Lead induced rise in intracellular free calcium is mediated through activation of protein kinase C in osteoblastic bone cells.

Lead characteristically perturbs processes linked to the calcium messenger system. This study was undertaken to determine the role of PKC in the Pb2+ induced rise of [Ca2+]i. [Ca2+]i was measured using the divalent cation indicator, 1,2-bis(2-amino-5-fluorophenoxy) ethane N, N,N',N'-tetraacetic acid (5F-BAPTA) and 19F-NMR in the osteoblast cell line, ROS 17/2.8. Treatment of cells with Pb2+ at 1 and 5 microM produced a rise in [Ca2+]i from a basal level of 125 nM to 170 nM and 230 nM, respectively, while treatment with phorbol 12-myristate 13-acetate (PMA) (10 microM), an activator of PKC, produced a rise in [Ca2+]i to 210 nM. Pretreatment with calphostin C, a potent and highly selective inhibitor of PKC activation failed to produce a change in basal [Ca2+]i and prevented any rise in [Ca2+]i in response to Pb2+. To determine whether Pb2+ acts directly on PKC, we measured the Pb2(+)-dependent activation of phosphatidylserine/diolein-dependent incorporation of 32P from ATP into histone and endogenous TCA precipitable proteins in the 100,000 X g supernatant from homogenized ROS 17/2.8 cells. The free concentrations of Pb2+ and Ca2+ were set using 5F-BAPTA; and [Ca2+] and [Pb2+] in the PKC reaction mixtures were confirmed by 19F-NMR. We found that Pb2+ activates PKC in the range of 10(-11)-10(-7) M, with an activation constant of 1.1 X 10(-10) M, whereas Ca2+ activates PKC in the range from 10(-8) to 10(-3) M, with an activation constant of 3.6 X 10(-7) M. These data suggest that Pb2+ activates PKC in ROS 17/2.8 cells and that Pb2+ activation of PKC mediates the documented rise in [Ca2+]i and, perhaps, other toxic effects of Pb2+.

Lead activation of protein kinase C from rat brain. Determination of free calcium, lead, and zinc by 19F NMR.

Lead (Pb2+) has been reported to activate calcium/phospholipid-dependent protein kinase C (PKC) at subnanomolar concentrations (Markovac, J., and Goldstein, G. W. (1988) Nature 334, 732-734); however, others have failed to find any Pb(2+)-induced activation of PKC (Murakami, K., Feng, G., and Chen, S. G. (1993) J. Pharmacol. Exp. Ther. 264, 757-761). In neither of these studies was the actual free Pb2+ or Ca2+ concentration measured. In this study, 1,2-bis(2-amino-5-fluorophenoxy)ethane N,N,N',N'-tetraacetic acid (5F-BAPTA) was used to buffer Pb2+ and Ca2+ concentrations in the PKC reaction mixture. The specific free ion concentrations of Pb2+ and Ca2+, as well as Zn2+ and other divalent cations contained in the PKC reaction mixtures, were determined by 19F NMR spectroscopy. Using this approach to set and confirm the free Pb2+ and Ca2+ concentrations, we measured the Pb(2+)-dependent and the Ca(2+)-dependent activation of phosphotydylserine/diolein-dependent incorporation of 32P from ATP into histone and endogenous acid precipitable proteins in the 100,000 x g supernatant from homogenized rat brain cortex. We found that free Pb2+ activates PKC in the range from 10(-11) to 10(-8) M, Kact = 5.5 x 10(-11) M, while Ca2+ activates PKC in the range from 10(-8) to 10(-5) M, Kact = 2.56 x 10(-7) M. These findings clearly resolve the activation of PKC by subnanomolar concentrations of free Pb2+ from activation induced by Ca2+ or other divalent cations. Furthermore, it documents the utility of 5F-BAPTA as buffer and indicator when resolving the contributions of multiple divalent cations in biochemical processes.

