Homologous Desensitization with Gonadotropin-Releasing Hormone (GnRH) also Diminishes Gonadotrope Responsiveness to Maitotoxin: A Role for the GnRH Receptor-Regulated Calcium Ion Channel in Mediation of Cellular Desensitization*

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Abstract
Maitotoxin (MTX) stimulates gonadotropin release from pituitary cell cultures. The time course and efficacy of LH release in response to GnRH and to MTX are similar; both secretagogues require extracellular Ca2+ and are inhibited by the selective Ca2+ ion channel antagonist methoxyverapamil (D600). LH release in response to either GnRH or MTX is not measurably inhibited by two other chemical classes of Ca2+ ion channel inhibitors represented by nifedipine and by diltiazem. The two secretagogues are nonadditive in their action on LH release when presented at high doses and prior studies indicate that MTX has no endogenous ionophoretic activity. These observations indicate that MTX likely stimulates LH release due to activation of the GnRH receptor associated Ca2+-ion channel in the gonadotrope. We have therefore assessed the functional state of this channel during the development of homologous desensitization of the gonadotrope to GnRH by measuring the ability of MTX to stimulate LH release. Cells were desensitized with GnRH in the presence of 3 mM EGTA. Under these conditions, the cells become refractory to GnRH in the absence of gonadotropin release since the latter process, but not the former, requires ectracellular Ca2+. Accordingly, this approach allows assessment of the degree of desensitization in the absence of the influence of gonadotropin depletion. Such desensitized cells are less responsive to GnRH. Desensitized pituitary cells also respond with diminished efficacy and potency to MTX three or more hours after GnRH treatment but not at an earlier time (1 h) when GnRH receptors are diminished. These data are consistent with a model in which homologous desensitization is viewed as developing in two phases. Initially, loss of responsiveness is due to receptor loss; subsequently it is maintained by loss of functional activity of the Ca2+ ion channel.
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