Suppression of follicle-stimulating hormone by the gonadal- and neurosteroid 3 alpha-hydroxy-4-pregnen-20-one involves actions at the level of the gonadotrope membrane/calcium channel.
We have previously shown that the gonadal- and neurosteroid 3 alpha-hydroxy-4-pregnen-20-one (3 alpha HP) suppresses FSH release in cultures of anterior pituitary cells. We undertook exploration of the mechanisms of this suppression by examining the possible sites of 3 alpha HP action in isolated anterior pituitary cells of rats. The specific objective of this study was to determine if 3 alpha HP suppresses FSH by action at the level of the gonadotrope membrane and/or calcium channels. Pituitary cells from adult randomly cycling female rats were precultured for 72 h and then treated for 4 h with 10 nM GnRH and 0.1 nM 3 alpha HP with or without Ca2+ channel agonists or antagonist. In other experiments, cells were treated with BSA-conjugated 3 alpha HP, progesterone, or 3 beta HP (the stereoisomer of 3 alpha HP). Levels of FSH were determined by RIA in media and cells. GnRH-stimulated FSH release and the total FSH (released plus cellular) were significantly suppressed by 3 alpha HP. The Ca2+ ionophore A23187 induced FSH release and 3 alpha HP significantly suppressed both released and total FSH in its presence. In combination with a high dose (100 microM) of the dihydropyridine-sensitive Ca2+ channel antagonist nifedipine, 3 alpha HP suppressed FSH secretion to a greater extent than the antagonist alone. Cellular content of FSH was also decreased by nifedipine (100 microM) and was further suppressed in the presence of 3 alpha HP. The phenylalkylamine-sensitive Ca2+ channel antagonist methoxyverapamil (D600) suppressed GnRH-induced FSH release, and 3 alpha HP significantly potentiated the suppression. Released and cellular FSH were increased by the dihydropyridine-sensitive agonist BAYK 8644, whereas 0.1 nM 3 alpha HP suppressed this agonist-induced FSH to a greater extent than the maximum dose (100 microM) of nifedipine. In order to test for direct action at the level of the gonadotrope membrane, 3 alpha HP was conjugated to BSA (3 alpha HP-BSA) and administered to cultured pituitary cells. The 3 alpha HP-BSA conjugate (but not progesterone-BSA or 3 beta HP-BSA) significantly suppressed release of FSH. The results of the study suggest that 3 alpha HP may be interacting with the Ca2+ channel component of the GnRH signal transduction mechanism; in addition, 3 alpha HP may also suppress FSH release (and possibly synthesis) through direct action at the level of the gonadotrope membrane.
Dhanvantari
S
S
Hormonal Regulatory Mechanisms Laboratory, University of Western Ontario, London, Canada.
Wiebe
J P
JP
D004092:20-alpha-Dihydroprogesterone / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects; D005260:Female; D005640:Follicle Stimulating Hormone / Q000037:antagonists & inhibitors*; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D007986:Luteinizing Hormone / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012710:Serum Albumin, Bovine / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004092:20-alpha-Dihydroprogesterone; C052610:3-hydroxypregn-4-en-20-one; D012710:Serum Albumin, Bovine; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone