L18 The phosphoinositide signalling pathway

give an example of a stimulus of the PI pathway

phenylephrine

• alpha-adrenergic receptor

how are Ca2+ spikes measured

with Ca2+ sensitive fluorescent indicator dye fura-2

Ca2+ triggers…

a series of calcium spikes

what happens if you double the concentration of the stimulus/hormone

frequency of the spikes increases

• amplitude of spikes stays the same

some calcium released from IP3 receptor binds to sensing proteins and triggers…

a downstream response

Calcium wave is formed by…

calcium released from IP3 receptor goes to the CICR and then through the ER, activating another IP3 receptor feeding forward and propagating a calcium wave

what does the CICR result in

a regenerative/augmentive Calcium release

• calcium triggers its own release

intracellular Ca2+ have both a…

• temporal spike

• spatial wave

what is the wave the spatial correlate of

the spike

what is the spike is the temporal correlate of

the wave

where does the PI signalling pathway occur

the liver

why bother with CICR and Ca2+ waves and spikes

if Calcium couldnt trigger its own release, Ca2+ movement is dependent solely upon diffusion which is very slow

• nearly all calcium released from ER will bind to buffer proteins (98-99%) (mopped up)

• no Ca2+ rise (as its been buffered) so no wave or spike

• because OFF mechanisms are so powerful protecting calcium overload

why is a regenerative Ca2+ signal needed

to saturate the buffers and generate Ca2+ waves and spikes

when we reach the peak of the calcium spike…

the cell is losing lots of energy

• due to the OFF mechanisms

when the peak of the calcium spike is reached, the cell stops releasing intracellular calcium (through IP3 receptor) which means…

OFF mechanisms remove calcium in the cell

• calcium in cell falls

when the OFF mechanisms turn off/relax what happens

the cell surprises itself with another calcium spike

• overpowering OFF mechanisms momentarily for another spike

what rate do Ca2+ waves propagate at

5-100micrometers/s

continual rise in Ca2+ in the cell triggers

apoptosis

having spikes means…

there is no continual rise in Ca2+

what can Ca2+ waves be propagated by

• IP3 receptors

• Ryanodine receptors (RyRs)

what is cADPR

cyclic adenosine diphosphate ribose

what does cADPR stimulate

RyR to release Ca2+

what do hepatocytes only contain

IP3 receptors

what do cardiac myocytes mostly contain

Ryanodine Receptors

which cells contain both IP3 and RyRs

• neurones

• secretory cells

• eggs

what does the IP3 receptor require

coagonists

• IP3

• Ca2+

in the presence of IP3 (agonist), the IP3 receptor can behave as a…

CICR channel

• regenerative Ca2+ release

in the presence of IP3, the IP3 receptors can release…

calcium by CICR

• regenerative Ca2+ release

what is IP3 also known as

cADPR

IP3 receptors and RyRs are regulated in a…

biphasic manner by cytosolic Ca2+

high concentrations of cytosolic calcium ions, inhibits…

Ca2+ release

what does RyR only require

Ca2+

• does not need cADPR

what is cADPR

cyclic ADP ribose

what does cADPR do with RyR

sensitises calcium release

• curve moves to the left (RyR looks more like IP3 curve)

• not an obligatory requirement

what do both IP3 receptors and RyRs require

ATP binding

• NOT hydrolysis

how does the cell sense low energy

use the ATP binding site of the IP3 receptors and RYRs

describe the biphasic curve

• any further rise in Ca2+ inhibits Ca2+ release

• any further rise in Ca2+ inhibits further CICR (called the limitation)