Acid base transport across the midgut of the tobacco hornworm, Manduca sexta

Larval Lepidoptera generate a large pH gradient across the midgut epithelium. The in vitro rate of luminal alkalinization (J(OH-)) and hemolymph acidification (J(H+)) under nominally CO2-free conditions was measured in the three morphologically distinct regions of the tobacco hornworm midgut. Under open-circuit conditions, the highest J(OH-) and J(H+) was observed in the anterior section and the lowest was in the middle section. In all three sections the J(H+) was equal to J(OH-) indicating transepithelial movement of acid or base equivalents. Furthermore, the rate at which the midgut transported acid or base was the same under open- and short-circuit conditions, indicating that acid/base transport is an active process. Although the inhibitors, acetazolamide and ethoxyzolamide, inhibited the activity of carbonic anhydrase in tissue homogenates, they had no effect on J(OH-), J(H+), or transepithelial potential. Therefore, under the nominally CO2-free conditions of this study, it is unlikely that hydration of CO, and the formation of HCO,is involved in luminal alkalinization. (C) 1999 Elsevier Science Ltd. All rights reserved.