DIETARY CALCIUM LEVELS AND TREATMENT INTERVAL DETERMINE THE EFFECTS OF LEAD INGESTION ON PLASMA 1,25-DIHYDROXYVITAMIN-D CONCENTRATION IN CHICKS

The combined effects of dietary calcium level and lead level on severa l indices of vitamin D endocrine function were examined in young, grow ing chicks. Day-old animals fed a nutritionally adequate diet for 2 wk were fed diets either adequate (1.2%) of low (0.1%) in calcium, and c ontaining 0, 0.2 or 0.8% lead for an additional 1 or 2 wk. In the calc ium-adequate group, lead ingestion significantly elevated intestinal c albindin-D-28k protein and mRNA levels as well as plasma 1,25-dihydrox yvitamin D concentration compared with the control animals fed a lead- free diet. The effect was apparent after 1 wk of treatment and continu ed through wk 2. In the calcium-deficient group, the early (1 wk) incr eases in plasma 1,25-dihydroxyvitamin D and calbindin-D-28k protein an d mRNA were significantly reversed by lead ingestion over the 2-wk tri al period in a dose-dependent fashion. In these circumstances, vitamin D endocrine function is severely compromised. Therefore, lead ingesti on may result in either enhanced or diminished circulating 1,25-dihydr oxyvitamin D concentrations and ensuing intestinal responses, dependin g of dietary calcium level and the duration of lead intake. These resu lts provide possible explanations for several apparently conflicting s ets of observations regarding lead-calcium interactions.