—The quantitative method of Ellman, Courtney, Andres and Featherstone (1961) was adapted to a differential assay for the determination of acetyl‐ and pseudocholinesterase activities of sympathetic ganglia of rats. The activities of the cholinesterases of superior cervical, stellate and thoracic chain ganglia and of the abdominal ganglionic complexes in apposition to the superior mesenteric and coeliac arteries (superior mesenteric, coeliac and cardiac ganglia) were measured. B.W.284C51 dibromide, 5 × 10−5m, and ethopropazine hydrochloride, 3·15 × 10−5m, were employed to inhibit selectively acetyl‐ and pseudocholinesterases, respectively. Linearity was shown to be maintained with enzyme concentrations corresponding to 0·12‐0·5 mg of ganglion (wet wt.)/incubation. Under the experimental conditions of this assay, the rates of the reaction of ganglionic acetyl‐ and pseudocholinesterases were linear for time periods greater than those employed for calculating the rates of hydrolysis in the homogenates of sympathetic ganglia. Several experimental approaches were used to ascertain the specificity of the inhibitors and of the reaction. Of the total cholinesterase activity of sympathetic ganglia of rats, 55‐63 per cent was due to acetylcholinesterase and 31‐39 per cent to pseudocholinesterase. On the basis of the specific enzyme activity, superior cervical, stellate and superior mesenteric ganglia contained higher acetyl‐ and pseudocholinesterase activities than did thoracic chain, coeliac and cardiac (abdominal) ganglia. The specific activity of acetylcholinesterase was similar in rat and cat superior cervical ganglia and sympathetic cervical trunks while the pseudocholinesterase activity of these two tissues was somewhat lower in cats than in rats.

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