1. The metabolism of methapyrilene (MPH) by rat, hamster and mouse liver microsomes in vitro was investigated together with the binding of 14C-MPH to calf thymus DNA after metabolic activation. 2. Both quantitative and qualitative differences in MPH metabolism were observed in these three species. Mouse liver microsomes catalyse the formation of two novel isomers of hydroxypyrdylmethapyrilene (hydroxypyridyl-MPH) as determined by mass spectral analysis. N,N'-Didesmethylmethapyrilene (didesmethyl-MPH) was formed in detectable quantities only when hamster liver microsomes were used. 3. Incubation of liver microsomes from all three species catalysed the binding of 14C-MPH to exogenous DNA, which was quantitatively similar for all three species. The effect of the cytochrome P-450 inhibitor, 2,4-dichloro-6-phenylphenoxyethylamine (DPEA), and methimazole, a flavin-dependent monooxygenase inhibitor, on binding differed significantly for the three species studied.

Methapyrilene ([14C]MPH) was found to bind to calf thymus DNA only after activation by both rat liver microsomes and NADPH. The cytochrome P-450 inhibitors 2,4-dichloro-6-phenylphenoxyethylamine, 2-diethylaminoethyl-2,2-diphenylvalerate and metyrapone inhibited binding, but methimazole, a flavin-dependent monooxygenase inhibitor, had no effect. However, 1,2-epoxy-3,3,3-trichloropropane, an epoxide hydrolase inhibitor, decreased binding by 30%. Pre-treatment of rats with isosafrole, pregnenolone-16 alpha-carbonitrile or phenobarbital had little or no effect on binding while 3-methylcholanthrene pretreatment decreased binding by 37%. Incubations in the presence of either N-acetylcysteine, glutathione, catalase or glutathione-peroxidase decreased binding to DNA while superoxide dismutase had no effect. These data suggest that MPH is metabolically activated to a species which binds to DNA and that this activation may be mediated by cytochrome P-450 isozymes.

Two types of mast cell can be identified histochemically in the dermis of the rat's external ear. One type is recognized by the binding of concanavalin A (con A) to the cytoplasmic granules (con A-positive cells) while in the other type (con A-negative cells), the granules do not bind con A. The granules in both types are stained metachromatically by toluidine blue. Antidromic stimulation of the great auricular nerve for 2 min results in an increased proportion of degranulating mast cells in the auricular dermis and both types of cell are affected to an approximately equal extent. In discussion of this observation, it is argued that both the con A-positive and the con A-negative mast cells are probably involved in the mediation of vasodilatation due to axon reflexes in injured skin. The proportions of degranulating mast cells determined in histological preparations varied with the fixatives (Carnoy and glutaraldehyde-formaldehyde) used, but the increased degranulation due to antidromic nervous stimulation could be detected after either fixation.

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