Direct inhibition of the pancreatic β-cell ATP-regulated potassium channel by α-ketoisocaproate

The ATP-regulated potassium (K(ATP)) channel plays an essential role in the control of insulin release from the pancreatic β-cell. In the present study we have used the patch-clamp technique to study the direct effects of α-ketoisocaproate on the K(ATP) channel in isolated patches and intact pancreatic β-cells. In excised inside-out patches, the activity of the K(ATP) channel was dose-dependently inhibited by α-ketoisocaproate, half- maximal concentration being approximately 8 mM. The blocking effect of α- ketoisocaproate was fully reversible. Stimulation of channel activity by the addition of ATP/ADP (ratio 1) did not counteract the inhibitory effect of α- ketoisocaproate. In the presence of the metabolic inhibitor sodium azide, α- ketoisocaproate was still able to inhibit single channel activity in excised patches and to block whole cell K(ATP) currents in intact cells. No effect of α-ketoisocaproate could be obtained on either the large or the small conductance Ca²⁺-regulated K⁺ channel. Enzymatic treatment of the patches with trypsin prevented the inhibitory effect of α-ketoisocaproate. Based on these observations, it is unlikely that the blocking effect of α- ketoisocaproate is due to an unspecific effect on K⁺ channel pores. Leucine, the precursor of α-ketoisocaproate, did not affect K(ATP) channel activity in excised patches. Our findings are compatible with the view that α- ketoisocaproate not only affects the β-cell stimulus secretion coupling by generation of ATP but also by direct inhibition of the K(ATP) channel.