Acute effects of oestradiol and progesterone on melittin- and gonadotrophin-releasing hormone-induced LH secretion

ABSTRACT It is well established that oestradiol and progesterone modulate gonadotrophin-releasing hormone (GnRH)-induced LH secretion from cultured rat pituitary cells. Short-term oestradiol and long-term progesterone treatment exert inhibition, while short-term progesterone and long-term oestradiol treatment lead to enhancement of GnRH-stimulated LH secretion. There are several lines of evidence to suggest that the steroid effects might be mediated via a mechanism involving modulation of the GnRH signal-transduction system. To evaluate the role of arachidonic acid, which serves as an intracellular signal transducer by itself or its lipoxygenase metabolites, in the mediation of oestradiol and progesterone actions, we examined their effects on melittin (activator of phospholipase A2)-stimulated LH secretion. When pituitary cells from adult female rats were treated for 48 h with 1 nmol oestradiol/l or 1 nmol oestradiol/l plus 100 nmol progesterone/l, GnRH (1 nmol/l)-induced LH secretion was stimulated or inhibited respectively. However, melittin (10–300 nmol/l)-stimulated LH secretion remained unaffected after such treatment. Short-term treatment with oestradiol inhibited GnRH-induced LH secretion while progesterone treatment of oestradiol-primed cells led to a stimulatory effect. Interestingly, melittin-stimulated LH secretion was influenced in the same way after the short treatment paradigm. Perifusion studies were performed to assess the kinetics of these acute steroid actions further. Four separate perifusion chambers were continuously perifused with medium and stimulated for 2 min with 1 nmol GnRH/l or 1 μmol melittin/l every 50 min in a pulsatile fashion. When 1 nmol oestradiol/l was added to the perifusion medium after the application of an initial control pulse, GnRH- and melittin-stimulated LH secretion were inhibited by 69 and 61% respectively. This effect was present after 50 min. When oestradiol-primed cells were treated with 100 nmol progesterone/l starting after the initial GnRH or melittin pulse, an acute stimulatory effect was observed in response to both stimuli after 50 min. LH release was enhanced by up to 279 (GnRH) or 419% (melittin) compared with the control pulse. The kinetics of inhibited or stimulated pulsatile LH secretion were virtually identical when GnRH or melittin were used as stimuli. These results demonstrate that short-term oestradiol or progesterone treatment modulate arachidonic acid-mediated LH secretion in a similar fashion to GnRH-induced LH secretion, while long-term oestradiol or progesterone treatment only affected GnRH-induced LH secretion. Journal of Endocrinology (1992) 132, 251–259