Bioscientifica Proceedings

Summary. Results of a detailed study involving 18 pregnant, 17 cyclic and 12 inseminated but non-pregnant Holstein heifers indicated the following. (1) Jugular plasma progesterone concentrations were higher (P < 0.05) in pregnant than in cyclic or in inseminated–non-pregnant animals from Days 10 through 18. (2) Jugular plasma progesterone concentrations of inseminated–non-pregnant animals could be related to heifers in which (a) fertilization may not have occurred (N = 5); (b) embryonic death may have occurred between Days 6 and 9 (N = 5); and (c) embryo mortality may have occurred shortly before Day 18 (N = 2). (3) Jugular plasma oestradiol concentrations were higher (P < 0.05) in pregnant than in cyclic animals between Days 10 and 18; the oestrogen values of the inseminated–non-pregnant animals resembled those of the pregnant animals. (4) The steroid- synthesizing capabilities of incubated luteal tissues from inseminated–non-pregnant heifers more closely resembled those of tissues from pregnant animals than those of cyclic animals. (5) At Day 18 PGF concentrations in endometrial and luteal tissues and ovarian arterial plasma did not differ among the 3 groups. (6) PGF and progesterone concentrations of luteal tissues were negatively correlated (r = -0.77, P < 0.05) in cyclic animals and positively correlated (r = 0.73, P < 0.05) in pregnant animals, implying a luteotrophic effect of PGF in pregnant animals.

These results suggested a luteotrophic effect of the embryo, beginning as early as Day 10 of pregnancy, and the results of further experiments suggested that (i) homogenates and extracts of Day 18 bovine embryos contain one or more substances capable of stimulating progesterone synthesis in dispersed bovine luteal cell preparations; (ii) the luteotrophic activity was proportional to the protein contents of the embryo homogenates; and (iii) the luteotrophic activity was heat labile and was removed from homogenates by dialysis, indicating a structure of one or more relatively small molecules.

© 1981 Journals of Reproduction & Fertility Ltd