Stable Ca Isotopes Research Articles

Calcium biogeochemical cycle at the beech tree-soil solution interface from the Strengbach CZO (NE France): insights from stable Ca and radiogenic Sr isotopes

Calcium (Ca) is the fourth most abundant element in mineral nutrition and plays key physiological and structural roles in plant metabolism. At the soil-water-plant scale, stable Ca isotopes are a powerful tool for the identification of plant-mineral interactions and recycling via vegetation. Radiogenic Sr isotopes are often used as tracers of Ca sources and mixtures of different reservoirs. In this study, stable Ca and radiogenic Sr are combined and analysed in several organs from two beech trees that were collected in June and September in the Strengbach critical zone observatory (CZO) (NE France) and in corresponding soil solutions.At the beech-tree scale, this study confirms the field Ca adsorption (i.e., physico-chemical mechanism and not vital effects) on carboxyl acid groups of pectin in the apoplasm of small roots. The analysis of the xylem sap and corresponding organs shows that although the Strengbach CZO is nutrient-poor, Ca seems to be non-limiting for tree-growth. Different viscosities of xylem sap between the stemwood and branches or leaves can explain δ44/40Ca values in different tree-organs. The bark and phloem 40Ca-enrichments could be due to Ca-oxalate precipitation in the bark tissues and in the phloem. The results from this study regarding the combination of these two isotopic systems show that the isotopic signatures of the roots are dominated by Ca fractionation mechanisms and Sr, and thus Ca, source variations. In contrast, translocation mechanisms are only governed by Ca fractionation processes.This study showed that at the root-soil solution interface, litter degradation was not the main source of Ca and Sr and that the soil solutions are not the complement of uptake by roots for samples from the 2011/2013 period. The opposite is observed for older samples. These observations indicate the decreasing contribution of low radiogenic Sr fluxes, such as recycling, alimenting the soil solutions. Such reduced importance of nutrient uptake and biomass production by the trees could be because the Strengbach trees are ageing and probably weakened by repeated storm events and drought episodes.

Calcium isotope fractionation between aqueous compounds relevant to low-temperature geochemistry, biology and medicine

Stable Ca isotopes are fractionated between bones, urine and blood of animals and between soils, roots and leaves of plants by >1000 ppm for the 44Ca/40Ca ratio. These isotopic variations have important implications to understand Ca transport and fluxes in living organisms; however, the mechanisms of isotopic fractionation are unclear. Here we present ab initio calculations for the isotopic fractionation between various aqueous species of Ca and show that this fractionation can be up to 3000 ppm. We show that the Ca isotopic fractionation between soil solutions and plant roots can be explained by the difference of isotopic fractionation between the different first shell hydration degree of Ca2+ and that the isotopic fractionation between roots and leaves is controlled by the precipitation of Ca-oxalates. The isotopic fractionation between blood and urine is due to the complexation of heavy Ca with citrate and oxalates in urine. Calculations are presented for additional Ca species that may be useful to interpret future Ca isotopic measurements.

Biological fractionation of stable Ca isotopes in Göttingen minipigs as a physiological model for Ca homeostasis in humans

ABSTRACTIn order to investigate fractionation of calcium (Ca) isotopes in vertebrates as a diagnostic tool to detect Ca metabolism dysfunction we analyzed the Ca isotopic composition (δ44/40Ca = [(44Ca/40Ca)sample/(44Ca/40Ca)reference]−1) of diet, faeces, blood, bones and urine from Göttingen minipigs, an animal model for human physiology. Samples of three groups were investigated: 1. control group (Con), 2. group with glucocorticosteroid induced osteoporosis (GIO) and 3. group with Ca and vitamin D deficiency induced osteomalacia (−CaD). In contrast to Con and GIO whose average δ44/40Cafaeces values (0.39 ± 0.13‰ and 0.28 ± 0.08‰, respectively) tend to be lower than their diet (0.47 ± 0.02‰), δ44/40Cafaeces of −CaD (−0.27 ± 0.21‰) was significantly lower than their δ44/40Cadiet (0.37 ± 0.03‰), but also lower than δ44/40Cafaeces of Con and GIO. We suggest that the low δ44/40Cafaeces of −CaD might be due to the contribution of isotopically light Ca from gastrointestinal fluids during gut passage. Assuming that this endogenous Ca source is a common physiologic feature, a fractionation during Ca absorption is also required for explaining δ44/40Cafaeces of Con and GIO. The δ44/40Caurine of all groups are high (>2.0‰) reflecting preferential renal reabsorption of light Ca isotopes. In Göttingen minipigs we found a Ca isotope fractionation between blood and bones (Δ44/40Cablood-bone) of 0.68 ± 0.15‰.

