Lack of synergism between Tenmo-DH₃₇ (DH37) and Tenmo-DH₄₇ (DH47). Diuretic peptides were tested at 0.2 nmol l^–1 and 40 nmol l^–1, approximately 1.5 times their respective EC₅₀ values, after which the Ringer’s solution was changed to one containing both peptides (at these same concentrations). No change in secretion rates was observed. Data are presented as means + 1 s.e.m. (N=8).

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.

Comparison of secretory response to cyclic AMP and Tenmo-DH₃₇ (DH37). No significant differences in the response time or maximum rates of secretion were observed when Malpighian tubules of Tenebrio molitor were stimulated with 1 mmol l^–1 cyclic AMP (broken line) or 100 nmol l^–1 Tenmo-DH₃₇ (solid line). Each point shows the mean ± s.e.m. for 8–12 tubules. The horizontal filled bar indicates the period during which tubules were exposed to either cyclic AMP or Tenmo-DH₃₇.

Dose–response curve for the effect of cyclic GMP on fluid secretion. Data are presented as means ± 1 s.e.m., N=5–11. The EC₅₀ value determined was 490 μmol l^–1 (r²=0.999, 95 % confidence intervals 99.7 μmol l^–1 to 2.4 mmol l^–1). Note that data for the highest cyclic GMP concentration have been 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.

Antagonistic effect of cyclic GMP on Malpighian tubules of Tenebrio molitor already stimulated with cyclic AMP. Each point shows the mean ± s.e.m. for 8–10 tubules. Responses are shown for 0.5 mmol l^–1 cyclic AMP in combination with 0.5 mmol l^–1 cyclic GMP (broken line), 0.5 mmol l^–1 cyclic AMP with 1 mmol l^–1 cyclic GMP (dashed line) and 0.25 mmol l^–1 cyclic AMP with 1 mmol l^–1 cyclic GMP (solid line). The horizontal dotted bar indicates the period during which tubules were exposed to cyclic AMP, and the horizontal filled bar indicates the period of exposure to cyclic GMP.

Antagonism between Tenebrio molitor peptides controlling Malpighian tubule secretion and between their second messengers. Effect of adding Tenmo-ADF (ADF) to tubules already stimulated with Tenmo-DH₃₇ (DH37) (both at concentrations of 100 nmol l^–1) compared with the effect of adding cyclic GMP to tubules already stimulated with cyclic AMP (both at concentrations of 0.5 mmol l^–1). Secretion rates were measured after 45 min of each treatment. Values are means + 1 s.e.m. for eight tubules (peptides) or seven tubules (second messengers). C, control rates.

Dose–response curve for the effect of CAP_2b on fluid secretion by Tenebrio molitor tubules. Data are presented as means ±1 s.e.m. for 8–9 tubules. The EC₅₀ value determined was 85 nmol l^–1 (r²=0.79, 95 % confidence intervals 4.3 nmol l^–1 to 1.7 μmol l^–1).

Antagonism between CAP_2b and Tenmo-DH₃₇ (DH37). Inhibition of fluid secretion by 1 μmol l^–1 CAP_2b and subsequent slow stimulation by Tenmo-DH₃₇ (10 nmol l^–1). Data are presented as means ±1 s.e.m. for five tubules. The horizontal filled bar indicates the period of exposure to CAP_2b, and the horizontal dotted bar indicates the period of exposure to Tenmo-DH₃₇.