Reduced Calcium Levels Research Articles

Malignant Hypercalcemia in Pregnancy

Hypercalcemia in pregnancy can have devastating effects on both mother and fetus. Pamidronate, a bisphosphonate and inhibitor of bone resorption, has been used to treat hypercalcemia, but it is considered contraindicated in pregnancy. A 40-year-old multigravida presented at 24 weeks of gestation with metastatic breast cancer. After unsuccessful chemotherapy, at 28 weeks of gestation she had deteriorating renal function, frequent contractions, and a calcium level of 17.6 mg/dL. When conventional treatment of hypercalcemia failed, pamidronate dramatically decreased both the calcium levels and the frequency of uterine contractions. Pamidronate in pregnancy successfully reduced severe maternal hypercalcemia and uterine contractions. Although the reduction in calcium level did not prevent the development of preeclampsia or further deterioration in the mother's condition, it was not associated with any adverse effects on the neonate.

Calcium is involved in EGF‐induced activation of ERK5 by mediating Lad‐MEKK2 interaction.

The ERK5 cascade is a MAPK pathway that transmits both mitogenic and stress signals, yet its mechanism of regulation is poorly understood. Here we undertook to elucidate the mechanism of activation of the ERK5 cascade in response to EGF stimulation. Using calcium chelators, we found that ERK5 activation by EGF is inhibited by the depletion of intracellular calcium levels. This calcium effect was found to occur at the MEKK2 level, upstream of the ERK5 cascade. Co-immunoprecipitation studies revealed that EGF increases the binding of MEKK2 to the adaptor protein Lad, which is known to mediate EGF signals to ERK5. This Lad-MEKK2 interaction was abolished by the calcium chelators, indicating that elevated calcium concentrations are required for the interactions, and thereby for the transmission of EGF signals to the ERK5 cascade. In addition, we found that while endogenous ERK5 and MEK5 are localized in the nucleus of resting and stimulated cells, MEKK2 is distributed all over resting cells, and upon EGF stimulation accumulates in the nucleus. The diffused cellular distribution of MEKK2 is probably caused by a free shuttle between the cytosol and the nucleus, as treatment with Leptomycin B resulted in a nuclear accumulation of the protein. Reduction in calcium levels inhibited the EGF-induced MEKK2 translocation in a Lad-independent manner, indicating that the effect of calcium on the translocation is probably due to the lack of MEKK2 activation. Taken together, these findings suggest that calcium is required for ERK5 activation by EGF by securing the proper interaction of the MEKK2 at the MAP3K level of this cascade with the upstream adaptor protein Lad.

Rapid Intraoperative Parathyroid Hormone Assay—More than Just a Comfort Measure

Minimally invasive radio-guided parathyroidectomy (MIRP) has been embraced as an acceptable therapeutic approach to primary hyperparathyroidism. Preoperative sestamibi scanning has facilitated this technique. Here we evaluate the addition of a rapid intraoperative parathyroid hormone (iPTH) assay for patients undergoing MIRP. A series of 51 patients underwent sestamibi localization of parathyroid glands followed by MIRP for primary hyperparathyroidism. Using peripheral venous samples, iPTH levels were measured prior to gland excision, as well as post-excision at 5, 10, and 15 minutes, taking a 50% reduction in iPTH level as indicative of complete excision. Next, changes in serum iPTH were compared with preoperative and postoperative changes in serum calcium, as well as levels of intraoperative ex-vivo radiation counts taken by hand-held gamma probe. In this series, a drop of greater than 50% in iPTH levels was observed in 94% of patients (n=48). Moreover, a significant drop in iPTH occurred within 10 minutes of excision in the majority (n=42) of cases (P<0.004). Changes in iPTH were comparable with the therapeutic reduction in calcium levels, as well as with the change in intraoperative ex-vivo gamma counts. This study demonstrates that the addition of an iPTH assay to MIRP provides a quick and reliable intraoperative diagnostic modality in confirming correct adenoma removal. Moreover, it precludes the requirement of frozen section.

Do ectomycorrhizas alter leaf‐litter decomposition in monodominant tropical forests of Guyana?

