
A
Miyake
Kazuaki
K
Bell
Graeme I
GI
Patterson
Bruce W
BW
Nichols
Colin G
CG
Polonsky
Kenneth S
KS
D018528:ATP-Binding Cassette Transporters / Q000235:genetics; D000328:Adult; D019943:Amino Acid Substitution; D003430:Cross-Sectional Studies; D005260:Female; D014644:Genetic Variation; D005838:Genotype; D015309:Glucose Clamp Technique; D005951:Glucose Tolerance Test; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D054086:KATP Channels / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D024661:Potassium Channels, Inwardly Rectifying / Q000235:genetics*; D011955:Receptors, Drug / Q000235:genetics; D012016:Reference Values; D064233:Sulfonylurea Receptors; D000099161:Kcnj11 Channel
D018528:ATP-Binding Cassette Transporters; D007328:Insulin; D054086:KATP Channels; D024661:Potassium Channels, Inwardly Rectifying; D011955:Receptors, Drug; D064233:Sulfonylurea Receptors; D000099161:Kcnj11 Channel