THE ROLE OF ION-MOTIVE ATPASES IN THE INSECT GUT

Abstract

The present set of studies examines the roles of two ion-motive enzymes, vacuolar-type H+-ATPase (VA) and Na+/K+ ATPase (NKA), in energizing transepithelial ion transport across the larval caecum and midgut epithelia of Drosophila melanogaster and Aedes aegypti. Even though both VA and NKA are expressed in insect epithelia, VA was considered the more important enzyme until the early 2000 because the ion transport was unaffected by the NKA inhibitor ouabain in many insect epithelia, a phenomenon termed the ‘ouabain paradox’. This paradox was resolved by the discovery of an organic anion transporter (OATP) that is colocalized with NKA and prevents the actions of ouabain on NKA. Since the resolution of the ouabain paradox, this is the first set of studies that investigates the role of NKA in energizing ion transport across the caeca and midgut of insects. First, I show that both VA and NKA are expressed in the caecum and the midgut. Moreover, the ATPase enzyme activities of VA and NKA are quantitatively similar within each region of the gut that was studied, suggesting that both ATPases may be important for establishing favourable electrochemical gradients for transport of ions across the gut. I used ATPase inhibitors to demonstrate that cation transport is dependent on the actions of both VA and NKA. Furthermore, this is the first set of studies that provides an insight into the ion transport mechanisms of the gastric caecum, an organ that is understudied in insects. In Aedes aegypti, I show that 5-hydroxytryptamine regulates the VA-rich cells of the gastric caecum, and therefore the rates of ion transport of these cells. Additionally, I also show that rearing salinity conditions for Aedes aegypti larvae alters the expression patterns of VA and NKA in the gastric caecum. In freshwater, increased activity of VA and NKA energizes transport of ions into the lumen of the caecum that likely maintains fluid volumes and ionic composition at levels appropriate for digestion and absorption. Overall, these studies provide novel information for caeca and midgut-specific actions of VA and NKA in insects, and present a number of new avenues for future research.