Expression of ryanodine receptors in the pituitary gland: evidence for a role in gonadotropin-releasing hormone signaling.

GnRH elicits secretion of LH and FSH from gonadotropes by activating an array of intracellular signals including the generation of inositol triphosphate and the release of intracellular calcium. Given the important role of calcium in the secretory responses to GnRH, we examined the expression and function of the ryanodine receptors, which are known to modulate calcium release from intracellular stores. Using RT-PCR analysis, we found that ryanodine receptor (RyR) types 2 and 3, but not type 1, are expressed in rat pituitaries. Pulses of GnRH were administered to perifused primary rat pituitary cells in the presence or absence of a ryanodine receptor antagonist, ruthenium red, to assess effects on GnRH-mediated LH secretion. Treatment with ruthenium red resulted in a 40% decrease in the spike phase of GnRH-induced LH release and a 35% reduction in the plateau phase. Ruthenium red also inhibited GnRH-mediated transcription of a transfected alpha-LUC reporter plasmid. RyR messenger RNA (mRNA) expression varied during the rat estrous cycle with maximal levels following increases of progesterone. The effects of gonadal steroids on pituitary RyR mRNA levels were examined directly in ovariectomized rats that were treated with estrogen (E), or estrogen and progesterone (P). In this paradigm, E decreased, whereas E + P increased RyR3 mRNA levels. These results indicate that RyR is expressed and hormonally regulated in the rat pituitary and suggest that it might play a role in mediating GnRH-induced gonadotropin synthesis and secretion.