Extrabranchial mechanisms of systemic pH recovery in hagfish (Eptatretus stoutii)

This study investigates the role of branchial and extrabranchial processes in acid–base regulation in the Pacific Hagfish (Eptatretus stoutii). Hagfish were injected with one of the following solutions: acid saline (250 mM HCl [pH = 0.60], 250 mM NaCl), alkaline saline (250 mM NaHCO3, 250 mM NaCl, [pH ≈ 8.43]) or control saline (500 mM NaCl) in order to achieve an acid/alkaline/saline load of 6000 μmol·kg− 1. Using a custom designed hagfish compartmentalizing flux chamber, we partitioned flux of net acid or base equivalents and ammonia into the anterior (gill + skin) and posterior (skin + intestinal/renal/cloacal) components. We found that Pacific hagfish excrete H+ primarily via branchial mechanisms but base excretion occurs through extrabranchial mechanisms located in the posterior region. In addition, we demonstrate that hagfish are able to excrete ammonia via the skin although this flux was not involved in compensation from an acid–base disturbance.