Nicotinamide does not protect islet B-cell metabolism against alloxan toxicity

Nicotinamide, a poly(ADP-ribose)synthetase inhibitor, protected NMRI mice against alloxan-induced hyperglycemia when given 10 min before, but not 10 min after, the injection of the drug. Pretreatment in vivo with nicotinamide induced hyperglycemia at the time of alloxan injection, and this could account for the protective action of nicotinamide against alloxan diabetes. Exposure of islets to alloxan (2 mM) in vitro caused a marked inhibition of both glucose-stimulated proinsulin biosynthesis and insulin release, and this was not affected by the action of nicotinamide. Alloxan-impaired islet glucose oxidation was partly restored by nicotinamide. The decreased islet content of NADH plus NAD, which was observed after alloxan treatment, could be prevented by nicotinamide. Glucose-stimulated islet oxygen uptake was abolished after treatment with alloxan, and nicotinamide had no protective effect in this process. Leucine (10 mM) plus glutamine (10 mM), however, were still able to evoke an islet respiratory response after alloxan exposure. Alloxan caused an immediate increase in the islet efflux of radiolabeled nucleotides, which was followed after about 5 min by a further increase. This latter increase of the radio efflux was inhibited by the addition of nicotinamide. The inability of nicotinamide to prevent the alloxan-induced impairment of proinsulin biosynthesis, insulin release, and oxygen uptake, together with the failure of nicotinamide to prevent the development of diabetes when given after alloxan, does not support a current hypothesis that the major cytotoxic effect of alloxan is primarily due to DNA damage. The present data suggest that organelles other than the nuclei, e.g., the mitochondria or the plasma membrane, are the primary sites of B-cell injury by alloxan.(ABSTRACT TRUNCATED AT 250 WORDS)