The effect of histamine on cAMP and cGMP levels in day 6 (144 h post coitum) rabbit blastocysts was determined. Histamine at 200 μM and 1000 μM concentrations stimulated the increased formation of cAMP in vitro, whereas stimulation of cGMP occurred only in the presence of 1000 μM histamine. Furthermore, intrauterine injection of RMI-12330A (50 μg or 500 μg/uterine horn), an inhibitor of adenylyl cyclase, on day 5 of pregnancy interrupted embryro development and implantation of the embryo. The drug was also effective in reducing the cAMP level in the endometrial cells in vitro. A relationship between histamine and cyclic nucleotide changes in embryo development and implantation is sugge
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