It's a tough life for a river lamprey. Not only do these primitive fish have to swim upstream from the sea to the freshwater streams where they spawn and then die, they also have to cope with changes in water composition along the way. Because they are immersed in water, any change in the environment's saltiness affects lampreys' body fluid volume. In salty surroundings the fish lose water by osmosis, while splashing in a freshwater stream has the opposite effect. Now a study by Anne Brown has shed light on how lampreys achieve this tricky balance (p. 223).
Most fish cope with changes in the environment's saltiness using the renin-angiotensin system, a protein network that regulates body fluid volume in virtually all vertebrates. When blood volume drops, renin triggers production of the active form of angiotensin. Angiotensin counteracts body fluid decreases by constricting blood vessels and encouraging the animal to drink. Until recently, lamprey osmotic regulation was a mystery, as they did not appear to have a renin-angiotensin system. When the system was finally discovered in lampreys, Brown was determined to find out how it responds to changes in body fluid volume and osmotic pressure.
Studying river lampreys in the UK is problematic as they are a protected species, but the Ringkøbing Fjord in Denmark provided an excellent source of experimental creatures. To see how the animals responded to changes in body fluid volume, Brown removed up to 40% of the lampreys' blood. She found that the renin-angiotensin system became activated, and circulating active angiotensin levels rapidly doubled. When she injected lampreys with extra blood-like fluid, angiotensin levels dropped. Intriguingly, she found that salty fluid injections did not affect angiotensin levels. This suggests that the effects of increased body fluid volume are counteracted to some extent by increased saltiness.
Brown also tested the effects of changing the environment's saltiness. Since the fjord's saltiness lies between freshwater and seawater, Brown could see how these migrating lampreys respond to osmotic pressure changes simply by moving the lampreys from saltwater to freshwater and vice versa. When she moved fish from freshwater to seawater, their angiotensin levels increased,but the reverse move caused angiotensin to drop. So the lamprey's renin-angiotensin system responds to both changes in volume and osmotic pressure, similar to the systems in bony fish like trout.
Brown suspects that volume detectors within the fish may regulate the lamprey renin-angiotensin system, and that osmotic receptors (detecting saltiness) modulate the volume detectors' effects. But while most vertebrates have a kidney-based renin supply, renin-releasing cells have not yet been found in the lamprey kidney. The location of the lamprey's renin-producing cells remains a mystery for now.
Since lampreys are primitive animals, Brown's results show that the renin-angiotensin system is an ancient and highly conserved blood volume regulation mechanism. After her adventures with lampreys in Danish fjords,Brown plans to study the renin-angiotensin system in the even more primitive hagfish. `Studying the hagfish would give us deeper insights into the origin of this system,' suggests Brown, `by taking us one step further back in evolutionary history.'