FERG

FERG
Fish Endocrinology Research Group

Endocrine Control of Teleost Osmoregulation
Teleost fish encounter very different osmotic challenges depending on the salinity of their external environment. In freshwater, fish face osmotic gain of water and compensate by excreting copious amounts of dilute urine. However, this strategy involves a loss of vital salts, which are replaced via ingested food and active uptake at the ‘chloride cells’ of the gill epithelia. In seawater, fish face osmotic water loss, coupled with salt gain. Drinking seawater helps to replace lost water, but this also causes the animal to take in large quantities of salt. This excess salt is excreted using specialised 'chloride cells' on the gill epithelium. Seawater adapted fish also produce small amounts of highly concentrated urine in order to conserve water.
Euryhaline teleost fish face different osmotic challenges as they migrate between freshwater and seawater environments. These fish therefore have to switch strategy, depending on their external environment. There are two principle types of euryhaline teleost fish: Anadromous teleost fish such as Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) spawn in freshwater and mature as adults in seawater. Catadromous teleosts fish such as the European eel, Anguilla anguilla, spawn in seawater and return to freshwater to mature. Euryhaline teleost fish must integrate the functions of gill, gut and kidney in order to adapt successfully to the dramatically different environments they face as part of their natural life cycle.
	Gill: In freshwater, ‘chloride cells’ remove divalent ions such as bicarbonate and ammonium, and the Na+K+ATPase pump on the basolateral membrane works to create an ionic gradient favoring the movement of Na+ and Cl- into the cell from the external environment. These chloride cells are primarily found at the base of the secondary lamellae of the gills and are the principle site for ionic exchange in euryhaline teleosts. In seawater-adapted fish, the chloride cells are larger, and function principally to remove excess Na+ and Cl- ions. Consequently, the Na+K+ATPase pump works to create a concentration gradient favoring the movement of these ions into the external environment. Recent research by FERG has demonstrated increased molecular expression of Na+K+ATPase in long-term seawater adapted eels.
Gut: As euryhaline teleost fish adapt to seawater, they must compensate for the water lost across the gills. This is achieved by drinking the external medium. One of the principal roles of the peptide hormone, Angiotensin II, is the control of drinking rate. Angiotensin II is the biologically active component of the renin angiotensin system (RAS). In mammals, renin, produced in the juxtaglomerular apparatus in the kidney, acts on a protein substrate angiotensinogen, produced in the liver, to form the decapeptide angiotensin I (Ang I). Angiotensin converting enzyme (ACE) then cleaves Ang I to form the octapeptide and biologically active component angiotensin II (Ang II). Further roles for the RAS include, vasopressor effects (constriction of blood vessels), anti-diuretic effects (reduction in urine production) and anti-natriuretic effects (reabsorption of sodium).
The presence of a renin angiotensin system in teleosts was detected as early as 1942, and confirmed in the early 1960's. However, it was not until recently that the biological activity of homologous Ang II was established in teleosts. Through pharmacological manipulation of the RAS, FERG has demonstrated some of the key functions of the RAS in teleosts, including cardiovascular and osmoregulatory roles, using the European Eel, Anguilla anguilla, as a model.
Movement between different environmental salinities is an energy demanding process which involves significant physiological stress to the animal. The stress of movement between freshwater and seawater may result in the animal being more susceptible to parasitic infection or disease. We have examined these effects in a range of teleost species using a number of laboratory techniques. These include the determination of plasma levels of a number of stress hormones by radioimmunoassay (RIA), including cortisol, ACTH and MSH. Stress and metabolic effects may also be determined by examination of the plasma concentrations of glucose and lactate, and measurement of liver glycogen levels by standard spectrophotometric and analytical techniques. Complete adaptation to freshwater or seawater involves an integration of physiological and molecular processes. Drinking rate is increased in the seawater environment to compensate for osmotic water loss and there is a concurrent increase in the expression of key molecular transporters, such as Na+K+ATPase, to remove excess sodium and chloride ions.