Calcium signalling

What is lower, resting cytoplasmic or extracellular [Ca]?

Cytoplasmic. 20,000 to 100,000 times lower

How is resting calcium conc maintained?

active transport to extracellular space, ER, and mitochondria

SERCA

sarcoplasmic/endoplasmic reticulum calcium ATPase, pumps calcium from cytosol → ER

Calsequestrin

storage protein in the ER that binds calcium to achieve high concentrations

Calcium signaling

Occurs when cells are stimulated to release calcium from stores or when calcium enters the cell.

calcium channel structure

tetramer with pore at centre

Na/Ca antiporter

cotransporter that uses the sodium concentration gradient to move Ca2+ against its gradient

How is phospholipase C activated?

activated by GTP-binding protein (G protein)

phospholipase C function

hydrolyses PIP2 → IP3 and DAG

What does IP3 do?

binds ER calcium channel → conformational change, opening channel → calcium released from ER

how is protein kinase C activated?

calcium ions and diacylglycerol (DAG) bind

Calcium ion role in PKC activation

binds C2 domain, changing PKC shape and encouraging binding to membrane by linking PKC to phosphatidylserine so PKC is localised to correct compartment

DAG role in PKC activation

binds C1B domain causing further conformational changes

What happens as a result of increased glucose uptake into beta cells via GLUT2 uniporters?

• glycolysis etc increases ATP:ADP

• ATP sensitive K+ channels close → depolarised membrane

• voltage gated Ca channels open → ca influx

• insulin secretory molecules fuse with membrane

types of Ca2+ channels

voltage gated, 2nd messenger operated or receptor operated

What are calcium channel blockers used for?

high blood pressure, irregular heartbeats and other syndromes

How do Ca channel blockers reduce heart rate?

• act on SAN

• LV fills completely reducing cardiac workload

How do Ca channel blockers reduce contractile force?

• act on cardiac and vascular smooth muscle

• arterial constriction reduced

• BP lowered

How does verapamil work?

plugs molecular pore of calcium channel preventing calcium from entering

Fluorescence reporters

Tools used to study calcium concentration changes in cells.

FRET (Fluorescence Resonance Energy Transfer)

measures proximity between two fluorophores, indicating calcium levels through changes in emission due to spectral overlap (as calcium has allosteric effects)

FRET and tropomyosin

when calcium increases tropomyosin bends → FRET. Decreased CFP and increased cpCitrine emission shown

dihydropyridine effects

exclusively vascular

phenylalkylamine and benzothiazepine effects

both vascular and cardiac

What other conditions can calcium channels be used against?

left ventricular diastolic dysfunction, Raynaud’s, migraine, preterm labour, oesophageal spasm, bipolar

common calcium channel blocker side effects

related to vasodilation properties: headache, flushing, dizziness, nausea