Parathyroid hormone-stimulated cadmium accumulation in Madin-Darby canine kidney cells

Cadmium-induced Inhibition of ADH-stimulated Ion Transport in Cultured Kidney-derived Epithelial Cells (A6)

An epithelial cell line (A6) derived from the distal tubule of toad kidney, was used to study the effect of cadmium (Cd2+) on the increase in active ion transport induced by antidiuretic hormone (ADH). Addition of Cd2+ (1mM) to the basolateral solution of A6 epithelia generated an immediate and transient increase in active ion transport, measured as short circuit current (SCC). This increase was not affected by prior addition of ADH. However, there was a distinct inhibition of ADH-induced stimulation of SCC in epithelia pre-treated with Cd2+. Since cAMP serves as an intracellular messenger for ADH by increasing the ion permeability of the apical membrane in A6 epithelial cells, the effects of Cd2+ on enzymes involved in cAMP metabolism were measured. The results showed that Cd2+ markedly inhibits cAMP production by inhibiting adenylate cyclase (which had been stimulated with forskolin, magnesium or a non-hydrolysed GTP-analog), indicating that Cd2+ inhibits the catalytic subunit of adenylate cyclase. Furthermore, degradation of cAMP by phosphodiesterase was not stimulated by Cd2+, also suggesting that the mechanism by which Cd2+ inhibits the ADH-induced ion transport could be through inhibition of adenylate cyclase. Taken together, these results indicate that, in addition to the well-known toxic effect on the proximal tubule, Cd2+ could also have an effect on the distal part of the kidney, where the important hormonal regulation of salt and water homeostasis takes place.

2-O-methyl PAF as a Ca2+ mobilizer in Madin Darby canine kidney cells

In Madin-Darby canine kidney (MDCK) cells, the effect of 2-O-methyl PAF, an inactive analogue of platelet activating factor (PAF), on intracellular Ca2+ concentration ([Ca2+]i) was measured by using the Ca2+-sensitive fluorescent dye fura-2. 2-O-methyl PAF (> or = 15 microM) caused a rapid rise of [Ca2+]i in a concentration-dependent manner. 2-O-methyl PAF-induced [Ca2+]i rise was partly reduced by removal of extracellular Ca2+. 2-O-methyl PAF-induced extracellular Ca2+ influx was also suggested by Mn2+ influx-induced fura-2 fluorescence quench. The 2-O-methyl PAF-induced Ca2+ influx was blocked by nifedipine, verapamil and diltiazem. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which 2-O-methyl PAF failed to increase [Ca2+]i; also, pretreatment with 2-O-methyl PAF depleted thapsigargin-sensitive Ca2+ stores. U73122, an inhibitor of phospholipase C, abolished ATP (but not 2-O-methyl PAF)-induced [Ca2+]i rise. These findings suggest that 2-O-methyl PAF evokes a rapid increase in [Ca2+]i in renal tubular cells by stimulating both extracellular Ca2+ influx and intracellular Ca2+ release.

Effects of parathyroidectomy on lead mobilization from bone in patients with primary hyperparathyroidism

Since lead (Pb) accrued from environmental exposure accumulates in bone with a half life time between 6 and 10 years, a release of bone Pb into the circulation and/or urine (PbU) should be expected in diseases with increased bone metabolism such as hyperparathyroidism. We studied 60 patients with primary hyperparathyroidism (pHPT, 50 women, 10 men, aged 61.4 +/- 10.6 and 64.1 +/- 9.9 years, respectively) (a) before, (b) 1-6 months, and (c) 6-12 months after parathyroidectomy. Besides lead in blood (PbB) and lead in 24-h urine samples (PbU), parathyroid hormone (PTH), serum Ca2+, osteocalcin (OC), phosphate (PO4), and serum pyridinoline cross-linked telopeptide (cTP) were determined. Control data were determined in 20 healthy age-matched subjects. As expected, Ca2+ decreased after parathyroidectomy. Mean PbB in patients with pHPT was in the same range as in controls. A decrease of PbB after parathyroidectomy was found in the interval beyond 6 months. In contrast, mean PbU initially increased after surgery (3.05 +/- 1.94 vs. 4.25 +/- 2.65 microg/l, P = 0.004) and was not different beyond 6 months in comparison with preoperative values at (c). Investigating only patients with PTH < 150 ng/l, no significant PbB or PbU alterations were detected before and after parathyroidectomy. In patients with PTH > 150 ng/l, the decrease of PbB at (c) was more pronounced as was the increase of PbU at (b). In these patients, PbB and OC as well as PbB and cTP were correlated preoperatively. In conclusion, our data show that in environmentally lead-exposed (by food or by pollution) hyperparathyroid individuals, there is no hazardous PbB release from bone. The preoperative correlation between PbB and OC in pHPT patients with PTH > 150 ng/l provides evidence that in fact there is a Pb release from bone into the blood-pool by bone remodeling. The increase of PbU after parathyroidectomy is suspected to be caused by PTH-dependent Pb accumulation in the kidney, which seems to be restored with decreasing PTH. Moreover, our data confirm prior findings that bone remodeling seems to be normalized 6 months after parathyroidectomy.

