Regulation of COX-2 mediates acid-induced bone calcium efflux in vitro.

Chronic metabolic acidosis induces net Ca efflux from bone; this osteoclastic bone resorption is mediated by increased osteoblastic prostaglandin synthesis. Cyclooxygenase, the rate-limiting enzyme in prostaglandin synthesis, is present in both constitutive (COX-1) and inducible (COX-2) forms. We report here that acidosis increases both osteoblastic RNA and protein levels for COX-2 and that genetic deficiency or pharmacologic inhibition of COX-2 significantly reduces acid-induced Ca efflux from bone. INTRODUCTION: Incubation of neonatal mouse calvariae in medium simulating physiologic metabolic acidosis induces an increase in osteoblastic prostaglandin E2 (PGE2) release and net calcium (Ca) efflux from bone. Increased PGE2 is necessary for acid-induced bone resorption, because inhibition of cyclooxygenase activity with indomethacin significantly decreases not only PGE2 production but also Ca release. Cyclooxygenase is present in both constitutive (COX-1) and inducible (COX-2) forms. Because COX-2 activity has been implicated in several forms of pathological bone resorption, we tested the hypothesis that COX-2 is critical for acid-induced, cell-mediated bone Ca efflux. MATERIALS AND METHODS: To determine the effect of metabolic acidosis on COX-2 RNA and protein, primary cells isolated from neonatal CD-1 mouse calvariae were cultured in neutral (Ntl) or physiologically acidic medium (Met). RNA levels for COX-2 and COX-1 were measured by quantitative real-time PCR. Levels of COX-2 and COX-1 protein were measured by immunoblot analysis. To determine the effect of acidosis on bone Ca efflux in genetically deficient COX-2 mice, mice heterozygous for the COX-2 knockout (strain B6;129S7-Ptgs2(tm1Jed)/J) were used as breeders, and neonatal calvariae were cultured in Ntl or Met. To determine the effects of the specific COX-2 inhibitor, NS398, on acid-induced bone resorption, CD-1 calvariae were incubated in Ntl or Met with or without NS398 (1 microM). Medium PGE2 was assayed by ELISA. RESULTS: Incubation of mouse calvarial cells in Met significantly increased COX-2 RNA and protein levels without a change in COX-1. Increased COX-2 protein levels in response to Met were also observed in cultured calvariae. Acid-induced, cell-mediated Ca efflux from B6;129S7-Ptgs2(tm1Jed)/J calvariae was dependent on genotype. From 0 to 24 h, when physicochemical Ca efflux predominates, Met significantly increased net Ca efflux in all genotypes. After 24 h, when cell-mediated Ca efflux predominates, Met induced greater Ca efflux from (+/+) than from (+/-), and there was no increase from (-/-). In calvariae from CD-1 mice, NS398 significantly inhibited both the acid-induced increase in PGE2 and Ca release. CONCLUSIONS: The specific acid-induced increase in COX-2 RNA and protein levels and the dependency of the increased Ca efflux on COX-2 activity, as determined by both genetic deficiency and pharmacologic inhibition, show that COX-2 is critical for acid-induced, cell-mediated bone resorption.     

Mechanical stretch induced calcium efflux from bone matrix stimulates osteoblasts

The mechanisms by which bone cells sense critically loaded regions of bone are still a matter of ongoing debate. Animal models to investigate response to microdamage involve post mortem immunohistological analysis and do not allow real-time monitoring of cellular response during the emergence of the damage in bone. Most in vitro mechanical stimulation studies are conducted on non-bone substrates, neglecting the damage-related alterations in the pericellular niche and their potential effects on bone cells. The current study reports spontaneous efflux of calcium ions (Ca(2+)) (1.924±0.742 pmol cm(-2)s(-1)) from regions of devitalized bone matrix undergoing post-yield strains, induced by a stress concentrator. When these samples are seeded with MC3T3-E1 osteoblasts, the strain-induced Ca(2+) efflux from bone elicits cell response at the stress concentration site as manifested by activation of intracellular calcium signaling (increase in fluorescence by 52%±27%). This activity is associated with extracellular calcium because the intracellular calcium signaling in response to mechanical loading subsides when experiments are repeated using demineralized bone substrates (increase in fluorescence by 6%±10%). These results imply a novel perspective where bone matrix acts as an intermediary mechanochemical transducer by converting mechanical strain into a chemical signal (pericellular calcium) to which cells respond. Such a mechanism may be responsible for triggering repair at locations of bone matrix undergoing critical deformation levels.     