31P-NMR study of transient ischemia in rat hippocampal slices in vitro.

Intracellular high energy phosphates (HEP) were monitored in rat hippocampal slices in vitro by 31P-NMR during continuous superfusion, no flow and reperfusion in order to model the changes which occur during cerebral ischemia and reperfusion in vivo. With continuous superfusion, stable intracellular HEP resonance signals were observed for over 4 h. When superfusion was stopped, there were rapid decreases in pH and phosphocreatine levels followed by slower loss of ATP. These changes are similar to those observed during cerebral ischemia in vivo by 31P-NMR. Upon reperfusion, the pH returned to normal, but the extent of HEP recovery depended on the length of time superfusion was halted. Following a 10 min ischemic period HEP levels returned to greater than 90% of preischemic values, while following a 16 min ischemic period there was only 60% recovery. Superfusion with low calcium, high magnesium medium significantly improved the recovery of HEP following 16 min of ischemia to 80% of preischemic levels. These data support the hypothesis that calcium influx during and following ischemia can disrupt energy metabolism in the hippocampus, and that magnesium can have a protective action on cellular energy status, perhaps by further blocking calcium influx.

Lead inhibits 1,25-dihydroxyvitamin D-3 regulation of calcium metabolism in osteoblastic osteosarcoma cells (ROS 17/2.8).

We have determined the dose-response of 1,25-dihydroxyvitamin D-3 (1,25-(OH)2D3) on the intracellular free calcium-ion concentration ([Ca2+]i) in the osteoblastic osteosarcoma cells, ROS 17/2.8, using 19F-NMR and the intracellular divalent cation indicator, 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA). The dose-response demonstrated an inverted U-shaped relationship with maximal elevation of [Ca2+]i at doses of 1 to 10 nM 1,25-(OH)2D3. At 10 nM, 1,25-(OH)2D3 elevated the [Ca2+]i from a control level of 118 +/- 4 nM to a peak value of 237 +/- 8 nM within 40 min. 1,25-(OH)2D3 also increased the initial rate of Ca2+ influx into ROS 17/2.8 cells, measured by 45Ca uptake, with a dose-response relationship which paralleled its effect on [Ca2+]i. Treatment of ROS 17/2.8 cells with Pb2+ at 1 and 5 microM significantly increased [Ca2+]i but significantly reduced the 1,25-(OH)2D3-induced elevation of [Ca2+]i. Simultaneous treatment of naive cells with 1,25-(OH)2D3 and Pb2+ produce little reduction of 1,25-(OH)2D3-induced 45Ca uptake while 40 min treatment with Pb2+ before addition of 1,25-(OH)2D3 significantly reduced the 1,25-(OH)2D3-induced increase in 45Ca influx. These findings suggest that Pb2+ acts by inhibiting 1,25-(OH)2D3-activation of Ca2+ channels and interferes with 1,25-(OH)2D3 regulation of Ca2+ metabolism in osteoblastic bone cells.

Effect of lead on parathyroid hormone-induced responses in rat osteoblastic osteosarcoma cells (ROS 17/2.8) using 19F-NMR.

Using 19F-NMR and the intracellular divalent cation indicator, 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid, we have recently demonstrated that Pb2+ treatment elevates the intracellular free calcium ion concentration ([Ca2+]i) of rat osteoblastic osteosarcoma cells (ROS 17/2.8) (Proc. Natl. Acad. Sci. USA (1989) 86, 5133-5135). In this study, we have examined the effects of Pb2+ on the basal and parathyroid hormone (PTH)-stimulated levels of [Ca2+]i and cAMP in cultured ROS 17/2.8 cells. PTH treatment (400 ng/ml) stimulated a 150% elevation in [Ca2+]i from a control level of 105 +/- 25 nM to a concentration of 260 +/- 24 nM. Treatment of ROS 17/2.8 cells with Pb2+ (5 microM) alone produced a 50% elevation in the [Ca2+]i to 155 +/- 23 nM. Pb2+ treatment diminished subsequent elevation in [Ca2+]i in response to PTH administration thereby limiting the peak increase in [Ca2+]i to only 25% or 193 +/- 22 nM. In contrast to the dampening effect of Pb2+ on the peak rise in [Ca2+]i produced by PTH, Pb2+ (1 to 25 microM) had no effect on PTH-induced increments in intracellular cAMP levels. Hence, Pb2+ dissociated the PTH stimulation of adenylate cyclase from PTH effects on [Ca2+]i and shifted the regulation of [Ca2+]i beyond the control of PTH modulation. These observations further extend the hypothesis that an early toxic effect of Pb2+ at the cellular level is perturbation of [Ca2+]i homeostasis.

