Studies on the control of luteal function

Ingamells, Susan (1986) Studies on the control of luteal function. University of Southampton, Doctoral Thesis.

Abstract

Factors involved in controlling luteolysis and luteal maintenance during early gestation were investigated in the guinea pig in vivo and in vitro. The mechanisms involved in gonadotrophin-stimulated progesterone secretion and its inhibition by cloprostenol were studied in rat dispersed luteal cells of different ages. In vivo luteal function was assessed by the radioimmunoassay of progesterone in the plasma of blood samples collected via an indwelling venous cannula. The acute effects of agonists and antagonists on luteal function in vitro were assessed by the measurement of the secreted progesterone. Human Chorionic Gonadotrophin (hCG) (10^-6 -10^-2 iU/ml) and dibutyryl cyclic AMP (d.b.cAMP) produced dose-dependent increases in progesterone secretion by rat dispersed luteal cells on day 3, 5, 7 and 9. Luteal sensitivity to hCG and d.b.cAMP changed with age. The hCG-stimulated progesterone secretion increased from day 3 to reach a maximum on day 7 (11x10^-8 M, per 4μg DNA) and fell to low levels on day 9. Maximum progesterone secretion (6x10^-8 M) in response to d.b.cAMP was found on day 5. The changes in luteal sensitivity with age are not accounted for by changes in extracellular calcium influx or arachidonic acid metabolism. Transmembrane influx of calcium is not an important step in the mechanism of gonadotrophin stimulation of steroidogenesis in the luteal cell. Preliminary studies revealed that the products of cyclooxygenase pathway of arachidonic acid metabolism may have a stimulatory role in gonadotrophin stimulated steroidogenesis and that they have a site of action distal to cAMP generation. The hysterectomized guinea pig was used as a model for studies on the effect of the luteolysin, cloprostenol (a prostaglandin F_2 analogue), in vivo at different stages of the oestrous cycle. Young (day 7-8) corpora lutea were resistant to the inhibitory effects of cloprostenol whereas older corpora lutea (day 13-16) were extremely sensitive. In the rat, young corpora lutea in vitro were insensitive to the acute inhibitory effect of cloprostenol (0.04μg/ml) on hCG and d.b.cAMP stimulated progesterone secretion; sensitivity to cloprostenol increased with luteal age. Cloprostenol had a site of action distal to cAMP generation in the steroidogenic pathway. The magnitude of inhibition of hCG-stimulated progesterone secretion by cloprostenol was greater than the inhibition of d.b.cAMP -stimulated progesterone secretion, in vitro. It is therefore suggested that cAMP is not the only second messenger mediating gonadotrophine stimulated steroidogenesis. Studies in calcium depleted medium revealed that the inhibitory action of cloprostenol is not mediated by extracellular calcium influx. Oxytocin is unlikely to be involved in the regulation of luteolysis in the guinea pig since it had no effect in long and short term in vivo or acute in vitro studies. A chorionic gonadotrophin-like factor with luteotrophic and anti-luteolytic properties was found in the placenta and subplacenta of the guinea pig. Extracts of placenta and subplacenta stimulated steroidogenesis by guinea pig luteal cells.

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