IS HISTAMINE PRODUCTION BY THE BLASTOCYST REQUIRED FOR IMPLANTATION IN THE RABBIT

The histamine producing capacity of rabbit blastocysts and uterine endometrial tissues was determined by measuring their histidine decarboxylase activities (HDC). The production rate of 14CO2 using carboxyl labeled L-histidine as substrate was used as the index of HDC. No enzyme was detectable in the 20.000 x g supernatant of endometrial extracts, but significant activity was found in the extracts from Day 5,6 or 7 blastocysts; the greatest activity was found in Day 6 embryos. Extracts of late Day 6 or Day 7 (Day 6 blastocysts cultured for 24 h) embryos appeared to contain an 'inhibitor' of the HDC when measured against the enzyme activity of fetal tissues. The intraluminal injection of DL-α-methylhistidine dihydrochloride (DL-α-MH), a specific inhibitor of HDC, on Day 5 of pregnancy, interrupted implantation; normal implantation rates were found in contralateral horns instilled with the same amount of L-histidine. The inhibiting action of DL-α-MH could be partially overcome by the co-injection of L-histidine. The results are consistent with the view that rabbit blastocysts have an active histamine forming systen and that inhibition of the enzyme can alter implantation.