1. In anaesthetized rats, intravenous administration of cannabis extract (10 mg/kg), Δ1-tetrahydrocannabinol (THC) (0·5 mg/kg) and Δ6-THC (0·5 mg/kg) caused a reduction in systemic blood pressure, pulse rate and respiratory rate.
2. Neither cannabinol (1 mg/kg, i.v.) nor cannabidiol (1 mg/kg, i.v.) had any observed effects on the cardiovascular and respiratory systems of the rat.
3. Pretreatment of rats with atropine (1 mg/kg, i.v.) reduced the hypotension and bradycardia caused by Δ1-THC or the extract.
4. In anaesthetized cats with autoperfused hindquarters, cannabis extract (10 mg/kg, i.v.) and Δ1-THC (0·2 mg/kg, i.v.) caused hypotension, bradycardia, depression of respiratory rate and reduction of hindlimb perfusion pressure.
5. Both cannabis extract and Δ1-THC potentiated reflex vasodilation and direct vasoconstriction in the hindlimb induced by intravenous noradrenaline in the cat; they reduced reflex hindlimb vasoconstriction elicited by histamine, acetylcholine or bilateral carotid occlusion.
6. Tolerance to these cardiovascular and respiratory effects of cannabis extract developed in rats which had been treated i.p. with the extract at (50 mg/kg) per day for 14 days.
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