Regulation of chloride permeability by endogenously produced tyramine in the Drosophila Malpighian tubule

The Malpighian (renal)tubule of Drosophila melanogaster is a useful model forstudying epithelial transport. The purpose of this study was toidentify factors responsible for modulating transepithelial chlorideconductance in isolated tubules. I have found that tyrosine and severalof its metabolites cause an increase in chloride conductance. The mostpotent of these agonists is tyramine, which is active at low nanomolarconcentrations; the pharmacology of this response matches that of thepreviously published cloned insect tyramine receptor. In addition, thetubule appears capable of synthesizing tyramine from applied tyrosine,as shown by direct measurement of tyrosine decarboxylase activity.Immunohistochemical staining of tubules with an antibody againsttyramine indicates that the principal cells are the sites of tyramineproduction, whereas previous characterization of the regulation ofchloride conductance suggests that tyramine acts on the stellate cells. This is the first demonstration of a physiological role for an insecttyramine receptor.