A CHOLERA TOXIN-SENSITIVE GUANYL NUCLEOTIDE-BINDING PROTEIN MEDIATES THE MOVEMENT OF PITUITARY LUTEINIZING-HORMONE INTO A RELEASABLE POOL -LOSS OF THIS EVENT IS ASSOCIATED WITH THE ONSET OF HOMOLOGOUS DESENSITIZATION TO GONADOTROPIN-RELEASING-HORMONE

Cholera toxin (CTX; 5 mug/ml), but not pertussis toxin (100 ng/ml), wh en preincubated with pituitary cells for 18 h, enhances the percentage of cellular LH released in response to continuous or pulsatile admini stration of 5 x 10(-9) m GnRH. This effect occurs without increasing t otal (intracellular plus extracellular) LH, indicating that it is best explained by redistribution of LH from a nonreleasable to a releasabl e pool. This site of action is consistent with the observation that CT X-pretreated cells are also sensitized to stimulation of LH release by the Ca2+ ionophore A23187. The observations that CTX stimulates the p roduction of cAMP in these cells and that the sensitizing action of CT X is mimicked by (Bu)2cAMP (1 mm) are consistent with the view that a CTX-stimulated guanyl nucleotide binding protein, capable of activatin g adenylyl cyclase, is mediating this sensitization. We used a perifus ed cell system to show that the movement of LH into a releasable pool is lost with the onset of homologous desensitization due to high pulse frequency or constant administration of GnRH (5 x 10(-9) M, continuou s or a pulse each 15 min). Sensitization to CTX is restored by stimula tion with a high concentration of GnRH (10(-6) M) or by resetting the pulse frequency to the rate measured in vivo (a pulse each 90 min). Bo th of these treatments also circumvent the desensitized state, restori ng LH release. These results identify a novel lesion associated with t he development of desensitization in the gonadotrope and support the r ole of a CTX-sensitive guanyl nucleotide binding protein in regulation of pituitary responsiveness to GnRH.