Cellular Aspects Flashcards
What is Ca and what is it needed for?
Divalent cation Ca2+.
Second messenger, essential enzyme cofactor and neurotransmitter.
Muscle contraction, fertilisation, bone formation, homeostasis and maintaining potential difference across the plasma membrane.
How does Ca act as a second messenger?
The binding of a hormone to a receptor (first messenger), causes the opening of the calcium channels which allow the influx of calcium into the cell.
Calcium then binds to inactive Calmodulin and together they are the second messenger.
The complex can then inactivate/activate an enzyme which causes the conversion of substrate -> product.
Compare concs of calcium inside and outside the cell. Where is Ca stored?
The conc of calcium is higher outside the cell and lower inside AT REST. Ca is stored in the ER/SR and mitochondria.
Conc are maintained by regulated calcium entry and efflux, and regulated exchange between the cytosol and stores.
List 3 ways calcium enters the cell
Voltage gated calcium channels
Ligand gated calcium channels
SOCE (store operated calcium entry)
Voltage gated:
-Open upon membrane depolarisation.
-Specific to calcium.
-5 types: L, T, N, P/Q and R.
-Long-lasting = hard to inactivate, found in cardiac muscle and smooth muscle/vasculature. Mainly for hypertension and cardiac arrhythmias (e.g., verapamil).
-Transient = easy to inactivate. Found in heart and neurones. Mainly for epilepsy (e.g., ethosuximide).
-Neither transient nor long-lasting = Analgesia (pain relief) and antivenom (e.g., ziconotide).
P/Q = Cerebellum e.g., agatoxin.
R = Resistance to Ca2+ channel blockers. Harder to activate. Less well understood.
Ligand gated:
-Poor specificity to Ca
-P2X receptor = ATP dependent. Only true LGR in smooth muscle
-GPCRs = indirectly affects Ca entry.
-NMDAR = binds to glutamate and glycine. high Ca permeability
SOCE:
-Allow entry when Ca2+ stores are depleted.
-Not sensitive to [Ca2+]i
-Depletion of Ca2+ from stores recruits a protein called STIM, which accumulates in the junction between the ER and PM.
-This traps and activates a second protein called ORAI1, which then allows the entry of Ca2+ from outside to inside the stores.
Extrusion vs release
Extrusion:
From the cytosol.
Bidirectional. Can be inwards towards the stores or outwards towards the PM.
Stores: ATPase enzyme = ATP dependent. E.g., thapsigargin.
PM: Na-Ca exchanger= no ATP. 3 Na in for every Ca out.
Release:
From the stores -> out.
Driven by IP3-IP3R, which is linked to GPCR action
Largely driven by RyR which detects an increase in intracellular calcium and so empties its stores. Calcium induced calcium release.
Dihydropyridines vs non-dihydropyridines
Dihydropyridines = vasculature. Potent vasodilators.
Non = cardiac tissue. Antiarrhythmia.
Allows for specificity.
What is an action potential and what types of cells is it associated with?
sudden and transient depolarization of the membrane.
Associated with excitable cells such as neurones, endocrine cells and muscle.
Importance of action potential
5 key ones:
-Inter and intra cellular signalling.
-Allows signal to propagate to all parts of cell membrane and to neighbouring cells.
-Responsible for communication over large distances at speed
-Maintenance of rhythmic activity
-Regulating secretion in gland cells.
charge of a resting cell and reasons
negative: -30 - - 80 mV. 2 reasons for this:
1) Membrane is impermeable to Na+
2) Na+ are actively removed via the action of the Na+-K+ pump.
- Net effect = intracellular conc. of Na+ is lower than K+. The K+ contribute a significant negative equilibrium potential. Other ions such as Cl- also contribute to the negative resting potential.
- Also at rest, K+ channels are often open, meaning the membrane is permeable to K+. -Resting potential settles -60 - -80mV, near to the resting potential of Ca2+ ions.
action potential
A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.
Lidocaine action
Lidocaine is a local anaesthetic that exerts its effects by blocking voltage gated sodium channels.
use dependent drugs
Drugs bind to inactive state and increase the refractory period = Use-dependent drugs binds an ion channel when it’s inactive but only functions when channel is activated (e.g., epileptic drugs) → inhibit high frequency action potentials while not affecting normal frequency action potentials.
what is the refractory period and what does it determine?
Time it takes from channels to go from inactive state to resting state.
Determines the freq of action potentials.
What types of muscle does contraction take place?
Smooth muscle (GI tract, Respiratory tract, Vasculature), Skeletal muscle and Cardiac muscle
smooth muscle contraction
Smooth:
-Spontaneous oscillation of calcium leads to rhythmic contractions.
-The membrane potential propagates through tissue slowly and less coordinated than other muscles.
-Through L-type gated VGCC.
- P2X receptor → allows Ca2+ entry dependent on ATP-binding.
- Agonist binds GPCR → activating IP3 → binds to IP3R on SR → SR release of Ca2+.
- Smooth muscle cells also express RyR → CICR can further elevate intracellular Ca2+ levels → cell hyperpolarisation → reducing Ca2+ entry into the voltage-gated channels.
After Ca accumulates:
Ca2+ binds to Calmodulin → Ca2+
Calmodulin complex activates MLCK → phosphorylates myosin light chain → increasing its ability to interact with the actin cytoskeleton → contraction. This is undone by action of MYPT. MLCK and MYPT are regulated by secondary messengers: cAMP & cGMP.
