Is Histamine Production by the Blastocyst Required for Implantation in the Rabbit?1

Abstract

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 carboxy labeled L-histidine as substrate was used as the index of HDC. No enzyme was detectable in the 20,000 .times. 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 HDC inhibitor when measured against the enzyme activity of fetal tissues. The intraluminal injection of DL-.alpha.-methylhistidine dihydrochloride (DL-.alpha.-MH), a specific HDC inhibitor, on day 5 of pregnancy, interrupted implantation; normal implantation rates in contralateral horns were instilled with the same amount of L-histidine. The inhibiting action of DL-.alpha.-MH could be partially overcome by the co-injection of L-histidine. Rabbit blastocysts evidently have an active histamine forming system. Enzyme inhibition can alter implantation.