Cadmium uptake in rat hepatocytes in relation to speciation and to complexation with metallothionein and albumin

Cadmium (Cd) uptake has been studied in primary cultures of rat hepatocytes focusing on the impact of inorganic and organic speciation. Uptake time-course studies over a 60-min exposure to 0.3 microM (109)Cd revealed a zero-time uptake and a slower process of accumulation which proceeds within minutes. (109)Cd uptake showed saturation kinetics (K(m) = 3.5 +/- 0.8 microM), and was highly sensitive to inhibition by Zn and Hg. There was no evidence for sensitivity to the external pH nor for any preferential transport of the free cation Cd(2+) over CdCl(n) (2-n) chloro-complexes. According to the assumption that only inorganic metal species are available, metal uptake decreased upon albumin (BSA) addition to the exposure media. In contrast, higher levels of (109)Cd accumulation were obtained under optimal conditions for Cd complexation by MT. Comparison among uptake data obtained under inorganic and organic conditions revealed that Cd-MT would be taken up 0.4 times as rapidly as Cd(inorg). We conclude that uptake of Cd in rat hepatocytes involves specific transport mechanism(s) subjected to Zn or Hg interactions. Uptake of inorganic Cd is not proportional to the levels of free Cd(2+) and does not involve the divalent cation transporter DCT1 nor the co-transporter Fe(2+)-H(+) NRAMP2. We found Cd-MT but not Cd-BSA to be available for the liver cells, and have estimated a binding affinity four orders of magnitude higher for Cd complexation with MT compared to BSA; MT may have a significant role in Cd delivery to the liver.

Short- and long-term influences of heavy metals on anionic drug efflux from renal proximal tubule

We recently demonstrated in isolated killifish renal proximal tubules that two classes of nephrotoxicants, aminoglycoside antibiotics and radiocontrast agents, rapidly decrease transport mediated by multidrug resistance protein 2 (Mrp2) by causing endothelin (ET) release and signaling through an ET(B) receptor and protein kinase C (PKC). In the present study, we used killifish proximal tubules, fluorescein methotrexate, a fluorescent model substrate for Mrp2, and confocal microscopy to examine the effects of two heavy metal salts (CdCl(2) and HgCl(2)) on Mrp2 function. Three patterns of effects were seen. First, exposing tubules to 10 microM CdCl(2) or 100 nM HgCl(2) for 30 min reduced Mrp2-mediated transport. This reduction was abolished by the ET(B) receptor antagonist, RES-701-1, and by the PKC-selective inhibitor, bis-indolylmaleimide I; neither of these pharmacological tools by itself affected transport. As with aminoglycoside antibiotics and radiocontrast agents, the acute effects of 10 microM CdCl(2) or 100 nM HgCl(2) on transport were also blocked by nifedipine, suggesting that Ca(2+) also initiated cadmium and mercury action. Second, exposure to higher concentrations of CdCl(2) and HgCl(2) appeared to be toxic. Third, exposing tubules for 6 to 24 h to lower levels of CdCl(2) increased Mrp2-mediated transport and Mrp2 immunostaining at the luminal membrane of the proximal tubule cells. Together, these findings indicate that exposure of renal proximal tubules to heavy metals initially leads to reduced Mrp2 function but is followed by an induction in Mrp2-mediated transport after long-term exposure.

