Branchial NH 4 + ‐dependent Acid‐base Transport Mechanisms and Energy Metabolism of Squid ( Sepioteuthis lessoniana ) Affected by Seawater Acidification

Abstract

CO2 induced pH fluctuation in the extracellular compartments are a unifying physiological phenomenon that all animals are confronted with. Cephalopods have evolved acid-base regulatory abilities to accommodate temporal pH fluctuations via highly pH sensitive hemocyanins. Interestingly, it has been suggested that metabolic energetic limitations may be a critical factor when exposed to acidified seawater. Here we show the effects of short-term (few hours) to medium-term (up to 168 h) seawater acidification on pelagic squids, Sepioteuthis lessoniana. Routine metabolic rates, NH4+ secretion and extracellular acid-base balance were monitored during exposure to control (pH 8.1) and acidified conditions of pH 7.7 and 7.3 along a period of 168h. Metabolic rates were significantly depressed by 40% after 168 hours in pH 7.3 conditions exposure. The extracellular pH is fully restored within 20 h and is maintained along the period of 168 h, and the accompanied acid-base transporter transcripts were up-regulated including V-type H+-ATPase (VHA), Rhesus protein (RhP), Na/HCO3- cotransporter (NBC) and cystolic carbonic anhydrase (CAc). Immunohistochemical analyses demonstrated co-localization of Na+/K+-ATPase (NKA), VHA in basolateral and Na+/H+-exchanger 3 (NHE3), RhP in apical membranes of the branchial epithelium. Protein levels of branchial VHA and RhP were increased in response to acidified conditions. The present work demonstrated the NH4+ based proton secretion via RhP may contribute to a well-developed acid-base regulatory machinery in branchial epithelia of cephalopods.