This work tested the hypothesis that ectomycorrhizas (EM) of Dicymbe corymbosa alter leaf-litter decomposition and residual litter quality in tropical forests of Guyana. Mass loss of leaf litter in litter bags was determined on three occasions, in two experiments, during a 12-month period. Paired root-exclusion plots were located randomly within a D. corymbosa forest. Both D. corymbosa and mixed-species leaf litters were reciprocally transplanted into their respective forest types. Elemental analysis was performed on the residual D. corymbosa leaf litter after 1 yr. Leaf litter mass loss in the D. corymbosa forest was not influenced by EM, despite high EM colonization. Elemental analysis of D. corymbosa leaf litter residues demonstrated reduced calcium levels in the presence of EM, which were negatively correlated with EM rootlet-colonizing mass. The lack of EM effect on the litter decomposition rate, coupled with high EM colonization, suggests an important but indirect role in mineral nutrient acquisition. Lowered Ca concentration in leaf litter exposed to EM may suggest a high Ca demand by the ectotroph system.

Perillyl Alcohol Inhibits a Calcium-Dependent Constitutive Nuclear Factor-κB Pathway

The cell death induced by the monoterpene anticancer agent perillyl alcohol correlates to the increased expression of certain proapoptotic genes known to influence cell survival. Whereas sequence-specific DNA-binding factors dictate the expression patterns of genes through transcriptional regulation, those transcriptional factors influencing constitutive cell survival with perillyl alcohol treatment are not well studied. Here, we investigated whether the monoterpenes can regulate the activity of nuclear factor-kappaB (NF-kappaB), a calcium-dependent transcription factor necessary for survival in the WEHI-231 B-lymphoma cells. Unique among the monoterpenes, perillyl alcohol short-term treatment induced a persistent decrease of calcium levels, whereas other various monoterpenes caused transient reductions in calcium levels. Perillyl alcohol treatment also rapidly elicited reductions of NF-kappaB DNA-binding activity and target gene induction, which was associated with an increase in apoptosis in these B-lymphoma cells. This apoptosis was directly due to NF-kappaB because its prior activation abolished the cell killing effects of perillyl alcohol treatment. Our findings suggest that perillyl alcohol can inhibit NF-kappaB function to modulate gene expression patterns and cell survival of certain B-lymphoma cells. The effects of perillyl alcohol were not limited to these B-lymphoma cells but were also observed in MDA-MB 468 cells, an estrogen receptor-negative breast cancer cell line. These results identify a calcium-dependent NF-kappaB pathway as a molecular target of perillyl alcohol activity in different cancer cell types.

(447) Response of Two Salt-stressed Cultivars of Sweetpotato to Changes in Calcium Levels under In Vitro Conditions

Two cultivars of sweetpotato, `Commensal' and `Salyboro', were subjected to salt stress using axillary bud cultures. The salt levels ranged from 0 to 10 g·L-1. After the initial experiments, levels of calcium in the medium were changed from 3 mm in the MS medium to 1.5, 6, and 12 mm. After 10 weeks of growth, plantlet shoot height, dry weight, number of nodes, levels of proline, soluble carbohydrate, and protein were measured. `Commensal' was tolerant to salt levels up to 4 g·L-1, but `Salyboro' was sensitive to concentrations of salt even at lower concentrations as evidenced by the growth and dry weight. Proline accumulation was higher in the shoot than in the root. The protein: carbohydrate ratios did not change much in `Commensal', but levels of carbohydrates increased in `Salyboro'. Reduction in calcium levels had a synergistic affect on salt-stressed cultivars. Enhanced levels of calcium reduced the inhibitory affects of salt stress. This was more pronounced in `Salyboro', which was susceptible. Proline levels were higher in plants subjected to salt stress and higher levels of calcium than controls, but lower than the plants subjected to salt stress. These and other metabolic changes suggest that calcium can reduce the adverse affects of salt stress in these two sweetpotato cultivars.

Treatment of bioprosthetic heart valve tissue with long chain alcohol solution to lower calcification potential.

