Exchangeable Calcium Pool Research Articles

Calcium balance in peritoneal dialysis patients treated by continuous ambulatory peritoneal dialysis (CAPD) and automated peritoneal dialysis (APD) cyclers

IntroductionAlthough vascular calcification is a recognised complication for haemodialysis patients, peritoneal dialysis (PD) patients are also at risk. As such we wished to review peritoneal and urinary calcium balance and the effect of calcium containing phosphate binders (CCPBs).MethodsTwenty-four-hour peritoneal calcium balance and urinary calcium were reviewed in PD patients undergoing their first assessment of peritoneal membrane function.ResultsResults from 183 patients, 56.3% male, 30.1% diabetic, mean age 59.4 ± 16.4 years, median 2.0 (2–6) months of PD, 29% treated by automated PD (APD), 26.8% continuous ambulatory (CAPD) and 44.2% APD with a day-time exchange (CCPD) were reviewed. Peritoneal calcium balance was positive in 42.6%, and remained positive in 21.3% after including urinary calcium losses. PD calcium balance was negatively associated with ultrafiltration (odds ratio 0.99 (95% confidence limits 0.98–0.99), p = 0.005. PD calcium balance was lowest with APD (APD − 0.45 (− 0.78 to 0.05) vs CAPD − 0.14 (− 1.18 to 0.59) vs CCPD − 0.03) − 0.48 to 0.5) mmol/day), p < 0.05, with 82.1% of patients with a positive balance prescribed icodextrin, when combining peritoneal and urinary losses. When considering CCPB prescription, then 97.8% of subjects prescribed CCPD had an over-all positive calcium balance.DiscussionOver 40% of PD patients had a positive peritoneal calcium balance. Elemental calcium intake from CCPB had a major effect on calcium balance, as median combined peritoneal and urinary calcium losses were < 0.7 mmol/day (26 mg), so caution is required to prevent excessive CCPB prescribing, increasing the exchangeable calcium pool and thus potentially increasing vascular calcification, particularly for anuric patients.Graphical abstract

Long-term changes in soil composition in unmanaged central European mountain spruce forests after decreased acidic deposition and a bark beetle outbreak

We studied the effect of tree mortality on nutrient concentrations and pools in soils of two unmanaged mountain forest catchments (Plešné, PL; and Čertovo, CT; Czech Republic) recovering from acidification due to decreasing acidic atmospheric deposition since the late 1980s. Both catchments were situated at similar elevation, and before a bark beetle outbreak were covered by healthy mature Norway spruce (Picea abies) forests. However, differing bedrock [granite (PL) vs mica-schist and quartzite (CT)] resulted in lower concentrations of base cations (except for magnesium), but higher concentrations of aluminium and iron oxyhydroxides in the CT soils, enabling higher phosphorus (P) accumulation. Despite these differences, soils were similarly acidic, with pHH2O from 3.5 to 4.3 and cation exchange capacity (CEC) dominated by exchangeable protons (H+ex) and aluminium in both catchments in 2000. In the PL catchment, >75 % of mature spruce trees died after a bark beetle infestation between 2004 and 2008, and all dead biomass was left on site. Forest damage in the CT catchment was minor. Soil concentrations and pools of exchangeable calcium (Ca2+ex), magnesium (Mg2+ex), potassium (K+ex), and H+ex were tightly related to concentrations of organic carbon (C). Following the tree dieback in the PL catchment, we observed significant (p < 0.05) increases in (1) concentrations and pools of Ca2+ex, Mg2+ex, and K+ex, (2) base saturation (from 30 to ∼ 40 %), (3) CEC:C ratios (i.e., CEC of soil organic carbon), (4) pHH2O, (5) mineral P forms, and (6) total nitrogen (N) in the upper (O and A) soil horizons. Similar changes were less pronounced or negligible in the less impacted CT soils. The elevated litterfall after tree dieback, resulting in the increased input of base cations to forest floor and increased CEC of soil organic C, played a more important role in the decreasing acidity of the PL soils than the continuing decrease in acidic deposition.

