Regulation of pancreatic β-cell mitochondrial metabolism: influence of Ca²⁺, substrate and ADP

To gain insight into the regulation of pancreatic β-cell mitochondrial metabolism, the direct effects on respiration of different mitochondrial substrates, variations in the ATP/ADP ratio and free Ca²⁺ were examined using isolated mitochondria and permeabilized clonal pancreatic β-cells (HIT). Respiration from pyruvate was high and not influenced by Ca²⁺ in State 3 or under various redox states and fixed values of the ATP/ADP ratio; nevertheless, high Ca²⁺ elevated pyridine nucleotide fluorescence, indicating activation of pyruvate dehydrogenase by Ca²⁺. Furthermore, in the presence of pyruvate, elevated Ca²⁺ stimulated CO₂ production from pyruvate, increased citrate production and efflux from the mitochondria and inhibited CO₂ production from palmitate. The latter observation suggests that β-cell fatty acid oxidation is not regulated exclusively by malonyl-CoA but also by the mitochondrial redox state. α-Glycerophosphate (α-GP) oxidation was Ca²⁺-dependent with a half-maximal rate observed at around 300 nM Ca²⁺. We have recently demonstrated that increases in respiration precede increases in Ca²⁺ in glucose-stimulated clonal pancreatic β-cells (HIT), indicating that Ca²⁺ is not responsible for the initial stimulation of respiration [Civelek, Deeney, Kubik, Schultz, Tornheim and Corkey (1996) Biochem. J. 315, 1015–1019]. It is suggested that respiration is stimulated by increased substrate (α-GP and pyruvate) supply together with oscillatory increases in ADP [Nilsson, Schultz, Berggren, Corkey and Tornheim (1996) Biochem. J. 314, 91–94]. The rise in Ca²⁺, which in itself may not significantly increase net respiration, could have the important functions of (1) activating the α-GP shuttle, to maintain an oxidized cytosol and high glycolytic flux; (2) activating pyruvate dehydrogenase, and indirectly pyruvate carboxylase, to sustain production of citrate and hence the putative signal coupling factors, malonyl-CoA and acyl-CoA; and (3) increasing mitochondrial redox state to implement the switch from fatty acid to pyruvate oxidation.