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Dietary Calcium Levels and Treatment Interval Determine the Effects of Lead Ingestion on Plasma 1,25-Dihydroxyvitamin D Concentration in Chicks^1,2,

Department of Physiology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853

The combined effects of dietary calcium level and lead level on several indices of vitamin D endocrine function were examined in young, growing chicks. Day-old animals fed a nutritionally adequate diet for 2 wk were fed diets either adequate (1.2%) or low (0.1%) in calcium, and containing 0, 0.2 or 0.8% lead for an additional 1 or 2 wk. In the calcium-adequate group, lead ingestion significantly elevated intestinal calbindin-D_28k protein and mRNA levels as well as plasma 1,25-dihydroxyvitamin D concentration compared with the control animals fed a lead-free diet. The effect was apparent after 1 wk of treatment and continued through wk 2. In the calcium-deficient group, the early (1 wk) increases in plasma 1,25-dihydroxyvitamin D and calbindin-D_28k protein and mRNA were significantly reversed by lead ingestion over the 2-wk trial period in a dose-dependent fashion. In these circumstances, vitamin D endocrine function is severely compromised. Therefore, lead ingestion may result in either enhanced or diminished circulating 1,25-dihydroxyvitamin D concentrations and ensuing intestinal responses, depending of dietary calcium level and the duration of lead intake. These results provide possible explanations for several apparently conflicting sets of observations regarding lead-calcium interactions.

KEY WORDS: • vitamin D • toxicity • calcium • lead • chicks

¹ Supported by NIH grant ES04072 from the National Institute of Environmental Health Sciences.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

Manuscript received 16 June 1994. Revision accepted 6 October 1994.