REGULATION OF PANCREATIC BETA-CELL MITOCHONDRIAL METABOLISM - INFLUENCE OF CA2+, SUBSTRATE AND ADP

To gain insight into the regulation of pancreatic beta-cell mitochondr ial metabolism, the direct effects on respiration of different mitocho ndrial substrates, variations in the ATP/ADP ratio and free Ca2+ were examined using isolated mitochondria and permeabilized clonal pancreat ic beta-cells (HIT). Respiration from pyru vate was high and not influ enced by Ca2+ in State 3 or under various redox states and fixed value s of the ATP/ADP ratio; nevertheless, high Ca2+ elevated pyridine nucl eotide fluorescence, indicating activation of pyruvate dehydrogenase b y Ca2+. Furthermore, in the presence of pyruvate, elevated Ca2+ stimul ated CO2 production from pyruvate, increased citrate production and ef flux from the mitochondria and inhibited CO2 production from palmitate . The latter observation suggests that beta-cell fatty acid oxidation is not regulated exclusively by malonyl-CoA but also by the mitochondr ial redox state. alpha-Glycerophosphate (alpha-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 incre ases in Ca2+ in glucose-stimulated clonal pancreatic beta-cells (HIT), indicating that Ca2+ is not responsible for the initial stimulation o f respiration [Civelek, Deeney, Kubik, Schultz, Tornheim and Corkey (1 996) Biochem. J. 315, 1015-1019]. It is suggested that respiration is stimulated by increased substrate (alpha-GP and pyruvate) supply toget her with oscillatory increases in ADP [Nilsson, Schultz, Berggren, Cor key and Tornheim (1996) Biochem. J. 314, 91-94]. The rise in Ca2+, whi ch in itself may not significantly increase net respiration, could hav e the important functions of (1) activating the alpha-GP shuttle, to m aintain an oxidized cytosol and high glycolytic flux; (2) activating p yruvate 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 stat e to implement the switch from fatty acid to pyruvate oxidation.