The selective kappa-compound U-50,488H, at doses producing strong central pharmacological effects (0.5-32 mg kg-1 i.p.) did not delay the intestinal transit of a charcoal meal and had little or no antagonist action against morphine-induced constipation. It appears that kappa-opioid receptors are probably not involved in the mechanisms responsible for opioid-induced constipation.
In rats intravenous aspirin was only slightly more effective an inhibitor of platelet thromboxane B2 (TxB2) than of aorta 6-keto-prostaglandin (PGF)1. generation (1.9 versus 2.1 mg/kg).In contrast, oral aspirin was about five times more effective on platelet than on aorta cyclooxygenase activity. The "biochemical selectivity" of aspirin as an inhibitor of platelet and vascular cyclooxygenase thus was not apparent after intravenous administration ofthe drug. However, this could be achieved by relatively low doses of oral (or intraduodenal) aspirin, on account of "presystemic" acetylation of platelet cyclooxygenase. Even in this condition, though, aspirin selectivity was relative to "systemic" peripheral vessels but not to the vessels of the enterohepatic circulation. Indeed after an oral or intraduodenal dose of 5 mg/ kg aspirin, genertion of portal vein 6-keto-PGFI. was inhibited to much the same extent as platelet TxB2, while inferior vena cava 6-keto-PGFIa formation was spared.
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