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EFFECTS OF KETAMINE ON VASCULAR SMOOTH MUSCLE FUNCTION

1 In vitro studies were undertaken on rat aortic strips and portal vein segments to determine whether or not the amine‐type anaesthetic, ketamine, can exert direct actions on vascular smooth muscle. 2 Ketamine was found to inhibit development of spontaneous mechanical activity and lower basal tension. This action took place with ketamine concentrations found in anaesthetic plasma concentrations, i.e., 1 × 10−5 to 2 × 10−4 m. 3 Ketamine (10−5 to 10−3 m) dose‐dependently attenuated contractions induced by adrenaline, noradrenaline, angiotensin II, vasopressin and KCl. These inhibitory actions were Observed with ketamine added either before or after the induced contractions. 4 Ca2+‐induced contractions of K+‐depolarized aortae and portal veins were also attenuated, dose‐dependently, by ketamine. 5 In contrast to the above inhibitory actions, ketamine (2 × 10−6 to 1 × 10−4 m) was found to potentiate specifically 5‐hydroxytryptamine(5‐HT)‐induced contractions of both aortic and venous smooth muscle. However, this was only observed if ketamine was added after 5‐HT had initiated a contractile response. 6 All of the inhibitory, as well as 5‐HT‐potentiating, effects were completely, and almost immediately, reversed upon washing out the anaesthetic from the organ baths. 7 A variety of pharmacological antagonists failed to mimic or affect the inhibitory effects induced by ketamine. 8 These data suggest that rat plasma concentrations of ketamine commonly associated with induction of surgical anaesthesia can induce, directly, relaxation and contractile potentiation of vascular muscle. 9 These diverse findings may aid in explaining the well‐known biphasic pressor actions of ketamine.