Neuronal and Cellular Signalling Lecture 8 Notes: Phospholipids and Calcium Part 3 Calcium - Intracellular, free Ca2+ is precisely controlled. [Ca2+]; is around 20nM whereas [Ca2+]ez is around 1.8nM. All cells use Ca2+ for intracellular signalling thus deregulated Ca2+ signalling causes death. - There are four mechanisms for regulating Ca2+ including: (a) Cell membranes (Ca2+ is charged thus cannot cross the membrane without a channel/pump) (b) Excretion systems (Ca2+ pumps and Na+- Ca2+ exchangers, keeps intracellular levels low) (c) Ca2+ channels (allow Ca2+ influx into the cell, e.g. NT receptors, VDCC) (d) Pumping into intracellular organelles (e.g. ER, keeps Ca2+ in the cytosol low) - Calcium signalling is based on Ca2+ concentration Gradients gradients. created by energy dependent Ca2+ transport across cellular membranes. For example, the Na+-Ca2+ exchanger will pump (1) Ca2+ up its concentration gradient and allows (2) sodium to move down its gradient. This Na+ will be pumped out via a Na *- K* ATPase pump. 10-3 M Ca2+ channel Ca2+-permeable cation channel Ca2+ Ca2+ Ca Na+ + H endoplasmic reticulum Ca2+ nucleus 10-7 M Ca2+ Ca2+ mitochondria Ca2+pump + Na+ Ca2+ Ca2+ Na+-Ca2+ exchanger - These processes are electrogenic (i.e. generate charge across the membrane) endoplasmic reticulum because pumping these ions in/out of the cell creates a membrane charge (i.e. electrochemical gradient). - Plasmalemmal/intracellular Ca2+ pumps and channels (Ca2+ ATPases) are opened by physiological stimulation, thus creating Ca2+ fluxes affecting [Ca2+] in various intracellular components. There are both plasma membrane Ca2+ ATPases (PMCA) and sarcoplasmic/endoplasmic reticulum ATPase (SERCA). These remove Ca2+ from the cytosol, and if pumped into the endoplasmic reticulum, allows cells to release a lot of Ca2+ very quickly because it is stored adjacently. - Thapsigargin raises cytosolic Ca2+ concentration. This blocks that ability of the cell to pump calcium into the SE/ER, opens InsP3-gated channels in the ER, and activates plasma membrane Ca2+ channels, elevating cytosolic Ca2+. - NMDA receptors are glutamate (neurotransmitter) receptors that are key for regulation of Ca2+ levels. They are permeable to Ca2+, Na+ and K *. Mg2+ block these receptors at the resting potential thus the loss of the Mg2 block during depolarisation leads to a large amount of Ca2+ entering the cell Neurotransmitters lonotropic Metabotropic VOC, Cyclic ADP-ribosc CICR_ _ @- - - RYR RYK ROCK * Pudins4,5P, >DAG Ca InsP CICR InsPR InsP.R --- lumen = Ca= Endoplasmic reticulum
allowing for downstream signalling. This process contributes to the molecular process of learning/memory. - Neurotransmitters will bind to ionotropic neurotransmitter receptors which open and allow Ca2+ to enter the cell through specific Ca2+ channels. Metabotropic receptors will bind to neurotransmitter and produce IP3 and DAG. IP3 will bind to calcium-regulated receptors on the ER, stimulating the release of more Ca2+. - InsP3 receptors on the ER membrane are homotetramers around 310kDa that have multiple isoforms. They are Ca2+-dependent (bell shaped response curve) and are activated by binding IP3, Mg2+ and ATP. They pump Ca2+ from the cytosol into the lumen of the ER, and InsP3, Ca2+, ATP and