Development of 19F NMR for measurement of [Ca2+]i and [Pb2+]i in cultured osteoblastic bone cells.

Lead (Pb) has been shown to perturb cellular calcium (Ca) homeostasis, altering sizes and flux rates of cellular pools of exchangeable Ca and impairing Ca-mediated cell processes. To date, however, a direct effect of Pb on intracellular-free Ca2+ has not yet been demonstrated. Heavy metals bind to the commonly used fluorescent Ca ion indicators with greater affinity than does Ca and thereby interfere with the expected Ca-dependent fluorescence. In this study, the fluorinated Ca ion indicator, 1,2-bis(2-amino-5-fluorophenoxy)ethane N,N,N',N'-tetraacetic acid (5F-BAPTA), and 19F NMR were used to measure the free intracellular Ca ion concentration ([Ca2+]i) in the rat osteoblastic bone cell line, ROS 17/2.8. Both Pb and Ca bind to 5F-BAPTA with high affinity, but the Pb-5F-BAPTA comple produces a 19F NMR signal at a chemical shift distinct from 5F-BAPTA and the Ca-5F-BAPTA complex. The apparent dissociation constants for Pb-5F-BAPTA and Ca-5F-BAPTA are 2 X 10(-10) M and 5 X 10(-7) M, respectively, at 30 degrees C, pH 7.1, and Mg2+ (0.5 mM). Thus, this methodology allows for the simultaneous identification and quantification of free Pb and free Ca ion concentrations. Determinations of [Ca2+]i were based on 19F NMR measurements of 5F-BAPTA-loaded ROS 17/2.8 osteoblastic bone cells that were attached to collagen-coated microcarrier beads. Cells were continuously superfused with freshly oxygenated medium at 30 degrees C. Under these conditions, the [Ca2+]i of ROS 17/2.8 cells was observed to be 128 +/- 14 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lead on intracellular free calcium ion concentration in a presynaptic neuronal model: 19F-NMR study of NG108-15 cells.

The intracellular free calcium ion concentration ([Ca2+]i) of the neuroblastoma x glioma hybrid cell line, NG108-15, was measured using the 19F-nuclear magnetic resonance divalent cation indicator, 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetra-acetic acid (5F-BAPTA). The basal [Ca2+]i was measured to be 106 +/- 14 nM. Treatment with 5 microM lead (Pb) for 2 h produced a 2-fold increase in [Ca2+]i to 200 +/- 24 nM and a measurable intracellular free Pb2+ concentration ([Pb2+]i) of 30 +/- 10 pM. Intracellular free Zn2+ concentrations ([Zn2+]i) were also observed in the presence of Pb. This represents the first direct demonstration that Pb elevates the [Ca2+]i in neurons, thus providing evidence for a role of [Ca2+]i in mediating the neurotoxicity of Pb.

Lead increases free Ca2+ concentration in cultured osteoblastic bone cells: simultaneous detection of intracellular free Pb2+ by 19F NMR.

