Part III THE ACCERETION AND RESORPTION OF CALCIUM IN THE SKELETON

Abstract

SummaryA method is described for the determination of digestive juices Ca, based on the continuous ingestion of Ca45 with stable Ca in the food. The true absorption of Ca can next be estimated.In the adult rat the digestive juices Ca was found to be in the range of 6.4 to 27.6 mg daily. The variability was considerable. When a number of consecutive periods were averaged in four vitamin D free rats, the variability was about ±15 per cent of the mean value of 11.2 mg Ca daily.Variations in serum Ca concentrations influence the digestive juices Ca as expected. When ionized Ca was calculated, a straight line relation was found between the ionized Ca and the amount of digestive juices Ca.Variations in the amount of food eaten resulted in proportional variations in the digestive juices Ca in rats supplied with vitamin D. In vitamin D free rats, variations in voluntary food consumption resulted in corresponding changes in the secretion of Ca with the digestive juices, not, however, to the same extent as in the rats supplemented with vitamin D.The method was found to be applicable with small errors only when the speed of Ca absorption was low, Serious errors may readily occur when the rate of Ca absorption is high or when high Ca diets are used.Some experiments in which the isotope was administered in a single dose by stomach tube revealed the serious weakness of such a procedure in contrast to the continuous feeding administration.Long‐term balance experiments were carried out to study the accumulation of Sr90 in rats with a constant level of Sr90 in their diet. Four groups were taken into experiment at different ages on a diet with 0.25 per cent Ca. A fifth group received a high Ca diet (0.71 per cent). The rats were kept in the experiment for 50 to 103 days.In the youngest group of rats the net absorption of Ca was nearly complete. The rats on the 0.71 per cent Ca diet did not absorb more Ca than rats of comparable age on the 0.25 per cent diet. The Ca absorption decreased with increasing age of the rats, as expected.Sr90 in faeces increased in the course of the experiments; the initial figures for faecal excretion in each experiment were definitely correlated to the efficiency of Ca absorption. An increase in the level of Ca in the diet resulted in a proportional depression of Sr90 absorption.The Observed Ratio (absorption) was calculated from the figures for net absorption of Ca and Sr90. Corrections have also been made for the secretion of Ca and Sr90 with the digestive juices. The O.R. (absorption) was found to be very high in the youngest rats, which is explained by the very high utilization of dietary Ca in the rats on the 0.25 per cent Ca diets.The urinary level of Ca was low in all the experiments. Sr90 in the urine remained within ten per cent of the intake in practically dl observations. There was a correlation between Sr90 absorbed and the urinary excretion. Within a given experiment, the Sr90 level in the urine became constant from the very early periods.The ratio of Sr90 in serum became constant after about eight days in young rats on continuous ingestion of Sr90.The Sr90/Ca ratio in the body was calculated at the end of each metabolic period. The resulting curves show a peak after approximately 33 days on the Sr90 diet. Next, some decrease followed, due to absorptive and urinary discrimination. The highest ratios for Sr90/Ca (body/diet) were found in the youngest rats, corresponding to the highest rates of absorption and the smallest body pool of Ca. The ratio of Sr90/Ca in the body was reduced in proportion to the increase of Ca in the diet.Following discontinuation of Sr90 ingestion, the faecal excretion was about twice as high as the urinary elimination.The figures for Ca and Ca45 retention were used in a calculation of the rate of bone accretion and resorption. The plasma Ca specific activity was constant in the periods used. The calculation rested on the presumption that the specific activity of Ca in accreted bone equals the specific activity of Ca in the plasma. In the first approximation zero specific activity of resorbed Ca was somewhat erroneously assumed. However a calculation with the aid of a set of figures for the specific activity of resorbed Ca results in: a) that accretion and resorption show a parallel increase, and b) gives a fairly good estimate of the probable size of the two variables under various experimental conditions. Experiments with vitamin D indicate that the vitamin acts primarily by increasing Ca absorption and bone accretion.