Interaction of land use history, earthworms, soil chemistry and tree species on soil carbon distribution in managed forests in Vermont, USA

Forest soils store a globally important pool of carbon (C) and reforestation has the potential to increase this pool. Past land use, tree species composition, and current management are known to affect carbon storage rates and amounts. Another important factor, especially in the vertical distribution of soil C, is the presence or absence of earthworms. We investigated these interactions in eighteen managed forest sites in Vermont that have varied land use history (cultivation, pasture, woodlot, continuous woodland) and a range of time since reforestation from 60 to 100+ years. Soils were sampled to about 1 m, or shallower if bedrock was reached, and both C and exchangeable cation pools quantified. Earthworms were enumerated to a depth of 20 cm and identified by species. Basal area by tree species was also measured and aboveground C calculated. Detailed land use history was documented using archival sources. Forest type was primarily northern hardwood, with a few lowland spruce-fir sites included. Eleven different species of earthworms were found, representing all ecotypes, and numbers ranged from 0 (at 8 of the 18 sites) to 319 individuals/m2. Carbon pools in the forest floor (1.5–30.1 Mg/ha) were greater with higher site elevation (range of 154–651 masl) and also greater with lower numbers of earthworms. Besides elevation, the best predictor of earthworm presence or absence, and overall numbers, was the soil exchangeable calcium (Ca) pool in the Oa and A horizons (range of 9.4–1288 cmolc/m2). Because soil exchangeable Ca and aluminum (Al) pools were inversely related, higher exchangeable Al was associated with lower earthworm numbers. In the mineral soil, the C pool (82.1–210.8 Mg/ha) was best explained by the thickness of the B horizons with the exchangeable Al pool as a positive, secondary factor. The thickness of the A horizon (0–16.3 cm), and its contribution to the entire profile C pool (0–54.7 Mg/ha), decreased relative to past land use in the order: cultivated < pasture < woodlot. No earthworm metric was strongly related to the C pool in either the mineral soil or the full soil profile. Dramatic differences in the vertical distribution of carbon were related to past land use and earthworm presence, which in turn was negatively correlated with elevation and the presence of conifers. While it is clear that earthworms and prior land-use have a long-lasting effect on the forest floor C pool, it is less clear whether or not there is an effect on full profile C sequestration over time.

Intradialytic Calcium Kinetics and Cardiovascular Disease in Chronic Hemodialysis Patients

Background/Objective: Calcium loading has been associated with cardiovascular risk in hemodialysis (HD) patients. However, it remains to be elucidated whether alterations of intradialytic calcium buffering add to the increased cardiovascular disease burden in this high-risk population. Methods: Intradialytic calcium kinetics was evaluated in a cross-sectional observational study by measuring dialysate-sided ionized calcium mass balance (iCa_MB), calcium buffer capacity, and change in serum calcium levels in 40 chronic HD patients during a routine HD session. A dialysate calcium of 3.5 mEq/L was used to adequately challenge calcium buffer mechanisms. Aortic pulse wave velocity and serum osteocalcin levels were measured prior to the HD session. Presence of cardiovascular disease and diabetes was assessed. Results: The mean dialysate-sided iCa_MB, extracellular fluid ionized calcium mass gain, and buffered ionized calcium mass were 469 (±154), 111 (±49), and 358 (±145) mg/HD, respectively. The mean ionized serum calcium increase (∆iCa) was 0.42 (±0.14) mEq/L per HD. The mean intradialytic calcium buffer capacity was 73 (±18)%. Multivariate regression analysis revealed significant independent association of (1) iCa_MB with the dialysate-to-blood calcium gradient at HD start and (2) intradialytic calcium buffer capacity with undercarboxylated osteocalcin. The presence of coronary heart disease was associated with higher ∆iCa but not iCa_MB in the multivariate model. Conclusions: In line with our proof-of-concept study, we provide clinical evidence for a rapidly accessible and exchangeable calcium pool involved in intradialytic calcium regulation and for the role of osteocalcin as a potential biomarker. Our findings argue for evaluating the prognostic potential of intradialytic calcium kinetics in prospective clinical trials.