The use of glutaraldehyde-treated biological tissue in heart valve substitutes is an important option in the treatment of heart valve disease. These devices have limited durability, in part, because of tissue calcification and subsequent tearing of the valve leaflets. Components thought to induce calcification include lipids, cell remnants, and residual glutaraldehyde. We hypothesized that treatment of glutaraldehyde-treated bioprosthetic heart valve material using a short and long chain alcohol (LCA) combination, composed of 5% 1,2-octanediol in an ethanolic buffered solution, would reduce phospholipid content and subsequently lower the propensity of these tissues to calcify in vivo. Phospholipid content of glutaraldehyde-treated porcine valve leaflets and bovine pericardium was found to be 10.1 +/- 4.3 (n = 7) and 3.9 +/- 0.48 (n = 2) microg/mg dry tissue, respectively, which was reduced to 0.041 +/- 0.06 (n = 7) and 0.21 +/- 0.05 (n = 4) microg/mg dry tissue, respectively, after LCA treatment. Calcification potential of the treated tissues was assessed using a rat subcutaneous implant model. After 60 days of implantation, calcium levels were found to be 171 +/- 32 (n = 11) and 83 +/- 70 (n = 12) mg/g dry weight for glutaraldehyde-treated porcine leaflets and bovine pericardium, respectively, whereas prior LCA treatment resulted in reduced calcium levels of 1.1 +/- 0.6 (n = 12) and 0.82 +/- 0.1 (n = 12) mg/g dry weight, respectively. These data, taken together, support the notion that treatment of glutaraldehyde-treated tissue with a short and long chain alcohol combination will reduce both extractable phospholipids and the propensity for in vivo calcification.

Modulations in Epidermal Calcium Regulate the Expression of Differentiation-Specific Markers

Mammalian epidermis normally displays a distinctive calcium gradient, with low levels in the basal/spinous layers and high levels in the stratum granulosum. Although changes in stratum granulosum calcium regulate the lamellar body secretory response to permeability barrier alterations, whether modulations in calcium also regulate the expression of differentiation-specific proteins in vivo remains unknown. As acute barrier perturbations reduce calcium levels in stratum granulosum, we studied the regulation of murine epidermal differentiation after loss of calcium accompanying acute barrier disruption and by exposure of such acutely perturbed skin sites to either low (0.03 M) or high (1.8 M) calcium. Three hours after acute barrier disruption, coincident with reduced calcium and ultrastructural evidence of accelerated lamellar body secretion, both northern analyses and in situ hybridization revealed decreased mRNA levels for loricrin, profilaggrin, and involucrin in the outer epidermis, but protein levels did not change significantly. Moreover, exposure of acutely disrupted skin sites to low calcium solutions sustained the reduction in mRNA levels, whereas exposure to high calcium solutions restored normal mRNA levels (blocked by the L-type calcium channel inhibitor, nifedipine). Finally, with prolonged exposure to a low (<10% relative humidity) or high (>80% relative humidity) humidity, calcium levels increased and declined, respectively. Accordingly, mRNA and protein levels of the differentiation-specific markers increased and decreased at low and high relative humidity, respectively. These results provide direct evidence that acute and sustained fluctuations in epidermal calcium regulate expression of differentiation-specific proteins in vivo, and demonstrate that modulations in epidermal calcium coordinately regulate events late in epidermal differentiation that together form the barrier.

The anticalcific effect of glutaraldehyde detoxification on bioprosthetic aortic wall tissue in the sheep model.