Microdamage induced calcium efflux from bone matrix activates intracellular calcium signaling in osteoblasts via L-type and T-type voltage-gated calcium channels

Mechanisms by which bone microdamage triggers repair response are not completely understood. It has been shown that calcium efflux ([Ca^2 +]_E) occurs from regions of bone undergoing microdamage. Such efflux has also been shown to trigger intracellular calcium signaling ([Ca^2 +]_I) in MC3T3-E1 cells local to damaged regions. Voltage-gated calcium channels (VGCCs) are implicated in the entry of [Ca^2 +]_E to the cytoplasm. We investigated the involvement of VGCC in the extracellular calcium induced intracellular calcium response (ECIICR). MC3T3-E1 cells were subjected to one dimensional calcium efflux from their basal aspect which results in an increase in [Ca^2 +]_I. This increase was concomitant with membrane depolarization and it was significantly reduced in the presence of Bepridil, a non-selective VGCC inhibitor. To identify specific type(s) of VGCC in ECIICR, the cells were treated with selective inhibitors for different types of VGCC. Significant changes in the peak intensity and the number of [Ca^2 +]_I oscillations were observed when L-type and T-type specific VGCC inhibitors (Verapamil and NNC55-0396, respectively) were used. So as to confirm the involvement of L- and T-type VGCC in the context of microdamage, cells were seeded on devitalized notched bone specimen, which were loaded to induce microdamage in the presence and absence of Verapamil and NNC55-0396. The results showed significant decrease in [Ca^2 +]_I activity of cells in the microdamaged regions of bone when L- and T-type blockers were applied. This study demonstrated that extracellular calcium increase in association with damage depolarizes the cell membrane and the calcium ions enter the cell cytoplasm by L- and T-type VGCCs.     

Selective stimulation of net calcium efflux from chick embryo tibiae by parathyroid hormone in vitro

Summary The purpose of this study was to measure in an in vitro system the movement of Ca and phosphate (P_i) out of bone when treated with parathyroid hormone (PTH). Tibiae from 13-day chick embryos were incubated for up to 8 h in a defined medium containing 1.8 mM Ca. Medium samples were collected every 2 h and were analyzed for Ca, P_i and lactate. Net effluxes from the bones were calculated. When bones were incubated with PTH in the medium (1 U/ml), net Ca efflux was increased 44, 60 and 100% at 4, 6 and 8 h, respectively. At no time was net P_i efflux affected by the hormone. The well known PTH-stimulated lactate production was not seen until 8 h. Lower doses of PTH (0.1 and 0.3 U/ml) were also effective. Comparing PTH (1 U/ml) responsiveness at higher (2.2 mM) and lower (0.9 mM) medium Ca concentrations, showed that with 2.2 mM Ca no increased Ca efflux was seen, while with 0.9 mM Ca significant elevation in medium Ca occurred 2 h sooner than in the experiments using 1.8 mM Ca. In another experiment, varying the medium P_i level from 1 to 2 mM had no effect on the Ca response to PTH. In neither experiment was P_i release affected by PTH. The results of this study have led to the following conclusions: (1) PTH acts on bone to cause an early dose related increase in net Ca efflux; (2) the effect is specific for Ca, since it is not accompanied by an increased P_i efflux, and may be saturated by raising the medium Ca level; and (3) PTH-stimulated Ca efflux in this system is not correlated with, and is probably not a result of increased lactate production.     

