An exciting new discovery has been made in the Atlantic stingray that has implications for ion regulation in elasmobranchs. In mammals, an active transport protein (HKα1) is present in the stomach and kidney that mediates proton (H+ or acid) secretion in exchange for potassium ions (K+) and functions in digestion, acid–base balance and ion regulation. Like mammals, elasmobranchs (sharks, skates and rays) are also believed to secrete acid into their stomachs, but unlike mammals, they primarily use their gills for acid excretion and ion regulation instead of their kidneys. Keith Choe, working with David Evans and his team, set out to determine whether an HKα1-like protein exists in the Atlantic stingray and to determine if it is expressed in the gill.
The group searched for HKα1 in the Atlantic stingray gill and stomach by using an antibody made against pig HKα1. With this antibody, the group identified HKα1 in gill cells rich in Na+/K+ATPase. These cells are important in acid secretion and ion absorption in fish. HKα1 immunoreactivity was not found in cells that stained for V-type H+-ATPase; cells that are thought to be important in base secretion in fish. Knowing that HKα1 is found in mammalian gastric glands, the team also found HKα1 immunoreactivity in the stomach of the stingray. Having found that the stingray produces HKα1, the next step was to determine whether its expression is regulated during freshwater exposure or ion regulation.
Characterising stingray HKα1 on a molecular level, Evan's team found that it is highly similar to rat HKα1, and that it is expressed in the stingray's stomach and gill but, surprisingly, not in the kidney. Using quantitative, real-time PCR, the team determined that the expression of gill HKα1 does not change with elevated environmental CO2 but is higher in fish acclimated to freshwater than those acclimated to seawater suggesting that it plays a role in freshwater acclimation.
That the expression of HKα1 in the gill is unaffected by the respiratory acidosis caused by high environmental CO2 suggests that this gene is not involved in acid–base balance. However, this finding does not exclude the involvement of pre-existing gill HKα1 proteins in response to acid–base disturbance. Importantly, HKα1 is upregulated in freshwater-acclimated stingrays, suggesting that it may act as a mechanism to increase active K+ uptake across the gill when environmental K+ levels are low, possibly making HKα1 vital for the physiological fitness of this freshwater organism.
This study is the first to provide direct evidence for the presence of HKα1 in any fish. From an evolutionary perspective, stomach acid secretion first appeared in elasmobranchs, which suggests that the Atlantic stingray may be one of the oldest living organisms to have HKα1.