Seasonal variations of carbon monoxide-induced polycythemia in rats

Fluctuations of the extent of polycythemia in rats due to repeated exposure to carbon monoxide prompted us to study whether the differences were caused by seasonal variations. Data from 18 independent experiments involving the hemoglobin values of 206 CO-exposed and 208 control rats over a period of nearly 3 years were available. Because the experiments were not designed for chronobiological studies, several methods of quasilinear and nonlinear regression analysis were applied to fit periodic models to the data. The best fit was obtained by a single cosine function with a period of nearly 12 months and with maximum values in winter and minimum values in summer. Experiments to elucidate mechanism(s) of seasonal variations showed that following a single CO exposure to the nocturnally active rodents, the rate of CO release from the body was higher in the dark span, that reduced food supply produced a potentiation of the CO-induced hemoglobin increase, and that the adaptive CO response was not affected by testosterone or thyroidectomy, but inhibited by L-thyroxine. Thus it seems that influences of light, CO elimination rate, food intake, metabolic rate and related hormones play a role for seasonal variations.