Acid–Base Regulation in Insect Haemolymph

Abstract

Insects regulate the acid–base balance of their haemolymph by ventilation, buffering and active uptake and excretion of acid–base equivalents with their gut. Haemolymph pH varies widely across ontogeny and between species, varying from acidic (~6.4) to alkaline (~8.0). Terrestrial and aquatic insects are exposed to different acid–base challenges by virtue of their environment and have evolved different mechanisms to cope. Terrestrial insects use bicarbonate buffering and change in ventilation to respond to metabolic acid loads, as well as clearing excess acid through excretion into the lumen of the hindgut. V-ATPase and Na+/K+-ATPase in the hindgut/Malpighian tubule complex generates the transmembrane electrochemical potential necessary to drive uptake and excretion of acid–base equivalents and ions. The activity of these transport processes is under hormonal control. While aquatic insects also use their hindgut for acid–base regulation, they possess additional ion-transporting chloride cells on parts of their cuticle that are in contact with the surrounding water. These cells provide an extra-renal pathway for regulating haemolymph pH and osmolality, relying on ion transport driven directly and indirectly by V- and Na+/K+-ATPases. While aquatic insects are among the most pH-tolerant animals on the planet, the mechanisms that allow them to tolerate chronic exposure to highly acidic or alkaline water across a wide range of ionic strengths are incompletely understood and require further investigation.