Cadmium (Cd2+) disrupts E-cadherin-dependent cell-cell junctions in MDCK cells

Previous studies from our laboratory have shown that Cd2+ can selectively disrupt E-cadherin-dependent cell-cell junctions in the porcine renal epithelial cell line, LLC-PK1. The objective of the present studies was to determine whether or not Cd2+ could produce similar effects in Madin-Darby canine kidney (MDCK) cells, an immortal epithelial cell line derived from dog kidney. This is an important issue because MDCK cells have been used extensively as a model system to study the basic mechanisms of E-cadherin-dependent cell-cell adhesion. MDCK cells on permeable membrane supports were exposed to Cd2+ by adding CdCl2 to either the apical or the basolateral compartment. The integrity of cell-cell junctions was assessed by morphologic observation of the cells and by monitoring the transepithelial electrical resistance. The results showed that exposure to 10-40 microM Cd2+ for 15 min-4 h caused the cells to separate from each other without detaching from the growing surface. The separation of the cells was accompanied by a marked drop in the transepithelial electrical resistance, a loss of E-cadherin from the cell-cell contacts, and a reorganization of the actin cytoskeleton. These effects were much more pronounced when Cd2+ was added basolaterally than when it was added apically. Moreover, the effects of Cd2+ were qualitatively similar to those observed when the cells were incubated in Ca(2+)-free medium. These results show that Cd2+ can disrupt E-cadherin-dependent cell-cell junctions in MDCK cells, and they indicate that this cell line would be an appropriate model for further mechanistic studies in this area.

Effects of cadmium on trophoblast calcium transport

Maternal exposure to cadmium (Cd) during pregnancy has been linked to low fetal birthweight, which may be attributed to placental damage and/or dysfunction in nutrient transport. Previous studies have suggested that Cd is accumulated in the placenta, and that placental transport of calcium (Ca) and zinc (Zn) is perturbed by Cd. To investigate the mechanism of Cd perturbation of Ca transport, we used JEG-3, a human choriocarcinoma cell line which exhibits trophoblastic properties, to analyse Cd effects in vitro. Treatment with Cd at low, physiologically relevant concentrations (e.g. 0.04 microM) did not result in obvious changes in cell morphology or integrity, whereas higher concentrations (> or = 0.16 microM) affected cell integrity. With lower concentrations of Cd treatment for 24 h, activities of cellular Ca uptake and transport, and Ca2+ binding were decreased, and intracellular [Ca2+] ([Ca2+]i) profile was also altered; however, membrane-associated Ca(2+)-activated ATPase activity remained relatively unchanged. Interestingly, cellular Ca uptake activity was unaffected by short-term (30 min) Cd pretreatment. The 24-h Cd treatment also resulted in elevated expression of the metal-binding protein, metallothionein, whereas the expression of a trophoblast-specific cytosolic Ca(2+)-binding protein (HCaBP) was drastically reduced. These results strongly suggest that Cd exposure significantly compromises the Ca handling ability of trophoblastic cells; this effect is probably not due to perturbations in Ca channel or membrane Ca pump activities, but rather a consequence of alterations in subcellular, cytosolic Ca2+ binding activities.

Effect of cadmium on calcium transport in a human fetal hepatic cell line (WRL-68 cells)

Toxic metals appear to use the transport pathways that exist for biologically essential metals. Calcium uptake in cells occurs through specific membrane channels. Since cadmium inhibits calcium uptake, this study was carried on to elucidate the mechanism of Cd interference with calcium transport using the fetal hepatic cell line WRL-68 as an in vitro model. Ca accumulation by WRL-68 cells presented an initial rapid phase, followed by a sustained phase of slower accumulation over a 60 min period. A concentration of 50 microM CdCl2 produced 39% inhibition of the uptake of CaCl2 (100 microM), while 100 microM nifedipine or verapamil decreased Ca accumulation by 35 and 63%, respectively. All Cd concentrations tested produced significant decrease in Ca uptake in a concentration-dependent manner at 1 min and thereafter, although with 10 microM CdCl2 no significant difference was found after 30 min of incubation. From the Lineweaver-Burk plot, we found that Cd exerted a competitive inhibition on Ca uptake, since there was no significant effect on the Vmax but an increased K(m). A second order rate constant of Cd inactivation of 0.061 mM-1.s-1 was determined from the course of Ca uptake during Cd inhibition. SH groups seemed to play an essential role in Ca inhibition uptake by Cd because the inhibition of Ca accumulation by 50 microM Cd was practically reversed after the addition of dithiothreitol.

