Model for parathyroid hormone secretion and metabolism in calves

A model for parathyroid gland response to differing calcium and magnesium concentrations is proposed based on direct determinations of parathyroid hormone (PTH) secretion rate. A proportional control seems adequate for physiological variations of calcium and magnesium levels. At extreme hypocalcemia, nonlinearities are observed: a model involving the depletion of a storage compartment is proposed, the size of which is calculated. PTH distribution was studied in another group of animals; sampling was made from the saphenous artery and the thoracic duct. Concentrations of plasma and lymph-intact PTH and carboxy-terminal fragments were determined by means of two different radioimmunoassays. The analysis of the results leads to the formulation of a five-compartment model, which shows that the metabolism of intact PTH is only partly due to the formation of carboxy-terminal fragments, most being directly secreted or catabolized. Linking these models for secretion and distribution of PTH, simulation studies were undertaken and compared with published data. This model is then discussed in comparison with previous work concerning other polypeptide hormones.