Biphasic effects of morphine on cardiovascular system of the cat

Morphine has been reported to both lower blood pressure (BP) and cause excitation in some species. These are seemingly contradictory effects. In this series of experiments, the actions of morphine on BP, heart rate (HR) and pupillary diameter were re-examined in sedated, paralyzed cats under 2 ranges of dosage and 3 types of sedative. Small doses (1--2 mg/kg) lowered BP and HR while larger doses (8--12 mg/kg) increased both BP and HR. The effect of morphine on pupillary diameter, however, was not biphasic in that increasing doses proportionally increased pupillary diameter. In adrenalectomized cats smaller doses of morphine decreased, but larger doses no longer increased, BP and HR as it has in the non-adrenalectomized animals. Morphine also no longer produced mydriasis in the adrenalectomized cats. The results suggest that, in the cat, morphine can have a biphasic effect and that the adrenals may play a role in the excitatory phase.