Gonadotropin-Releasing Hormone Stimulation of Gonadotropin Subunit Transcription: Evidence for the Involvement of Calcium/Calmodulin-Dependent Kinase II (Ca/CAMK II) Activation in Rat Pituitaries

The intracellular pathways mediating GnRH regulation of gonadotropin subunit transcription remain to be fully characterized, and the present study examined whether calcium/calmodulin-dependent kinase II (Ca/CAMK II) plays a role in the rat pituitary. Preliminary studies demonstrated that a single pulse of GnRH given to adult rats stimulated a transient 2.5-fold rise in Ca/CAMK II activity (as determined by an increase in Ca/CAMK II phosphorylation), with peak values at 5 min, returning to basal 45 min after the pulse. Further studies examined the α, LHβ, and FSHβ transcriptional responses to GnRH or Bay K 8644+KCl (BK+KCl) pulses in vitro in the absence or presence of the Ca/CAMK II-specific inhibitor, KN-93. Gonadotropin subunit transcription was assessed by measuring primary transcripts (PTs) by quantitative RT-PCR. In time-course studies, both GnRH and BK+KCl pulses given alone increased all three subunit PTs after 6 h (2- to 4-fold). PT responses to GnRH increased over time (3- to 8-fold over basal at 24 h), although BK+KCl was ineffective after 24 h. KN-93 reduced the LHβ and FSHβ transcriptional responses to GnRH by 50–60% and completely suppressed the αPT response. In contrast, KN-93 showed no inhibitory effects on basal transcriptional activity or LH or FSH secretion. In fact, KN-93 tended to increase basal α, LHβ, and FSHβ PT levels and enhance LH secretory responses to GnRH. These results reveal that Ca/CAMK II plays a central role in the transmission of pulsatile GnRH signals from the plasma membrane to the rat α, LHβ, and FSHβ subunit genes.