In relation to their environment, what are the osmotic challenges experienced by marine invertebrates, hagfish, sharks, and bony fishes?
- marine invertebrates and hagfish face no major problems with osmoregulation
- sharks and bony fishes have lower salt concentrations than their environments, causes water to leave their bodies to dilute the ocean: dehydration + NaCl diffusing into their bodies
How do these different animals (marine invertebrates, hagfish, sharks, and bony fishes) make up for the “osmotic gap”?
- sharks: accumulation of urea/TMAO prevents large water fluxes, accumulate amino acids in internal fluids, rectal gland secretes excess NaCl
- marine invertebrates and hagfish: their internal composition is very similar to the composition of seawater, no significant osmotic gap
- bony fishes: drink seawater and excrete NaCl through gills, some fish (EX: flounder) do intestinal carbonate precipitation
What is the main role of urea in sharks? How is it produced?
- prevents large water fluxes
- convert amino acids from high-protein diet to ammonium
- ammonium, alanine, and glutamate are transported through the blood to the liver and urea is created in the urea cycle
What is a potentially serious side effect of accumulating urea? How do sharks prevent this from happening?
- urea denatures proteins and binds to them in the denatured state, impairing their function
- accumulates TMAO to counteract effects of urea
- TMAO interacts with the protein, urea and water, excluding urea and favoring folding
Why is a carnivorous diet essential for sharks?
- the protein is the source of the amino acids used to create urea and ammonium is created in the process
- allows shark to osmoregulate
What is the rectal gland, and what is its physiological function?
- the rectal gland uses ATP to secrete excess NaCl
- receives blood supply and opens to the rectum, then the cloaca => SW
- made up of thousands of tubules that fuse making larger and larger tubules
What proteins are present in rectal gland cells and are important for NaCl excretion? Where in the cell is each protein found?
- NKA, NKCC, and K+ channel are on basolateral membrane
- CFTR (Cl- channel) on apical membrane
Why do rectal gland cells have abundant mitochondria?
-ATP is used to power the NKA and pump Na+ out of the cells, so mitochondria are necessary to provide this energy
Describe the mechanism for NaCl excretion in rectal gland cells, step by step.
- NKA keeps intracellular [Na+] very low
- Excessive K+ accumulation is prevented by the basolateral K+ “leak” channel
- Low [Na+] drives Cl- import into the cell (together with Na+and K+) through NKCC
- Cl- accumulates inside the cell
- “Cl-wants to leave” the cell bc many negative charges
- Cl- leaves the cell through the only rout it has: the apical CFTR, into the lumen of the rectal gland
- Na+ follows through the paracellular pathway (in between cells) bc opposite charges attract
- NaCl is excreted
What are the osmotic and ionic challenges experienced by marine bony fish, and how do they deal with them?
- Higher osmolarity in seawater, so dehydration
- More NaCl in seawater, so NaCl gain and disruption of homeostasis
- drink water and secrete NaCl through gills
What is the cellular mechanisms for intestinal water absorption in marine bony fish? Describe it step by step.
- More solutes inside the intestinal lumen (higher osmolarity than in the blood) so water will move from the blood into the intestinal lumen by osmosis
- HCO3- combines with Ca2+ and precipitates as carbonate crystals, making fewer but larger molecules and decreasing the osmolarity
- water moves from the intestines to the blood
- calcium carbonate is pooped out
What happens with the calcium carbonate that is produced during intestinal water absorption?
-he mucous degrades, the carbonate crystals are released into the environment (pooped out)
How do marine bony fishes they deal with the excess NaCl load?
-the excess NaCl is excreted across the gills by specialized cells using the same mechanism as shark rectal gland cells
