Efferent loop small intestinal vitamin D receptor concentration and bone mineral density after Billroth II (Polya) gastrectomy in humans

Animal studies have demonstrated that the highest concentration of vitamin D receptors (and greatest capacity for active calcium absorption) occurs in the proximal duodenum. By passing the duodenum following Polya/Billroth II gastrectomy could result in the development of a metabolic bone disease and low bone mineral density (BMD). We thus compared the vitamin D receptor (VDR) concentration in mucosal biopsies taken at endoscopy from two functionally corresponding areas of the small intestine: the jejunum (or efferent loop) in 21 patients with a history of Polya/Billroth II gastrectomy and the second part of the duodenum in age/sex-matched control subjects. We also measured the BMD by dual energy X-ray absorptiometry. The mean VDR concentration was not significantly different between the two groups (patients vs controls, fmol/mg protein, mean +/- SE: 34.99 +/- 2.57 vs 34.67 +/- 3.71; P = 0.22), even when subgrouped as males (36.22 +/- 3.16 vs 31.2 +/- 4.24; P = 0.351) or females (31.93 +/- 4.7 vs 43 +/- 6.76; P = 0.193). In Polya/Billroth II gastrectomy patients, the VDR concentration in the efferent loop declined with age (r = -0.78, P = 0.02). In the same group, BMD, as compared with matched controls, was significantly reduced at the lumbar spine (Z-score: patients vs controls: -1.138 vs 0.099, P = 0.01), but not at the femoral neck (Z-score: -0.69 vs 0.7, P = 0.084). There was no correlation between VDR and time since operation or BMD. These results suggest that following Polya/Billroth II gastrectomy, the functional capacity of the jejunal efferent loop in reference to VDR concentration is similar to that of the second part of the duodenum in normal subjects. Therefore, the reduced BMD in our patients, also a common finding in other studies, may not be secondary to the reduced capacity of the VDR system that facilitates the active calcium transport pathway in the proximal small intestine.

Ursodeoxycholic acid enhances fractional calcium absorption in primary biliary cirrhosis

Bone disease is a frequently reported complication in primary biliary cirrhosis (PBC), but its pathogenesis is poorly understood. Calcium malabsorption has been considered as an important contributing factor. Ursodeoxycholic acid (UDCA) is the treatment of choice in PBC, improving survival, but its effect on calcium absorption is unknown. In this study, we have measured fractional calcium absorption, using a single isotope method, in a group of female PBC patients (median age: 60 years, range: 46-78 years) and age-matched female controls (median age: 58 years, range: 36-74). Bone mineral density (BMD) in PBC patients was significantly lower than age-matched controls (g/cm(2) +/- SEM; lumbar spine: controls 1.139+/-0.028, PBC patients 1.004+/-0.026, p = 0.0028; femoral neck: controls 0.944+/-0.034, PBC patients 0.819+/-0.023, p = 0.0032). Twenty two PBC patients, who were not vitamin D-deficient, were off and on UDCA for approximately 1 month and approximately 8 weeks, respectively. Fractional calcium absorption in PBC patients prior to UDCA treatment (mean +/- SEM, 33.8+/-2.6%) was significantly lower than controls (52.0+/-2.4%, p<0.001). Following UDCA therapy, fractional calcium absorption increased significantly (Off UDCA: 33.1+/-2.6%, On UDCA: 36.6+/-2.5%, p<0.0058). Osteocalcin levels were significantly raised in the PBC group (mean +/- SEM, ng/ml, 41.4+/-2.02) compared to controls (31.1+/-2.64, p = 0.002). There were no differences in parathyroid hormone (PTH) or 25-hydroxyvitamin D levels between these two groups or following UDCA therapy. In conclusion, we found that PBC patients display low spinal and femoral neck BMD, reduced fractional calcium absorption, and elevated plasma osteocalcin. The calcium malabsorption is corrected partially by UDCA therapy. Long-term studies are required to determine whether this effect can be sustained, and whether a sustained increase in fractional calcium absorption can translate into a favorable change in bone strength in patients with PBC.

