Alloxan effects on mitochondria: study of oxygen consumption, fluxes of Mg2+, Ca2+, K+ and adenine nucleotides, membrane potential and volume change in vitro

Isolated mouse liver mitochondria incubated with alloxan showed stimulated resting (state 4) respiration with succinate, and inhibited resting respiration with pyridine-linked substrates, whereas active (state 3) respiration was decreased with both kinds of substrates. The effects were dependent on the concentration of alloxan, on the energy state, and on transport of inorganic phosphate and uptake of Ca2+. Using succinate as substrate, the effects of alloxan on endogenous Mg2+, K+ and adenine nucleotides, uptake of K+, accumulated Ca2+, membrane potential and volume were studied in liver mitochondria, and in addition efflux of endogenous K+ and accumulated Ca2+ were investigated in mouse islet mitochondria. High concentrations of alloxan (⩾ 3 mmol/l) induced efflux of endogenous Mg2+, K+ and adenine nucleotides, efflux of accumulated Ca2+, inhibition of uptake of K+, loss of membrane potential, and swelling. Low concentrations of alloxan (< 3 mmol/l) had similar effects only in the presence of added Ca2+ and inorganic phosphate. The influence of potentially protective agents was studied mainly with regard to alloxan induced swelling. Complete or partial protection was offered by antimycin A, malonate, La3+, Ni2+, ruthenium red, mersalyl and N-ethylmaleimide, suggesting requirement for energized transport of Ca2+ and uptake of inorganic phosphate. The start of the respiratory changes, decrease of membrane potential and loss of Mg2+ preceded the release of accumulated Ca2+, which occurred in parallel with efflux of K+ and swelling. The loss of Ca2+ in association with swelling agrees with data previously obtained using qualitative and quantitative electron microscopy and X-ray microanalysis of islet β cells from alloxan-treated mice. Since preceding studies in vivo have shown that alloxan passes across plasma membranes and is taken up in mitochondria of islet β cells and hepatocytes, the combined data support the view that alloxan diabetes may be due to mitochondrial damage.