Effects of phenytoin on sympathetic reflex responses in the cat

Phenytoin (DPH), in a low dose (2 mg/kg) was found to potentiate reflex elevations in blood pressure and responses of the nictitating membrane induced either by the electrical stimulation of the sciatic nerve or by intravenously applied pentetrazol in light anaesthetized cats. Pressor responses to carotid occlusion under chloralose anaesthesia were also potentiated. In contrast a high dose (20 mg/kg) of DPH exerted an inhibitory effect on these responses. These effects of DPH on sympathetic reflex responses proved to be long-lasting. Intravenously applied picrotoxin (0.05 mg/kg) or the same dose of ouabain, injected into the vertebral artery, partially reversed the inhibitory effect of DPH (20 mg/kg). Our data support the suggestion that DPH exerts its inhibitory action by effecting ionic fluxes in the CNS, mostly in inhibitory structures.