Ion and CO₂ Regulation in the Freshwater Water Boatman, Cenocorixa blaisdelli (Hung.) (Hemiptera, Corixidae)

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Abstract
Isolated Malpighian tubules of the freshwater Cenocorixa blaisdelli secrete an alkaline urine following stimulation of fluid flow with cAMP. To determine the importance of this avenue of CO₂ loss for pH and ion regulation, corixids raised in fresh water were compared to those acclimated to alkaline (pH 9.8) conditions, with respect to whole organism ion balance. The high-pH insects exhibited an increase in hemolymph chloride concentration and a reduced rate of drinking. Urinary loss of ions and CO₂ could not balance the higher intake of sodium and CO₂ in the alkaline-acclimated insects, although hemolymph concentrations of these components were not significantly different from those concentrations in insects maintained in fresh water. Isolated Malpighian tubules from the two groups of insects did not exhibit differences in their ability to secrete ions or CO₂. Extrarenal avenues may account for the increased chloride concentration and the constant sodium and CO₂ concentrations. It is suggested that chloride cells which are present on corixids may be important for Na⁺/H⁺ and Cl⁻/HCO₃⁻ exchange under acute high-pH stress. However, other mechanisms may be functional in corixids adapted to alkaline conditions.

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