Calcium channels in cell signalling

Question 1

Ca2+ waves

Answer 1

Post-fertilisation wave in Medaka fish egg synchronises development of the egg (cell cycles and other events trigger at the same time)
Prevents polyspermy

Question 2

Ca2+ gradient

Answer 2

Polarised cell growth

Long tubular growth of pollen tubes (plant cells) associated with tip-high Ca2+ gradient (induces exocytosis)

Question 3

Ca2+ oscillations

Answer 3

Transient and localised Ca2+ oscillations in smooth muscle.

Associated with excitation/contraction coupling

Question 4

Ca2+ transport energetics

Answer 4

Cytosolic Ca2+ maintained at 100-200nM (resting)
Always a driving force for Ca2+ influx into cytosol (via ion channels)
Efflux requires ATP hydrolysis (pumps) or driver ion gradients (carriers)

Question 5

Sources of Ca2+

Answer 5

Extracellular
Intracellular from discrete compartments
ER, SR, nucleus, vacuoles, chloroplasts, mitochondria

Question 6

Endomembrane channels

Answer 6

Inositol triphosphate receptor (IP3R)
Ryanodine receptor (RyR)

Question 7

Plasma membrane channels

Answer 7

Voltage-gated Ca2+ permeable channels
TRP channels
CNGCs
HCN

Question 8

Structure of IP3R

Answer 8

N-terminus forms the ligand (IP3) binding site
Large central domain forms a regulatory domain (Ca2+, CAM, ATP binding)
C terminus contains 6TMS domains. Forms channel pore
Forms a tetramer

Question 9

Properties of IP3R

Answer 9

Calcium to potassium selectivity of 4:1
Blocked by heparin
Single channel conductance of 350pS
IP3 dependent gating. IP3 binding affinity modulated by Ca2+, ATP and kinases

Question 10

IP3R isoforms

Answer 10

IP3R-1 expressed in the CNS
IP3R-2 expressed in hepatocytes and lymphocytes
IP3R-3 expressed in cardiomyocytes

Different responses to IP3 and Ca2+
Ca2+ acts as an antagonist for IP3R-1 but doesn’t inhibit IP3R-3 activity

Question 11

VGCC alpha-subunit

Answer 11

4 repeats of S4 family domain (voltage sensor)
P-region (pore forming)
DHP binding site
Beta-subunit binding site
RyR binding site

Question 12

VGCC properties

Answer 12

Activated by membrane depolarisation
Allow extracellular Ca2+ influx
Present in all excitable cells

Converts action potential to effect via elevated cytosolic Ca2+
Leads to excitation, exocytosis or gating of ion channel activity

Question 13

Ion channel gating

Answer 13

Regulation of smooth muscle tone by BKca channels
Ca2+ influx leads to contraction
This activates BKca channels which mediate K+ efflux leading to hyperpolarisation
This reduces VGCC activity and counteracts contraction

Question 14

VGCC isoforms

Answer 14

L-type common in muscle and brain (DHP sensitive)
N-type in neurons, endocrine cells (sensitive to conotoxin)
P-type in purkinje cells (sensitive to FTX)
T-type in heart cells

Question 15

Heparin

Answer 15

Competitive antagonist of IP3 for all subtypes
Inhibits receptor function
Also activates RyR