Increasing concentrations of glutaraldehyde (GA) lead to a decreased rather than increased calcification of bioprosthetic aortic wall tissue. This study determined to what extent the benefit of better cross-linking is masked by the intrinsic propensity of GA towards calcification. Porcine aortic roots were immediately fixed at the abattoir at three different concentrations of GA (0.2%, 1.0%, and 3.0% for 1 week at 4 degrees C). Subsequently, roots underwent a GA extraction process using high volumes of Urazole solution (acetic acid buffer, pH 4.5, 37 degrees C, 1 week) followed by NaBH4 reduction (2 days, 37 degrees C). Roots were implanted in the distal aortic arch of young sheep for 6 weeks and 6 months. Calcium analysis was quantitatively done by atomic absorption spectrophotometry and qualitatively assessed by light microscopy on Von Kossa stains. There was a distinct anticalcification effect of GA detoxification after 6 weeks (56.8% to 97.9%; 95% confidence interval [CI]), which stabilized on a more moderate level after 6 months of implantation (19.1% to 31.6%; 95% CI). The most pronounced effect of GA extraction was seen in 0.2% fixed tissue, where aortic wall calcification was mitigated by 97% and 32% after 6 weeks and 6 months, respectively. Mitigation of aortic wall calcification was 71% (6 weeks) and 21% (6 months) in the 3.0% GA group. The combined effect of higher cross-link density and detoxification achieved an 82% (6 weeks) and 48% (6 months) reduction of calcium levels in the 3.0% GA group. In long-term implants (6 months), detoxification alone on top of standard 0.2% GA fixation was as effective (from 174.1 +/- 11.9 microg/mg without detoxification to 119.3 +/- 19.3 microg/mg with detoxification) as 3.0% fixation (114.8 +/- 10.0 microg/mg without detoxification to 91.3 +/- 11.5 microg/mg with detoxification). We were able to determine in the circulatory sheep model to what degree the intrinsic procalcific effect of GA counteracts the protective effect of higher cross-link density. Our study also established that the effect of detoxification is particularly pronounced in commercial low-grade fixation.

Calcium concentration in seawater and exoskeletal calcification in the blue crab, Callinectes sapidus

The blue crab, Callinectes sapidus Rathbun, supports valuable commercial fisheries in the temperate areas of the coastal Atlantic states and US Gulf of Mexico (GOM). Soft crabs form a small part of the total United States blue crab landings, but have a higher market value on a per pound basis than do hard crabs. The soft crab industry depends upon the capture of premolt crabs which are held in open or closed seawater systems until they molt. A major factor limiting profitability is the labor-intensive nature of the shedding operation. Extending the length of time crabs remain in the softshell stage would significantly reduce labor requirements. Exoskeletal calcification in blue crabs is achieved predominantly with calcium absorbed from seawater. In the present study, seawater at a salinity of 12‰, with calcium levels reduced to 60–80% of normal, decreased the calcification rate without increasing shedding mortality. Salinity did not influence calcification rates for crabs shed at 5‰, 12‰, and 25‰, provided calcium concentrations were within the normal ranges for their respective salinities: x̄=54, 139, and 281 mg/l. Crabs molting over the range of test salinities in waters with normal calcium levels remained soft for ∼4 h. At test salinities with reduced calcium levels, the rate of exoskeletal calcification decreased as total ionic strength of the seawater increased. Crabs shed in low-calcium seawater at 25‰ remained longer in the softshell stage than crabs shed at 5‰ or 12‰. Modifying existing closed system shedding facilities to incorporate low-calcium seawater technology would reduce labor requirements, increase profitability, and produce a higher quality soft crab.

Mucoadhesive DL-Lactide/Glycolide Copolymer Nanospheres Coated with Chitosan to Improve Oral Delivery of Elcatonin

The purpose of this work was to develop a novel mucoadhesive DL-lactide/glycolide copolymer (PLGA) nanosphere system to improve peptide absorption and prolong the physiological activity following oral administration. The desired PLGA nanospheres with elcatonin were prepared by the emulsion solvent diffusion method to coat the surface of the resultant nanospheres with a mucoadhesive polymer such as chitosan, poly(acrylic acid), and sodium alginate. Their mucoadhesive properties were evaluated by measuring the nanospheres adsorbed to a rat everted intestinal sac (in vitro). The chitosan-coated nanospheres showed higher mucoadhesion to the everted intestinal tract in saline than the other polymer-coated nanospheres. There was no mucoadhesion site-specificity of the chitosan-coated nanospheres between duodenal, jejunal, and ileal sacs. The payload of drug in the chitosan-coated nanospheres was successfully increased by using the solvent diffusion method in oil. The pattern of drug release of the resultant nanospheres did not differ markedly from that of uncoated nanospheres. The chitosan-coated nanospheres with elcatonin were administered intragastrically to fasted Wistar rats. The chitosan-coated nanosphere reduced significantly the blood calcium level compared with elcatonin solution and uncoated nanospheres, and the reduced calcium level was sustained for a period of 48 hr. Even under nonfasting conditions, the mucoadhesion of chitosan-coated nanospheres was unaltered and the reduction in blood Ca levels was maintained satisfactorily.

