Lecture 13- Intracellular signalling and calcium homeostasis Flashcards
how much calcium in the body
1kg
where is most calcium found
in bones (99%)
concentration of calcium in the blood serum
8-10 mg/dL (50% free calcium)
whole body calcium homeostasis is regulated via
• Intestinal calcium uptake • Ca2+ reuptake by the kidney • Bone calcium regulation
how is calcium homeostasis controlled hormonally
• Ca2+ sensing receptors in the parathyroid (GPCR) • Parathyroid hormone receptor • Calcitonin receptors • Vitamin D3- nuclear receptors
concentration of calcium in the cytoplasm
low 1 x 10^-7 M
concentration of calcium outside the cell
high (same conc as blood) 1-2 x10^-3M
concentration of calcium outside in the endoplasmic reticulum/ SR
high 2-3 x 10^-4
why is calcium important
• Muscle contraction • NT/ stimulus-secretion coupling • Fertilisation • Cell death (apoptosis) • Regulation of metabolism • Learning and memory
plasma membrane is
impermeable to calcium
specific pumps and transporters
move calcium in and out of the cell
what decrease the free level of calcium
buffer proteins within the cytoplasm
channels which decrease intracellular [calcium]
- PMCA 2. SERCA 3. NXC
PMCA
plasma membrane calcium ATPase uses ATP to pump calcium out of the cell
SERCA
sarcoplasmic reticulum calcium ATPase Uses ATP to pump calcium in the SR from the cytoplasm
NCX
sodium calcium exchanged calcium exchanged for sodium
channels which increase levels of calcium in the cell (Influx)
• Ligand gated ion channel • Voltage gated channels
how is calcium moved out of the SR
- Calcium induced calcium release (CICR)- ryanodine receptors - IP3R receptors (think Gq)
effectors stimulated by GPCR G protein can be
enzymes (2nd messengers) or ionic channels
example of ionic channels
• Voltage gated calcium channels (VOCCs) • G-proteins evaluated inwardly rectifying K+ channels (GIRKs)
example of 2nd messengers
• Adenylyl cyclise (ATP —> cyclicAMP) • Phospholipase C (PIP2–> IP3 + DAG) • Phosphoinositide 3-kinase (P13K) • cGMP phosphodiesterase (cyclic GMP —> 5’-GMP)
outline agonist binding to Gs GPCR
- Agonist binds to GPCR
- Causes GDP for GTP exchange in G-protein
- AlphaS and BY subunits dissociate
- AlphaS-GTP activates AC
- AC converts ATP to cAMP
- cAMP activates PKA
cAMP exerts the majority of its effects via
PKA
PKA structure
- x2 C subunit contain protein kinase subunit
- x2 R subunit is where CAMP binds
how many cAMP bind to each regulatory region
2 (4 in total)
when cAMP binds to PKA
this causes it to release the catalytic subunit (covalent change) , so it can act as a PK and phosphorylate other proteins to activate them (usually phosphorylation serine or threonine residues) - Therefore when CAMP increases so does the activity of PKA
outline agonist binding to Gi GPCR
- Agonist binds to GPCR 2. Causes GDP for GTP exchange in G-protein 3. AlphaI and BY subunits dissociate 4. AlphaI inhibits adenyl cyclase 5. No production of cAMP 6. No activation of second messengers- no signal transduction
which G protein is activated to regulate phospholipase C (PLC)
Gq
outline agonist binding to Gq GPCR
- Agonist binds to GPCR 2. GDP for GTP exchange in G protein 3. Alphaq – GTP activates phosoholipase C (PLC) 4. PLC catalyses the hydrolysis of PIP2 5. PIP2 DAG (still associated with membrane)+ IP3 6. IP3 diffuses through cytoplasm and binds to IP3 receptros and causes them to open 7. Allows movement of calcium out of SR/ER 8. DAG has its own PK and phosphoryklates PKC
IP3 binds to
IP3R
IP3R activation can
increase cytoplasmic [CA2+] by 5-10fold within a few seconds of agonist addition
key feature of signalling pathways
signal amplification
signal amplification
- allows small amount of ligand e.g. hormone to cause a relatively large response within the cells
• E.g. For a few molecules of adrenaline binding to surface of B-adrenoreceptors may cause a massive cellular response
inotropy
strength of contraction
chronotrophy
heart rate
outline how B1- adrenoreceptors can be stimulated to cause positive inotropy
- Adrenaline binds
- Activates alphas (can also directly activate VOCC) activates AC
- AC produces cAMP
- cAMP activates PKA
- PKA phosphorylates VOCC (voltage gated calcium channels)
- Increases intracellular calcium
- Increase contractility of the heart
sympathetically released noradrenaline interacts with alpha1 adrenoreceptors of smooth muscle
vasoconstriction
parasympathetic ally release ACh can interact with bronchiolar smooth muscle M3 receptors causing
bronchoconstriction
smooth muscles all utilise
Gq- Phospholipase C (PLC)
smooth muscle utilisation of Gq
Phospholipase C (PLC) • IP3 causes release of calcium from SR (triggers contraction) • PKC activated (sustained contraction) • Leads to prolonged contraction of the smooth muscle
At the presynaptic neurone, GPCRs
can regulate NT release
BY subunit of G inhibits specific types of
Can VOCCs reducing calcium influx and NT release at the synaptic cleft
U-opiod reeptor
- Morphine binds to U-opioid receptor
- Gi protein
- Activates GI (GDP for GTP exchange)
- Causes release of both alpha and BY subunits
- BY binds to VOCC and inhibit VOCC
- This means that the next time there is a depolarisation, the VOCC is inhibited
- Meaning less NT is released into synaptic cleft
- Reduced transmission of AP from neurone to neurone
• 80-90% of inhibition