The in vitro elution characteristics of antifungal-loaded PMMA bone cement and calcium sulfate bone substitute

The use of antimicrobial-loaded delivery vehicles, most often as antibiotic beads, is common practice for the treatment of deep musculoskeletal infections. The elution of antibacterial drugs from various bone cements has been extensively studied. However, much less is known about the elution of other antimicrobials from these materials. In particular, the use of this approach for fungal infections has not been well studied despite growing concern about these difficult-to-treat organisms. Voriconazole is a broad-spectrum and highly effective antifungal that has been used in the treatment of resistant fungal pathogens. We examined the in vitro elution characteristics of voriconazole from nonabsorbable polymethylmethacrylate (PMMA) beads and from absorbable calcium sulfate beads. Voricanazole-containing beads were immersed in a 5-mL bath of phosphate-buffered saline at room temperature and placed on an orbital shaker. Eluent samples were collected over the course of 2 weeks. Concentrations of the antifungal drug in solution were measured using high-performance liquid chromatography. To verify biologic activity of the eluted antifungal, collected samples were also tested against control yeasts. We found that samples collected out to 2 weeks contained relatively high voriconazole concentrations and enough active antifungal activity to inhibit growth of the control yeasts. These data demonstrate that voriconazole retains its antifungal activity when mixed into either PMMA or calcium sulfate beads, and elutes out of beads at biologically effective concentrations over a time period of at least 2 weeks. Therefore, incorporation of voriconazole into either absorbable or nonabsorbable beads appears to be a reasonable strategy for the local delivery of a potent, broad-spectrum antifungal agent to an infected wound bed.     

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 Ca2+ by outward and inward Ca2+ 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 Ca2+ 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 Ca2+ concentration gradient (10 microm spatial resolution) were performed on metatarsal bones of weanling mice after drilling a 100-mum 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 Ca2+ 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 (>or=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 Ca2+ 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.     

磷酸钙骨水泥复合微小颗粒骨的体外实验研究

[目的]进行磷酸钙骨水泥复合微小颗粒骨的实验,探讨磷酸钙骨水泥(calcium phosphate cement,CPC)和微小颗粒骨最佳混合比例。[方法]取大耳白兔双侧股骨制作颗粒骨,实验组为颗粒骨和CPC分别以1∶1、1∶2、1∶3、1∶4、1∶5混合固化的材料;对照组为CPC固化的材料。测定两组的固化时间及最大抗压强度;间充质干细胞培养于材料浸提液中,MTT检测细胞增殖;间充质干细胞与各材料共培养5 d后,电镜观察材料表面的细胞量及形态。[结果]MTT检测:比例为1∶2和1∶3组明显促进细胞的增殖,优于其他组,有统计学意义(P0.05)。电镜扫描:1∶2、1∶3组材料表面的细胞生长良好,其他组材料表面的细胞量明显少于这两组。1∶3组材料的固化时间及最大抗压强度适中,与CPC组比较,有统计学意义(P0.05)。[结论]1∶3在可操作性、生物力学、细胞相容性方面是最合适的混合比例。     

Effect of acute calcium influx after mechanical stretch injury in vitro on the viability of hippocampal neurons

We use a new in vitro model to examine the effect of mechanical deformation on neurons. We examined acute changes in cytosolic calcium concentrations ([Ca(2+)](i)) caused by a rapid stretch of cultured hippocampal neurons, using mechanical loading conditions that mimic brain deformations during trauma. We found that stretch-injury of neurons induces a strain-dependent increase in [Ca(2+)](i). Remarkably, the extent of this calcium response exceeded the levels initiated by chemical toxicity with NMDA (100 microM) or glutamate (5 mM) exposure. Propidium iodide labeling at 24 h following stretch showed neuronal death occurred only at the most severe level of mechanical injury. Although NMDA-induced toxicity could be inhibited in calcium free media or by treatment with MK-801, stretch-induced neuronal death was not similarly reduced with either treatment. Unexpectedly, reduction of the acute stretch-induced calcium transient with calcium-free media or MK-801 resulted in an increase in neuronal death at lower stretch levels. These data suggest that mechanical stretch can initiate calcium influx in hippocampal neurons, but substantially modulating the early calcium flux from the extracellular space or through the NMDA channel does not provide an effective means for improving neuronal survival.     

