Fig. 2. Proctolin enhances the amplitude of high (30 mmol l^-1) K⁺-induced contractures of Idotea muscle fibres. (A) Membrane potential (top) and tension (bottom) measured simultaneously before application of proctolin (control), in the presence of 1 µmol l^-1 proctolin, and after washing off proctolin for 30 min. Control shows one of three K⁺-contractures prior to the peptide tests. (B) Normalised membrane potentials and tensions (N=8) of the experiments as shown in A, indicating no significant change in membrane potential (top) and a 47% increase in the amplitudes of K⁺-contractures with the peptide (bottom). (C) K⁺-induced contractures measured before exposure to proctolin (control), after a 5 min application of 1 µmol l^-1 proctolin in the presence of the PKA inhibitor H89 (1 µmol l^-1) for 5 min, and after washing off proctolin and H89 for 30 min. (D) Summary of independent experiments showing the maximal amplitude of contractures normalised to the controls. 1 µmol l^-1 proctolin increases the amplitudes of contractures significantly (N=8). In the presence of 20 µmol l^-1 H89, proctolin increases the amplitudes of contractures significantly compared to controls without H89 (N=4). H89 alone significantly increases the amplitudes of contractures (N=6). ^*P<0.05.