The pressor effects of paraoxon in the pithed rat

1 In pithed rats paraoxon 825 microgram/kg induced a short-lasting pressor effect. Lower doses of the drug were ineffective. 2 The pressor effect was prevented by N-methylatropine, dexetimide and alpha-receptor blocking agents but not by mecamylamine. 3 When blood pressure of pithed rats was elevated either by the continuous infusion of vasopressin or by electrical stimulation of the pithing rod, both 275 and 825 microgram/kg paraoxon induced further pressor effects. The effectiveness of various receptor blocking agents was similar to that observed in pithed rats without vasopressin. 4 It is concluded that the pressor effect of paraoxon is mediated by ganglionic muscarinic receptors. Stimulation of these receptors by accumulated acetylcholine results in an increase in postganglionic sympathetic activity and causes pressor effects. 5 The peripheral action of paraoxon is compared with its action in intact anaesthetized animals.