Acute calcium kinetics in haemodialysis patients.

To avoid excessive calcium loading in haemodialysis (HD) patients, current guidelines suggest a dialysate calcium concentration (dCa) of 2·5 mEq/L based on relatively stable intradialytic serum calcium levels. However, the latter do not account for possible calcium storage in acutely accessible pools. A rapidly exchangeable calcium pool located at the bone level has been previously proposed to be involved in acute (minute-to-minute) extracellular calcium regulation. To evaluate the contribution of this pool in the maintenance of serum calcium levels, acute calcium buffer capacity was assessed by measuring intradialytic dialysate-sided ionized calcium mass balance (iCaMB ) and change in extracellular fluid calcium mass in chronic HD patients using a dCa of 3·5 (n = 28) and 2·5 (n = 10) mEq/L. Serum osteocalcin, the most abundant noncollagenous bone protein, was measured before the HD session. iCaMB was invariably positive for both 2·5 and 3·5 mEq/L dCa, with a mean of 434 (±125) and 725 (±162) mg/HD, respectively (P < 0·001). Buffered intradialytic calcium load was 410 (±116) and 565 (±130) mg/HD, and acute calcium buffer capacity was 95 (±8)% and 78 (±7)% (mean values at 2·5 and 3·5 mEq/L dCa, respectively) (P < 0·001). Using 3·5 mEq/L dCa, an independent association of acute calcium buffer capacity with undercarboxylated osteocalcin (β = 0·512, P = 0·002) was demonstrated. Our data strongly suggest the existence of a rapidly exchangeable calcium pool that counteracts acute serum calcium deviations in HD patients. This study provides, for the first time, experimental evidence for the involvement of bone in acute extracellular calcium regulation in vivo.

Long-term soil calcium depletion after conventional and whole-tree harvest

Whole-tree harvest (WTH) may compromise tree productivity and health and lead to soil and surface water acidification. The aim of this study was to assess the long-term change (up to circa 40years) in soil exchangeable calcium (Ca2+) pools following conventional (CH) and WTH at three Swedish coniferous sites. A second aim was to evaluate how well the results could be reproduced by the dynamic model MAGIC (Model of Acidification of Groundwater in Catchments). Soil Ca2+ pools (down to 20cm) decreased at all three sites from stand age 15–16 to 37–38years. The depletion ranged from 2.6 to 8.6kEqha−1 (26.5–52.7%) and 0.2 to 5.0kEqha−1 (2.3–49.1%) in the CH and WTH treatment, respectively. The presence of an interaction effect indicated that the main effect of time was not statistically significant at all three sites. Over the course of time, soil Ca2+ pools have also become more similar between the CH and WTH treatments, but the Ca2+ pools were still significantly lower (p<0.05) after WTH at stand age 37–38years. The measured declines in Ca2+ pools were generally greater than what has been found in other studies and were largely explained by high soil Ca2+ availability and high tree Ca2+ uptake, especially in the CH-plots, as indicated by the MAGIC mass balance budgets. Model simulations by MAGIC partly agreed with the measured data. However, the model exaggerated the soil Ca2+ losses between 1990 and 2013 (CH=3.6–9.9kEqha−1; WTH=3.0–8.3kEqha−1), especially at the spruce sites. Furthermore, MAGIC could not reproduce the rapid diminishing differences between CH and WTH. Uncertainties in model parameters, underestimated soil Ca2+ pools or biological feed-back mechanisms could explain this discrepancy. Until these have been resolved, interpretations of Ca2+ changes related to CH or WTH using dynamic modelling or mass balance budget calculations should be done with caution.

