Fig. 6. (A,B) The effects of GABA (Ai) and histamine (Bi) on heartbeat of the isolated heart. (Aii, Bii) Dose–response curves for the effects on the heart rate and contraction force of the agents are plotted (values are means ± S.D., N=5). GABA and histamine were applied between the arrows. Heart rate just before GABA or histamine treatment was 35.3±12.74 or 31.9±13.9, respectively. (C) The effects of picrotoxin on cardiac inhibition induced by stimulation of the CI nerve at 70 Hz in normal saline (control; top trace), 8 min afterapplication of 10^–4 mol l^–1 picrotoxin (middle trace) and 30 min after washing out picrotoxin (wash; bottom trace). The bars show a period of repetitive stimulation of the CI nerve. The effects of picrotoxin in abolishing neurally induced cardiac inhibition and the recovery from the effects were observed in five preparations.