PROBLEMS ASSOCIATED WITH ESTIMATION OF NET CALCIUM-UPTAKE DURING ENAMEL FORMATION USING CA-45

Ca-45 uptake in mineralizing tissues may occur by net Ca uptake or by isotopic exchange. It is rarely possible to differentiate between thes e effects, making interpretation of the findings difficult. Unfortunat ely, this problem is not often considered, and Ca-45 uptake is usually regarded as representative of only net calcium uptake. The study repo rted here was undertaken to estimate the extent to which Ca-45 uptake in mineralizing enamel is due to net Ca deposition or to isotopic exch ange, and to consider the implications. The enamel surfaces of the low er incisors of adult rats were notched at the gingival Line, and the e ruption distance over 16 hours was measured. This distance was used to establish the position of a 0.3-mm-wide increment of enamel at the be ginning and end of the 16-hour period, during which it passed through the early-maturation stage of enamel formation. The rate of Ca uptake was determined by chemical assay. Other rats were injected with Ca-45, mean plasma specific activity values for the experimental period dete rmined, and the rate of Ca uptake through the same area of enamel form ation was estimated. The estimates were from two- to nearly ten-fold g reater than those established by chemical assay, indicating that from 50 to 90% of the Ca-45 uptake occurred by isotopic exchange. Ca-45 upt ake may indicate more about the labile state of Ca in mineralizing ena mel than about the rate of mineral deposition.