Gastrointestinal absorption of lead in chicks: involvement of the cholecalciferol endocrine system

The role of dietary calcium and phosphorus in modifying the intestinal absorption of lead and also the effect of lead ingestion on the metabolism of cholecalciferol were studied in chicks. The efficiency of absorption of 203Pb and 47Ca was increased when the animals were fed a low calcium diet and treated with cholecalciferol. The synthesis of the vitamin D-induced calcium-binding protein (CaBP) was correspondingly increased. When the chicks were depleted of vitamin D and repleted with 1,25-dihydroxycholecalciferol [1,25(OH)2D3] as their only source of the vitamin, the absorption of both 47Ca and 203Pb was unaffected by dietary calcium levels, and no change in CaBP levels occurred. Low dietary intake of phosphorus resulted in an increase in 47Ca and 203Pb absorption and in CaBP synthesis when the animals were treated with cholecalciferol. However, when the birds were repleted with 1,25(OH)2D3, the intestinal absorption of 47Ca and of 203Pb was increased, as well as the intestinal CaBP levels. Intracardial injection of increasing doses of 1,25(OH)2D3 to rachitic chicks resulted in a concomitant increase in 203Pb absorption in a manner that correlated with the degree of synthesis of CaBP. Ingestion of lead by the chicks was found to impair growth and renal production of 1,25(OH)2D3, resulting in lowered circulating and intestinal content of the hydroxylated metabolites of cholecalciferol.