Assay of substance P on the fowl rectal caecum.

The optimum conditions for the assay of substance P on fowl rectal caecum have been studied. Effective concentrations vary from 0.01 to 0.5 u./ml.; it is remarkably insensitive to other polypeptides, such as bradykinin. The test can be made more specific by using a bath fluid containing antagonists for known interfering substances. A suitable antagonist for acetylcholine is atropine or hyoscine, for 5-hydroxytryptamine, methysergide and for catecholamines, ephedrine. The effects of histamine and adenosine compounds can be abolished by specific tachyphylaxis, in which the bath fluid contains an excess of the active substance itself. The assay is reasonably accurate. Woolf's index of precision (L) was estimated as 15.5 with a range (9) of 9.2 to 29.4. Simplified methods of calculating L, and the fiducial range, are described.