Differential effects of interleukin-6 receptor activation on intracellular signaling and bone resorption by isolated rat osteoclasts

The effects of the related cytokines interleukin-6 (IL-6), leukemia inhibitory factor (LIF) and oncostatin-M on bone resorption and cytosolic Ca(2+) signaling were compared in isolated rat osteoclasts. In the traditional disaggregated osteoclast (pit) assay, IL-6 and LIF, but not oncostatin-M, conserved the bone resorption otherwise inhibited by high extracellular [Ca(2+)] (15 mM). It produced a paradoxical, concentration-dependent stimulation of resorption by elevated extracellular Ca(2+). In the micro-isolated single osteoclast resorption assay, IL-6, high [Ca(2+)] or IL-6 plus high [Ca(2+)] all increased pit formation. In contrast, the IL-6 receptor (IL-6R)-specific agonist antibody MT-18 inhibited bone resorption in a concentration-dependent manner (1:500 to 1:500 000). MT-18 triggered cytosolic Ca(2+) signals in fura 2-loaded osteoclasts within approximately 10 min of application. Each cytosolic Ca(2+) transient began with a peak deflection that persisted in Ca(2+)-free, EGTA-containing extracellular medium, consistent with a release of intracellularly stored Ca(2+). This was followed by a sustained elevation of cytosolic [Ca(2+)] that was abolished in Ca(2+)-free medium, as expected from an entry of extracellular Ca(2+), and by the Ca(2+) channel antagonist Ni(2+). The inclusion of either IL-6 or soluble human (sh) IL-6R specifically reversed both the above effects of MT-18, confirming that both effects were specific for the IL-6R. The findings suggest that IL-6R activation by IL-6 stimulates osteoclastic bone resorption either by reversing the inhibitory effect of high extracellular Ca(2+) in stromal-containing systems or itself stimulating bone resorption along with Ca(2+) by micro-isolated osteoclasts. In contrast, activation of the IL-6R by an agonist antibody produces an inhibition of bone resorption and an associated triggering of the cytosolic Ca(2+) signals previously associated with regulation of bone resorptive function in other situations.

New insights into the regulation of cathepsin K gene expression by osteoprotegerin ligand

Cathepsin K plays a key role in bone resorption. We provide the first evidence that osteoprotegerin ligand (OPGL), a critical pro-resorptive cytokine, acutely stimulates the expression of cathepsin K in osteoclasts. We used in situ RT-PCR and real time quantitative RT-PCR to analyze cathepsin K gene expression. OPGL enhanced cathepsin K mRNA levels in mature osteoclasts isolated from rat neonatal long bones. OPGL together with macrophage colony-stimulating factor (M-CSF) also stimulated cathepsin K gene expression in monocytic cells and multinucleate osteoclasts in bone marrow cultures. Real time quantitative RT-PCR demonstrated high levels of cathepsin K mRNA in bone marrow cultures, paralleling the degree of osteoclastogenesis. We therefore suggest that OPGL enhances bone resorption, at least in part, by inducing cathepsin K gene expression.

Macrophage colony-stimulating factor suppresses osteoblast formation

We provide the first evidence that the bone marrow-derived cytokine, macrophage colony-stimulating factor (M-CSF), inhibits the formation of bone-forming osteoblasts. We examined both osteoclast and osteoblast formation in primary rat bone marrow cultures. As expected, M-CSF together with osteoprotegerin ligand (OPGL) markedly accelerated osteoclastogenesis. In contrast, treatment with M-CSF alone yielded no osteoclasts at any time. The most striking and novel observation was that M-CSF with or without OPGL dramatically suppressed osteoblast formation. In separate experiments, estradiol markedly suppressed osteoclast formation in the M-CSF/OPGL-treated cultures independently of osteoblasts. Consistent with this was the expression of estrogen receptor-alpha (ERalpha) and ERbeta mRNA in osteoclast precursors. We therefore conclude that in addition to the well-known action of M-CSF to modulate osteoclastogenesis, this cytokine may also regulate osteoblast formation.

A two year global evaluation of the susceptibility of Candida species to fluconazole by disk diffusion

The in-vitro activity of fluconazole against 46,831 yeast isolates collected over a two-year period from 57 laboratories in 33 countries worldwide was assessed using a disc diffusion method. Candida albicans was the organism isolated most frequently, accounting for 68.6% of the total number of isolates. C. glabrata, C. tropicalis, C parapsilosis and C. krusei and Cryptococcus neoformans represented 9.9, 4.7, 4.3, 1.9, and 1.4% of isolates respectively during the 2 year period and rates varied markedly between countries. In 1999 data blood isolates represented 4.9% of all isolates and intensive care unit isolates represented 9.9%. In both the 1998 and 1999 data, 99% of C. albicans were fully susceptible (S) to fluconazole, and 95.6% of all species of yeasts tested were S or susceptible-dose dependent (S-DD) to fluconazole. No emerging trends of resistance were noted with any of the Candida spp. tested as 96% of all isolates retained susceptibility (S or S-DD) to this agent.

