Mechanism of the Insulin-Releasing Action of α-Ketoisocaproate and Related α-Keto Acid Anions

α-Ketoisocaproate directly inhibits the ATP-sensitive K+ channel (KATP channel) in pancreatic β-cells, but it is unknown whether direct KATP channel inhibition contributes to insulin release by α-ketoisocaproate and related α-keto acid anions, which are generally believed to act via β-cell metabolism. In membranes from HIT-T15 β-cells and COS-1 cells expressing sulfonylurea receptor 1, α-keto acid anions bound to the sulfonylurea receptor site of the KATP channel with affinities increasing in the order α-ketoisovalerate < α-ketovalerate < α-ketoisocaproate < α-ketocaproate < β-phenylpyruvate. Patch-clamp experiments revealed a similar order for the KATP channel-inhibitory potencies of the compounds (applied at the cytoplasmic side of inside-out patches from mouse β-cells). These findings were compared with the insulin secretion stimulated in isolated mouse islets by α-keto acid anions (10 mM). When all KATP channels were closed by the sulfonylurea glipizide, α-keto acid anions amplified the insulin release in the order β-phenylpyruvate < α-ketoisovalerate < α-ketovalerate ≈ α-ketocaproate < α-ketoisocaproate. The differences in amplification apparently reflected special features of the metabolism of the individual α-keto acid anions. In islets with active KATP channels, the first peak of insulin secretion triggered by α-keto acid anions was similar for α-ketoisocaproate, α-ketocaproate, and β-phenylpyruvate but lower for α-ketovalerate and insignificant for α-ketoisovalerate. This difference from the above orders indicates that direct KATP channel inhibition is not involved in the secretory responses to α-ketoisovalerate and α-ketovalerate, moderately contributes to initiation of insulin secretion by α-ketoisocaproate and α-ketocaproate, and is a major component of the insulin-releasing property of β-phenylpyruvate.