Acute Severe Hypercalcemia After Traumatic Fractures and Immobilization in Hypophosphatasia Complicated by Chronic Renal Failure

Hypophosphatasia (HPP) features deficient activity of the "tissue-nonspecific" isoenzyme of alkaline phosphatase (TNSALP) due to loss-of-function mutation(s) within the TNSALP gene. Consequently, inorganic pyrophosphate, a TNSALP substrate and inhibitor of mineralization, accumulates extracellularly. This can cause rickets or osteomalacia. We report a 55-year-old man with HPP and chronic renal failure (CRF) requiring hemodialysis who developed severe hypercalcemia acutely after traumatic fractures and immobilization. He manifested HPP in childhood and in middle age received hemodialysis for CRF attributed to hypertension and anti-inflammatory medication. He took 2 g of calcium carbonate orally each day to bind dietary phosphorus, but never aluminum hydroxide or any form of vitamin D. Pretrauma serum levels of calcium spanned 8.4-10.7 mg/dL (normal [Nl], 8.6-10.3), inorganic phosphate 5.8-6.4 mg/dL (Nl, 2.5-4.5), and PTH 63-75 pg/mL (Nl, 10-55). Rapid succession falls fractured multiple major bones. Six hours later, he became confused. Serum calcium was 14.9 mg/dL, ionized calcium was 7.4 mg/dL (Nl, 4.5-5.1), and PTH was 16 pg/mL. Hemodialysis quickly corrected his hypercalcemia and confusion. Low serum alkaline phosphatase persisted, and follow-up skeletal histopathology showed that his osteomalacia was severe. Hemodialysis does not heal the skeletal disease of HPP. During sudden fracture immobilization in HPP, sufficient calcium can emerge from bone, perhaps from a rapidly exchangeable calcium pool, to cause acute severe hypercalcemia if the kidneys cannot compensate for the mineral efflux. Hence, we worry that acute hypercalcemia might accompany sudden immobilization in CRF patients without HPP if they have adynamic bone disease.

Chemical properties of upland forest soils in the Catskills region

The Catskills forest provides a valuable array of ecosystem services for local and regional populations, including the provision of forest products, wildlife habitats, and high-quality water. These services depend on chemical and biological processes that occur in forest soils. In 2011, we sampled soils in 25 headwater catchments in the Catskills region to quantify the pools of soil nutrients and examine the variation in soil properties in the region. The average soil depth in the 50 excavated pits was 56.6 cm. Average soil mass was 205 kg/m(2). The pools of soil carbon and nitrogen averaged 58.5 and 3.95 Mg/ha, respectively. The thin organic horizons accounted for less than 1% of soil mass, but included 14% of the soil carbon and 11% of soil nitrogen. Catskills forest soils are highly acidic, with mean pH ranging between 3.9 and 4.75. Base saturation was high (>60%) in organic horizons and low (12-31%) in mineral soils. The pool of exchangeable calcium is approximately equivalent to 20 years of calcium export from headwater streams, raising concerns regarding the ability of these catchments to maintain current stream calcium concentrations. The data and samples collected in this study provide a baseline for future soil monitoring in the region.

The effect of harvest intensity on long-term calcium dynamics in soil and soil solution at three coniferous sites in Sweden