Identification and Characterization of a Sodium/Calcium Exchanger, NCX-1, in Osteoclasts and Its Role in Bone Resorption

We provide the first demonstration for a Na+/Ca2+ exchanger, NCX-1, in the osteoclast. We speculate that by using Na+ exchange, NCX-1 couples H+ extrusion with Ca2+ fluxes during bone resorption. Microspectrofluorimetry of fura-2-loaded osteoclasts revealed a rapid and sustained, but reversible, cytosolic Ca2+ elevation upon Na+ withdrawal. This elevation was abolished by the cytosolic introduction (by gentle permeabilization) of a highly specific Na+/Ca2+ exchange inhibitor peptide, XIP, but not its inactive analogue, sXIP. Confocal microscopy revealed intense plasma membrane immunofluorescence with an isoform-specific monoclonal anti-NCX-1 antibody applied to gently permeabilized osteoclasts. Electrophysiological studies using excised outside-in membrane patches showed a low-conductance, Na+-selective, dichlorobenzamil-sensitive, amiloride-insensitive channel that we tentatively assigned as being an NCX. Finally, to examine for physiological relevance, an osteoclast resorption (pit) assay was performed. There was a dramatic reduction of bone resorption following NCX-1 inhibition by dichlorobenzamil and XIP (but not with S-XIP). Together, the results suggest that a functional NCX, likely NCX-1, is involved in the regulation of osteoclast cytosolic Ca2+ and bone resorption.

Molecular cloning, expression, and functional characterization of a novel member of the CD38 family of ADP-ribosyl cyclases

We report the molecular cloning and functional characterization of a novel member of the CD38 family of cyclic ADP-ribose (cADPr)-generating cyclases. We cloned a cDNA insert that encoded a 298-amino-acid-long protein (M(w) approximately 39 kDa). The predicted protein displayed 69, 61, and 58% similarity, respectively, to mouse, rat, and human CD38. Rabbit CD38 was also 28% homologous to Aplysia ADP-ribosyl cyclase and leukocyte CD157 (another ADP-ribosyl cyclase); the three cyclases shared 10 cysteine and 2 adjacent proline residues. We then transfected CD38-negative NIH3T3 cells with cDNA encoding a CD38-EGFP fusion protein. Epifluorescence microscopy showed intense EGFP fluorescence confirming CD38 expression. We finally confirmed the ADP-ribosyl cyclase activity of the expressed CD38 by measuring its ability to catalyze the cyclization of the nicotinamide adenine dinucleotide (NAD(+)) surrogate, NGD(+), to its fluorescent nonhydrolyzable derivative, cGDPr.

Novel biochemical and functional insights into nuclear Ca(2+) transport through IP(3)Rs and RyRs in osteoblasts

We report the first biochemical and functional characterization of inositol trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) in the nuclear membrane of bone-forming (MC3T3-E1) osteoblasts. Intact nuclei fluoresced intensely with anti-RyR (Ab(34)) and anti-IP(3)R (Ab(40)) antisera in a typically peripheral nuclear membrane pattern. Isolated nuclear membranes were next subjected to SDS-PAGE and blotted with isoform-specific anti-receptor antisera, notably Ab(40), anti-RyR-1, anti-RyR-2 (Ab(129)), and anti-RyR-3 (Ab(180)). Only anti-RyR-1 and Ab(40) showed bands corresponding, respectively, to full-length RyR-1 ( approximately 500 kDa) and IP(3)R-1 (approximately 250 kDa). Band intensity was reduced by just approximately 20% after brief tryptic proteolysis of intact nuclei; this confirmed that isolated nuclear membranes were mostly free of endoplasmic reticular contaminants. Finally, the nucleoplasmic Ca(2+) concentration ([Ca(2+)](np)) was measured in single nuclei by using fura-dextran. The nuclear envelope was initially loaded with Ca(2+) via Ca(2+)-ATPase activation (1 mM ATP and approximately 100 nM Ca(2+)). Adequate Ca(2+) loading was next confirmed by imaging the nuclear envelope (and nucleoplasm). Exposure of Ca(2+)-loaded nuclei to IP(3) or cADP ribose resulted in a rapid and sustained [Ca(2+)](np) elevation. Taken together, the results provide complementary evidence for nucleoplasmic Ca(2+) influx in osteoblasts through nuclear membrane-resident IP(3)Rs and RyRs. Our findings may conceivably explain the direct regulation of osteoblastic gene expression by hormones that use the IP(3)-Ca(2+) pathway.

