Fig. 6. 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.