Homeostasis part 2

Question 1

What is the site on the gills where active osmoregulation occurs in freshwater and marine teleosts?

Answer 1

gill filaments

Question 2

What are the three types of cells in marine teleosts. Which cells are present for freshwater teleosts?

Answer 2

Marine - CC, AC, PC

Freshater - CC, PC (NO AC!)

Question 3

Describe what a PC is in a marine teleost.

Answer 3

Support structure, pavement cell

Question 4

Describe what a CC is in a marine teleost.

Answer 4

Role in ion movement, most of the ions.
Contains basal elements:
Na/K ATPase
Na/K/Cl co-transporter
K channel
Calcium active transporter

Apical elements:
chloride channel
calcium channel
(potassium channel)

Question 5

Describe what an AC is in a marine teleost.

Answer 5

Provides a transcellular gap from which sodium can move from the blood to the outside.

Question 6

Describe the mechanism for ion movement in marine teleosts.

Answer 6

Co-transporter is technically the first step assuming a sodium/potassium gradient has been generated by the Na/K ATPase.

1) Chloride (2), sodium (1) and potassium (1) travel from the blood through a co-transporter into the CC
- chloride goes against its concentration gradient
- sodium and potassium go with their concentration gradient (K less so)
2) Sodium (3) pumped out into the blood and potassium (2) pumped into the cell by the basal Na+/K+ ATPase to concentrate sodium in the blood (for step 1)
3) Potassium passively moves along its concentration gradient through basally located potassium channels
4) Built up chloride ions at the basal surface of CC diffuse towards the apical side and passively diffuse through open channels on the apical surface of the CC
5) Concentrated sodium in the blood near basal side of CC passes through gap between CC and AC and follows the negative charged electric gradient of chloride which then goes out into the seawater
6) Calcium passively flows from the seawater to the CC through open channels on the apical surface, diffuses towards the basal surface of the cell and are actively pumped into the blood by basal active transporters in the CC

Question 7

Describe the difference in function of the freshwater filament cells.

Answer 7

No AC!
CC has mainly the same function except in different directions.
Apical:
- proton channels
- chloride-bicarbonate antiporters
- calcium channels
- sodium channels

Basal:

• Na/K ATPase
• Calcium active transporter
• potassium channel
• chloride channel

PC
Apical
- bicarb/Cl antiporter

Basal
- chloride channel

Question 8

Describe the mechanism for freshwater teleosts osmoregulation.

Answer 8

Sodium comes into the chloride cell from the environment by exchange for a hydrogen proton, simultaneously setting up an electrical gradient, the cell is more negative.

Sodium gets pumped into the blood by the Na/K ATPase

Chloride is exchanged with the environment with bicarbonate (chloride in, bicarb out), also sets up an electrical gradient.
This can occur in the pavement cell and in the CC.
chloride diffuses into the blood through basally located chloride channels.
Calcium goes into the CC by an open channel on the apical side and is actively transported basally into the blood.

Question 9

What is the importance of calcium?

Answer 9

Otoliths, scales, bones

Question 10

Are freshwater and saltwater CCs the same?

Answer 10

No

Question 11

What is the most likely explanation for the difference in PC activity between freshwater and saltwater teleosts?

Answer 11

Up and downregulation of channels/transporters.

Question 12

Describe the movement from saltwater to freshwater and how the cells of the gill filaments are affected.

Answer 12

1) Receptors in the blood detect a decrease in salinity when approaching brackish waters.
2) Space between the CC and AC closes off
3) Prolactin is produced/upregulated
4) Prolactin kills off salwater CC and upregulates freshwater CC

Question 13

Describe the movement from freshwater to saltwater and how the cells of the gill filaments are affected.

Answer 13

1) When moving towards saltwater, ion change is detected and the proton pomp gets downregulated, stopping sodium uptake
2) cortisol production from the brain increases
3) Cortisol, along with IGF growth factor and growth hormone downregulate the freshwater CC and upregulate the saltwater CC

Question 14

Describe the marine teleost GI tract and the role it plays in osmolarity.

Answer 14

Main role is in water uptake (osmosis) and monovalent ion uptake.
Esophagus is mainly filled with seawater and active pumps take up salt (mostly), which then draws in water.
Pumps located in esophageal wall.
Extra salts are then bled out of the CC but, the water is retained.
There is a clear decrease in osmolarity from the esophagus to the rectum.

Question 15

Differentiate the marine teleost kidneys and the freshwater teleost kidneys.
In your answer refer to GRF, UFR, permeability changes and which one can be aglomerular.

Answer 15

Marine fish:
Low GFR
Low UFR
Can be aglomerular to prevent water loss
- however in these cases, need active secretion of ions into the tubules
- extreme case to prevent water loss
Bladder permeable to water
- secrete larger ions into tubules

Freshwater fish:
High GFR
High UFR
Bladder impermeable to water
- do not secrete larger ions into tubules
- most ions are taken up from the tubules, water is not

Question 16

Shark blood has a higher osmolarity than that of its environment. How?

Answer 16

Sodium and chloride are lower but total osmotic concentration is higher.
Store urea in the blood to increase osmotic concentration.
Water actively comes into the shark, doesnt dehydrate.
TMAO, enzyme, surrounds urae and detoxifies it so it doesn harm the cells.

Question 17

How are ions taken from the blood and released into saltwater in marine chondricthyes?

Answer 17

Rectal gland is a small outpocket near the intestine.
Blood runs past it.
It has a series of Na/K ATPases and a series of co-transporters (K/Na/Cl)
Na and Cl are actively pumped from the blood into the rectal gland through the rectal gland epithelium.
Excess sodium and chloride are excreted into the environment from the rectal gland.

Question 18

Describe the chondricthyes kidney and how it concentrates urea.

Answer 18

Long nephron with a large glomeruli.
Very long and convoluted tubules in the nephron.
These flow in one direction whilst the closely surrounding blood flow (capillary) goes in the opposite direction, termed countercurrent.
Urea is exchanged from the tubules to the blood through countercurrent exchange, which is a physiological concentration mechanism.
Osmotic gradient favours this exchange b/c urea will always be of higher concentration in the tubules compared to the blood.
The kidney is also permeable to water for transport of salts and urea.

Question 19

How are bigger ions dealt with in the chodricthyes kidney?

Answer 19

Bigger ions flow are selective secreted from the blood into the tubules for excretion.
Too big for transporters in the rectal gland.

Question 20

Where is urea absorbed along the tubles?

Answer 20

Everywhere. More and more permeable towards the distal end.

Question 21

How are freshwater sharks different from saltwater sharks?

Answer 21

Dont want to keep urea
higher UFR
hgiher GFR
reduced rectal gland activity
shorter nephron