IP(3), IP(3) receptor, and cellular senescence

Herein we demonstrate that replicative cellular senescence in vitro results in sharply reduced inositol 1,4,5-trisphosphate (IP(3)) receptor levels, reduced mitogen-evoked IP(3) formation and Ca(2+) release, and Ca(2+) store depletion. Human diploid fibroblasts (HDFs) underwent either 30 mean population doublings [mean population doublings (MPDs) thymidine labeling index (TI) >92% ("young") or between 53 and 58 MPDs (TI < 28%; "senescent")]. We found that the cytosolic Ca(2+) release triggered by either ionomycin or by several IP(3)-generating mitogens, namely bradykinin, thrombin, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF), was attenuated markedly in senescent HDFs. Notably, the triggered cytosolic Ca(2+) transients were of a smaller magnitude in senescent HDFs. However, the response latency seen with both PDGF and EGF was greater for senescent cells. Finally, a smaller proportion of senescent HDFs showed oscillations. In parallel, IP(3) formation in response to bradykinin or EGF was also attenuated in senescent HDFs. Furthermore, senescent HDFs displayed a sharply diminished Ca(2+) release response to intracellularly applied IP(3). Finally, to compare IP(3) receptor protein levels directly in young and senescent HDFs, their microsomal membranes were probed in Western blots with a highly specific anti-IP(3) receptor antiserum, Ab(40). A approximately 260-kDa band corresponding to the IP(3) receptor protein was noted; its intensity was reduced by approximately 50% in senescent cells. Thus, we suggest that reduced IP(3) receptor expression, lowered IP(3) formation, and Ca(2+) release, as well as Ca(2+) store depletion, all contribute to the deficient Ca(2+) signaling seen in HDFs undergoing replicative senescence.

[Epidemic of pneumonia associated with mechanical ventilation in Mérida, Yucatán]

OBJECTIVE

To determine the main epidemiological, clinical, and microbiologic characteristics of an outbreak of ventilator-associated pneumonia at an intensive care unit in Yucatan.

MATERIAL AND METHODS

An 11-month prospective and observational study was designed to determine incidence, mortality, potential reservoirs, etiologic agents and antibiotic susceptibility patterns.

RESULTS

The incidence of ventilator-associated pneumonia was 74%. The crude mortality rate was 88% compared to a 19.5% expected-mortality rate. Gram-negative bacteria were isolated from 98% of the cultures, of which 46% were susceptible to third generation cephalosporins, 59% to fourth generation cephalosporins, 70% to ciprofloxacin and 100% to imipenem. Klebsiella pneumoniae and Pseudomonas aeruginosa were isolated from some of the ventilator circuits and the sink.

CONCLUSIONS

The high incidence of pneumonia and associated mortality in our intensive care unit may be attributed to the absence of infection control measures and the high prevalence of multiresistant organisms which is related to antibiotic abuse.

A new function for CD38/ADP-ribosyl cyclase in nuclear Ca2+ homeostasis

Nucleoplasmic calcium ions (Ca2+) influence nuclear functions as critical as gene transcription, apoptosis, DNA repair, topoisomerase activation and polymerase unfolding. Although both inositol trisphosphate receptors and ryanodine receptors, types of Ca2+ channel, are present in the nuclear membrane, their role in the homeostasis of nuclear Ca2+ remains unclear. Here we report the existence in the inner nuclear membrane of a functionally active CD38/ADP-ribosyl cyclase that has its catalytic site within the nucleoplasm. We propose that the enzyme catalyses the intranuclear cyclization of nicotinamide adenine dinucleotide to cyclic adenosine diphosphate ribose. The latter activates ryanodine receptors of the inner nuclear membrane to trigger nucleoplasmic Ca2+ release.

