• Calcium(2+) physiology in primary vagal sensory neurons

      Hoesch, Robert Eugene; Kao, Joseph P. Y. (2002)
      Microfluorimetric, electrophysiological, pharmacological, and intracellular photorelease techniques were used to study Ca2+ physiology in rabbit vagal sensory neurons (nodose ganglion neurons, NGNs). NGNs exhibit robust Ca2+-induced Ca2+ release (CICR) that can be triggered by caffeine, the classic CICR agonist. A caffeine-induced increase in cytosolic free Ca2+ concentration ([Ca2+ ]i) was traditionally taken as diagnostic of the existence of CICR. However, when CICR was disabled through depletion of intracellular Ca2+ stores or pharmacological blockade of intracellular Ca 2+ release channels (ryanodine receptors, RyRs), caffeine still elicited a significant rise in [Ca2+]i in ∼50% of NGNs. We demonstrated that this rise in [Ca2+]i results from Ca2+ influx, and that Ca2+ influx is one component of a non-selective cation current that is activated by caffeine. Therefore, in approximately half of all NGNs, caffeine elicits both CICR and Ca2+ influx. We determined that d-myo-inositol 1,4,5-trisphosphate receptors (IP3Rs), another type of intracellular Ca2+ release channel, coexist with RyRs in NGNs. ATP, an extracellular physiological signaling molecule, consistently evoked robust transient increases in [Ca 2+]i (Ca2+ transients) that have a dual origin: (1) Ca2+ influx via P2X receptors and voltage-gated Ca2+ channels; and (2) intracellular Ca2+ release, via IP3Rs and RyRs, that requires activation of P2Y receptors and phospholipase C. We determined that Ca2+ transients evoked by IP3 photorelease can gate a Ca2+-activated K+ current (I IP3) in NGNs. IIP3 is unaffected by three common antagonists of Ca2+-activated K+ currents: iberiotoxin, apamin, and 8-Br-CAMP. Using caffeine to selectively deplete intracellular Ca2+ stores, we show that while CICR does not contribute significantly to global IP3-evoked Ca2+ transients, surprisingly, ∼20% of IIP3 is activated by CICR. We propose a model of Ca2+ signaling microdomains that rationalize these observations. We showed that ATP can evoke a Ca2+-activated K + current in 57% of NGNs. ATP-evoked Ca2+ influx activates ∼75% of this current, while ATP-evoked intracellular Ca2+ release activates the other ∼25%. We also determined that NGNs express P2X receptors that mediate robust influx, therefore our data suggest that ATP can exert both excitatory and inhibitory effects in NGNs.
    • Mercury metabolism in rabbits for human risk assessment from dental amalgams

      Van de Wal, Katherine Mary; Fowler, Bruce A. (1998)
      The controversy surrounding the use of dental amalgam restorations has sparked a stronger interest in the toxicity of low-level mercury exposures. This was a two phase dose response study conducted to determine the influence of form on the uptake of mercury into hard and soft tissues in rabbits. This study was designed to examine the potential risk of exposure to mercury from various types of dental amalgam restorations, and to serve as a basis for future studies in biomarkers of mercury exposure. The first phase of this study was an intravenous (I.V.) dosing study. Three forms of mercury, (organic, inorganic, and mixed organic and inorganic) were injected intravenously into the marginal ear veins of rabbits. A range of doses from 0 mug to 25 mug mercury was used to simulate exposure from dental amalgams. The second phase of the study was placement of three types of amalgam restorations used in general dentistry in the teeth of rabbits. The restorations were allowed to mechanically wear and then were replaced in a manner that allowed for no wear. Blood and urine samples were taken through out the time of exposure. Tissues including brain, liver, kidney, and bone were harvested at the time of sacrifice. These specimens were digested and analyzed for the mercury content using cold vapor atomic fluorescence. The results of the I.V. study showed clear dose response relationships in mercury accumulation in all tissues examined. There were statistically significant differences in the absorption patterns between the different forms of mercury. At low doses, the organic and inorganic mercury mix appeared to have a synergistic affect on the accumulation of mercury in the liver. At the higher dose levels, the synergistic affect of the combined dose was present in the bone. Results from the amalgam exposure studies showed no significant differences in the uptake of mercury in the kidney and liver from un-bonded versus bonded amalgam restorations. There was, however, a significant increase in the amount of mercury found in the brain and cortical bone of the rabbits with bonded amalgams. Blood and urine samples taken during this time also reflected differences in mercury release rates when amalgams were bonded. These results lead to the recommendation that the use of bonding agents for mercury amalgams should be studied in more detail. The potential for health hazards from mercury from all dental amalgams is of concern.