Calcium metabolism in birds: computer simulation of response to 1,25-dihydroxycholecalciferol

A simulation model of calcium metabolism in the chick has been used to evaluate the response of the systems that regulate plasma calcium to a continuous input of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] from the intestine. The results of simulation showed that the concentration of the vitamin D metabolite in the intestine increased during the first 24 h and thereafter decreased slightly due to suppression of endogenous hormone production. Metabolite accumulation was accompanied by an increase in calcium absorption, with a lag period of about 4 h. Most of the increase in absorbed calcium was excreted in the urine, with a lesser change in net bone calcium uptake. Plasma calcium increased with time, in proportion to the intake of the exogenous hormone. The validity of the predicted steady-state plasma calcium levels, and bone and kidney calcium flows, is discussed. The results of the simulation demonstrate the importance of the vitamin D-intestinal axis in regulation of plasma calcium.