STUDIES ON THE DISTRIBUTION AND METABOLISM OF THIOPENTAL IN CATTLE, SHEEP, GOATS AND SWINE

¹ Department of Physiology and Pharmacology, College of Veterinary Medicine, University of Minnesota, St.Paul, Minnesota

Thiopental caused surgical anesthesia of short duration in cattle, sheep, goats and swine after a single i.v. injection. The anesthesia was followed by a varying duration of depression, which was very long in cattle. The drug disappeared from the plasma of all species at an initially rapid rate followed by a more gradual disappearance. Distribution of the drug seemed to play a greater role than metabolism in lowering the plasma concentration during the first two hours after injection and in terminating thiopental anesthesia in these species of large animals. The longer duration of postanesthetic depression in cattle is probably due to a slower metabolism of the drug in this species. Thiopental accumulated in various body tissues, the concentration being greatest in adipose tissue. It was not directly related to the concentration of the unbound drug in plasma. Cattle tissues, with the possible exception of adipose tissue, had lower concentrations of thiopental than the tissues of the other three species. The extent of protein binding of thiopental was different in the plasma of various species. Thiopental was metabolized in vitro by the liver microsomal fraction of all four species, but bovine liver microsomes were the least active in this respect. Thiopental seems to be metabolized in the body to a large extent, since little was excreted unchanged in the urine even when plasma levels were held constant for long periods.

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