Ion Transport and Acid–Base Balance in Freshwater Bivalves

Blood acid–base and ionic balance in freshwater bivalves is affected by the relative activities of epithelial Na⁺ and Cl⁻ transporters. In the unionid Carunculina texasensis, the Na⁺/H⁺ exchanger is the predominant epithelial transporter that affects acid–base state, while Cl⁻/HCO₃⁻ exchange is of lesser importance. In the corbiculid Corbicula fluminea, Cl⁻ and Na⁺ transport are both significant components affecting acid–base state. Serotonin (5-hydroxytryptamine, 5-HT) stimulates Na⁺ and Cl⁻ transport in both species. In C. texasensis, the effect of exogenous serotonin is four times greater on Na⁺/H⁺ exchange than on Cl⁻/HCO₃⁻ transport, resulting in an increase in acid secretion and a rise in blood pH. In a Na⁺-free environment, serotonin had no effect on blood acid–base state in C. texasensis. In C. fluminea, the acid–base consequences of serotonin stimulation of Na⁺/H⁺ exchange were offset by similar increases in Cl⁻/HCO₃⁻ exchange and by alterations in blood in medium containing Na⁺. In Na⁺-free medium, stimulation of the Cl⁻ transporter with 5-HT resulted in a decrease in blood pH. The differences between these two species are related to the reliance of C. fluminea on Cl⁻ as the major anion in the blood, requiring high levels of epithelial Cl⁻ transport. In C. texasensis, the anionic component of the blood consists of both Cl⁻ and HCO₃⁻ and these ions are interchangeable over a wide concentration range. Extracellular acid–base balance in freshwater bivalves is governed, in part, by epithelial ion transporters.