Regulation of pancreatic beta-cell mitochondrial metabolism: influence of Ca2+, substrate and ADP.
To gain insight into the regulation of pancreatic beta-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 beta-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 beta-cell fatty acid oxidation is not regulated exclusively by malonyl-CoA but also by the mitochondrial redox state. alpha-Glycerophosphate (alpha-GP) oxidation was Ca(2+)-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 beta-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 (alpha-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 alpha-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.
Civelek
V N
VN
Diabetes and Metabolism Unit, Evans Department of Medicine, Boston University Medical Center, MA 02118, USA.
Deeney
J T
JT
Shalosky
N J
NJ
Tornheim
K
K
Hansford
R G
RG
Prentki
M
M
Corkey
B E
BE
D000244:Adenosine Diphosphate / Q000378:metabolism* / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002999:Clone Cells; D004533:Egtazic Acid / Q000494:pharmacology; D005994:Glycerophosphates / Q000378:metabolism; D007340:Insulinoma; D007515:Islets of Langerhans / Q000378:metabolism*; D007656:Ketoglutaric Acids / Q000494:pharmacology; D007700:Kinetics; D008316:Malonyl Coenzyme A / Q000378:metabolism; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D010084:Oxidation-Reduction; D010101:Oxygen Consumption / Q000187:drug effects; D010190:Pancreatic Neoplasms; D011768:Pyruvate Dehydrogenase Complex / Q000378:metabolism; D051381:Rats
D005994:Glycerophosphates; D007656:Ketoglutaric Acids; D011768:Pyruvate Dehydrogenase Complex; D008316:Malonyl Coenzyme A; D004533:Egtazic Acid; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; C029620:alpha-glycerophosphoric acid; D002118:Calcium