Fig. 4. cGMP levels are stimulated by capa peptides in Diptera. Basal and capa-stimulated cGMP levels in tubules from several species were measured by radioimmunoassay (RIA). Tubules were stimulated with capa-1 (red), AngCAPA-QGL (blue) and AngCAPA-GPT (green) peptides (10^-7 mol l^-1) for 10 min. Data for each sample were calculated as fmol cGMP µg^-1 protein (± S.E.M.; N=4–6) in order to normalise the data across species. Protein estimations were conducted by the Bradford assay. In order to aid comparison with other insects, values for G. morsitans tubules have been under-represented on the graph: levels of cGMP in G. morsitans tubules stimulated by A. gambiae capa peptides were 10±0.2 fmol µg^-1 protein (AngCAPA-QGL) and 9.8±0.2 fmol µg^-1 protein (AngCAPA-GPT) compared with 0.610±0.025 fmol µg^-1 protein for control tubules. No increase in cGMP content was observed upon stimulation of S. gregaria tubules with either capa-1, AngCAPA-QGL or AngCAPA-GPT. ^*Statistically significant data compared with untreated tubules, where P<0.05 (Student's t-test, unpaired samples).