Characterization of Na+ transport to gain insight into the mechanism of acid-base and ion regulation in white sturgeon (Acipenser transmontanus)

Abstract

Freshwater fish actively take up ions via specific transporters to counter diffusive losses to their hypotonic environment. While much is known about the specific mechanisms employed by teleosts, almost nothing is known about the basal fishes, such as white sturgeon (Acipenser transmontanus) which may offer insight into the evolution of osmo- and ionoregulation in fishes. We investigated Na⁺ uptake in juvenile white sturgeon in the presence and absence of transporter inhibitors. We found that sturgeon acclimated to 100μmoll^-1 Na⁺ have Na⁺ uptake kinetics typical of teleosts and that a Na⁺/H⁺ exchanger (NHE) is the predominant transporter for Na⁺ uptake. White sturgeon are tolerant to hypercarbia-induced respiratory acidoses and recover blood pH (pH_e) at 1.5kPa PCO₂ but not at higher PCO₂ (6kPa PCO₂) where they preferentially regulate intracellular pH (pH_i). It was hypothesized that during exposure to hypercarbia Na⁺ uptake would increase at CO₂ tensions at which fish were capable of pH_e regulation but decrease at higher tensions when they were preferentially regulating pH_i. We found that Na⁺ uptake did not increase at 1.5kPa PCO₂, but at 6kPa PCO₂ Na⁺ uptake was reduced by 95% while low water pH equivalent to 6kPa PCO₂ reduced Na⁺ uptake by 71%. Lastly, we measured net acid flux during hypercarbia, which indicates that net acid flux is not associated with Na⁺ uptake. These findings indicate Na⁺ uptake in sturgeon is not different from freshwater teleosts but is sensitive to hypercarbia and is not associated with pH_e compensation during hypercarbia.

Keywords: Acid-base regulation; Acipenser transmontanus; Fish physiology; Hypercarbia; Ionoregulation; Na(+) uptake.