LEC20: Channel Regulation by Second Messengers

Question 1

what is faster: ligand-gated channels response to trasmitters, or signaling pathways involving second messengers?

Answer 1

ligand-gated channels are faster

signaling patways involving second messengers take seconds to minutes

Question 2

where is Ca2+ sequestered in cells?

Answer 2

storage organelles, particularly sarcoplasmic reticulum and endoplasmic reticulum

Question 3

what are the different triggering mechanisms to release Ca2+ into the ytoplasm?

Answer 3

1) IP3 receptor (in the ER)
2) Ca2+-responsive receptor, ryanodine receptor (both ER & SR)

Question 4

what is the relatino of IP3 and ryanodine Ca2+ receptors?

Answer 4

they are proteins that form homologous Ca2+ channels

otherwise, the receptors are unrelated

Question 5

how is IP3 produced?

how does it trigger Ca2+ release?

Answer 5

1) GPCRs or RTKs stimulate PLC, phospholipase C, which cleaves PIP2 into DAG (remains in lipid bilayer), and IP3, (diffuses into cytoplasm)
2) IP3 reaches the ER membrane, binds to its receptor
3) IP3 binding induces conformational change, opens the ER channel
4) Ca2+ floods out of ER into cytoplasm

Question 6

what kind of Ca2+ release do ryanodine receptors participate in?

Answer 6

CICR, Ca2+-induced-Ca2+ release

Question 7

what do the ryanodine receptors provide? what do they work with?

Answer 7

amplification of Ca2+; provide the additional Ca2+ needed to supplement Ca2+ signals from voltage-gated Ca2+ channels, nAChRs, and NMDA-type glutamate receptors

Question 8

how is the ryanodine receptor mechanism controlled?

Answer 8

by positive feedback:

ryandodine receptor has high affinity binding site, which activates the channel, and low affinity site, which inhibits the channel

initial Ca2+ comes in, opens ryanodine channels, binds high affinity site, increases chance channel will be open

explosive positive feedback

once there’s sufficient [Ca2+], low affinity site is bound, shuts down the channel b/c of **inhibitory binding of Ca2+ **

thus receptor’s inhibited by high Ca2+ concentrations, so when Ca2+ level is sufficiently high, inhibits ryanodine receptor for releasing more

do not get excessive Ca2+ increase

Question 9

when are Ca2+ stores replenished in ryanodine receptor?

Answer 9

pds of relaxation

Question 10

what depletes PIP2?

Answer 10

PLC, phospholipase C

Question 11

when PIP2 is abundant, is PLC activity high/low, and what is speed of is inactivation of K+ channels?

when PIP2 is depleted, how is PLC activity, and how is inactivation of K+ channels affected?

Answer 11

low **PLC **activity = high PIP2 = retarded inactivation of K+ channels

**high PLC **activity = depleted PIP2 = rapid inactivation of K+ channels (b/c there’s no competition for ball-and-chain)

Question 12

how does PIP2 impact voltage gated K+ channel inactivation?

when does this action get stymied?

Answer 12

PIP2 is on inner leaflet of cell membrane

PIP2 has **negatively charged head group **

PIP2’s head group interacts w/ positively charged residues of proteins near the membrane, such as K+ channel’s “ball and chain,” positively charged region at cytoplasmic N-terminus of each subunit

PIP2 (-) thus attracts N-terminal region of voltage gated K+ channel (+)

this **slows channel inactivation **

HOWEVER, when PLC is active, PIP2 is depleted, channel can inactivate rapidly

Question 13

what is the structure of a photoreceptor cell?

Answer 13

inner segment: leak K⁺ channel (hyperpolarizing), Na⁺/K⁺ ATPase

outer segment: Na⁺ and Ca²⁺ cGMP-gated channel, Na⁺/Ca²⁺ Exchanger

Question 14

which inner segment channel provides predominant membrane conductance? what is the effect?

Answer 14

Leak type K+ channel

allows K+ to leave

is a hyperpolarizing channel

Question 15

when is the cGMP gated chanel open?

what is this called? explain

what is its effect on the Vm of the cell?

Answer 15

**open **when cGMP levels are high in the cell

this means **in the dark **

this is the dark current: inward current in outer segment, outward current in inner segment; get **net movement of positive charges **from outer segment –> inner segment –> out of cell

-40 mV is the Vm

Question 16

what is the dark current?

Answer 16

contributions of diff photoreceptor channels to overall membrane conductance, which keep Vm at -40 mV in darkness

current flow:

1) charges enter through Na⁺/Ca²⁺ channels

these repel K⁺ ions from inner segment

2) Na⁺/K⁺ ATPase in inner segment maintains the K⁺ and Na⁺ gradients
3) Na⁺/Ca²⁺ antiporter removes Ca²⁺ from the cell

Question 17

what does light do biochemically w/in the photoreceptor?

Answer 17

reduces cellular [cGMP]

decreases current flowing through the cGMP-gated channels in outer segment of the rod

dark current is diminished

membrane hyperpolarizes as V_m moves toward K⁺ equilibrium potential

this is a graded response

Question 18

describe pathway of cGMP-mediated decrease in dark current

Question 19

what happens to membrane when dark current diminishes?

Answer 19

membrane hyperpolarizes as inner current’s contribution to membrane potential is reduced, and Vm moves toward K+ equilibrium potential

Question 20

what is photoreceptor’s response to a stimulus ?

Answer 20

graded, not all or none like depolairzing action potentials of nerve/muscle

Question 21

what is the cGMP-gated channel of photoreceptor an example of ?

Answer 21

nucletide-gated ion channels; play role in olfactory (smell) neurons

Question 22

what is the effect of beta/gamma subunits of G protein on the membrane?

Answer 22

membrane-delimited effect; do not communicate w/ cytoplasmic enzymes

Question 23

what mediates slowing of the heart when body is at rest?

Answer 23

G-beta/gamma-mediated channel via GPCRs, muscarinic receptors, that couple to Go and Gi

Question 24

what is the muscorinic receptor?

Answer 24

an Ach receptor, GPCR, that couples to Gi and Go (all beta/gamma, no alpha subunit to Go)

Question 25

what happens when muscorinic receptor is activated?

Answer 25

G-alpha-I stays in the membrane

but G-beta-gamma subunits are liberated, they can directly interact w/ a Kir, GIRK

they increase its probability of being open

the beta/gamma subunits associated w/ it drive the membrane to be more negative upon repolarization

Question 26

what is GIRK?

Answer 26

G-protein-regulated inward rectifying K+ channel; channels open rarely, for short durations w/o a drug; adding Ach directly into the GIRK increases probability the channels open, showing effect of the drug was membrane-delimited (b/c did not have response when Ach was just injected into bathing solution - had to be directly into the GIRK)

Question 27

how does Ach activate GIRK?

Answer 27

very rapidly; through a membrane-delimited mechanism

Question 28

what controls heart rate?

Answer 28

the Ach-activated GIRK

Question 29

what dominates the membrane conductance of pacemaker cells between action potentials?

Answer 29

GIRK channel

Question 30

why shut off inward-rectifying K+ channels during long cardiac action potential?

Answer 30

prevents what would be a large K+ current from repolarizing the membrane prematurely

Question 31

what happens when action potential ends, cell begins to repolarize?

Answer 31

GIRK channels return to high conductance state; Vm thus becomes more negative; takes long time for cell to spontaneously depolarize to threshold for triggering next action potential

Question 32

what is Ach’s effect on intervals btwn pacemaker action potentials?

Answer 32

Ach increases interval, b/c activates GIRKs through membrane-delimited G-beta/gamma pathway