Teleosts

Question 1

How do freshwater fish regulate their ions?

Answer 1

• Freshwater fish are hyper-osmotic to their environment.
• Freshwater fish drink little water.
• Water enters the fish and ions exit the body by diffusion across thin membranes (gills/pharynx/skin/diet)
• Fish gain water and lose ions by diffusion
• Fish excrete large volumes of dilute water
• Ions are consumed through diet
• Monovalent ions (Na+, Cl-) are reabsorbed in nephron
• Active uptake of salts via gill epithelial (chloride cells and opercular epithelium)

Question 2

How does the Chloride Cell work in freshwater teleost?

Answer 2

• active uptake of ions from environment
• Na+ from seawater into cell in exchange for H+ in chloride cell (H+/Na+ exchange pump) by active transport.
• Cl- from seawater is exchanged for HCO3- in chloride cell (Cl-/HCO3- exchange pump) by active transport.
• H+ and HCO3- are produces when CO2 from metabolism and H20 from body fluid combine.
• The Na+K+ATPase pump exchanges 3Na+ into extracellular for 2K+ into the chloride cell.
• The Cl- moves out through calcium channels on the basolateral membrane

Question 3

How do chloride cells work in marine teleost?

Answer 3

• There is active transport of Na+ and Cl- out of the chloride cells.
• The NKCC co-transporter brings Na+, K+ and 2Cl- into the chloride cell from the extracellular fluid using the Na+ electrochemical gradient.
• The NKCC ions separate
  The 2Cl- ions exit the chloride cell into seawater via chloride channels (down their concentration gradient) on the apical membrane.
• K+ exits the cell back into the extracellular fluid through K+ channels (diffusion).
• Na+ exits the chloride cell via the Na+K+ATPase pump; 3NA+ into extracellular and 2K+ into chloride cell.
• High concentrations of Na+ in the extracellular cause Na+ to move from ECF to seawater via tight junctions - paracellular route.

Question 4

How do marine teleost regulate their internal ion concentrations?

Answer 4

• Marine teleost are hypo-osmotic to their environment.
• Marine teleosts drink lots of seawater.
• Water moves out and ions move in by diffusion.
• Low volume of urine is produced.
• Urine mainly contains divalent ions (Mg2+, SO2-).
• Water is reabsorbed from the gut.
• There is active transport of monovalent ions out into seawater by chloride cells.

Question 5

Why are chloride cells rich in mitochondria?

Answer 5

There is lots of active transport needed in a chloride cell and mitochondria produce ATP, which is needed.

Question 6

Which bits of chloride cells are highly permeable and to which ions

Answer 6

The apical cleft is highly permeable to Cl- ions, and tight junctions are highly permeable to Na+

Question 7

Give some features of Gills that make them perfect for ion exchange

Answer 7

• Short diffusion distance across epithelial
• Surface area of epithelial is large
• counter-current system for optimal exchange
• chloride cells and gas epithelial cells located on primary filament
• all cardiac output through gills

Question 8

Where is the chloride cell located?

Answer 8

On the base of the secondary lamellae and operculum within the columnar cell (which spans the width of the epithelial membrane connecting the ECF to water)

Question 9

What creates the ‘leaky junction system’?

Answer 9

Accessory cells

Question 10

Which fluid is universal?

Answer 10

ICF is universal in ion composition but organic osmolytes vary.