Fig. 6. Graph summarizing the results of pre-incubation experiments for Rhodnius tubules. All amino acids were tested at 20 mmol l^-1 except for threonine (10 mmol l^-1) and tyrosine (0.5 mmol l^-1). Each bar represents the mean secretion rate (+ S.E.M.) as a percentage of the corresponding control mean. The broken line indicates the control rate (100%). N=7-16 tubules for each amino acid and corresponding control group. Asterisks indicate significant (P<0.05) increases or decreases in secretion rate relative to controls. The inset shows the extent to which the inhibition produced by cysteine could be reversed by subsequent addition of 20 mmol l^-1 glutamine.