Erythrocyte voltage-dependent calcium influx is reduced in hemodialyzed patients.

BACKGROUND: Uremia displays increased cytosolic free calcium ([Ca2+]i) in many different cell types, supporting the hypothesis of an altered Ca2+ transport modifying the functional activity of calcium signaling pathway. METHODS: Thirty-five hemodialyzed patients and 20 age-matched subjects were studied. Erythrocyte resting [Ca2+]i and Ca2+ influx were measured by the fluorescent Ca2+-sensitive dye fura-2. RESULTS: We found an increase of resting [Ca2+]i in erythrocytes from uremic hemodialyzed patients compared with matched healthy controls (103 +/- 2.5 nM, N = 20, vs. 90 +/- 4, N = 20, P < 0.01). Moreover, we found an altered voltage-dependent Ca2+ influx showing a reduced transport rate (0.42 +/- 0.03 nM/second vs. 0.74 +/- 0.08, P < 0.01). High levels of plasma parathyroid hormone (PTH) were related to augmented Ca2+ entry (r = 0.511, P < 0.05), contributing to maintain a high level of [Ca2+]i. Hemodialysis had no effect on cell calcium level and Ca2+ influx indices. The therapy with Ca2+ antagonists did not modify the values of resting [Ca2+]i or Ca2+ influx indices, but the correlation between PTH and influx indices was lost. CONCLUSIONS: In conclusion, we found evidence for an alteration of erythrocyte Ca2+ influx caused by uremic toxicity that could be related to some organ disorders in uremia. The chronic increase of cellular calcium may contribute to influx derangement.     

Mechanism of amphotericin B stimulation of net calcium efflux from neonatal mouse calvariae

Amphotericin B is a polyene antifungal agent that binds to membrane sterols, creating aqueous pores that permit ion fluxes sufficient to cause cell lysis. It has also been shown to alter ion transport in mammalian cells, including proton secretion from renal tubular cells. The latter effect can lead to distal renal tubular acidosis in patients treated for systemic fungal infections. Based on the understanding that osteoclast-mediated bone resorption is dependent on proton secretion, we examined the effect of amphotericin B on calcium efflux from neonatal mouse calvariae in organ culture. Amphotericin B (5 micrograms/ml) stimulated net calcium efflux from calvariae within 24 h to a level almost as great as that produced by a maximally effective concentration of parathyroid hormone. The stimulated calcium efflux was completely inhibited by both 10 ng/ml salmon calcitonin, a physiologic inhibitor of osteoclast activity, and 4 x 10(-4) M acetazolamide, a specific inhibitor of carbonic anhydrase, the enzyme necessary for substantial proton generation by osteoclasts. These results indicate a direct effect of amphotericin B on bone in vitro to stimulate osteoclast-mediated calcium efflux.     

Oxalate influx rate in red blood cells in calcium oxalate nephrolithiasis

We established a method of human red blood cell oxalate influx rate under the condition of steady state exchange. Using this method we measured the influx rate in 10 patients with recurrent calcium oxalate nephrolithiasis and in 18 controls. DIDS inhibited oxalate flux across the human red blood cell membrane. This result suggested that band 3 protein mediates oxalate transport. Oxalate influx rate depended on reaction temperature and pH of reaction buffer. Although the oxalate influx rate in 4 degrees C could not be determined under an initial condition rate, the measurable condition was in 0 degrees C of reaction temperature. Consequently we measured the oxalate influx rate under the condition of 0 degrees C reaction temperature and 20 ml volume of washing buffer. The mean oxalate influx rate was significantly higher in patients with nephrolithiasis than in controls (-1.00 +/- 0.19 vs. -0.78 +/- 0.14).     