Novel mechanisms of calcium handling by the osteoclast: A review-hypothesis

The osteoclast is a cell that is unique in its ability to resorb bone and, in doing so, becomes exposed to unusually high millimolar Ca2+ concentrations. It is generally accepted that, during resorption, osteoclasts can "sense" changes in their ambient Ca2+ concentration. This triggers a sharp cytosolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The change in cytosolic Ca2+ is transduced finally into inhibition of bone resorption. It has been shown that a type 2 ryanodine receptor isoform, expressed uniquely in the plasma membrane, functions as a Ca2+ influx channel and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily Ca2+ release channels that have a microsomal membrane location in a wide variety of eukaryotic cells, including the osteoclasts. However, only recently has it become obvious that ryanodine receptors are also expressed in osteoclast nuclear membranes, at which site they probably gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+ in turn regulates key nuclear processes, including gene expression and apoptosis. Here, we review the potential mechanisms underlying the recognition, movement, and effects of Ca2+ in the osteoclast. We will also speculate on the general biological significance of the unique processes used by the osteoclast to handle high Ca2+ loads during bone resorption.

Direct microsensor measurement of nitric oxide production by the osteoclast

Nitric oxide (NO) triggers marked osteoclast retraction which closely resembles that due to Ca2+. The effect of Ca2+ has been attributed to a stimulated release of NO. Here, we show for the first time, by direct measurement with a microsensor, that osteoclasts do indeed produce NO and that this production is enhanced by a high Ca2+. We also show that the Ca2+ ionophore, A23187, mimics the latter. Furthermore, osteoclasts on dentine produce more NO than osteoclasts on glass and NO release from dentine-plated osteoclasts is much less sensitive to stimulation by Ca2+. Finally, the microsomal Ca2+ store-depleting agent, thapsigargin, attenuates NO release only from osteoclasts on glass, suggesting that stored Ca2+ has the dominant effect in modulating NO release from non-resorbing cells. NO is a powerful inhibitor of bone resorption: a direct demonstration of its production is therefore strong evidence for a role in modulating osteoclast function.

Emerging insights into the role of calcium ions in osteoclast regulation

Osteoclasts are exposed to unusually high, millimolar, Ca2+ concentrations and can "sense" changes in their ambient Ca2+ concentration during resorption. This results in a sharp cystolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The rise in cystolic Ca2+ is transduced finally into an inhibition of bone resorption. We have shown that a type 2 ryanodine receptor isoform, expressed uniquely in the osteoblast plasma membrane, functions as a Ca2+ influx channel, and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily microsomal membrane Ca2+ release channels. They have only recently been shown to be expressed a other sites, including nuclear membranes. At the latter site, ryanodine receptors gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+, in turn, regulates key nuclear processes, including gene expression and apoptosis. Here, we review potential mechanisms underlying the recognition, movement, and actions of Ca2+ in the osteoclast.

An unusual case of orbital hydatid cyst: a surgical emergency

A rare case of a 19-year-old man with an orbital hydatid cyst is presented. The lesion caused rapid deterioration of vision and was not responsive to the systemic use of mebendazole. It was treated successfully with emergency surgery. Early detection, surgical excision, and the systemic use of albendazole are suggested for the treatment of orbital hydatid cysts.

Retrograde Ca2+ signaling in C2C12 skeletal myocytes in response to mitochondrial genetic and metabolic stress: a novel mode of inter-organelle crosstalk

We have investigated the mechanism of mitochondrial-nuclear crosstalk during cellular stress in mouse C2C12 myocytes. For this purpose, we used cells with reduced mitochondrial DNA (mtDNA) contents by ethidium bromide treatment or myocytes treated with known mitochondrial metabolic inhibitors, including carbonyl cyanide m-chlorophenylhydrazone (CCCP), antimycin, valinomycin and azide. Both genetic and metabolic stresses similarly affected mitochondrial membrane potential (Deltapsim) and electron transport-coupled ATP synthesis, which was also accompanied by an elevated steady-state cytosolic Ca2+ level ([Ca2+]i). The mitochondrial stress resulted in: (i) an enhanced expression of the sarcoplasmic reticular ryanodine receptor-1 (RyR-1), hence potentiating the Ca2+ release in response to its modulator, caffeine; (ii) enhanced levels of Ca2+-responsive factors calineurin, calcineurin-dependent NFATc (cytosolic counterpart of activated T-cell-specific nuclear factor) and c-Jun N-terminal kinase (JNK)-dependent ATF2 (activated transcription factor 2); (iii) reduced levels of transcription factor, NF-kappaB; and (iv) enhanced transcription of cytochrome oxidase Vb (COX Vb) subunit gene. These cellular changes, including the steady-state [Ca2+]i were normalized in genetically reverted cells which contain near-normal mtDNA levels. We propose that the mitochondria-to-nucleus stress signaling occurs through cytosolic [Ca2+]i changes, which are likely to be due to reduced ATP and Ca2+ efflux. Our results indicate that the mitochondrial stress signal affects a variety of cellular processes, in addition to mitochondrial membrane biogenesis.

