title: Desensitization to gonadotropin-releasing hormone in perifused chicken anterior pituitary cells.
abstract: A perifusion method consisting of dispersed chicken anterior pituitary cells suspended in columns of Bio-Gel was developed to monitor the dynamics of LH release. The perifused cells responded to chicken I GnRH (Gln8-GnRH) in a dose-dependent manner. The ED50 was 3 X 10(-10) M, and maximal LH release occurred in response to 4 X 10(-9) M Gln8-GnRH. Continuous administration of 10(-7) M Gln8-GnRH and agonist stimulated an initial 8- to 10-fold increase in LH release within minutes. LH release then declined rapidly, reaching basal levels within 100 min. A biphasic response was noted. Calcium ionophore A23187 was effective in releasing additional LH from cells desensitized to 10(-7) Gln8-GnRH and agonist, indicating that total cellular LH was not depleted. In contrast, delivery of 2-min pulses of 10(-7) M and 10(-9) M Gln8-GnRH at a frequency of one pulse every 30 or 60 min for 3-5 h maintained pituitary responsiveness. Exposure to 10(-7) M Gln8-GnRH for 20 min was sufficient to desensitize pituitary cells to subsequent Gln8-GnRH stimulation. However, 20-min exposure to 10(-7) M GnRH antagonist neither evoked LH release nor had a desensitizing effect on subsequent stimulation by 10(-7) M Gln8-GnRH, indicating that receptor activation, not merely receptor binding, is necessary for Gln8-GnRH-mediated homologous desensitization. Pituitary cells desensitized by 20-min exposure to 10(-8) M Gln8-GnRH maintained responsiveness to a higher dose (10(-6 M) of Gln8-GnRH, suggesting that down-regulation of pituitary GnRH receptors might play a part in desensitization. Calcium ionophore A23187 partially desensitized pituitary cells to subsequent stimulation with Gln8-GnRH, probably due to depletion of releasable LH or desensitization of calcium-coupled secretory mechanisms. In calcium-free medium, 10(-7) M Gln8-GnRH did not evoke LH release, but nevertheless partially desensitized cells to subsequent 10(-7) M Gln8-GnRH stimulation. Thus desensitization is partially calcium-dependent. These findings demonstrate that the GnRH-mediated desensitization of gonadotrophs is a characteristic of chicken pituitary cells as in the mammal. However, chicken pituitary cells differ from mammalian cells in that desensitization is more rapid and partially dependent on extracellular calcium.
mesh_terms: D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002645:Chickens; D004361:Drug Tolerance; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives* / Q000494:pharmacology; D007986:Luteinizing Hormone / Q000378:metabolism*; D010477:Perfusion; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011761:Pyrrolidonecarboxylic Acid / Q000031:analogs & derivatives
chemical_list: D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; C037700:LHRH, Gln(8)-; D007986:Luteinizing Hormone; D002118:Calcium; D011761:Pyrrolidonecarboxylic Acid