Fig. 3. Dose–response curve for the effect of cyclic AMP on fluid secretion by Tenebrio molitor Malpighian tubules. Data are presented as means ± S.E.M. of 7–12 tubules. The EC₅₀ was 350 µmol l^–1 (r²=0.999 for the curve fit). Note that data for the two highest cyclic AMP concentrations tested are shown in the graph, but were excluded from the non-linear regression analysis because they indicate some receptor desensitisation at high levels, and this cannot be accommodated by the algorithm used.