Lecture 16 - Calcium controlling levels, function, measurement and effect 4 mechanisms for regulating Ca2+ to maintain conc. difference Key: - Calcium signalling is based on Ca2+ conc. gradients - Gradients created by energy dependent Ca2+ transport across cellular membranes Mechanisms: - Cell membrane - Excretion systems (Ca2+ pumps and Na+- Ca2+ exchangers) o Ca2+ pumped out of cell -> keeps intracellular levels low - Calcium channels (NT receptors, VDCC) 10-3 M Ca2+ channel Ca2+-permeable cation channel Ca2+ Ca2+ Na+ endoplasmic reticulum nucleus 10-7 M Ca2+ Ca2+pump + H Na+ Ca2+ A Ca2+ mitochondria o Could be voltage or neurotransmitter calcium channels o Allows calcium to go back into cell - intracellular organelles such as endoplasmic reticulum Sodium- calcium exchangers - Direction governed by membrane potential - Exchanges Na+ for Ca2+ - Important for the removal of Ca2+ - Uses electrochemical Na+ gradient provided by the Na+/K+ ATPase - Electrogenic -> creates charge across the membrane Calcium Pumps Ca Na+-Ca2+ exchanger Ca endoplasmic reticulum Ca++ Na+ K 1 2 10 3 Ca* Cat Na K+ 1 = ATP-dependent Ca ** pump 2 = Na'/Ca" exchanger (3:1) 3 = Na*/K *- ATPase pump (3:2) Physiological stimulation opens intracellular Ca2+ channels - Creates Ca2+ fluxes affecting [Ca2+] in various intracellular compartments. 1. ATP ADP Ca2+ ADP ER/SE Ca2+ 2. ATP Removal of calcium from cell cytosol 1. Plasma membrane Ca2+-ATPases (PMCA) -> pump calcium outside of the cell 2. Sarcoplasmic/endoplasmic reticulum ATPase (SERCA) > pump into vesicles Thapsigargin (inhibitor) raises cytosolic Ca2+ concentration: - Blocking the ability of the cell to pump calcium into the SE/ER - Opening InsP3-gated channels in the ER - Activating plasma membrane Ca2+ channels Thapsigargin linked dyes can locate the pumps
Neurotransmitter receptors are critical for calcium entry Cal+ N Glutamats - NMDA receptor is glutamate (neurotransmitter) receptor > key for regulating Ca2+ levels - Permeable to Na+, K+ and Ca2+ - Mg2+ block at resting potential - Loss of Mg2+ block during depolarisation -> ion flow - Large amount of Ca2+ enters cell o -> downstream signalling o > contributes to molecular processes of learning/memory Channel- pore Mg? binding site Glycine Mgª Basic schematic overview: Neurotransmitters Ionotropic Metabotropic VOC ROC Calcium PtdIns4,5P2 DAG release from stores Cyclic ADP-ribose CICR Ca2+ ® 0 , 1 ER membrane - intracellular InsP CICR RYR RYR In Ca2+ Ca2+, ATP, Mg2+ Ca lumen 2+ InsP3 ADP + Pi + + cytosolic mode + Endoplasmic reticulum - - InsP3 Receptor - Homotetramer -> made of 4 identical proteins ATP Ca2+ o Activated by binding InsP3 and in presence of Ca 2+, ATP and Mg 2+ - Multiple isoforms - Ca2+ dependency, bell shaped response curve o Pumping calcium from the cytosol into the lumen of the ER o Binding of IP3, calcium and ATP will release the calcium o = CICR: calcium induced calcium Lumen of blood vessel release -> product activates channel to release more product Acetylcholine 4 1 Acetylcholine GPCR luminal mode ER 2 Phospholipase C Calcium and smooth