Fig. 5. Secretion rates of 4th instar Rhodnius Malpighian tubules set up using the pre-incubation protocol. Values are means ± S.E.M. (N=8 for both groups). The experimental tubules (filled circles) were preincubated for 2.5 h in 20 mmol l^-1 glutamine, whereas the control tubules (open circles) were pre-incubated for 2.5 h in amino-acid-free control saline. All tubules were then transferred to amino-acid-free control saline and stimulated with 5-hydroxytryptamine (5-HT) at the time indicated by the arrow.