Aims/hypothesis
We have previously reported that glucose-stimulated insulin secretion (GSIS) is induced by glucagon-like peptide-1 (GLP-1)
in mice lacking ATP-sensitive K+ (KATP) channels (Kir6.2
−/−
mice [up-to-date symbol for Kir6.2 gene is Kcnj11]), in which glucose alone does not trigger insulin secretion. This study aimed to clarify the mechanism involved in the induction
of GSIS by GLP-1.
Methods
Pancreas perfusion experiments were performed using wild-type (Kir6.2
+/+
) or Kir6.2
−/−
mice. Glucose concentrations were either changed abruptly from 2.8 to 16.7 mmol/l or increased stepwise (1.4 mmol/l per step)
from 2.8 to 12.5 mmol/l. Electrophysiological experiments were performed using pancreatic beta cells isolated from Kir6.2
−/−
mice or clonal pancreatic beta cells (MIN6 cells) after pharmacologically inhibiting their KATP channels with glibenclamide.
Results
The combination of cyclic AMP plus 16.7 mmol/l glucose evoked insulin secretion in Kir6.2
−/−
pancreases where glucose alone was ineffective as a secretagogue. The secretion was blocked by the application of niflumic
acid. In KATP channel-inactivated MIN6 cells, niflumic acid similarly inhibited the membrane depolarisation caused by cAMP plus glucose.
Surprisingly, stepwise increases of glucose concentration triggered insulin secretion only in the presence of cAMP or GLP-1
in Kir6.2
+/+
, as in Kir6.2
−/−
pancreases.
Conclusions/interpretation
Niflumic acid-sensitive ion channels participate in the induction of GSIS by cyclic AMP in Kir6.2
−/−
beta cells. Cyclic AMP thus not only acts as a potentiator of insulin secretion, but appears to be permissive for GSIS via
novel, niflumic acid-sensitive ion channels. This mechanism may be physiologically important for triggering insulin secretion
when the plasma glucose concentration increases gradually rather than abruptly.
Keywords cAMP - GLP-1 - Incretin - Insulin secretion - KATP channel -
Kir6.2
- Niflumic acid