The Effect of a Whey Protein Supplement on Bone Mass in Older Caucasian Adults.

It has been assumed that the increase in urine calcium (Ca) that accompanies an increase in dietary protein was due to increased bone resorption. However, studies using stable Ca isotopes have found that dietary protein increases Ca absorption without increasing bone resorption. The objective of the study was to investigate the impact of a moderately high protein diet on bone mineral density (BMD). This was a randomized, double-blind, placebo-controlled trial of protein supplementation daily for 18 months. The study was conducted at two institutional research centers. Two hundred eight older women and men with a body mass index between 19 and 32 kg/m(2) and a self-reported protein intake between 0.6 and 1.0 g/kg participated in the study. Subjects were asked to incorporate either a 45-g whey protein or isocaloric maltodextrin supplement into their usual diet for 18 months. BMD by dual-energy x-ray absorptiometry, body composition, and markers of skeletal and mineral metabolism were measured at baseline and at 9 and 18 months. There were no significant differences between groups for changes in L-spine BMD (primary outcome) or the other skeletal sites of interest. Truncal lean mass was significantly higher in the protein group at 18 months (P = .048). C-terminal telopeptide (P = .0414), IGF-1 (P = .0054), and urinary urea (P < .001) were also higher in the protein group at the end of the study period. There was no difference in estimated glomerular filtration rate at 18 months. Our data suggest that protein supplementation above the recommended dietary allowance (0.8 g/kg) may preserve fat-free mass without adversely affecting skeletal health or renal function in healthy older adults.

Placental vitamin D receptor (VDR) expression is related to neonatal vitamin D status, placental calcium transfer, and fetal bone length in pregnant adolescents

The purpose of the study was to identify determinants of placental vitamin D receptor (VDR) expression and placental calcium (Ca) transfer among pregnant adolescents. Placental tissue was obtained in 94 adolescents (≤18 yr) at term. In 12 of these teens, stable Ca isotopes were given intravenously ((42)Ca) and orally ((44)Ca) early in labor. Placental VDR expression was assessed via Western blot and validated by RT-PCR. Maternal-to-fetal Ca transfer was calculated as the enrichment in cord blood at delivery relative to maternal serum enrichment 2 h postdosing. Isotopic study outcomes were examined in relation to fetal long bone length, placental VDR, serum 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)2D], and parathyroid hormone (PTH) in maternal circulation and cord blood at delivery. Placental VDR expression was inversely associated with neonatal 25(OH)D (P=0.012) and positively with neonatal 1,25(OH)2D (P=0.006). Placental VDR was a positive predictor of fetal femur length Z score (P=0.018; R(2)=0.06) and was positively correlated with maternal-to-fetal transfer of intravenous (42)Ca (P=0.004; R(2)=0.62). The fetus may regulate placental VDR expression given the significant associations with neonatal vitamin D metabolites. The association between placental VDR and fetal long bone length may indicate a role for VDR in fetal bone development, potentially by mediating transplacental Ca transfer.