Inhibition by nitrendipine of 86Rb+ fluxes in subconfluent MDCK cells

Part of the natriuretic mechanism of dihydropyridine Ca2+ channel antagonists involves the inhibition of renal tubular sodium reabsorption. To identify the membrane ion transport system involved in this natriuretic action, we tested nitrendipine on unidirectional 86Rb+ fluxes in Madin-Darby canine kidney (MDCK) cells. To dissect between direct and indirect effects (via cytosolic Ca2+) of nitrendipine, the compound was re-examined on ion fluxes in human erythrocytes. In MDCK cells, external Ca2+ (3 mM), adrenalin (100 microM) and the Ca2+ ionophore A23187 (20 microM) strongly and transiently stimulated 86Rb+ efflux. All these stimulatory actions were fully inhibited by quinine (1 mM) suggesting that they reflect the opening of Ca(2+)-sensitive K+ channels. Nitrendipine was able to inhibit these Ca(2+)-sensitive K+ channels, bit this inhibitory action required concentrations of the compound (approximately 100 microM). Regarding 86Rb+ influx, the most significant result with nitrendipine was a partial inhibition of bumetanide-sensitive 86Rb+ influx. This effect represented a maximal flux inhibition of about 70% and required very low nitrendipine concentrations (IC50 approximately 1 nM). The Ca2+ ionophore A 23187 strongly stimulated bumetanide-sensitive 86Rb+ influx in MDCK cells. Conversely, a very important reduction (approximately 79%) of this influx component was found in Ca2+ depleted cells. In human red blood cells, Na+, K+, Cl- cotransport fluxes were resistant to nitrendipine, even at high concentrations of the compound (100-500 microM). Conversely, Ca(2+)-sensitive K+ channels were inhibited by nitrendipine with IC50 = 6 +/- 3 microM (mean +/- S.E.M., n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Surface binding and uptake of cadmium (Cd2+) by LLC-PK1 cells on permeable membrane supports

Recent studies have shown that Cd2+ has relatively specific damaging effects on cell-cell junctions in the renal epithelial cell line, LLC-PK1. The objective of the present studies was to examine the surface binding and uptake of Cd2+ by LLC-PK1 cells in relation to the disruption of cell-cell junctions. LLC-PK1 cells on Falcon Cell Culture Inserts were exposed to CdCl2 containing trace amounts of 109Cd2+ from either the apical or the basolateral compartments, and the accumulation of 109Cd2+ was monitored for up to 8 h. The integrity of cell-cell junctions was assessed by monitoring the transepithelial electrical resistance. The results showed that the cells accumulated 3-4 times more Cd2+ from the basolateral compartment than from the apical compartment. The accumulation of Cd2+ from the basolateral compartment occurred in two phases: a rapid, exponential phase that occurred in 1-2 h and coincided with a decrease in transepithelial resistance, and a slower, linear phase that continued for 6-8 h. The Cd2+ that accumulated during the rapid phase was easily removed by washing the cells in EGTA, indicating that most of it was bound to sites on the cell surface. By contrast, most of the Cd2+ that accumulated during the slower phase could not be removed by EGTA, indicating that it had been taken up by the cells. Additional studies showed that the rapid phase of Cd2+ accumulation was enhanced when Ca2+ was present at low concentrations (0.1 mM), and was greatly reduced when Ca2+ was present at high concentrations (10 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc alters actin filaments in Madin-Darby canine kidney cells

Exposure of semiconfluent cultures of Madin-Darby canine kidney cells to 10 microM zinc leads to a change in the organization of the actin filament system. Most of the stress fibers at the basal end of the cell are lost and the actin associated with the lateral membrane and junctional regions appears to retract into the cytoplasm. In addition, at the base of the cell in regions of cell-substratum contact, dense, actin-rich plaques appear. These alterations in actin filaments are associated with a change in cell shape. Microtubules were unaffected by exposure to 10 microM zinc. At zinc concentrations greater than or equal to 50 microM the microtubules depolymerized. Exposure to cadmium alters the actin filaments as well but the effect is different from the change seen with zinc. When the cells are exposed simultaneously to zinc and cadmium the cells appear the same as they would if exposed to zinc alone. Exposure of MDCK cells to either metal, individually or in combination, results in a significant and similar increase in F-actin content as determined spectrofluorometrically. The changes in organization and amount of F-actin are associated with a reduction in the ability of the cells to remain attached to the substrate, a toxic effect of these metals with regard to epithelial function. The results indicate that zinc, an essential metal, and cadmium, a highly toxic metal, interact with the actin cytoskeleton in intact cells.