Chronic Acidosis in the Long-Horned Sculpin (Myoxocephalus Octodecimspinosus); Effect of Low External Sodium or Chloride on Acid-Base Transfers Across the Gills

¹Department of Biology, Georgia Southern University, Statesboro, GA 30460 and ²The Mount Desert Island Biological Laboratory, Salsbury Cove, ME 04672

(Bulletin of the Mount Desert Island Biological Laboratory 34: 63, 1995)

We have recently shown that acid-base transfers in the long-horned sculpin are impaired when the fish is exposed to dilutions of the ambient water ( Claiborne, Walton & Compton-McCullough, J. Exp. Biol. 193:79-95, 1994) and have hypothesized that Na⁺/H⁺ and Cl^-/HCO3^- exchanges are working in opposition across the gills (Claiborne, Perry, & Bellows, Bull. MDIBL 32:95-97, 1993; Claiborne & Bellows, Bull. MDIBL 33:99-100, 1994). In a preliminary attempt to characterize the dependency of NH4⁺, HCO3^- and H⁺ transfers on variations in external [Na⁺] and [Cl^-], three groups of 20% seawater pre-adapted sculpin were subjected to a series of 4 intraperitoneal acid infusions (0.75 mmol kg^{- 1}; infused over ~1 minute at the start of the period). At the beginning of each two hour post-infusion period, fish either remained in 20% seawater (~100 mM NaCl; Control series) or were exposed to low external sodium (^Na⁺ series) or chloride (^Cl^- series). Starting with an initial concentration of 2-4 mM, the [Na⁺] or [Cl^-] was then increased in each subsequent 2 hour infusion period.

Figure 1.

Figure 1 shows the net transfers of acid (^H⁺) between the animals and the water in each of the three series. Control animals began to excrete the infused load following the second infusion, while the ^Na⁺ group only began to show a positive ^H⁺ when external Na⁺ had increased to 26 mM after the third infusion. The ^Cl^- group immediately excreted H⁺ after the first infusion when external Cl^- was ~4 mM. The transfers measured in all three groups were mainly due to adjustments in the measured ^HCO3^- , while ^NH4⁺ played only a minor role (not shown). Thus it appears that 20-30 mM external Na⁺ is required for these animals to maintain a positive net H⁺, and these results support our hypothesis that low external [Cl^- ] inhibits Cl^-/HCO3^- transfers (HCO3^- from animal to water). H⁺ efflux linked to Na⁺ uptake then becomes the predominant exchange driving the measured net acid excretion. This study was funded by NSF DCM 86-02905 and a Georgia Southern University research stipend to JBC and a Burroughs Wellcome Fellowship to SB.