Blood and gill responses to HCl infusions in the Pacific hagfish (Eptatretus stoutii)

Pacific hagfish (Eptatretus stoutii (Lockington, 1878)) were serially infused with HCI (6000 mu mol center dot kg(-1)) every 6 It over a 24 h period to explore branchial acid secretion mechanisms. Blood pH recovered from a value -2 pH units below that of resting pH. This pH regulatory mechanism was activated by 6 It and persisted over the 24 It time course. Western-blot analysis revealed an increased abundance (similar to 2.3x) of NHE2-like immunoreactivity (NHE2 L-IR) in whole gill fractions from HCl-infused fish compared with control NaCl-infused fish. Membrane fractions isolated from the gills of HCl-infused fish also showed an increased abundance (similar to 1.8x). Immunohistochemistry results demonstrated a significant redistribution of NHE2 L-IR to the apical region in HCl-infused fish. V-H+-ATPase abundance did not significantly increase in HCl-infused fish, while levels of Na+/K+-ATPase expression were unchanged. Immunohistochemistry of these transporters did not show any changes in cellular localization. We propose that the Pacific hagfish overcomes a metabolic acidosis via an increased synthesis of an NHE-like protein and an increased density of the transporter at the apical region of the mitochondria-rich cells of the gill.