Ca isotope fractionation in a high-alkalinity lake system: Mono Lake, California

Precipitation of calcium carbonate minerals from aqueous solutions causes surface-controlled kinetic stable Ca isotope fractionation. The magnitude of fractionation depends on the relative rates of ion attachment to and detachment from the mineral surface, which in turn is predicted to depend on both the saturation state and the solution stoichiometry or the Ca2+:CO32- activity ratio. Experimental studies have not directly investigated the effects of varying solution stoichiometry on calcium isotope partitioning during calcite or aragonite growth, but natural alkaline lake systems such as Mono Lake, California provide a test bed for the hypothesized stoichiometry dependence. Mono Lake has a Ca2+:CO32- activity ratio of about 0.0001, seven orders of magnitude lower than ocean water and typical terrestrial freshwater. We present chemical and isotopic measurements of streams, springs, lake water, and precipitated carbonates from the Mono Basin that yield evidence of stoichiometry-dependent Ca isotope fractionation during calcite, aragonite and Mg-calcite precipitation from the alkaline lake water. To estimate the Ca isotope fractionation factors, it is necessary to characterize the lake Ca balance and constrain the variability of lake water chemistry both spatially and temporally. Streams and springs supply Ca to the lake, and a substantial fraction of this supply is precipitated along the lake shore to form tufa towers. Lake water is significantly supersaturated with respect to carbonate minerals, so CaCO3 also precipitates directly from the water column to form carbonate-rich bottom sediments. Growth rate inhibition by orthophosphate likely preserves the high degree of supersaturation in the lake. Strontium isotope ratios are used to estimate the proportions of fresh and alkaline lake water from which each solid carbonate sample precipitated. Carbonate minerals that precipitate directly from lake water (low Ca2+:CO32-) experience relatively large Ca isotope fractionation during growth. Tufa and shoreline carbonates that precipitate from lake water with a significant fraction of spring water (higher Ca2+:CO32-) are considerably less fractionated, as predicted from theory.The behavior of the Mono Lake Ca isotope system is similar in some ways to that of the global oceans, in that the average δ44/40Ca of lake water is positive (estimated average of +1) and both riverine inputs and precipitated carbonates are isotopically light (δ44/40Ca between −0.5 and 0). We present a calcium isotope budget of the lake to constrain the long-term average lake water Ca isotope composition. Archived water samples indicate that the lake δ44/40Ca varied by over 2‰ between 1995 and 2010. The most extreme excursions are toward higher δ44/40Ca, and are probably caused by carbonate precipitation events induced by breakdown of chemostratification. This variability indicates that the lake is out of steady state with respect to calcium isotopes, and that unlike the ocean, calcium isotopes in Mono Basin carbonate sediments likely do not record the balance between weathering and carbonate mineralization fluxes to and from the lake.

Biogeochemistry of stable Ca and radiogenic Sr isotopes in a larch-covered permafrost-dominated watershed of Central Siberia

Stable Ca and radiogenic Sr isotope compositions were measured in different compartments (stream water, soil solutions, rocks, soils and soil leachates and vegetation) of a small permafrost-dominated watershed in the Central Siberian Plateau. The Sr and Ca in the area are supplied by basalt weathering and atmospheric depositions, which significantly impact the Sr isotopic compositions. Only vegetation significantly fractionates the calcium isotopes within the watershed. These fractionations occur during Ca uptake by roots and along the transpiration stream within the larch trees and are hypothesised to be the result of chromatographic processes and Ca oxalate crystallisations during Ca circulation or storage within plant organs. Biomass degradation significantly influences the Ca isotopic compositions of soil solutions and soil leachates via the release of light Ca, and organic and organo-mineral colloids are thought to affect the Ca isotopic compositions of soil solutions by preferential scavenging of 40Ca. The imprint of organic matter degradation on the δ44/40Ca of soil solutions is much more significant for the warmer south-facing slope of the watershed than for the shallow and cold soil active layer of the north-facing slope. As a result, the available stock of biomass and the decomposition rates appear to be critical parameters that regulate the impact of vegetation on the soil–water system in permafrost areas. Finally, the obtained δ44/40Ca patterns contrast with those described for permafrost-free environments with a much lower δ44/40Ca fractionation factor between soils and plants, suggesting specific features of organic matter decomposition in permafrost environments. The biologically induced Ca isotopic fractionation observed at the soil profile scale is not pronounced at the scale of the streams and large rivers in which the δ44/40Ca signature may be controlled by the heterogeneity of lithological sources.

The effect of selective amino acid supplementation on calcium absorption during a low protein diet

Increasing dietary protein intake within a physiologic range stimulates intestinal Ca absorption but it is not known if specific amino acids (AA) or dietary protein as a whole is responsible for this effect. We therefore selectively supplemented a low protein (0.7 g/kg) diet with either aromatic (Trp, Phe, His) or dibasic (Arg, Lys) AA to achieve intakes comparable to the content of a high protein diet (2.1 g/kg) and measured Ca absorption. Fourteen young women were randomized to a placebo‐controlled, doubleblind, cross‐over feeding trial in which each subject ingested, in random order, a 6‐day low protein diet supplemented with aromatic AA, dibasic AA, or methylcellulose capsules (control) after a 10‐day adjustment period. All subjects ingested all 3 diets. Intestinal Ca absorption was measured between days 5 and 6 using dual stable Ca isotopes. There was no difference in Ca absorption between the diet supplemented with aromatic AA and the control diet (22.9±2.0% vs.22.3±1.4% respectively, p = 0.322). However, Ca absorption was significantly higher with dibasic AA supplementation (25.2±1.4% vs. 22.3±1.4%, p = 0.047). Individuals with protein intakes below the RDA may benefit from Arg and Lys supplementation to improve Ca economy.

