M&R session 5: control of cytosolic Ca2+ Flashcards
Ca2+ concentrations of extracellular fluid and cytoplasm?
ECM: 1-2 mM
Cytoplasm: 100 nm
difference of 100000 fold so large inward gradient
How is the concentration difference between ECM and cytoplasm maintained?
- Relative membrane impermeability (ion channels open or closed)
- Ability to expel calcium using PMCA and NCX (see previous information on these)
- Ca2+ buffers such as calsequestrin
- Intracellular Ca2+ stores: rapidly-releasable and non-rapidly-releasable
Cellular events dependent on Ca2+
Fertilisation Proliferation Secretion Neurotransmission Metabolism Contraction Learning and memory Apoptosis Necrosis Bone mineralisation
Advantages and disadvantages of basal resting Ca2+ gradient
Adv: rapid change in [Ca2+]in with little movement of Ca2+ and little has to be removed to re-establish resting conditions
Disad: energy expensive, inability to deal with Ca2+ means easy overload–>loss of regulation + cell death
What is the function of calmodulin in transmitting a Ca2+ signal to cellular components?
Small polypeptide with 4 Ca2+ binding sites
When [Ca2+] increases, Ca2+ binds to calmodulin and this complex binds to Ca2+-ATPase which removes Ca2+
How do Ca2+ buffers work?
E.g. parvalbumin, careticulin, calsequestrin
Limit Ca2+ diffusion before encountering a binding molecule
Ca2+ diffusion depends on [binding molecule] and their level of saturation: communication between extracellular and intracellular levels to help coordinate direction Ca2+ moves
What are trigger proteins?
Bind to Ca2+ but are not buffers
They regulate activity and/or subcellular location
E.g. calmodulin, troponin, synaptotagmin
How can altered permeability causes Ca2+ influx?
i. VOCCs: different types open and close for a different time, regulated differently so allows flexibility of signalling. Allow Ca2+ influx due to depolarisation; driving force is concentration gradient
ii. Ionotropic receptors: driving force is concentration gradient +/- electrical gradient. Ligand-gated. E.g. NMDA/AMPA receptors for glutamate, some nAChRs
What are the rapidly-releasable calcium stores?
Intracellular stores in the endoplasmic/sarcoplasmic reticulum. Mediate release using GPCRs or CICR
How can GPCRs be used by rapidly-releasable Ca2+ stores?
G protein binds to receptor and is activated, due to stimuli such as hormones, neurotransmitters, ions, taste, light
Ligand binds to GPCR-activates Gq alpha subunit
Subunit binds to PIP2 (membrane phospholipid
This releases IP3, which binds to IP3 receptor (ligand gated ion channel) on SR/ER, triggering Ca2+ release down conc. gradient into cell
Explain calcium-induced calcium release
Some Ca2+ enters through VOCCs/ionotropic receptors
Ca2+ binds to ryanodine receptor on side of SR/ER
Ca2+ is released down conc gradient into cell
RYR is structurally similar to IP3 receptors, but activated by Ca2+ not IP3 (conformational change opens pore)
Describe the role of Ca2+ as an agonist at the RYR in cardiac myocytes
Depolarisation causes entry of Ca2+ through VOCCs
Ca2+ acts on RYR of SR
Explosive release of large amounts of Ca2+ from intracellular stores
Ensures a strong and coordinated contractile event
Describe Ca2+ handling by cardiac myocytes
- Early AP (height of depolarisation): conditions favour reversal of NCX so a small amount of Ca2+ enters
- intracellular Ca2+ increases, repolarisation starts, NCX reverts back to Ca2+ extrusion
- Ca2+ is also moved back into the SR by SERCA
How do the properties of cardiac cell ion channels allow prolongation of depolarisation?
Ca2+ channels have similar voltage-sensitive activation and inactivation to Na+ channels, but much slower
Low K+ conductance at depolarised potentials
Role of Ca2+ in skeletal muscle contraction
Ca2+ binds to troponin
Conformational change in tropomyosin (to which it is bound) moves and reveals binding sites on actin for the myosin head groups
Presence of ATP: myosin undergoes cycles of attachment that cause sliding of actin along the myosin bundles and a shortening of the myocyte
