Mechanism of the insulin-releasing action of α-ketoisocaproate and related α-keto acid anions

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