Removal of logging residues may lead to soil and surface water acidification by lowering the amount of buffering base cations. Long-term treatment differences in soil exchangeable calcium pools (down to 20cm) and soil solution calcium concentrations at 50cm soil depth were examined at three coniferous sites in Sweden following conventional and whole-tree harvesting in 1974–1976. Results from the statistical analysis (p<0.05) showed that soil water concentrations of calcium were 17μeql−1 (or 40%) lower in whole-tree harvested plots compared with conventional harvested plots, 27–30years after harvest. The main treatment differences had largely disappeared 32–35years after harvest although site specific treatment differences (ΔWTH–CH: −24μeql−1) were still measurable at the well-buffered site in northern Sweden. These results are in agreement with soil data showing that previously found treatment differences in calcium pools had diminished in the forest floor but remained in deeper soil layers (−0.29, −0.37 and −0.24kmolcha−1 in the 5–10, 10–15 and 15–20cm soil layer, respectively). The presence of an interaction effect in the 10–15cm soil layer indicates, however, that the treatment response on calcium pools is much less at the southern sites. These results indicate that the effect of WTH on soil and soil solution concentrations is temporary but site specific. Contrary to common beliefs, the greatest soil and soil water effect was observed at the well-buffered site where the loss of calcium during WTH is less likely to lead to acidification effects. The treatment effects on soil solution at the more acidic sites in southern Sweden were much smaller and probably not large enough to fully counterbalance the general recovery from acidification during the study period.

Serum 1,25-dihydroxyvitamin D and calcium intake affect rates of bone calcium deposition during pregnancy and the early postpartum period

Factors affecting bone calcium deposition across pregnancy and lactation are not well characterized. The impact of maternal age, calcium intake, race-ethnicity, and vitamin D status on the rate of bone calcium deposition (VO+) was assessed across pregnancy and lactation. Stable calcium isotopes were given to 46 women at pre- or early pregnancy (trimester 1), late pregnancy (trimester 3), and 3-10 wk postpartum. Three cohorts were included: 23 adolescents from Baltimore (MD), aged 16.5 ± 1.4 y (mean ± SD; Baltimore cohort); 13 adults from California, aged 29.5 ± 2.6 y (California cohort); and 10 adults from Brazil, aged 30.4 ± 4.0 y (Brazil cohort). The total exchangeable calcium pool, VO+, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D [1,25(OH)₂D], parathyroid hormone, and calcium intake were evaluated. At trimester 3, inverse associations between 1,25(OH)₂D and VO+ were evident in the Baltimore (P = 0.059) and Brazil (P = 0.008) cohorts and in the whole group (P = 0.029); calcium intake was not a significant determinant of VO+ in any group during pregnancy. At postpartum, a significant positive association was evident between VO+ and calcium intake (P ≤ 0.002) and between VO+ and African ethnicity (P ≤ 0.004) in the whole group and within the Baltimore and Brazil cohorts. Elevated 1,25(OH)₂D was associated with decreased rates of bone calcium deposition during late pregnancy, a finding that was particularly evident in pregnant adolescents and adult women with low calcium intakes. Higher dietary calcium intakes and African ethnicity were associated with elevated rates of bone calcium deposition in the postpartum period.

The exchangeable calcium pool: physiology and pathophysiology in chronic kidney disease

Excessive soft tissue and vascular calcifications are typical complications of chronic kidney disease (CKD) and disorders of phosphate homeostasis are considered to be a major contributor to the pathogenesis. However, at least in some individuals, calcium administration also increases the risk, and furthermore, it is widely accepted that there is a link between bone disease and vascular pathology. In this review, we discuss the role of the bone exchangeable calcium pool (ECP) in the acute regulation of the serum calcium concentration (Ca(s)) in health and CKD. This pool is able to buffer an acute calcium load as well as to maintain a stable Ca(s) during acute calcium deprivation. Indeed, the minute-to-minute regulation of Ca(s) appears to depend exclusively on this mechanism without any obvious contribution of other factors like parathyroid hormone, which nonetheless define the Ca(s) steady state set point. It is tempting to speculate that a reduction of the bone ECP plasticity in some patients with CKD leads to short-lasting increases in Ca(s) above the individual mid- to long-term set point as observed during haemodialysis or after the ingestion of calcium-containing phosphate binders. This could contribute to and partially explain the propensity of these subjects to develop extraosseous calcifications. An improved understanding of the processes involved and the availability of new techniques to assess the capacity of this pool, at least in dialysis patients, will make this area an attractive target for new investigations.