High-Precision Measurement of Variations in Calcium Isotope Ratios in Urine by Multiple Collector Inductively Coupled Plasma Mass Spectrometry

We describe a new chemical separation method to isolate Ca from other matrix elements in biological samples, developed with the long-term goal of making high-precision measurement of natural stable Ca isotope variations a clinically applicable tool to assess bone mineral balance. A new two-column procedure utilizing HBr achieves the purity required to accurately and precisely measure two Ca isotope ratios ((44)Ca/(42)Ca and (44)Ca/(43)Ca) on a Neptune multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS) in urine. Purification requirements for Sr, Ti, and K (Ca/Sr > 10 000; Ca/Ti > 10 000 000; and Ca/K > 10) were determined by addition of these elements to Ca standards of known isotopic composition. Accuracy was determined by (1) comparing Ca isotope results for samples and standards to published data obtained using thermal ionization mass spectrometry (TIMS), (2) adding a Ca standard of known isotopic composition to a urine sample purified of Ca, and (3) analyzing mixtures of urine samples and standards in varying proportions. The accuracy and precision of δ(44/42)Ca measurements of purified samples containing 25 μg of Ca can be determined with typical errors less than ±0.2‰ (2σ).

Daily Supplementation with 25 µg Cholecalciferol Does Not Increase Calcium Absorption or Skeletal Retention in Adolescent Girls with Low Serum 25-Hydroxyvitamin D1–4

In healthy adolescents, cross-sectional studies show either no or negative relationships between serum 25-hydroxyvitamin D [25(OH)D] and calcium (Ca) absorption. Using a 2-period metabolic balance study, the effect of vitamin D supplementation on Ca absorption and retention in adolescent girls was investigated. Eleven girls aged 12–14 y with a mean entry serum 25(OH)D of 35.1 nmol/L consumed a controlled intake (providing 5 µg vitamin D and 1117 mg Ca/d) for two 3-wk metabolic balance periods separated by a 1-wk washout period. Sunlight exposure was minimized by sunscreen with a sun protection factor ≥ 15. After the first metabolic balance period, participants received 25 µg/d cholecalciferol supplementation for 4 wk. Fractional Ca absorption was measured in each metabolic balance period using a stable Ca isotope method. All urine and fecal samples were collected and analyzed to measure net Ca absorption and Ca retention. Paired t tests and correlations were used to analyze the data. Daily supplementation with 25 µg vitamin D resulted in a mean increase in serum 25(OH)D of 13.3 nmol/L (P < 0.01) but a decrease in fractional Ca absorption of 8.3% (P < 0.05) and no significant change in fasting serum 1,25-dihydroxyvitamin D, parathyroid hormone, net Ca absorption, or Ca skeletal retention. In pubertal girls with vitamin D status considered insufficient in adults, vitamin D supplementation of 25 µg/d for 4 wk did not improve fractional Ca absorption, net Ca absorption, or Ca retention.

Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

We measured Ca stable isotope ratios (δ 44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m −2) is present as sulfates (2.5 kmol m −2), and to a lesser extent carbonates (0.4 kmol m −2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ 44/40Ca E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ 44/40Ca in the top six horizons resembles that of sulfate-δ 44/40Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ 44/40Ca values observed on Earth. Linear correlation among δ 44/40Ca, δ 34S and δ 18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ 34S values are positively correlated with sulfate-δ 18O values ( R 2 = 0.78) and negatively correlated with sulfate δ 44/40Ca E values ( R 2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ 44/40Ca fractionation factor of −0.4‰ in CaSO 4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.

Pygmy dipole resonance in stable Ca isotopes

The properties of the low-lying electric dipole strength in the stable 40 Ca, 44 Ca and 48 Ca isotopes have been calculated within the Extended Theory of Finite Fermi Systems (ETFFS). This approach is based on the random phase approximation (RPA) and includes the single-particle continuum as well as the coupling to low-lying collective states which are considered in a consistent microscopic way. For 44 Ca we also include pairing correlations. A good agreement with the existing experimental data for the gross properties of the low-lying strength has been obtained. We conclude that for the detailed understanding of the low-lying dipole strength an approach like the present one is absolutely necessary.

