Fig. 1. Effects of amino acids on secretion rates of Drosophila melanogaster tubules. The continuous-exposure protocol was used (see Materials and methods). Amino acids were tested at 20 mmol l^-1 except for threonine and tyrosine, which were tested at 10 mmol l^-1 and 0.5 mmol l^-1, respectively, due to their lower solubility in Drosophila saline. For each amino acid, a corresponding set of controls was run in amino-acid-free saline. Each bar represents the mean secretion rate (+S.E.M.) as a percentage of the corresponding control mean. N=8-16 tubules for each amino acid and corresponding control group. Asterisks represent significant (P<0.05) increases or decreases in secretion rate relative to the controls. The inset shows a dose—response curve (mean ± S.E.M.) for glutamine.