Biogeochemical cycling and chemical fluxes in a managed northern forested wetland, Michigan, USA

Forest harvesting and subsequent regeneration treatments may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation pools and fluxes, and to construct a hydrogen ion (H+) mass balance to identify major processes controlling acidity production and consumption 14 years following whole-tree harvesting and regeneration in a northern forested wetland with underlying mineral soils derived from calcareous glacial drift. Results for soil elemental and nutrient pools in the harvested/regenerated stand were compared to an adjacent non-harvested stand and a riparian zone. The riparian zone had the highest soil total C, total N, and exchangeable calcium (Ca) and magnesium (Mg) pools; however, no difference in exchangeable potassium (K) was evident among stand types. Moreover, no differences between the harvested/regenerated and uncut stands were evident in any of the soil chemical pools.Net export of base cations was minimal and the H+ mass balance indicated that net cation exchange was not a significant process in H+ production or consumption in either the uncut or harvested/regenerated stands. The most striking differences in the H+ mass balance were (1) eight times the H+ consumption from sulfate (SO42−) reduction in the harvested/regenerated stand compared to that in the uncut condition and (2) nearly twice the H+ production due to N immobilization in the harvested/regenerated stand. However, both stand types were net H+ sinks and increases in H+ export due to whole-tree harvesting were not evident.The riparian zone was a net exporter of base cations. This finding was attributed to a combination of base cation exchange and carbonate mineral weathering; data suggested the importance of the latter. More research, however, is required to isolate the contributions of cation exchange and carbonate weathering on base cation export from the riparian zone. Stream chemistry was consistent with that of the riparian zone, indicating a strong linkage between the riparian zone stream chemistry, and whole-tree harvesting had no intermediate term (i.e., 14 years) effects on stream acidification in this managed northern wetland ecosystem.

Synthetic human calcitonin in Paget's disease of bone and osteoporosis (author's transl)

Synthetic human calcitonin was used in the treatment of 26 patients over a period of 1-14 months. 17 patients had Paget's disease of the bone, 6 postmenopausal osteoporosis and 3 Sudeck's syndrome. Subjective improvement (reduction of pain, improvement of mobility) was found in 15 patients with Paget's disease, in 4 females with postmenopausal osteoporosis and in all 3 patients with Sudeck's syndrome. Radiographic improvement of bone changes developed only very slowly. These results were confirmed by diminution of the exchangeable calcium pool indicating reduction of rates of osseous degradation. Calcitonin tolerance was acceptable. Transitory nausea and occasional vomiting occurred in 3 patients.

Bone as an ion exchange organ: Evidence for instantaneous cell-dependent calcium efflux from bone not due to resorption

The current study tests the hypothesis that basal level and minute-by-minute correction of plasma Ca 2+ by outward and inward Ca 2+ fluxes from and into an exchangeable ionic pool in bone is controlled by an active partition system without contributions from the bone remodeling system. Direct real-time measurements of Ca 2+ fluxes were made using the scanning ion-selective electrode technique (SIET) on living bones maintained ex vivo in physiological conditions. SIET three-dimensional measurements of the local Ca 2+ concentration gradient (10 μm spatial resolution) were performed on metatarsal bones of weanling mice after drilling a 100-μm hole through the cortex to expose the internal bone extracellular fluid (BECF) to the bathing solution, whose composition mimicked the extracellular fluid (ECF). Influxes of Ca 2+ towards the center of the cortical hole (15.1 ± 4.2 pmol cm −2 s −1) were found in the ECF and were reversed to effluxes (7.4 ± 2.9 pmol cm −2 s −1) when calcium was depleted from the ECF, mimicking a plasma demand. The reversal from influx to efflux and vice versa was immediate and fluxes in both directions were steady throughout the experimental time (≥2 h, n = 14). Only the efflux was nullified within 10 min by the addition of 10 mM/L Na-Cyanide ( n = 7), demonstrating its cell dependence. The timeframes of the exchanges and the stability of the Ca 2+ fluxes over time suggest the existence of an exchangeable calcium pool in bone. The calcium efflux dependency on viable cells suggests that an active partition system might play a central role in the short-term error correction of plasma calcium without the contribution of bone remodeling.

