GnRH-stimulated Luteinizing Hormone Secretion Research Articles

In vitro effect of ethanol exposure on basal and GnRH-stimulated LH secretion from pituitary cells.

The question of whether ethanol's (ETOH's) known suppressive effect on serum luteinizing hormone (LH) could be mediated directly at the anterior pituitary level was addressed by examining the effects of ETOH in vitro on release of LH from cultured male rat pituitary cells. The impact of added ethanol concentrations ranging from 50 to 400 mg% on LH release was examined in the basal state and after stimulation by gonadotropin-releasing hormone (GnRH) at a dose of 5 x 10(-10) M. While ETOH did not significantly suppress basal LH release, secretion stimulated with GnRH was noted to be attenuated with higher doses of ETOH (greater than or equal to 100 mg%) compared to stimulated control cells. It is concluded that ETOH exposure in vitro alters stimulated LH secretion by acting directly on pituitary gonadotropes.

Estrogenic effects of phenol red on rat pituitary cell responsiveness to gonadotropin-releasing hormone

This study investigated whether phenolsulfonphtalein (PR), a common pH indicator in tissue culture media, affects luteinizing hormone (LH) secretion from rat pituitary cells or 17β-estradiol (E 2) augmentation of pituitary responsiveness to gonadotropin-releasing hormone (GnRH). PR enhanced GnRH-stimulated LH secretion and shifted the GnRH dose-response curve leftward with a relative potency ratio of 0.24 ± 0.09 (± SE; p < 0.01). The effect of E 2 on LH release was significantly diminished by PR, which elevated GnRH-stimulated LH secretion in the absence of E 2. This phenomenon was elicited by PR from different sources and was inhibited by the antiestrogen C1628. Thus, PR exerted estrogen-like effects on rat pituitary cells and caused an underestimation of the degree to which E 2 enhanced GnRH-stimulated LH secretion.

Cyclic AMP indirectly mediates the extracellular Ca 2+-independent release of LH

This study was undertaken to determine whether cyclic AMP mediates the extracellular Ca 2+-independent component of luteinizing hormone (LH) release. Gonadotropin releasing hormone (GnRH) increased cAMP production in female pituitaries incubated in Ca 2+-free medium containing 3-isobutyl-1-methylxanthine (IBMX), but was ineffective in male pituitaries. The increases in female pituitaries were inhibited by flufenamate (flu). GnRH-stimulated LH secretion from male pituitaries was completely inhibited in Ca 2+-free medium, whereas only a partial inhibition was obtained from female pituitaries, a response prevented by cycloheximide. Infusions of flu completely inhibited the extracellular Ca 2+-independent release of LH, while dibutyryl cAMP (dbcAMP) partially restored this component of LH secretion. The dbcAMP-restored response was dependent upon protein synthesis. These results suggest that (i) the extracellular Ca 2+-independent component of LH release is indirectly mediated by cAMP through the stimulation of de novo protein synthesis, and (ii) extracellular Ca 2+ is required for the activation of adenylate cyclase in male but not in female gonadotrophs.

Evidence for cAMP as a mediator of gonadotropin secretion from male pituitaries.

The purpose of this study was to use sodium flufenamate, a compound that inhibits gonadotropin-releasing hormone (GnRH)-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) production in the pituitary, to evaluate the potential role of cAMP as a mediator of GnRH-stimulated gonadotropin secretion from male pituitaries. Quartered male pituitaries were perifused at 37 degrees C and sequential effluent fractions collected every 10 min. Infusions of GnRH resulted in a twofold increase in luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion. Cycloheximide, 5 microM, completely inhibited the GnRH-stimulated LH and FSH secretion. Infusions of 0.1 mM flufenamate had similar effects on gonadotropin secretion as cycloheximide, whereas the administration of 5 mM dibutyryl cAMP in combination with GnRH and flufenamate restored the secretory responses of both hormones. The flufenamate-inhibited GnRH stimulated LH and FSH release, which was restored by DBcAMP and appeared to be protein synthesis dependent and specific for cAMP. These results suggest an indirect role for cAMP as a mediator of gonadotropin secretion from male pituitaries. However, in contrast to female pituitaries, the secretion of these hormones from male pituitaries is completely dependent on cAMP and de novo protein synthesis.

