2-Oxocarboxylic acids and function of pancreatic islets in obese–hyperglycaemic mice. Insulin secretion in relation to 45Ca uptake and metabolism

Abstract

The effects of aliphatic 2-oxocarboxylic acids, at concentrations of up to 40mm, on the function of pancreatic islets from ob/ob (obese-hyperglycaemic) mice were investigated. 1. 2-Oxopentanoate, dl-3-methyl-2-oxopentanoate, 4-methyl-2-oxopentanoate and 2-oxohexanoate all induced insulin release by isolated incubated islets and a biphasic insulin-secretory pattern in perfused mouse pancreas. The last two substances were similar in potency to glucose. Pyruvate, 2-oxobutyrate, 3-methyl-2-oxobutyrate and 2-oxo-octanoate did not induce insulin release significantly. 2. 2-Oxocarboxylic acids with significant insulin-secretory potency also induced significant (45)Ca uptake by isolated incubated islets. 3. The rates of decarboxylation of [1-(14)C]pyruvate, 3-methyl-2-oxo[1-(14)C]butyrate and 4-methyl-2-oxo[1-(14)C]pentanoate were twice as high as the rates of oxidation of the corresponding U-(14)C-labelled compounds. However, whereas the rates of metabolism of labelled pyruvate and 3-methyl-2-oxobutyrate steadily increased over the concentration range 1-40mm, those of labelled 4-methyl-2-oxopentanoate and d-[U-(14)C]glucose levelled off at concentrations above 10mm. 4. Omission of (40)CaCl(2) from the incubation medium reduced the rate of oxidation of the insulin secretagogue [U-(14)C]4-methyl-2-oxopentanoate, but left that of the non-(insulin secretagogue) [U-(14)C]3-methyl-2-oxobutyrate unaffected. 5. Only glucose, and not pyruvate, 3-methyl-2-oxobutyrate and 4-methyl-2-oxopentanoate, significantly inhibited oxidation of endogenous fatty acids. 6. It is suggested that stimulus-secretion coupling and the resulting exocytosis of insulin in pancreatic beta-cells may modulate both fuel oxidation and (45)Ca uptake.

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

“…6A). In mouse islets, ␣-ketoisovalerate is decarboxylated at a high rate by the branched-chain keto acid dehydrogenase (Lenzen and Panten, 1980) but does not provide acetyl-CoA (MacDonald, 2002). Moreover, at an early step in the degradation of ␣-ketoisovalerate, 3-hydroxyisobutyrate is formed, substantial amounts of which probably leave the ␤-cells, as observed for other cell types (Corkey et al, 1982;Letto et al, 1990).…”

“…ATP-production by ␣-ketoisovalerate metabolism was apparently so low that no or only insignificant insulin release was caused in the absence of any other fuel or secretagogue (Fig. 5B) (Panten et al, 1972;Matschinsky et al, 1973;Lenzen and Panten, 1980). ␣-Ketovalerate produced first and second phases of insulin secretion which were much weaker than the corresponding responses to ␣-ketocaproate (Fig.…”

“…At these concentrations, transamination of ␣-keto acid anions with endogenous glutamate and glutamine enhances the availability of ␣-ketoglutarate in the ␤-cell mitochondria and thereby increases the capacity of the citrate cycle (Hutton et al, 1979;Malaisse et al, 1981Malaisse et al, , 1983Lenzen et al, 1984Lenzen et al, , 1986MacDonald et al, 2005). The resulting promotion of the oxidation of exogenous and endogenous fuels rapidly activates the ␤-cell energy metabolism (Panten et al, 1972;Hutton et al, 1979;Lenzen and Panten, 1980;Duchen et al, 1993). These findings and the ␣-ketoisocaproate-induced inhibition of K ATP channels in intact ␤-cells (Ashcroft et al, 1987) led to the view that ␣-ketoisocaproate and related ␣-keto acid anions trigger insulin release by enhancing the ATP production in ␤-cell mitochondria.…”

“…6A, results not shown). Pyruvate enters the ␤-cells as indicated by the high rate of pyruvate decarboxylation in mouse pancreatic islets (Lenzen and Panten, 1980).…”

“…As soon as insulin release is initiated, the secretory response is enhanced by an amplifying pathway requiring the metabolism of the fuel secretagogue (Henquin, 2000). The ATP/ADP ratio in the ␤-cell cytosol has been suggested to serve as amplification signal.␣-Ketoisocaproate (4-methyl-2-oxopentanoate), the transamination product of L-leucine, and some related ␣-keto acid anions (␣-ketocaproate, ␣-ketovalerate, and ␤-phenylpyruvate) stimulate insulin secretion by pancreatic islets in the absence of any other fuel or secretagogue (Panten et al, 1972;Matschinsky et al, 1973;Hutton et al, 1980;Lenzen and Panten, 1980). This effect requires millimolar extracellular concentrations (Ͼ10-fold higher than the plasma concentrations of ␣-ketoisocaproate in healthy humans) (Schauder et al, 1985), which lead to millimolar concentrations in the cytosol and mitochondria of ␤-cells (Hutton et al, 1979;Malaisse et al, 1983;Hutson et al, 1990).…”