University of Florida

Michael J. Fields, Ph.D.

Intracellular events leading to the expression and secretion of luteal secretory products and their subsequent effect on reproductive events is the focus of research in this program. Identification of luteal secretory products, determining their regulatory control, and elucidating their function is important to understanding how the corpus luteum regulates the estrous cycle, sustains pregnancy and influences parturition. The morphology of the bovine corpus luteum has been characterized and at least two populations of steroidogenic large luteal cells have been identified. Both populations have oxytocin-containing secretory granules that are released during the estrous cycle in response to prostaglandin F2-alpha. This luteal oxytocin drives the bovine endometrium to secrete prostaglandin F2-alpha which in turn contributes to the regression of the corpus luteum. However, during pregnancy the secretory granule contents are retained to be released at a later time. Oxytocin secreted during prei-implantion of pregnancy is believed to serve two roles.  One is to maintain pregnancy via secretion of placental/uterine PGE2.  The other role may serve to regress the corpus luteum via uterine secretion of PGF2-alpha in the case of a degenerating embryo. Two populations of secretory granules have been identified. Those of the cycle that contain oxytocin and those of later stages of pregnancy the contents of which are unknown.

Molecular and in vitro synthesis techniques have been utilized to identify luteal genes and their proteins across the estrous cycle and pregnancy. Secreted proteins identified include apolipoprotein E and A-I and tissue inhibitor of metalloproteinases 1 and 2. Apolipoprotein E (the major lipoprotein of low density lipoprotein, LDL) was found to be expressed and secreted only by the very early developing corpus luteum, indicating it may have a role in providing cholesterol for the expanding membrane synthesis and the needed cholesterol for use as a precursor for progesterone.

To complement the luteal secretory protein studies we have pursued the ontogeny of oxytocin receptors and its role in regulating the prostanoid system in the uterus and cervix in preparing the female for a new opportunity for ovulation as well as its role in sustaining pregnancy and insuring an optimal outcome at the time of parturition. In addition to oxytocin, LH is also being examined for its role in driving uterine prostanoid synthesis during the estrous cycle.

Representative Publications:
Shemesh M, Mizrahci D, Gurevich M, Shore LS, Reed J, Chang SM-T, Thatcher WW, Fields MJ. Expression of functional Luteinizing Hormone (LH) receptor and its messenger ribonucleic acid in bovine endometrium: LH augmentation of cAMP and inositol phosphate in vitro and human chorionic gonadotropin (hCG) augmentation of peripheral prostaglandin in vivo. Reprod Biol2002; 1:13-32. 

Fuchs AR, Graddy LG, Kowalski AA, Fields MJ. Oxytocin induces PGE2 release from bovine cervical mucosa in vivo. Prostaglandins 2002; 70:119-129.  

Lucy MC, Billings HJ, Butler WR, Ehnis LR, Fields MJ, Kesler DJ, Kinder JE, Mattos RC, Short RE, Thatcher WW, Wettemann RP, Yelich JV, Hafs HD. Efficacy of an intravaginal progesterone insert and an injection of PGF2α for synchronizing estrus, and shortening the interval to pregnancy in postpartum beef cows, peripubertal beef heifers, and dairy heifers. J Anim Sci 2001; 79:982-995. 

Shemesh M, Mizrachi D, Gurevich M, Stram Y, Shore LS,  Fields MJ. Functional importance of bovine myometrial and vascular LH receptors and cervical FSH receptors.  Sem Reprod Med 2001; 19:87-96. 

Fuchs AR, Ivell R, Ganz N, Fields MJ.  Secretion of oxytocin in pregnant and parturient cows: corpus luteum may contribute to plasma oxytocin at term. Biol Reprod 2001; 65:1135 -1141.

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fields

Michael J. Fields, Ph.D.
Professor Emeritus
Department of Animal Sciences
Ph.D., Texas A & M University