Connexin-Aequorin Chimerae Report Cytoplasmic Calcium Environments along Trafficking Pathways Leading to Gap Junction Biogenesis in Living COS-7 Cells

The cytoplasmic calcium environments along membrane trafficking pathways leading to gap junction intercellular communication channels at the plasma membrane were studied. Connexins, the constitutive proteins of gap junctions, were fused at their carboxyl terminus to the calcium-sensitive photoprotein aequorin. The cellular location of the chimeric proteins was determined by immunolocalization and subcellular fractionation. The generation of functional gap junctions by the connexin chimerae was monitored by the ability of the cells to exchange small dyes. Although aequorin fused to connexin-26 was nonfunctional, its ability to report Ca2+ and to form functional gap junctions was rescued by replacement of its cytoplasmic carboxyl tail with that of connexin-43. In COS-7 cells expressing these connexin-aequorin chimerae, calcium levels below the plasma membrane were higher (approximately 5 microM) than those in the cytoplasm (approximately 100 nM); gap junctions were able to transfer dyes under these conditions. Cytoplasmic levels of free calcium surrounding the ERGIC/Golgi reported by connexin-43 chimera (approximately 420 nM) were twice those measured by connexin-32 chimera (approximately 200 nM); both chimerae measured calcium levels substantially higher than those reported by a connexin-26 chimera (approximately 130 nM). Dispersion of the ERGIC and Golgi complex by brefeldin A led to a marked reduction in calcium levels. The results show that the various connexin chimerae were located in spatially different subcellular stores and that the ERGIC/Golgi regions of the cell maintain heterogeneous cytoplasmic domains of calcium. The implications of the subplasma-membrane Ca2+ levels on the gating of gap junctions are discussed.

Transforming growth factor alpha treatment alters intracellular calcium levels in hair cells and protects them from ototoxic damage in vitro

To determine if transforming growth factor alpha (TGFα) pretreatment protects hair cells from aminoglycoside induced injury by modifying their intracellular calcium concentration, we assayed hair cell calcium levels in organ of Corti explants both before and after aminoglycoside (i.e. neomycin, 10−3M) exposure either with or without growth factor pretreatment. After TGFα (500ng/ml) treatment, the intracellular calcium level of hair cells showed a five-fold increase as compared to the levels observed in the hair cells of control cultures. After ototoxin exposure, calcium levels in hair cells of control explants showed an increase relative to their baseline levels, while in the presence of growth factors pretreatment, hair cells showed a relative reduction in calcium levels. Pretreatment of organ of Corti explants afforded significant protection of hair cell stereocilia bundle morphology from ototoxic damage when compared to explants exposed to ototoxin alone. This study correlates a rise in hair cell calcium levels with the otoprotection of hair cells by TGFα in organ of Corti explants.

Analysis of secretory dynamics and development of media for the controlled secretion of insulin-related peptides from βTC-3 insulinoma cells

Controlled secretion of proteins from endocrine-derived cell lines has been proposed as a means to produce some classes of post-translationally modified proteins in bioreactors. Under the right biological and environmental conditions it may be possible to improve the product purity or quality relative to that obtained through steady (constitutive) secretion. The pancreatic-islet-derived cell line, betaTC-3, was selected as a model system to explore the secretory dynamics of insulin under various combinations of stimulatory or inhibitory environmental conditions. The betaTC-3 cells exhibited a glucose-mediated stimulus-response pattern which was saturated above 1 mM glucose and with an apparent "Kg" of 0.1 mM glucose. However, the kinetics of insulin synthesis were closely coupled to those of secretion such that betaTC-3 cells cycled between saturating and basal levels of glucose were never perturbed far from an intracellular synthesis-secretion equilibrium. When more powerful and selective agents were used to control secretion, the system performance improved markedly. A combination of 1 mM isobytylmethylxanthine (IBMX) and 1 microM carbachol (with saturating levels of glucose) could discharge 75% of stored insulin in 2 h. When this treatment was followed by incubation in media adjusted to attenuate the influx of calcium into the cells, intracellular pools were efficiently replenished within 24 h. Calcium attenuating treatments included hyperpolarization with reduced potassium (1 mM), calcium channel blockade with the dihydropyridine verapamil (1 microM), and the direct mass-action effect of reduced environmental calcium (0.5 mM versus 1.8 mM). Other inhibitory treatments were explored, but these tended to reduce both insulin synthesis and secretion. The best recharging treatment found was a combination of verapamil (1 microM) with reduced calcium level (0.5 mM).To demonstrate the feasibility of a controlled secretion process, betaTC-3 T-flask cultures were grown to confluence, then cycled through two periods of discharging (2 h) and recharging (20 h) with the best combinations of secretagogues and calcium attenuators. The overall process was quite efficient: Only 15% of the overall insulin secretion took place during the recharging episodes, and this residual secretion represented only 10% of the net insulin synthesis during these episodes. Discharging was very effective in the first episode (80% recovery of stored insulin), but slightly less efficient in subsequent discharging episodes, possibly due to a desensitization effect of the calcium attenuating media. Nevertheless, the regulated secretory pathway of betaTC-3 cells could be successfully harnessed to a controlled secretion process for the selective recovery of stored insulin.

