Altered mitochondria oscillations and circadian changes in NADH levels in freshwaters mussels exposed to cadmium

Abstract

The effects of contaminants on biological rhythms are largely unknown. The purpose of this study was to examine the effects of cadmium (Cd) and time of day on levels of defence mechanism (metallothionein-MT), mitochondria NADH oxidase activity and oscillations in NADH levels in freshwater mussels Dreissena bugensis. Following exposure of mussels to 50 and 100 μg/L Cd for 7 days, mussels were sampled at various times of the day (9, 12, 15, 18, 21 and 24 h) and immediately stored at −85 °C. Tissues were analyzed for MT levels, NADH oxidase and oscillatory changes in NADH in mitochondria assessments. The data revealed that exposure to Cd readily increased MT and NADH oxidase activity at each time of day. In control mussels, NADH oxidase activity changed during the day reaching a minimum and maximum at 15 h and 24 h respectively as with MT levels but the maximum occurred at 12 h. Exposure to Cd displaced the maximum values in NADH oxidase towards the night (≥18 h) while no change in the pattern in MT changes during the day albeit remained at high levels. NADH oscillations in mitochondria were complex but main changes occurred at periods of 60–90 s in control mussels. The strength of association (β) between amplitudes and frequencies of these oscillations was negatively associated with MT levels suggesting that activation of defence mechanism limited the propagation of amplitude changes at higher times. The study revealed that Cd exposure could disrupt not only the intensity of responses but at the levels of rhythms commonly found in biological systems.