Estimation of bone mineral transfer rate by the measurement of long-term retention of SR 85

Standardisation of 85Sr with digital anticoincidence counting and half-life determination of the 514 keV level of 85Rb

The activity of a ⁸⁵Sr solution was determined by means of fully digital 4πβ(LS)-γ anticoincidence counting. The measurements were carried out in a custom-built, combined TDCR / 4πβ(LS)-γ coincidence system, utilising a commercially available CAEN N6751C digitizer. The analysis of the experimental data, collected in list-mode format, was performed off-line by using the SoftKAM computer code developed at PTB. The data were also used to determine the half-life of the 514 keV level of ⁸⁵Rb which was found to be (1020.2 ± 6.0) ns.

Strontium substitution for calcium in lithogenesis

PURPOSE

Strontium has chemical similarity to calcium, which enables the replacement of calcium by strontium in biomineralization processes. Incorporating strontium into human bone and teeth has been studied extensively but little research has been performed of the incorporation of strontium into urinary calculi. We used synchrotron based x-ray fluorescence and x-ray absorption techniques to examine the presence of strontium in different types of human kidney stones.

MATERIALS AND METHODS

Multiple unique human stone samples were obtained via consecutive percutaneous nephrolithotomies/ureteroscopies. A portion of each stone was sent for standard laboratory analysis and a portion was retained for x-ray fluorescence and x-ray absorption measurements. X-ray fluorescence and x-ray absorption measurements determined the presence, spatial distribution and speciation of strontium in each stone sample.

RESULTS

Traditional kidney stone analyses identified calcium oxalate, calcium phosphate, uric acid and cystine stones. X-ray fluorescence measurements identified strontium in all stone types except pure cystine. X-ray fluorescence elemental mapping of the samples revealed co-localization of calcium and strontium. X-ray absorption measurements of the calcium phosphate stone showed strontium predominately present as strontium apatite.

CONCLUSIONS

Advanced x-ray fluorescence imaging identified strontium in all calcium based stones, present as strontium apatite. This finding may be critical since apatite is thought to be the initial nidus for calcium stone formation. Strontium is not identified by standard laboratory stone analyses. Its substitution for calcium can be reliably identified in stones from multiple calcium based stone formers, which may offer opportunities to gain insight into early events in lithogenesis.

Physiologically based models for bone-seeking elements. II. Kinetics of lead disposition in rats

For toxicants with long residence times in the body, body burden is determined largely by exposure history rather than by current exposure. There is a need for physiologically based toxicokinetic models capable of integrating exposure over time by incorporating growth, development, and aging. Such a model is presented for lead kinetics in the growing rat from birth to adulthood. The model incorporates age dependence of the physiologic and metabolic processes that control lead distribution to bone and soft tissues, as well as age dependence of lead absorption from the gastrointestinal tract and age dependence of elimination of lead. The essential features of bone structure and metabolism are integrated into a framework that determines bone lead kinetics. Parameter values used in the model are taken from the literature or are estimated from the best visual fit of the model to data from chronic and short-term studies of lead exposure in rats. The model accommodates any pattern of lead exposure. The uptake of lead by bone varies with the age at which exposure occurs. The predictions of the model are compared with data from a chronic study in rats in which lead exposure was discontinued after exposure periods varying from 3 to 12 months.

A physiologically based model of skeletal growth in the rat

A model of the growing small animal skeleton has been developed. The model accurately reproduces a variety of different kinds of measurements made at different ages in different species, as well as reproducing a precise and detailed series of measurements in the growing guinea pig. Of the multiple mechanisms responsible for transfer of bone-seeking elements from blood to bone and back again, several are quantitatively insignificant on a skeletal scale. Surface (rapid) exchange, diffuse (slow) exchange, and secretion of new bone during growth are the principal determinants of skeletal metabolism. Bone blood flow can probably by satisfactorily modeled as a combination of a basal flow rate plus an increment that is directly proportional to formation rate of new bone.

The actions of parathyroid hormone on bone: relation to bone remodeling and turnover, calcium homeostasis, and metabolic bone disease. Part I of IV parts: mechanisms of calcium transfer between blood and bone and their cellular basis: morphological and kinetic approaches to bone turnover

The supracellular organization of living bone enables the study of isolated cellular and subcellular systems to be related to the study of the whole organism. Bone is formed by osteoblasts in successive stages, separated in both time and space, of matrix formation and primary mineralization. Osteoblasts are joined by tight junctions and largely cover the osteoid seam which separates them from mineralized bone. Secondary mineralization is not completed for several months and is not regulated by the osteoblast. Bone is resorbed by osteoclasts which simultaneously accomplish mineral dissolution and matrix digestion. Active osteoblasts occupy about 5% of the free bone surface, osteoid seams with less active osteoblasts about 10%, active osteoclasts about 0.5%, and Howship's lacunae at which bone remodeling is either quiescent or arrested about 5%. The remaining 80% of the free bone surface is covered by a leaky envelope of thin flattened cells, termed surface osteocytes. Some osteoblasts become permanently buried in the bone as deep osteocytes, around which a specialized and metabolically active perilacunar bone is formed. This bone is less highly mineralized and can temporarily lose or gain calcium in accordance with homeostatic needs. Deep osteocytes maintain contact with each other and with the surface osteocytes, their cell processes within canaliculi being joined by gap junctions. Remodeling of cortical bone proceeds with the excavation by osteoclasts of a longitudinal tunnel which is refilled by osteoblasts to form a new osteon. The anatomically discrete longitudinally oriented structure consisting of a cutting cone of osteoclasts in front and a closing cone of osteoblasts behind is termed a cortical remodeling unit. The events of centrifugal resorption and centripetal formation which occur in a single cross section is termed a cortical remodeling cycle. Normally each new cycle is slightly out of phase with its predecessor. The quantities which characterize cortical remodeling are the birth rate of new remodeling cycles or activation frequency (mu), and the durations of the resorptive period (sigma r), the quiescent interval (sigma q) and the formation period (sigma f). The average distances traveled by the osteoclast and osteoblast are indicated respectively by the mean cement line diameter and mean wall thickness of completed osteons. These quantities show little interindividual variation. Because of this constancy the magnitude of bone turnover (the bone formation rate) is almost entirely a function of mu, the activation frequency of new remodeling cycles. Variations in the velocity of advance of osteoclasts (the linear resorption rate) or of osteoblasts (the appositional rate) alter inversely both the extent of surface engaged in resorption or formation and the time taken to replace a particular moiety of bone, but in a steady state do not influence the rate of turnover of the skeleton as a whole...