Participation of voltage-dependent calcium channels in the action of gonadotropin-releasing hormone.

The roles of calcium ions and voltage-sensitive calcium channels in gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release were investigated in rat anterior pituitary cells. The calcium ionophores A23187 and ionomycin stimulated LH release from cultured pituitary cells, in which ionomycin (5 X 10(-5) M) was as effective as a maximal concentration of GnRH (10(-8) M). In addition, a concentration of the calcium channel agonist methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine- 5-carboxylate (BK 8644) (10(-8) M), which did not alter basal LH release, potentiated the ability of submaximal (10(-10) and 10(-9) M) doses of GnRH to increase LH secretion. Furthermore, concentrations of the calcium channel antagonist nitrendipine (10(-8)-10(-5) M) that did not alter basal LH release, inhibited GnRH-stimulated LH secretion by 30-40%. Nitrendipine (10(-8) M) also blocked the potentiation of GnRH-induced LH secretion by BK 8644. During column perifusion of pituitary cells, 10(-8) M nitrendipine reduced the LH response to a 2-min pulse of 10(-8) M GnRH by about 40%. In Quin-2-loaded pituitary cells, supramaximal doses (10(-6) M) of a potent GnRH agonist rapidly elevated cytoplasmic Ca2+ by 50-100 nM. Concomitant treatment with nitrendipine decreased the GnRH agonist-induced Ca2+ signal by 40-60%. These results indicate that increases in intracellular Ca2+ via voltage-sensitive calcium channels partially reproduce GnRH action, and also that GnRH causes activation of such channels. However, the increase in cytoplasmic Ca2+ concentration during GnRH action must originate in part from mobilization of internal Ca2+ stores, and its relatively small magnitude may be consistent with the concomitant activation of protein kinase C as an intermediate step in GnRH action.

In vivo and in vitro studies on the effect of adrenocorticotropic hormone or cortisol on the pituitary response to gonadotropin releasing hormone.

Experiment I: Hyperadrenal states were induced in intact heifers (N = 3) or adrenalectomized (ADRX) heifers (N = 3) by constant infusion of ACTH (20.8 micrograms, 1-24 ACTH/h) or hydrocortisone succinate (HS) (30 mg/h), respectively. Control infusions consisted of the saline vehicle. All infusions began on Day 2 of a normal estrous cycle. Exogenous gonadotropin releasing hormone (GnRH) was given as a 100-micrograms bolus i.v. on Days 7, 9, and 11 (intact) or 5, 7, and 9 (ADRX) of the cycle. In intact heifers, the cumulative luteinizing hormone (LH) response was reduced (P less than 0.05) by the ACTH treatment. In ADRX heifers, the HS treatment did not alter the cumulative response but did alter the qualitative response with a time X treatment interaction (P less than 0.01). The LH response in the HS-ADRX animals had a slower onset and lower peak concentrations with a more prolonged response. Experiment II: Dispersed bovine pituitary cells were prepared and incubated at concentrations of 2 X 10(6) viable cells in 2.0 ml per dish. Cells were exposed to cortisol at concentrations of 0.01, 0.10, 0.21 and 1.03 X 10(-6) M for time periods of 8, 14, 20 or 26 h for basal LH secretion studies and 10, 16, 22 and 28 h for GnRH-stimulated LH secretion. Both dosage of cortisol and length of exposure had a depressing effect on basal LH release. The cortisol pretreatment also decreased (P less than 0.001) the LH release following addition of GnRH (8.5 X 10(-8) M) in cultures at all dosages and exposure times of cortisol. However, there was no decrease in LH or protein content of cells. These experiments indicate a direct action of cortisol on the pituitary gland to depress both basal and stimulated LH release.