Cell Volume Regulation

Question 1

what’s the purpose of establishing ion gradients in/outside cells?

Answer 1

to maintain cell volume

Question 2

what is Vm?

Answer 2

membrane potential

Question 3

is cell membrane permeable to water?

Answer 3

yes

Question 4

what can challenge cell volume?

Answer 4

osmosis

Question 5

what has been a major limiting factor in evolution?

Answer 5

osmoregulation

Question 6

what does each ion generate?

Answer 6

an osmotic potential

Question 7

for there to be no osmosis extracellular osmolarity must be the same as

Answer 7

intracellular osmolarity

Question 8

what was the first hypothesis of cell volume maintenance?

Answer 8

pump-leak hypothesis

Question 9

what is the pump-leak hypothesis?

Answer 9

the idea that Na+ and K+ are being pumped/leaked in and out to maintain cell volume and osmolarity

Question 10

what other ion was more recently found to have a key role in cell volume regulation?

Answer 10

Cl-

Question 11

what 2 things happen to Cl-?

Answer 11

either accumulated or extruded (forced out)

Question 12

what does chloride move alongside to control cell volume?

Answer 12

K+

Question 13

what’s it called when extracellular osmolarity is greater than intracellular?

Answer 13

hypertonic

Question 14

what’s it called when extracellular osmolarity is lower than intracellular?

Answer 14

hypotonic

Question 15

when the extracellular environment is hypertonic, what happens to the cell?

Answer 15

it shrinks (volume decreases)

Question 16

what happens when the extracellular environment is hypotonic to the cell?

Answer 16

it grows (volume increases)

Question 17

why don’t cell volumes match theoretical levels in hyper or hypo tonic conditions (2)?

Answer 17

1- there’s 15-30% of cell volume that doesn’t respond to osmotic challenge- water won’t enter or leave
2- cell volume regulatory mechanisms are activated in most cells before a full volume change is complete (correcting mechanisms)

Question 18

what 2 ions do cell regulatory mechanisms use?

Answer 18

K+ and Cl-

Question 19

what happens when cell volume drops due to osmosis?

Answer 19

there’s a regulatory volume increase (RVI)

where K+ and Cl- enter cell and water follows by osmosis

Question 20

what happens when cell volume rises due to osmosis?

Answer 20

there’s a regulatory volume decrease (RVD) where K+ and Cl- leave the cell and water follows by osmosis

Question 21

what’s the mechanism of RVI?

Answer 21

NKCCl (Na+, 2Cl-, K+ cotransporter) - transports all these into the cell by secondary active transport (using Na+ gradient)
exchangers- Na+ in and H+ out- NHEI
- Cl- in and HCO3- out- AE2
net GAIN of potassium and chloride in cell (increasing cell volume by osmosis)
sodium entering by these mechanisms is removed by Na+/K+ pump- Na+ pumped out
Cl- is being accumulated

Question 22

what’s the mechanism of RVD?

Answer 22

some cells use K+ Cl- cotransporter- transporting out of cell
some cells use K+ and Cl- channels- out of cell
transport driven by K+ and/or Cl- gradient
- most cells use the K+ and Cl- channels
- the cells that use the cotransporter are labeled atypical cells

Question 23

what are atypical cells?

Answer 23

• mature neurons, pancreatic a-cells and skeletal muscle cells
• in these cells there’s not an accumulation of intracellular Cl- therefore can’t simply diffuse through a channel out of the cell
• Cl- / K+ are actively extruded (if Cl- channels were opened Cl- would flood in)

Question 24

is Cl- low in all cells?

Answer 24

NO- most cells have higher Cl-

in most cells opening Cl- channels would cause efflux and depolarisation

Question 25

how can cancerous glioma squeeze between CNS cells during metastasis?

Answer 25

they can reduce their cell volume by losing Cl- and K+ (water follows by osmosis) so they’re small enough to squeeze through small spaces