Modulation of calcium uptake in cadmium-pretreated tilapia (Oreochromis mossambicus) larvae

Tilapia larvae were exposed to 0 (control), 50 (50-Cd) or 100 (100-Cd) µg l-1 cadmium for 4 days and then transferred to cadmium-free fresh water for 3 days of detoxification. Total length and weight, calcium influx and total body calcium and cadmium content were examined at various times during detoxification. All the groups grew normally with regards to total length and body weight. Within the first 12h of detoxification the 50- and 100-Cd exposed groups released cadmium at the similar rate of about 24 ng mg-1 h-1 (or 140 ng larva-1 h-1). Later, however, this rate declined to only 4–16% of the initial level. Calcium influx in the control group showed a 10–26% increase during the detoxification period. Calcium influx in the 50-Cd group increased by about 280% and reached it peak at 12h. Calcium influx in the 100-Cd group increased by 440% and did not peak until 24h after transfer. After peaking, the influxes in both 50- and 100-Cd groups declined to the level of control at the end of the experiment. Calcium contents in 50- and 100-Cd groups increased more rapidly than that in control group within first 24h of the detoxification period. However the rate of increase in calcium content in three groups was the same after 24h. The changes in calcium influx appeared to be correlated with those in calcium content, and these suggested that tilapia larvae regulate the mechanism of calcium balance to compensate for the reduced calcium level in the body.

Growth cone calcium elevation by GABA

Cytoplasmic calcium plays a key role in neurite growth. In contrast to previous work suggesting that gamma aminobutyrate's (GABA) role in regulating growth cone calcium is primarily to antagonize the effects of glutamate, we report that GABA can act in an excitatory manner on developing hypothalamic neurites, independently raising calcium in growing neurites and their growth cones. Time-lapse digital video and confocal laser microscopy with the calcium-sensitive dyes fluo-3 and fura-2 were used to study the influence of GABA on neurite calcium levels. GABA (10 μM) evoked a calcium rise in both bicarbonate- and Hepes-based buffers. The calcium rise was greatly reduced after chloride transport was blocked. GABA raised calcium by stimulating the cell body, resulting in an increase in calcium throughout the neuronal cell body and dendritic arbor. GABA also acted locally, stimulating a neuritic calcium rise only in a single dendrite or growth cone. In some neurites and growth cones during early development, GABA generated a greater calcium rise than did glutamate. Bicuculline, a GABAA receptor antagonist, reduced calcium levels in neurites of young synaptically coupled neurons, indicating that ongoing synaptic release of GABA raised neuritic calcium. These data suggest that during early development, GABA may play a significant role in regulating process growth and modulating the formation of early connections in the hypothalamus. Our data support the hypothesis that GABA receptors are functionally active and may play a calcium regulating role similar to that of glutamate in neuronal development. This is particularly true in early development, as later in development GABA's role becomes more inhibitory, and glutamate plays the primary excitatory role. © 1996 Wiley-Liss, Inc.

Growth cone calcium elevation by GABA.

