HARRISON, J.F.: Renal and Respiratory Mechanisms of Acid-base Regulation in Grasshoppers
In 1961, J.E. Phillips first provided data to support the hypothesis that active, regulated secretion of acid by the grasshopper hindgut functions as a pH regulatory mechanism. Subsequent studies by Phillips and co-workers, and our laboratory have strongly supported this hypothesis. First, it is clear that insects in general, and grasshoppers in particular have a strong need for acid-base regulation. Grasshoppers experience strong variation in acid-base loading due to variation in diet, body size (metabolic rate) and feeding state, yet regulate hemolymph acid-base status closely. Ingestion of plant secondary compounds (terpenes) also strongly modifies acid-base excretion, probably due to metabolism and subsequent excretion of ingested compounds. Second, the hindgut appears to have a dominant role in acid-base regulation of grasshoppers. In grasshoppers, the ventilatory system regulates internal PCO2, and feedback mechanisms associated with this process contribute to acid-base homeostasis. However, the bulk of the recovery in extracellular pH in response to nonvolatile acid-base challenges occurs without changes in ventilation or hemolymph PCO2. Diets which are associated with increased fecal acid excretion are associated with more acidic hindgut lumens, but similar pH’s in other sections of the gut lumen. Injection of acid into the hemolympH is associated with compensatory variation in the gut lumen of the Malpighian tubules, ileum and rectum. We used chromium and cobalt EDTA as markers of in vivo fluid flow, and then estimated acid secretion by the various gut segments from fluid flow rates, lumen buffer values, and the pH change associated with acid injection. These estimates suggest that the bulk of acid-base regulation in response to injection of acid into the hemocoel can be attributed to variation in hindgut acid-base transport.
