Fig. 1. The effects of proctolin, F₂ and glutamic acid on force development of lobster cardiac ostia. In (A) and (B) the threshold concentration of each peptide was continuously perfused for 4 min, in the other four traces (C-F) the peptide or glutamic acid (Glu) was present for 30 s. Tetani were produced by trains of stimuli (5 ms pulses, 20 Hz, 200 ms train duration, 2 trains min^-1).