Lead (Pb) has been shown to perturb Ca-mediated cellular processes. However, to date, a direct effect of Pb on intracellular free Ca2+ concentration ([Ca2+]i) has not been demonstrated. 19F NMR in combination with 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA) was used to simultaneously measure [Ca2+]i and intracellular free Pb2+ concentration ([Pb2+]i) in the rat osteoblastic bone cell line ROS 17/2.8. The basal concentration of [Ca2+]i in ROS 17/2.8 cells was measured to be 128 +/- 24 nM. Treatment with Pb2+ at 5 and 25 microM produced sustained 50% and 120% increases in [Ca2+]i, respectively, over a time course of 5 hr. At a medium Pb2+ concentration of 25 microM, the entry of Pb2+ into ROS 17/2.8 cells yielded measurable [Pb2+]i in cultured cells. Collectively, these findings advance the hypothesis that Pb toxicity is mediated, in part, through perturbations in [Ca2+]i.

Hippocampal long-term potentiation increases mitochondrial calcium pump activity in rat.

45Ca2+ pump activity was measured in hippocampal homogenates after the induction of long-term potentiation (LTP) of perforant path-dentate gyrus synapses in vivo. The mitochondrial poison, sodium azide, was used to separate mitochondrial from non-mitochondrial 45Ca2+ uptake. High-frequency stimulation of the perforant path input produced a selectively increased mitochondrial uptake of calcium. This increased pump activity may permit granule cells to more effectively buffer higher intracellular calcium levels associated with LTP.

Liver plasma membrane calcium transport. Evidence for a Na+-dependent Ca2+ flux.

Plasma membrane vesicles isolated from rat liver exhibited an azide-insensitive Mg2+-ATP-dependent Ca2+ pump which accumulated Ca2+ at a rate of 5.1 +/- 0.5 nmol of calcium/mg of protein/min and reached a total accumulation of 33.2 +/- 2.6 nmol of calcium/mg of protein in 20 microM Ca2+ at 37 degrees C. Equiosmotic addition of 50 mM Na+ resulted in a loss of accumulated calcium. Measurement of Mg2+-ATP-dependent Ca2+ uptake in the presence of 50 mM Na+ revealed no effect of Na+ on the initial rate of Ca2+ uptake, but a decrease in the total accumulation. The half-maximal effect of Na+ on Ca2+ accumulation was achieved at 14 mM. The Ca2+ efflux rate constant in the absence of Na+ was 0.16 +/- 0.01 min-1, whereas the efflux rate constant in the presence of 50 mM Na+ was 0.25 +/- 0.02 min-1. Liver homogenate sedimentation fractions from 1,500 to 105,000 X g were assayed for azide-insensitive Mg2+-ATP-dependent Ca2+ accumulation. Na+-sensitive Ca2+ uptake activity was found to specifically co-sediment with the plasma membrane-associated enzymes, 5'-nucleotidase and Na+/K+-ATPase, whereas Na+-insensitive Ca2+ uptake was found to co-sediment with the endoplasmic reticulum-associated enzyme, glucose-6-phosphatase. The plasma membrane Ca2+ pump was also distinguished from the endoplasmic reticulum Ca2+ pump by its sensitivity to inhibition by vanadate. Half-maximal inhibition of plasma membrane Ca2+ uptake occurred at 0.8 microM VO4(3-), whereas half-maximal inhibition of microsomal Ca2+ uptake occurred at 40 microM.

Alcohol-dependent liver cell necrosis in vitro: a new model.

In alcoholic liver injury, necrosis is involved in the progression from benign fatty liver to alcoholic hepatitis and cirrhosis. However, there is no practical model of alcohol-dependent liver cell necrosis. The calcium-dependent killing of cultured rat hepatocytes by two different membrane-active hepatotoxins, galactosamine and phalloidin, is potentiated by ethyl alcohol. This indicates that some general physical effect of alcohol on cellular membranes renders cells susceptible to otherwise nonlethal injuries. The in vitro model described in this report may thus be used to search for a general mechanism underlying alcohol-related tissue injury.

