Fig. 3. Effect of proctolin on the cGMP concentration of Idotea muscle fibres. (A) The cGMP concentration is not reduced significantly after 15 min exposure to 1 µmol l^-1 proctolin (N=6). In the presence of the PKC inhibitor BIM-1 (10 nmol l^-1), 15 min application of proctolin has no effect on the cGMP concentration (N=6). The cGMP concentration is not reduced in the presence of BIM-1 (N=8). (B) After a preincubation for 10 min with the unselective phosphodiesterase inhibitor IBMX (0.5 nmol l^-1), application of proctolin in the presence of IBMX for 3 min reduces the cGMP concentration in the fibres significantly compared to the cGMP concentration of control with IBMX (N=4, N=3, respectively). ^*P<0.05.