The turnover time of calcium in the exchangeable pools of bone in man and the long-term effect of a parathyroid hormone fragment.

(1) A non-compartmental method is described for analysing the kinetics of the exchange of calcium between the bloodstream and the exchangeable bone pools. (2) This method has been applied to studies on osteoporotic patients treated with low doses of an active fragment of human parathyroid hormone. The data of Phang et al. (1969), who studied the effects of dietary calcium peturbation on normal volunteers, were also analysed. (3) It was found that in response to chronic PTH treatment the transit time of calcium in the exchangeable pools decreased. This could not be explained by the observed changes in the calcium accretion rate to the fixed bone pools, and it was necessary to postulate an alteration in the ratio of the rate constants governing the interchange of calcium between bloodstream and exchangeable pools. (4) The changes in the behaviour of the exchangeable pools in response to dietary perturbation could not be explained by the postulated increase in endogenous PTH secretion rate in response to dietary calcium restriction.