Mycorrhizal weathering in base-poor forests

Minerals in soil contain many small pores, which has led to the suggestion that trees are able to 'mine' essential nutrients such as calcium through their association with symbiotic mycorrhizae, thereby bypassing the exchangeable calcium pool in the soil1,2. On the basis of the calcium-to-strontium (Ca/Sr) ratios in the foliage of trees at Hubbard Brook (an experimental forest), Blum et al. suggest that these trees have direct access to calcium from apatite (calcium phosphate) in lower soil horizons through their mycorrhizal associations3. But because Ca/Sr ratios can vary greatly in different parts of a plant, Ca/Sr ratios in foliage are poor indicators of the source of calcium for trees. Differences in Ca/Sr ratios in tree foliage should be interpreted with caution until more is known about the cycling of these elements in forests.

The study of skeletal calcium metabolism with 41Ca and 45Ca

The living skeleton can be labeled for life by the administration of radiologically trivial amounts of 41Ca tracer. After initial elimination of tracer from the readily exchangeable calcium pools subsequent skeletal calcium turnover maintains and modulates the urine 41Ca content. Uniquely, bone calcium metabolism may then be studied with tracer in near equilibrium with the body's calcium and resorbing calcium directly measured by accelerator mass spectrometry (AMS) of excreta. Our experiments with 25 41Ca labeled subjects demonstrate excellent diurnal stability and remarkable response to intervention of the urine signal. Thus the tracer method may prove a competitive means of measuring the effects of antiresorptive osteoporosis treatments, for therapy development or even clinical monitoring. Novel studies of long-term skeletal evolution are also possible. We realize that routinely administered short-lived calcium radiotracers contain 41Ca impurities and that thousands of experimental participants have been historically inadvertently 41Ca labeled. The 41Ca urine index might now rapidly further be characterized by contemporary measurements of these one-time subjects, and with their by now thoroughly skeleton-equilibrated tracer they might be ideal participants in other new experiments. We are also investigating 45Ca AMS. It may prove preferable to label the skeleton with this radiotracer already familiar to bioscientists, but new to AMS.

Effect of growth hormone treatment on calcium kinetics in patients with osteogenesis Imperfecta Type III and IV

Using a dual stable isotope technique, the effect of growth hormone (GH) on whole body calcium (Ca) metabolism was studied in children (ages 5–14 years) with type III (n = 9) and IV (n = 8) osteogenesis imperfecta. Each subject was studied twice: at baseline and following a GH (0.1–0.2 U/kg per day) treatment period of 1–1.5 years. Subjects were given 42Ca intravenously and 44Ca orally. The sera and urine 42Ca and 44Ca isotopic enrichments were followed over 7 days using thermal ionization mass spectrometry. The S aam program was used to fit a three-compartment model to the tracer data. No significant differences were observed between: (1) children with type III and IV disease; or (2) baseline studies of boys and girls within each disease type. However, GH treatment significantly increased: (1) the exchangeable calcium pool (EP) in type III patients (2086 vs. 4422 mg/day, p = 0.02); and (2) the parameter associated with bone calcium accretion in type IV patients (Vo +: 973 vs. 1560 mg/day, p = 0.03) with boys responding with a significantly greater increase than girls (p = 0.008). Although not statistically significant, a trend toward an increase in Vo + in type III patients and in EP in type IV was observed following treatment. Our observations imply that more Ca was available for bone mineralization following GH treatment in these subjects.