改良复合磷酸钙骨水泥诱导分化成骨作用的体外研究

目的 通过体外实验观察兔骨髓基质干细胞(BMSCs)与改良复合磷酸钙骨水泥(BCPC)共培养的生物相容性及诱导成骨活性,探讨其可能的机制.方法 配制相同大小规格的BCPC及普通磷酸钙骨水泥(CPC)块,实验分为4组:空白对照组(不加载体)、普通CPC组、BCPC组、加BMP组(加BMP的BCPC).将各组材料与兔BMSCs复合培养,倒置显微镜下观察细胞生长情况,进行细胞计数,碱性磷酸酶(ALP)试剂盒检测细胞合成ALP活性变化,扫描电镜观察细胞在材料表面的生长及长入情况,real-time PCR检测成骨及成软骨相关基因mRNA表达情况并比较表达差别.结果 复合培养后倒置显微镜观察各组细胞与载体边缘接触良好,各组细胞增殖差异无统计学意义(P＞0.05);扫描电镜观察细胞在BCPC表面生长增殖良好,逐渐伸出伪足并长入材料孔隙;复合培养7 d后,BCPC组及加BMP组比CPC组和空白对照组表达更高的ALP活性,差异有统计学意义(P＜0.05);复合培养第10天,BCPC组及加BMP组分别与CPC组及空白对照组比较,其成骨及成软骨相关基因mRNA表达差异有统计学意义(P＜0.05).结论 配制的BCPC具有良好的生物相容性和一定的成骨诱导能力,其疏松的结构及更大的孔径适宜细胞长入材料内部.     

Humoral factor in children with neonatal Bartter syndrome reduces bone calcium uptake in vitro

The neonatal Bartter syndrome (NBS) is associated with a complex disorder of mineral metabolism in children, including hypercalciuria, nephrocalcinosis, and diminished bone mineral density. Although cyclooxygenase inhibition usually brings about improvement in these findings, there is a variable component which is resistant to such therapy in many children. The factor mediating this disorder has not been identified. Blood and urine from 12 children with NBS were examined. When compared with samples from normal children and adults, all (NBS) sera reduced bone calcium uptake in a bone disc bioassay. This effect persisted in the presence of parathyroid hormone (PTH) antibody and PTH receptor blockade, indicating that neither PTH nor PTH related peptide was responsible. It was eliminated by indomethacin, suggesting that prostanoid generation was essential. Protamine was also inhibitory, as was the addition of ecteola, an anion binder. Activity could be recovered from ecteola by elution with hypertonic buffer. Urine samples from children with NBS had the same calcitropic effect. The agent was removed by ecteola and recovered by hypertonic elution. Activity was eliminated by protamine and by heparinase, but not by trypsin digestion. Size exclusion centrifugation showed that the activity was associated with a material between 10 and 30 kilodaltons. Finally, urine ecteola eluates from NBS patients raised serum concentrations of calcium after intraperitoneal injection in rats. These data suggest that children with NBS have a calcitropic substance in their serum and urine which is not found in normal individuals. The substance is heparin like, and mediates its effects through prostanoid production. These studies provide additional evidence against a direct renal cause of the urinary calcium disturbance characteristic of the disorder. Received February 18, 1997; received in revised form September 16, 1997; accepted September 19, 1997     

Estimating total body calcium from peripheral bone measurements

In a heterogeneous group of 12 male and 30 female patients, total body calcium (TBCa) was measured by neutron activation analysis, and bone mineral content of the distal femur and six regions of the distal radius and ulna were measured by photon absorptiometry. TBCa was an average of 71 g (9%) lower than normal in the females and 119 g (11%) lower than normal in the males. Of the forearm measurements, those of the radial shaft provided the best estimate of TBCa with a standard error of estimation (SEE) of 88 g (14% of the mean value) for female patients alone, and 113 g (16%) for the whole group. The measurement on the femur provided a better estimate with SEE 80 g (13%) for female patients and 90 g (13%) for the whole group. These SEE are comparable with the variability in TBCa between normal subjects, which has a coefficient of variation of the order of 20% in both sexes. A slightly better estimate of TBCa can be obtained by using a multiple regression equation involving both the femur and radial shaft measurements with a SEE of 74 g (12%) for female patients and 81 g (11%) for the whole group. The correlation between the estimated deficit of TBCa and the estimated deficit of bone mineral from the forearm and the femur has been calculated.     