Cytoplasmic calcium plays a key role in neurite growth. In contrast to previous work suggesting that gamma aminobutyrate's (GABA) role in regulating growth cone calcium is primarily to antagonize the effects of glutamate, we report that GABA can act in an excitatory manner on developing hypothalamic neurites, independently raising calcium in growing neurites and their growth cones. Time-lapse digital video and confocal laser microscopy with the calcium-sensitive dyes fluo-3 and fura-2 were used to study the influence of GABA on neurite calcium levels. GABA (10 microM) evoked a calcium rise in both bicarbonate- and Hepes-based buffers. The calcium rise was greatly reduced after chloride transport was blocked. GABA raised calcium by stimulating the cell body, resulting in an increase in calcium throughout the neuronal cell body and dendritic arbor. GABA also acted locally, stimulating a neuritic calcium rise only in a single dendrite or growth cone. In some neurites and growth cones during early development, GABA generated a greater calcium rise than did glutamate. Bicuculline, a GABAA receptor antagonist, reduced calcium levels in neurites of young synaptically coupled neurons, indicating that ongoing synaptic release of GABA raised neuritic calcium. These data suggest that during early development, GABA may play a significant role in regulating process growth and modulating the formation of early connections in the hypothalamus. Our data support the hypothesis that GABA receptors are functionally active and may play a calcium regulating role similar to that of glutamate in neuronal development. This is particularly true in early development, as later in development GABA's role becomes more inhibitory, and glutamate plays the primary excitatory role.

The biochemistry of the neuron. Neurosecretory habituation to repetitive depolarizations in PC12 cells.

Habituation in response to repetitive depolarization of PC12 cells can be used as a model for memory processes at the molecular level. In response to depolarization by high external potassium, a triphasic elevation in internal calcium levels occurred. Calcium elevation was maximal immediately after addition of the stimulus (phase 1), followed by a 2-min period in which the calcium level decreased (phase 2), leading to a new steady-state level which was higher than in the unstimulated cell (phase 3). In response to repetitive depolarizations, the calcium level in phase 1 was reduced by as much as 43%, and phase 3 was reduced by as much as 40%. By measuring the relationship between calcium elevation and secretion, it was shown that measured reductions in calcium levels were correlated with neurosecretory habituation. One of the components responsible for the reductions in calcium levels was a tetraethylammonium-sensitive potassium channel, and the habituation of this channel was reversed by addition of 4 beta-phorbol 12-myristate 13-acetate.

Calcium Carbonate as a Phosphate Binder: Is There a Need to Adjust Peritoneal Dialysate Calcium Concentrations for Patients Using CaCO3?

The widespread use of calcium carbonate as a phosphate binder is limited by its tendency to develop hypercalcemia in some patients using effective dosages needed to control hyperphosphatemia. Most common continuous ambulatory peritoneal dialysis (CAPD) regimens using dialysis solutions containing 3.5 mEq/L of calcium result in net absorption of calcium from the dialysis solution and, hence limit the amount of oral calcium that can be administered. Peritoneal dialysis solutions with reduced calcium levels are needed for effective use of CaCO3 to control hyperphosphatemia in some dialysis patients.

A quantitative study of the dependence of feline cold receptor activity on the calcium concentration

An isolated preparation of the cat tongue was developed to study quantitatively the relation between single cold receptor activity and calcium concentration. The activity recorded from lingual cold fibres of the isolated preparation was identical to that observed by previous authors in lingual cold receptors of whole animals. A perfusing solution containing excess calcium (5 mmol/l) suppressed discharge at constant temperatures as well as the dynamic response to rapid cooling. Reduced calcium levels (0.5 mmol/l) enhanced cold fibre activity at adapting temperatures above 15 degrees C. Cold receptor activity was linearly inversely related to the logarithm of calcium concentration at adapting temperatures above 20 degrees C. The linear relation was valid for both the discharge at constant temperatures and the dynamic peak frequency during cooling steps. From this relation it was calculated that at 25 degrees C an elevation of 0.1 mmol/l external calcium above the control value (1.53 mmol/l) reduces static firing rate by 0.35 impulses per second. It is concluded that neither the in vitro conditions nor the application of various external calcium concentrations impair cold receptor function. The data are consistent with the hypothesis of a calcium-stimulated outward current controlling the static and dynamic response characteristics of cold receptors.