Four compounds with H1 anti-histamine activity and four adrenoceptor stimulants, each given to rats prior to passive peritoneal anaphylaxis (PPA), inhibited extravasation of serum proteins into the peritoneal fluid at doses which had no effect on histamine release. In contrast, aminophylline and some non-steroidal anti-inflammatory agents inhibited extravasation only at doses which inhibited histamine release; they showed a similar type of activity to that of disodium cromoglycate (DSCG) and a nitroindanedione (BRL 10833), although they were much less potent. Predosing with DSCG reduced the potency of subsequent doses of DSCG, BRL 10833 and indomethacin, but not of aminophylline or phenylbutazone, and therefore DSCG, BRL 10833 and indomethacin may share a common pathway to produce activity. In the rat PPA system, no evidence was found for histamine ‘feedback’ inhibition of histamine rele
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