Assessing soil calcium depletion following growth and harvesting of Sitka spruce plantation forestry in the acid sensitive Welsh uplands

Abstract. A simple mass balance has been used to estimate soil calcium depletion during the growth of a 50 year old Sitka spruce crop on acid, base-poor peaty podzol soils in upland Wales. Growth of the crop will deplete the soil calcium reserve by an amount (205 kg Ca ha-1) approximately equivalent to the exchangeable calcium pool to the bottom of the profile and equal to 14% of the total soil calcium reserve to the bottom of the B horizon. Despite these predictions, measurements of exchangeable calcium show no differences beneath mature forest and acid grassland, implying that i) weathering rates in forest soils are greater than long-term estimates and predictions by the PROFILE soil chemistry model ii) the trees can access other sources of calcium or iii) there are significant errors in the mass balance. Following stem-only harvesting, growth of a 50 year old second rotation crop will lead to further depletion of soil calcium, but this amount (79 kg Ca ha-1), is less than for a second rotation crop following whole-tree harvesting (197 kg Ca ha-1). After the first crop, stem-only harvesting would allow a further 18 rotations before depletion of the total calcium reserve to the bottom of the B horizon. Whole-tree harvesting would allow for seven rotations after the first crop. These calculations assume that all sources of calcium are equally available to the crop. This can only be resolved by dynamic modelling of the calcium cycle at the ecosystem scale based on appropriate field measurements. The potential for significant soil acidification is therefore greater following whole-tree harvesting and, in line with current recommendations (Nisbet et al., 1997), this technique should probably be avoided on acidic, nutrient-poor soils unless remedial measures are included to enhance the soil base cation status.

Protein Kinase C Activation Stimulates Calcium Transport in Adrenal Zona Glomerulosa Cells

Adrenal zona glomerulosa (ZG) cells produce aldosterone in response to angiotensin II and extracellular potassium through different mechanisms which involve changes in cytosolic free calcium (Cai). Protein kinase C (PKC) activation is part of the angiotensin II signalling cascade but its effects on Caiare unknown. PKC activation with 1 μM phorbol 12-myristate 13-acetate (PMA) and 8 mM Kosignificantly increased the rate of calcium influx (P < 0.001). Both the PKC- and the Ko-induced calcium influx occurred via a nifedipine-sensitive pathway. When both were combined, PKC activation and 8 mM Kowere not additive over either agent alone. PKC activation and 8 mM Koalso stimulated calcium efflux (P < 0.01). When combined together PKC activation and 8 mM Kohad additive effects on calcium efflux (P < 0.05). PKC activation did not increase Cainor the exchangeable calcium pool in contrast to 8 mM Kowhich significantly increased both (P < 0.001). Thus, PKC activation in ZG cells induces a pattern of calcium transport characterized by accelerated calcium recycling across the cell membrane without increasing cell calcium content.

Pathways of cerebral calcium accumulation in a model of focal ischemia in rats

Focal cerebral ischemia was produced in anesthetized rats by a minimally invasive photothrombic procedure. Rose bengal was injected into a tail vein and the right middle cerebral artery region irradiated for 5 min through the skull with the right common carotid artery temporarily occluded. This resulted in focal cerebral infarction which was restricted to the cortex as shown by autoradiography and histopathology. Edema and the uptake of 45Ca were determined 1, 3 or 24 hours after ischemia in different regions of the brain, ipsilateral and contralateral to the ischemic injury, the tracer uptake at three time points after administration. The values of 45Ca uptake and edema were the highest at the center of the infarction. Simulation of the 45Ca uptake kinetics in the 24 h post-ischemic group, enabled -the determination of the contributions of different physiological pathways to cerebral calcium flux. The results indicated the breakdown of the blood-brain barrier to be primarily responsible for the increased uptake of the tracer by the ischemic cortex. A concomitant, presumably intracellular, sequestration resulted in a ca. 76-fold increase in the tissue pool ofexchangeable calcium. Simulation such as proposed here would be of value in predicting the outcome of tracer accumulation in pathological situations [Neural Res 1998; 20: 169–177]