Fig. 8. (A) Potentiation of tetani produced by prolonged electrical stimulation with 2 ms stimuli, 50 Hz for 3.5 s. The single tetani before and following prolonged stimulation were produced by 2 ms stimuli, 50 Hz and 200 ms train duration. Caffeine-induced contractures (arrows, 1 mmol l^-1 for 30 s) applied before and following a (B) proctolin (1 µmol l^-1 for 30 s) and a (C) F₂ (1 µmol l^-1 for 30 s) induced contracture. (D) Caffeine pulses (arrows, 10 mmol l^-1 for 60 s) applied before and following a glutamic acid pulse (10 mmol l^-1 for 60 s). There were no electrical stimuli delivered during PR-, F₂- and Glu-induced contractures.