Progesterone-induced calcium influx in cynomolgus monkey (Macaca fascicularis) spermatozoa

For in vitro capacitation to occur in cynomolgus monkey (Macaca fascicularis) spermatozoa, there is an absolute requirement for exogenous stimulation with the sperm activators, caffeine (1 mM) and db-cyclic adenosine monophosphate (dbcAMP) (1 mM), which are known to induce capacitation-related hyperactivated motility. Tyrosine phosphorylation of sperm tail proteins is an integral component of this caffeine- and dbcAMP-stimulated hyperactivated motility. In both capacitated and noncapacitated human spermatozoa, progesterone (P4) has been reported to elicit an immediate, potent increase in intracellular calcium ion concentrations [Ca2+]i. The objective of this study was to examine the effects of progesterone on requisite events in macaque fertilization, including [Ca2+]i, hyperactivated motility, and the concomitant tyrosine phosphorylation of sperm tail (STTP) proteins after treatment with caffeine and dbcAMP. The effect of 1 microM of progesterone on [Ca2+]i was determined by spectrofluorometry with the fluorescent indicator, fura-2/AM, on hyperactivated motility using computer analysis (HTM-IVOS) with the sorting criteria lateral head amplitude (> or = 8.0 microm), curvilinear velocity (> or = 150 microm/s), linearity (< or = 60%), and on STTP by immunocytochemistry. The results showed that progesterone elicited a significant increase in [Ca2+]i in caffeine- and dbcAMP-activated macaque sperm with maximal stimulation at 30 minutes after activation. The response in nonactivated sperm was dramatically reduced compared with the response in activated sperm. Basal [Ca2+]i increased as a function of time in both activated and nonactivated control sperm although basal levels were significantly increased in activated sperm. Progesterone stimulation resulted in a small but significant increase in both hyperactivation and STTP when sperm were first pretreated with caffeine and dbcAMP. Our results provide evidence that macaque sperm activation with caffeine and dbcAMP is required for a progesterone-elicited response, which results in calcium influx, hyperactivated motility, and sperm tail tyrosine phosphorylation.     

Bradykinin induces a calcium-store- dependent calcium influx in mouse mesangial cells

Bradykinin (BK) elicits extracellular-dependent [Ca2+](i) elevations in mouse mesangial cells (MMC) that are not blocked by verapamil, nifedipine, L-nicardipine, NiCl(2), or LaCl(3). The aim of the present study was to evaluate the mechanisms involved in calcium influx induced by BK in MMC. [Ca2+](i) was analyzed through spectrofluorometry employing fura-2-AM, and the data were expressed as [Ca2+](i )obtained/[Ca2+](i )basal ratio. Heparin (IP(3), a receptor antagonist) almost abolished the effects of BK in MMC (1.85 +/- 0.15 vs. 1.13 +/- 0.02, n = 4, p = 0.001). Following external and intracellular calcium store depletion, BK's effect was absent even after successful extracellular calcium replenishment. ML-7 (a myosin light chain kinase inhibitor) blocked responses to thapsigargin (2.62 +/- 0.13 vs. 1.11 +/- 0.04, n = 4, p < 0.001), but not those of BK (6.51 +/- 0.39, n = 6, vs. 5.86 +/- 1.17, n = 4, p = 0.39). On the other hand, genistein (a tyrosine kinase inhibitor) was able to inhibit thapsigargin (3.12 +/- 0.22, n = 5, vs. 1.28 +/- 0.16, n = 4, p < 0.001) as well as BK responses (6.46 +/- 0.66 vs. 2.89 +/- 0.61, n = 4, p < 0.05). Econazole (a P-450 monooxygenase inhibitor) inhibited the responses to both thapsigargin (3.45 +/- 0.16 vs. 1.03 +/- 0.03, n = 4, p < 0.001) and BK (6.49 +/- 0.83, n = 6, vs. 1.17 +/- 0.08, n = 4, p = 0.01). Finally, responses to BK were not affected by indomethacin (6.69 +/- 0.66 vs. 6.57 +/- 0.87, n = 4, p = 0.916). Thus, BK promotes an IP(3)-sensitive store-dependent calcium influx in MMC. This phenomenon seems to involve tyrosine kinase and P-450 monooxygenase products in its transduction pathway.     

