Changes to Intestinal Transport Physiology and Carbonate Production at Various CO2 Levels in a Marine Teleost, the Gulf Toadfish (Opsanus beta).

Most marine teleosts defend blood pH during high CO2 exposure by sustaining elevated levels of HCO3(-) in body fluids. In contrast to the gill, where measures are taken to achieve net base retention, elevated CO2 leads to base loss in the intestine of marine teleosts studied to date. This loss is thought to occur through transport pathways previously demonstrated to be involved with routine osmoregulation in marine teleosts. The main objective of this study was to characterize the intestinal transport physiology of the gulf toadfish (Opsanus beta) when exposed to varied levels of CO2: control, 5,000, 10,000, and 20,000 μatm CO2 (0.04, 0.5, 1, and 2 kPa, respectively). Results of this study suggest that intestinal apical anion exchange is highly responsive to hypercarbia, evidenced by a dose-dependent increase in intestinal luminal HCO3(-) (mmol L(-1)) that was mirrored by a reduction in Cl(-) (mmol L(-1)). Despite activation of HCO3(-) transport pathways typically used during osmoregulation, fractional fluid absorption was only significantly lower at the highest level of CO2. Although increased HCO3(-) excretion could provide more substrate for intestinally produced carbonates, carbonate production was not significantly increased during hypercarbia at the levels tested. This study is among the first to thoroughly characterize how compensation for elevated CO2 affects transport physiology and carbonate production in the marine fish intestine. This deeper understanding may be particularly relevant when considering the impacts of future predicted ocean acidification, where prolonged base loss may alter the energetic cost of acid-base balance or osmoregulation in marine fish.