Seasonal changes in lead absorption in laboratory rats.

A retrospective study of the relationship of season to the absorption of radiolead in laboratory rats was performed using data representing 305 animals from 36 experiments over 6 calendar years. Male Wistar rats weighing 200 to 250 g were given 1 microgram of radiolabeled lead in an aqueous solution, pH 4.0, in isolated small intestine, and absorption of the radiolead was quantified after a 4-hour interval using whole-body counting. Similar values of absorption occurred in the summer (June-August) and fall (September-November), 20.51 +/- 1.11% (1 SEM) and 23.0 +/- 1.23% of the test dose, respectively, but significantly lower values occurred in the winter (December-February) and spring (March-May): 16.51 +/- 0.77%, p less than 0.01, and 11.87 +/- 0.99%, p less than 0.01, respectively. Harmonic analysis yielded an excellent approximation of the mean quarterly absorption data. The resulting cosine function had a period of 4.08 +/- 0.05 quarter-years with an amplitude of 7.32 +/- 1.06%; predicted peak absorption values fell precisely between summer and fall. The relationships of these observations to possible mechanisms of lead absorption and to summertime epidemics of lead poisoning in children are discussed.