Mechanisms of transport of Na+, K+ and CI- in Malpighian tubules of Rhodnius prolixus and Drosophila melanogaster

Most insects regulate their haemolymph composition and volume within narrow ranges even when exposed to extreme or variable environmental conditions. Osmotic and ionic balance are maintained by the actions of the excretory system which consists of the Malpighian tubules and hindgut. In this thesis I examine the mechanisms of ion secretion involved in fluid secretion by Malpighian tubules of species faced with excess sodium or potassium in the diet. Malpighian tubules of the blood-feeder Rhodnius prolixus secrete Na + -rich fluid, whereas tubules of Drosophila melanogaster, which feeds on yeast growing on rotting fruit, secrete K+ -rich fluid. Of particular interest is the means by which tubules ofthese two species control the ratio of Na+ to K+ in the secreted fluid. My results show that K+ and Cl- are actively transported into the cells across the basolateral membrane of Malpighian tubule cells of both species through a bumetanidesensitive cotransporter driven by the electrochemical potential favouring Na+ entry. In Mapighian tubules of Drosophila most of the Na+ that enters through the bumetanidesensitive transporter is recycled back to the haemolymph through a Na+/K+-ATPase, resulting in secretion of a K+ -rich fluid. I hypothesize that the Na + /K+ ratio of the fluid secreted by tubules of Drosophila is modulated by the activity of the Na+ /K+ -ATPase. Serotonin-stimulated Malpighian tubules of Rhodnius do not recycle Na+ and secrete Na+-rich fluid. The secreted fluid Na+/K+ ratio varies with haemolymph composition, so that less K+ is secreted as haemolymph K+ concentration declines. This replacement of K+ by Na+ in the secreted fluid contributes to homeostasis by regulating the concentrations ofK+ and Na+ in the haemolymph.