Molecular and functional evidence for calcineurin-A alpha and beta isoforms in the osteoclast: novel insights into cyclosporin A action on bone resorption

We provide the first molecular evidence for the presence of a functional serine/threonine phosphatase, calcineurin-A (CN-A), in the osteoclast. Polymerase chain reaction (PCR) of an osteoclast cDNA library, together with restriction mapping, revealed two isoform sequences, alpha and beta. We then examined the functionality of the detected CN-A by assessing the effect of a classical antagonist, cyclosporin A (CsA), in the osteoclast resorption (pit) assay. CsA (0.1 and 1 microg ml-1) potently inhibited bone resorption. The presence of lymphocytes, with or without prior exposure to CsA in vivo, failed to reverse the CsA-induced resorption-inhibition. Expectedly, CsA had no direct effect on cytosolic Ca2+ levels in fura-2-loaded osteoclasts. These studies are a prelude to further investigations into the possible role of CN-A in osteoclast regulation. Finally, mechanistic studies on the bone effects of CsA, a widely used immunosupressant, should proceed from these observations.

The effect of extracellularly applied divalent cations on cytosolic Ca2+ in murine leydig cells: evidence for a Ca2+-sensing receptor

1. The effect of extracellularly applied divalent cations upon cytosolic Ca2+ levels ([Ca2+]) was investigated in fura-2-loaded mouse Leydig (TM3) cells. 2. The extracellular application of Ca2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) elicited concentration-dependent elevations in cytosolic [Ca2+] that were followed by decays to baseline levels. Extracellular Mg2+ (0.8-15 mM) failed to influence cytosolic [Ca2+]. 3. Conditioning applications of Ca2+ (2.5-10 mM), Mg2+ (2.5-15 mM) or Ni2+ (0.5-5 mM) all attenuated the cytosolic Ca2+ response to a subsequent test application of 5 mM [Ni2+]. 4. The amplitude of Ni2+-induced cytosolic Ca2+ signals remained constant in low-Ca2+ solutions. Such findings suggest a participation of Ca2+ release from intracellular stores. In parallel, depletion of Ca2+ stores by either ionomycin (5 microM, in low-Ca2+ solutions) or thapsigargin (4 microM) abolished or attenuated Ni2+-induced Ca2+ transients. 5. Ionomycin (5 microM) elevated cytosolic [Ca2+] in Ca2+-free solutions even after prior Ni2+ application, indicating the presence of Ni2+-insensitive stores. 6. Caffeine (250 and 500 microM) elevated cytosolic [Ca2+] and attenuated Ni2+-induced Ca2+ release. Furthermore, TM3 cells stained intensely with a specific anti-ryanodine receptor antiserum, Ab34. These findings suggest that Ca2+ release is regulated by ryanodine receptors. 7. Both membrane depolarization and hyperpolarization, brought about by changes in extracellular [K+] ([K+]e) in the presence of valinomycin (5 microM), altered the waveform of the Ni2+-induced cytosolic Ca2+ signal. Hyperpolarization, in addition, diminished the response magnitude. Such voltage-induced response modulation localizes the regulatory events to the Leydig cell plasma membrane. 8. We propose the existence of a cell surface divalent cation (Ca2+) receptor in Leydig cells, the activation of which triggers Ca2+ fluxes through ryanodine receptors.

Mode of action of interleukin-6 on mature osteoclasts. Novel interactions with extracellular Ca2+ sensing in the regulation of osteoclastic bone resorption

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti- IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 microg/l), but not IL-11 (10 and 100 microg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 microg/l). Additionally, IL-6 (5 pg/l to 10 microg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.