Contribution of calcium influx in mediating glucose-stimulated oxygen consumption in pancreatic islets

In brain, muscle, and pancreatic islets, depolarization induces an increase in respiration, which is dependent on calcium influx. The goal of this study was to assess the quantitative significance of this effect in islets relative to glucose-stimulated ATP turnover, to examine the molecular mechanism mediating the changes, and to investigate the functional implications with respect to insulin secretion. Glucose (3-20 mmol/l) increased steady-state levels of cytochrome c reduction (32-66%) in isolated rat islets, reflecting an increased production of NADH, and oxygen consumption rate (OCR) by 0.32 nmol/min/100 islets. Glucose-stimulated OCR was inhibited 30% by inhibitors of calcium influx (diazoxide or nimodipine), whereas a protein synthesis inhibitor (emetine) decreased it by only 24%. None of the inhibitors affected cytochrome c reduction, suggesting that calcium's effect on steady-state OCR is mediated by changes in ATP usage rather than the rate of NADH generation. 3-isobutyl-1-methylxanthine increased insulin secretion but had little effect on OCR, indicating that the processes of movement and exocytosis of secretory granules do not significantly contribute to ATP turnover. At 20 mmol/l glucose, a blocker of sarcoendoplasmic reticulum calcium ATPase (SERCA) had little effect on OCR despite a large increase in cytosolic calcium, further supporting the notion that influx of calcium, not bulk cytosolic calcium, is associated with the increase in ATP turnover. The glucose dose response of calcium influx-dependent OCR showed a remarkable correlation with insulin secretion, suggesting that the process mediating the effect of calcium on ATP turnover has a role in the amplification pathway of insulin secretion.     

Blockade of calcium influx through L-type calcium channels attenuates mitochondrial injury and apoptosis in hypoxic renal tubular cells

In hypoxia, ATP depletion causes cellular Ca(2+) increase, mitochondrial injury, and apoptosis in renal tubular cells. However, the molecular basis of these observations is incompletely delineated. IRPTC, a rat renal proximal tubular cell line, was treated with antimycin A, and disturbances in cytoplasmic calcium ([Ca(2+)]c) and mitochondrial calcium ion concentration ([Ca(2+)]m), dissipation of mitochondrial membrane potential (DeltaPsi(m)), cytochrome c release, and resultant apoptosis were examined. Pharmacologic targeting of L-type Ca(2+) channels in vitro and in vivo was used to clarify the involvement of voltage-dependent Ca(2+) channels during this process. In vitro studies indicated that ATP depletion-induced apoptosis was preceded by increased [Ca(2+)]c and [Ca(2+)]m before activation of mitochondrial signaling. Antagonizing L-type Ca(2+) channels offset these findings, suggesting [Ca(2+)]c and [Ca(2+)]m involvement. Azelnidipine administration ameliorated cellular and mitochondrial Ca(2+) accumulation, mitochondrial permeability transition, cytochrome c release, caspase-9 activation, and resultant apoptosis (15.8 +/- 0.8% versus 8.9 +/- 0.7%; P < 0.01). Similar effects of azelnidipine were substantiated in an in vivo ischemia/reperfusion injury model. There were fewer terminal-deoxynucleotidyl transferase mediated dUTP nick-end labeling-positive cells in the azelnidipine-treated group (0.322 +/- 0.038/tubule) as compared with the vehicle-treated group (0.450 +/- 0.041; P < 0.05), although the antiapoptotic effect was smaller in vivo than in vitro, partly as a result of distinct levels of Bax expression. It is proposed that voltage-dependent Ca(2+) channels are involved in cellular and mitochondrial accumulation of Ca(2+) subsequent to ATP depletion and play an important role in regulating mitochondrial permeability transition, cytochrome c release, caspase activation, and apoptosis.