Calcium dynamics: a model system approach.

A computerized model used to simulate calcium metabolism in growing chicks combines growth equations with differential equations that account for the amount and action of various components of the plasma calcium regulating subsystems--intestine, kidney and bone. These in turn are modulated by the calcium-regulating hormones: parathyroid hormone and 1,25-dihydroxycholecalciferol. Simulation with this model indicated oscillations in the plasma calcium concentration in growing chicks under normal dietary conditions. The oscillations diminish in amplitude and finally disappear when dietary calcium concentrations are either reduced or elevated. These oscillations, triggered by the perturbation imposed by growth, are the result of the dual action of parathyroid hormone on bone on the one hand and on intestinal calcium absorption via the 1,25-dihydroxycholecalciferol synthesizing system on the other and the difference in the response time between the two subsystems. Simulation also predicts that at high or low intakes of calcium, the capacity of the control systems is exceeded and oscillation in plasma. calcium diminish and finally disappear. Bone calcium, simulated for different calcium concentrations, mimics documented experimental results.