Microscopic description of the low lying and high lying electric dipole strength in stable Ca isotopes

Abstract The properties of the low lying and high lying electric dipole strength in the stable 40Ca, 44Ca and 48Ca isotopes have been calculated within the Extended Theory of Finite Fermi Systems (ETFFS). This approach is based on the random phase approximation (RPA) and includes the single particle continuum as well as the coupling to low lying collective states which are considered in a consistent microscopic way. For 44Ca we also include pairing correlations. We obtain good agreement with the existing experimental data for the gross properties of the low lying and high lying strength. It is demonstrated that the recently measured A-dependence of the electric dipole strength below 10 MeV is well understood in our model: due to the phonon coupling some of the strength in 48Ca is simply shifted beyond 10 MeV. The predicted fragmentation of the strength can be investigated in ( e , e ′ ) and ( γ , γ ′ ) experiments. The isovector dipole strength below 10 MeV is small in all Ca isotopes. Surprisingly, the proton and neutron transition densities of these low lying electric dipole states are in phase, which indicate isoscalar structure. We conclude that for the detailed understanding of the structure of excited nuclei e.g. the low lying and high lying electric dipole strength an approach like the present one is absolutely necessary.

Microscopic Nature of the Pygmy Dipole Resonance: The Stable Ca Isotopes

The electric dipole strength distribution in 44Ca has been measured up to 10 MeV in high resolution photon scattering experiments for the first time. The data obtained have been compared to earlier measurements on (40,48)Ca in order to view the evolution of the electric pygmy dipole resonance (PDR). Calculations that were performed within the framework of the microscopic extended theory of finite Fermi systems, which adds contributions of the quasiparticle-phonon coupling to random phase approximation calculations, give a qualitative agreement with the experimental data for all three isotopes. We have shown that it is necessary to include this coupling to describe the PDR.

Protocol for assessing bone health in humans by tracing long-lived 41Ca isotope in urine, serum, and saliva samples

2 Slow bone turnover rate and large day-to-day variation in excreted bone biomarkers create a challenge in assessing bone health in humans [1]. The use of the longlived radioisotope 41-calcium (41Ca; t1/2D»10 years), which has a rare natural abundance (41Ca/CaD»10i15), and accelerator mass spectrometry (AMS)1, which is able to detect 41Ca tracer, show great promise in resolving bone health issues [2–4]. However, sample processing for AMS measurements is a barrier to widespread Ca– AMS applications [5]. This communication summarizes an improved protocol for quantifying 41Ca concentrations in biological specimens. Calcium isotope tracers are present in multiple body pools. Saliva sampling is proposed as a noninvasive alternative to blood sampling in a week-long study of two subjects using stable Ca isotopes [6]. Urine is the ultimate noninvasive sample and is known to reXect average plasma levels [4], but high temporal resolution requires direct blood sampling. The previous AMS preparation protocol [4] did not address measurements of either saliva or serum specimens. Our protocol increases sample throughput at a reduced cost and expands routine 41Ca tracer determinations to a wider range of biological specimens including urine, serum, and saliva samples. Urine, fasting blood, and fasting saliva samples were collected at diVerent time points from a healthy post-

Calcium kinetics are altered in clinically stable girls with cystic fibrosis.

Reduced bone mass in individuals with cystic fibrosis (CF) may result from alterations in calcium metabolism. Bone calcium deposition and resorption rates, calcium balance, and markers of bone turnover were assessed using stable isotopes of calcium in 22 prepubertal and pubertal girls with CF. Bone calcium deposition was associated with the availability of dietary calcium, total serum osteocalcin, and leptin concentrations. Reduced bone mass in individuals with CF may result from inadequate bone calcium (Ca) deposition, and excessive resorption, although these parameters have not been directly assessed in children with CF. We used stable Ca isotopes to measure rates of bone Ca deposition (Vo+), resorption, and retention in 22 clinically stable girls with CF (aged 7-18 yr). Rates of bone Ca deposition were determined by mathematically modeling the disappearance of iv Ca stable isotope ((42)Ca) for 6 d post dosing. Indirect markers of bone turnover and hormones associated with pubertal development were also assessed. Rates of bone Ca deposition and retention were highest during early puberty (Tanner stages 2 and 3). Calcium deposition rates in prepubertal (Tanner 1) and postmenarchal girls (Tanner stages 4 and 5) did not support substantial bone Ca retention. Net absorption of dietary Ca and serum osteocalcin and leptin concentrations were positively associated with Vo+. Time post menarche and serum leptin concentrations explained 91% of the variability in Vo+ (P = 0.0007). Serum total osteocalcin was low (10.9 +/- 5.4 ng/ml), and a substantial percentage of osteocalcin was undercarboxylated (54.3 +/- 11.8%). We concluded that increased calcium absorption and serum leptin concentrations were significantly associated with rates of bone Ca deposition, demonstrating an impact of nutritional status on this process. Rates of bone Ca deposition were lower than typically reported in healthy children, as were indirect markers of bone formation. These alterations in bone turnover contribute to reduced bone mass in girls with CF.