Effects of electromagnetic stimulation on the functional responsiveness of isolated rat osteoclasts

We report the effects of pulsed electromagnetic fields (PEMFs) on the responsiveness of osteoclasts to cellular, hormonal, and ionic signals. Osteoclasts isolated from neonatal rat long bones were dispersed onto either slices of devitalised cortical bone (for the measurement of resorptive activity) or glass coverslips (for the determination of the cytosolic free Ca2+ concentration, [Ca2+]). Osteoclasts were also cocultured on bone with osteoblastlike, UMR-106 cells. Bone resorption was quantitated by scanning electron microscopy and computer-assisted morphometry. PEMF application to osteoblast-osteoclast cocultures for 18 hr resulted in a twofold stimulation of bone resorption. In contrast, resorption by isolated osteoclasts remained unchanged in the presence of PEMFs, suggesting that osteoblasts were necessary for the PEMF-induced resorption simulation seen in osteoblast-osteoclast cocultures. Furthermore, the potent inhibitory action of the hormone calcitonin on bone resorption was unaffected by PEMF application. However, PEMFs completely reversed another quite distinct action of calcitonin on the osteoclast: its potent inhibitory effect on the activation of the divalent cation-sensing (or Ca2+) receptor. For these experiments, we made fura 2-based measurements of cytosolic [Ca2+] in single osteoclasts in response to the application of a known Ca2+ receptor agonist, Ni2+. We first confirmed that activation of the osteoclast Ca2+ receptor by Ni2+ (5 mM) resulted in a characteristic monophasic elevation of cytosolic [Ca2+]. As shown previously, this response was attenuated strongly by calcitonin at concentrations between 0.03 and 3 nM but remained intact in response to PEMFs. PEMF application, however, prevented the inhibitory effect of calcitonin on Ni2+-induced cytosolic Ca2+ elevation. This suggested that the fields disrupted the interaction between the calcitonin and Ca2+ receptor systems. In conclusion, we have shown that electromagnetic fields stimulate bone resorption through an action on the osteoblast and, by abolishing the inhibitory effects of calcitonin, also restore the responsiveness of osteoclasts to divalent cations.

A possible new role for vitamin D-binding protein in osteoclast control: inhibition of extracellular Ca2+ sensing at low physiological concentrations

Upon removal of its sialic acid or galactose residue, vitamin D-binding protein (DBP) becomes a potent macrophage-activating factor, DBP-MAF. Here we document a new function of DBP-MAF and its parent molecule, DBP, in osteoclast control. We show that all DBPs potently inhibit extracellular Ca2+ (cation) sensing at low nanomolar concentrations with the following rank order of potency: native DBP = sialidase-treated DBP > beta-galactosidase-treated DBP. This attenuation remains unaffected despite co-incubation either with the native DBP ligand, 1,25-dihydroxyvitamin D3, or with an asialoglycoprotein receptor modulator, asialoorosomucoid. Taken together, the results suggest that circulating DBP may play a role in the systemic control of osteoclastic bone resorption, a hitherto unrecognized action of the protein.

Functional and molecular evidence for P2X receptors in LLC-PK1 cells

Extracellular ATP affects a wide variety of cells via purinergic membrane receptors. One class of purinergic receptors, P2X, consists of ATP-gated, calcium-permeable, cation-selective channels. We performed whole cell patch-clamp studies, intracellular calcium concentration ([Ca2+]i) measurements, and reverse transcription-polymerase chain reaction (RT-PCR) to determine whether P2X receptors are expressed in LLC-PK1 cells. First, in patch-clamp studies, 100 microM ATP depolarized the cell membrane and increased the whole cell conductance of LLC-PK1 cells. This response was dose dependent and inhibited by 100 microM suramin, a P2 receptor antagonist. The ATP-induced conductance was cation selective but did not discriminate between Na+ and K+. ADP, alpha,beta-methylene-ATP, and beta,gamma-methylene-ATP had no effect on the whole cell conductance. Next, 10 microM ATP caused a rapid rise in [Ca2+]i in LLC-PK1 cells. This effect of ATP was inhibited by the absence of extracellular calcium and by suramin but not by pretreatment with pertussis toxin. ADP and beta,gamma-methylene-ATP had little or no effect on [Ca2+]i. Finally, RT-PCR produced a 330-bp fragment from LLC-PK1 cell RNA, whose sequence was 80% identical to the rat P2X1 receptor. We conclude that LLC-PK1 cells express purinergic receptors of the P2X class, which mediate depolarization and calcium entry when activated.