Augmentation of phenyldiguanide-induced bradycardia by Buthus tamulus venom in adult rats.

Intravenous injection of phenyldiguanide (PDG) in anaesthetized rats produced dose-dependent (1-10 micrograms/kg) decrease in heart rate for a period of time (time-response area). The maximum response occurred at 10 micrograms/kg PDG. Administration of B. tamulus (BT) venom as low as 20 micrograms/kg augmented the PDG-induced bradycardia response by 2.5 times the initial PDG (10 micrograms/kg) response. The maximal augmentation was observed after 60 min of venom injection. Increasing the BT venom concentration to 40 micrograms/kg failed to enhance the reflex response (1.7 times the initial response). The threshold concentration of BT venom was 4 micrograms/kg. BT venom (100 micrograms/kg) alone, decreased the heart rate significantly only after 90 min. Results indicate that, even the sublethal concentrations of BT venom sensitize the reflexes elicited by PDG.