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1 Chapter 24 Calcium Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
2 FIGURE 24-1: A. Many chemical indicators of [Ca 2+ ] are based on EGTA. A. One example, Indo-1, has two Ca-binding sites, and when excited with 355 nm light the resulting fluorescence spectra is the sum of the curves for Ca-Indo-1 (peak, 398 nm) and apo-Indo-1 (peak, 482 nm). The ratio of the fluorescence at these wavelengths is used to estimate free Ca 2+. All spectra regardless of [Ca 2+ ] cross at 447 nm, the isobestic point (arrow). B. Membrane-permeant Indo-1 AM is free to diffuse across membranes and is trapped inside cells and their organelles by endogenous esterases. The fluorescence, shown emanating from the cytoplasm and the endoplasmic reticulum, can be used to estimate the [Ca 2+ ] in each region. AM, acetoxymethyl; Ex, excitation; Em, emission. Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
3 FIGURE 24-2: Ca 2+ can enter a cell through store-operated channels (SOC), ligand-operated channels (LOC) and voltage- operated channels (VOC). Ca 2+ is transported out of the cytoplasm into the endoplasmic reticulum (ER) by the sarcoplasmic– endoplasmic reticulum Ca 2+ -ATPase (SERCA), or to the extracellular space by the plasma membrane Ca 2+ -ATPase (PMCA) and the Na + –Ca 2+ exchanger (NCX). Ca 2+ is released from the ER via either ryanodine receptor (RyR) activation by Ca 2+, or inositol-trisphosphate (IP 3 ) activation of the inositoltrisphosphate receptor (IP 3 R). The IP 3 is generated by phospholipase C (PLC) upon its activation by G- protein–coupled receptors in the plasma membrane. Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
4 FIGURE 24-3: Mitochondria are often positioned in the cytoplasm at sites of high transient [Ca 2+ ] near the plasma membrane or ER, and elevated [Ca 2+ ] serves as a signal for energy production. In the case of the ER the close association with the mitochondria is maintained by tethering proteins (a), or by binding both the ER and the mitochondria to cytoskeletal elements such as microtubules (MT) (b). Ca 2+ released from the ER via activation of the RyR or IP 3 R in close proximity to the mitochondria create local domains of elevated Ca 2+. The voltage-dependent anion-selective channels (VDAC) in the outer mitochondrial membrane (OMM) supply Ca 2+ to the electrogenic uniporter in the inner mitochondrial membrane (IMM) that accumulates Ca 2+ into the mitochondria. The elevated mitochondrial [Ca 2+ ] m activates the rate limiting steps in the TCA cycle, elevates oxidative phosphorylation and promotes ATP synthesis. ATP is ultimately exported from the mitochondria through the adenine nucleotide translocase (ANT) in exchange for ADP. Ca 2+ exits the mitochondria by Na + –Ca 2+ or H + –Ca 2+ exchangers (N(H)CX), and can be sequestered in the ER by the SERCA. Necrosis occurs when high [Ca 2+ ] m is combined with oxidative or nitrosative stress that promotes the opening of the mitochondrial permeability transition pore M(PTP) with a subsequent loss of ATP. Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
5 FIGURE 24-4: Astrocytic filopodia envelop neuronal pre- and postsynaptic processes to form a tripartite synapse. The release of glutamate from the presynaptic neuron activates AMPA and NMDA receptors in the postsynaptic neuron, and diffuses to the G-coupled receptors in the astrocyte leading to an activation of phospholipase C and the creation of IP 3. Binding of IP 3 to its receptor in the ER leads to Ca 2+ elevation in the filopodia and the release of D-serine into the synaptic space where it binds to NMDA receptors. Current evidence suggests that Ca 2+ elevation in the astrocyte leads to glutamate release and activation of extrasynaptic NMDA receptors, as well as the release of ATP that is converted by ectonucleotidases to adenosine to gate presynaptic A1 receptors. Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
6 FIGURE 24-5: Elevating Ca 2+ in astrocyte end-feet leads to either arteriole dilation or constriction by way of Ca 2+ -sensitive phospholipase A 2 (PLA2) production of arachidonic acid (AA). A. Under conditions of reduced nitric oxide (NO), AA production leads to dilation of the vessel following production of epoxyeicosatrienoic acid (EET) or prostaglandin E 2 (PGE 2 ), both of which increase the K + channel open probability which hyperpolarizes and relaxes the vascular smooth muscle. B. In elevated NO the production of EET is inhibited in the astrocyte relative to the generation of 20-hydroxyeicosatetraenoic acid (20-HETE) in the smooth muscle, and on balance the K + channel open probability decreases leading to depolarization and contraction of the smooth muscle. Copyright © 2012, American Society for Neurochemistry. Published by Elsevier Inc. All rights reserved.
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