TSUP]41[/TSUP]Ca in Presolar Graphite of Supernova Origin

Five low-density graphite grains from the Murchison meteorite have large 41K excesses that can be attributed to the decay of 41Ca. The inferred 41Ca/40Ca ratios, which range up to 1.7 × 10-2, and other isotopic signatures,15N,18O, and 28Si excesses and high 26Al/27Al ratios, indicate a Type II supernova origin for these grains and are evidence for heterogeneous mixing of supernova ejecta.41Ca seems to mainly originate from neutron capture in the C-rich and O-rich supernova zones, with only minor contributions from the O-burning zone. Admixture of material from the O-rich zone to outer C-rich layers is required to explain the abundance patterns of the stable Ca isotopes. In contrast, the Ti isotopic compositions reflect that of the C-rich zone and do not require substantial amounts of material from the O-rich zone.

Ion microscopic imaging of calcium during 1,25-dihydroxyvitamin D-mediated intestinal absorption.

A combination of ion microscopic and conventional radionuclide techniques was employed to investigate the temporal-spatial dynamics of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-stimulated intestinal calcium (Ca) absorption. At varying times following the administration of a single intravenous dose of 1,25(OH)2D3 to vitamin D-deficient chicks, transepithelial transport and tissue retention of Ca were quantitated in vivo, using the ligated duodenal loop technique and 47Ca as the tracer. The localization of Ca in the intestinal tissue during absorption was monitored by ion microscopy, using the stable Ca isotope, 44Ca, as the absorbed species. There was little transepithelial absorption of Ca in the vitamin D-deficient animals despite a substantial tissue accumulation of luminally derived Ca, the latter localizing predominantly in the brush border region of the enterocyte, as shown by the 44Ca-ion microscopic images. The early (30 min-1 h) response to 1,25(OH)2D3 was an increased tissue uptake of luminal 47Ca, which also primarily associated with the brush border region, again as shown by ion microscopy. At 2-4 h after the 1,25(OH)2D3 dose, there was a progressive redistribution of Ca from the brush border region throughout the cytoplasm and into the lamina propria. At 8-16 h, 47Ca absorption was maximal and 44Ca was sparsely distributed in the intestinal tissue. 47Ca absorption gradually declined and reached pre-dose levels by 72 h. At this time, tissue 44Ca was again largely limited to the brush border region. These results provide support for the multiple actions of 1,25(OH)2D3 on the intestinal Ca absorption process. The ion microscopic images provided unique information on the specific time-dependent changes in the tissue localization of Ca during the process of its intestinal absorption as affected by 1,25(OH)2D3.

Saturation of the Ca intercombination line and its application to spectroscopy and frequency standards

Experiments were carried out using a Ca atomic beam and a tunable dye laser. Lamb dips were obtained in the fluorescence line of various Ca isotopes. It is shown that the narrow signals associated with saturation of the 4s2 1S0–4s4p 3P1, intercombination transition are potentially useful for frequency standards and highresolution spectroscopy. Measurements were made of the nuclear charge radii of six stable Ca isotopes. Radiofrequency transitions between excited state sublevels were studied. The nuclear quadrupole moment of the rarely encountered isotope 43Ca was determined.

Isotope shift in the Ca I resonance line and changes in mean-square nuclear charge radii of the stable Ca isotopes

The isotope shift of all stable Ca isotopes was studied in the Ca I resonance line. Enriched isotopes were used in an atomic beam, passing through the center of a spherical Fabry-Perot interferometer. The measured isotope shifts were separated into mass shift and field shift by comparing the optical isotope shift with δ〈r2〉 values derived from recent muonic x-ray measurements. The results are discussed together with known data on the isotope shift in the Ca I intercombination line and data from Hartree-Fock calculations. The following mean values are obtained for the change in nuclear charge distribution δ〈r2〉 [fm2]∶ [40, 42] 0.23(3); [40, 43] 0.13(3); [40, 44] 0.28(4); [40, 46] 0.14(7); [40, 48] 0.00(2).