Galactosamine-induced cell death in primary cultures of rat hepatocytes.

Primary cultures of rat hepatocytes were exposed to 0.5 mM D-galactosamine. After 36 hours, only 10-20% of the original cells were viable, as assessed by trypan blue exclusion. In the absence of galactosamine, there was no loss of viability over this same period. The addition of 3 mM uridine to the culture medium completely prevented the cell death produced by galactosamine. Glucosamine had no effect on the viability of the hepatocytes. The extent of galactosamine-induced cell death was dependent upon the concentration of Ca++ ions in the culture medium. With the only source of Ca++ that added with the fetal calf serum, galactosamine had only a very slight effect on viability. With higher Ca++ than with the fetal calf serum, galactosamine had only a very slight effect on viability. With higher Ca++ concentrations, from 0.9 to 3.6 mM, the viability ranged from 75% to 31% 18 hours after treatment with galactosamine. The addition of 1.4 microM chlorpromazine to culture medium containing 1.8 mM Ca++ decreased the extent of the galactosamine-induced cell death. This protective effect was progressively reduced by raising the Ca++ concentration to 3.6 and 5.4 mM. Chlorpromazine given to intact rats 2 hours after treatment with 400 mg/kg galactosamine prevented the appearance of liver cell necrosis. At the same time, chlorpromazine prevented the increases in liver cell Ca++ content. These results indicate that many of the features of the effect of galactosamine on intact rat liver cells can be reproduced in primary cultures of these same cells. The data also support the hypothesis that a disturbance in intracellular Ca++ homeostasis leading to accumulations of these ions is causally related to the cell death produced by galactosamine.

Calcium dependence of phalloidin-induced liver cell death.

The role of Ca2+ in toxic liver cell death was studied with primary cultures of adult rat hepatocytes. Within 1 hr of exposure to phalloidin, a bicyclic heptapeptide isolated from the mushroom Amanita pahlloides, at 50 micrograms/ml, 60--70% of the cells were dead (trypan blue stainable). There was no loss of viability of the same cells exposed to phalloidin in culture medium devoid of Ca2+. A marked structural alteration of the surface of the phalloidin-treated hepatocytes characterized by innumerable evaginations seen by scanning electron microscopy occurred in the presence or absence of Ca2+. Pretreatment of the cells with cytochalasin B at 10 micrograms/ml prevented the surface alteration and the death of the cells in Ca2+ medium. Exposure of the cells to phalloidin in the absence of Ca2+ followed by exposure to cytochalasin B and then to Ca2+ also prevented the cell death. These results suggest a two-step mechanism by which phalloidin causes liver cell death. Initially phalloidin interacts in a Ca2+-independent process with cell membrane-associated actin. The second step is a Ca2+-dependent process that most likely represents an increased influx of Ca2+ across a compromised cell membrane permeability barrier and down the steep concentration gradient that exists between the outside and inside of the cell. These results strengthen the hypothesis that disturbances in Ca2+ homeostasis induced in vivo by a variety of hepatotoxins are causally related to liver cell death.

Calcium dependence of toxic cell death: a final common pathway.

Primary cultures of adult rat hepatocytes were treated in the presence or absence of extracellular calcium with ten different membrane-active toxins. In all cases more than half the cells were killed in 1 to 6 hours in the presence but not in the absence of extracellular calcium. An effect of calcium on the primary mechanism of membrane injury by any of the agents cannot be implicated. Viability, as determined by trypan blue exclusion correlated well with other indices of viability such as plating efficiency and the hydrolysis of fluorescein diacetate. It is concluded that the cells are killed by processes that involve at least two steps. In each type of injury, disruption of the integrity of the plasma membrane by widely differing mechanisms is followed by a common functional consequence involving extracellular calcium, and most likely representing an influx of calcium across the damaged plasma membrane and down a steep concentration gradient. This later step represents, or at least initiates, a final common pathway for the toxic death of these cells.