Fluoride mobilizes intracellular calcium and promotes Ca2+ influx in rat proximal tubules

Abstract

In the renal proximal tubule, external Ca2+ ([Ca2+]o) is required for parathyroid hormone to elevate cytosolic Ca2+ ([Ca2+]i). However, other hormones increase [Ca2+]i in the absence of [Ca2+]o. These differences may arise from a diversity of signal transduction pathways acting on external and internal Ca2+ pools. However, Ca2+ influx may be necessary to expedite and maintain the rise of [Ca2+]i for a period after the initial surge. In this study, F- was used to probe the roles of intracellular Ca2+ mobilization, Ca2+ influx, and phosphoinositide (PI) hydrolysis on the surge of [Ca2+]i in rat proximal tubules. In the presence of external Ca2+; 1-20 mM F- evoked incremental rises of [Ca2+]i in tubules loaded with aequorin. Whereas 10 mM F- increased [Ca2+]i in the absence of [Ca2+]o, the time constant for the [Ca2+]i surge was increased. These findings are consistent with a role of Ca2+ influx on the effect of F- on [Ca2+]i. Indeed, 10 mM F- also enhanced the uptake of 45Ca2+, and promoted Ca2+ influx in aequorin- and fura-2-loaded, Ca(2+)-deprived tubules. In tubules, F- also activated PI hydrolysis with a time course that paralleled Ca2+ mobilization. The effect of F- on [Ca2+]i was not altered when the 39-kDa pertussis toxin substrate was inactivated with the toxin. This G protein was most likely Gi, because prostaglandin E2, an activator of Gi in tubules, dissociated the pertussis toxin-sensitive protein. The results support the notion that activation of a signal-transduction complex, the F- substrate, causes Ca2+ influx, mobilizes internal Ca2+, and activates PI hydrolysis in rat proximal tubules.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium fluoride induces apoptosis in mouse embryonic stem cells through ROS-dependent and caspase- and JNK-mediated pathways

“…Intracellular calcium ions can play critical roles in fluoride-induced apoptosis (Dominguez et al, 1991; Kubota et al, 2005). Intracellular calcium homeostasis is also critical for maintaining cellular functions in response to extra- and/or endogenous stimuli.…”

Studies of renal injury. II. Activation of the glucose transporter 1 (GLUT1) gene and glycolysis in LLC-PK1 cells under Ca2+ stress.

“…Cytosolic calcium (Ca 2 ϩ i ) was measured in LLC-PK1 cells by a modification of the fura2 method as reported by our laboratory (30). Cells were loaded with 12.5 M fura2-AM and 0.0025% pluronic acid in MEM for 40 min at 37 Њ C, washed in PBS, and resuspended for fluorescence determinations in assay buffer (in mM): NaCl, 145; KCl, 5; MgCl 2 , 1; CaCl 2, 1.…”

Molecular Mechanisms of Cytotoxicity and Apoptosis Induced by Inorganic Fluoride

“…Lately, the role of Gα i protein in execution of toxic fluoride effects was demonstrated in the clonal RINm5F pancreatic β-cells and rat Langerhans islets, where apoptosis induced by 5 mM NaF was markedly enhanced by pretreatment with pertussis toxin (Ptx) modifying the α-subunit of Gα i/o family [35,36,58]. The rat proximal tubules also respond to fluoride through Gα i protein [59]. In contrast, Ptx did not augment the apoptosis of the lung A549 cells triggered by 5-10 mM NaF [26].…”

“…An increase in Ca 2+ concentration might be one of the common features for cytotoxicity and cell death either as a direct target or as indirect consequence of altered cellular processes [68,69]. Millimolar fluoride concentrations have been reported to increase intracellular Ca 2+ concentration in the cells of many types including erythrocytes [44,70], osteoblasts [71], proximal tubules [59], human neuroblastoma SH-SY5Y cells [72], and renal epithelial cell line NRK-52E [73] via release from intracellular Ca 2+ stores, suppression of Ca 2+ -pump, or activation of Ca 2+ channels. In the pulmonary artery endothelial cells, activation of pertussis toxin-insensitive G proteins by NaF can initiate a series of events leading to increases in phosphoinositide hydrolysis and Ca 2+ mobilization from intracellular stores [66].…”