Fig. 9. Secretion rate and secreted fluid Na⁺ and K⁺ concentrations for 4th instar Rhodnius Malpighian tubules set up in a pre-incubation experiment. Values are means ± S.E.M. (N=8 tubules for each group). Filled circles represent tubules pre-incubated with 20 mmol l^-1 glutamine, and open circles represent control tubules pre-incubated in amino-acid-free saline. Asterisks represent significant (P<0.05) increases or decreases relative to controls. Tubules were stimulated with 5-hydroxytryptamine (5-HT) at t=0 min.