Quantitation of LH subunits released by rat anterior pituitary cells in primary culture

Intra-pituitary regulation of gonadotrophs in male rodents and primates

Paracrine and autocrine regulation is well established in many organs including the gonads, but the notion of communication among pituitary cells is a relatively new concept. The FSH-beta and GnRH-receptor genes are up-regulated by pituitary activin and down-regulated by pituitary follistatin, and circulating inhibin disrupts this local regulation by functioning as an endogenous competitor of the activin receptor. Activin and follistatin production by folliculostellate cells may play a central role in these responses. alpha-Subunit expression is maintained at high levels in the absence of GnRH through unknown mechanisms. There is evidence that the intra-pituitary regulation of FSH-beta and GnRH-receptor gene expression may activate pubertal maturation in male rats. Finally, there are marked differences in follistatin expression and its regulation by GnRH and androgens in male primates and rats that appear to explain species differences in the differential secretion of FSH and LH, although the physiological significance of these differences is not yet known.

Release of gonadotropin alpha subunit from rat pituitary cultures in response to GnRH

Gonadotropin releasing hormone (GnRH)-stimulated release of the alpha subunit common to the gonadotropins and to thyrotropin was studied in rat pituitary cell cultures. In these studies we took advantage of a recently prepared antiserum specific for the alpha subunit. We show that pituitary cells treated with GnRH released alpha subunit in a similar pattern to intact luteinizing hormone (LH) during short-term incubations (0-12 h); during prolonged incubations (12-48 h), however, release of alpha subunit did not desensitize in response to the releasing hormone and the pattern became different from that measured for intact LH. Further, we assessed the relative requirement for Ca2+ in the release of LH and alpha subunit. When pituitary cells were treated with 10(-8) M GnRH in the presence of a range of concentrations of the Ca2+ ion channel antagonist, methoxyverapamil (D-600), release of both LH and alpha subunit was inhibited in a similar and dose-dependent manner; 10(-4) M D-600 showed maximum inhibitory efficacy (IC50 = 10(-5) M). The calmodulin antagonist, pimozide, also inhibited both GnRH-stimulated LH and alpha subunit release (IC50 = 0.75 microM). These data suggested that although the Ca2+/calmodulin system appears to mediate both the release of LH and alpha subunit in response to GnRH, these processes appear differentially regulated during long-term exposure to the releasing hormone.

Ontogeny of gonadotropic and thyrotropic cells in fetal mouse anterior pituitary. Comparison between two species C57 BL6 and Balb/C

The fetal period constitutes a determinant stage in the ontogenesis of the hypothalamo-hypophysial-gonadal system. This work mainly concerns gonadotropic and thyrotropic functions and compares their different aspects in two strains of fetal mice. Balb/c and C57 BL6 fetal mice were studied at 16, 17, 18 and 19 days of gestation. The appearance and distribution of immunoreactive gonadotropic and thyrotropic cells of the anterior pituitary were observed by immunocytology using an indirect method with anti-porcine luteinizing hormone beta serum, anti-bovine thyrotrope hormone serum after saturation with bovine luteinizing hormone and anti-rat luteinizing hormone serum. In the two strains and in both sexes, LH gonadotropes appeared at 17 days of gestation and preferentially localized in the ventral part of the anterior lobe; a similar distribution was noted at 18 days and there was an increase in the number and staining intensity of labeled cells. By 19 days of gestation the gonadotrophs seemed more numerous, more generally distributed throughout the gland and often abutted to sinusoidal capillaries. An account of immunoreactive cells with anti-porcine luteinizing hormone serum and statistical evaluation of the results performed by variance analysis showed significant differences between the two strains. LH gonadotropic cells were always more numerous at each day of gestation in Balb/c fetuses especially in female fetuses. The possibility of a different evolution and/or differentiation for this cell population is discussed. Comparison of gonadotropic function between rat fetuses, mouse fetuses and human fetuses lead us to conclude that mouse fetuses appeared as an experimental model more closely related to human fetuses.

LHRH-receptor-regulated Ca2+-ATPase activity in murine pituitary gland

Addition of luteinizing hormone releasing hormone (LHRH) in vitro (10(-5) -5 X 10(-9) M) to murine pituitary membranes resulted in a dose-related decrease in Ca2+-ATPase activity within 15 min. Inhibitory effects of LHRH (10(-7) M) occurred after 90 sec, and appeared maximal by 120 sec. Eadie-Hofstee analysis at 10(-7) M LHRH, at varying [Ca2+]free, resulted in a Km = 0.89 +/- 0.06 microM and a Vmax = 18.8 +/- 0.71 nmol/mg per 2 min, compared to a Km = 0.69 +/- 0.06 microM and a Vmax = 32.8 +/- 1.21 nmol/mg per 2 min for controls. Pre-incubation for 5 min with LHRH antagonist (10(-8) M) significantly attenuated (50%) the inhibitory effects of 10(-7) M LHRH on pituitary Ca2+ ATPase activity with a Km = 0.97 +/- 0.24 microM and a Vmax = 28.1 +/- 2.8 nmol/mg per 2 min. The addition of LHRH (10(-7) M) to pituitary homogenates significantly increased luteinizing hormone (LH) release already at 10 and up to 40 sec compared to basal LH release. Systemic administration of 50 ng LHRH (i.p.), significantly (P less than 0.05) reduced pituitary Ca2+-ATPase after 30, 60 and 90 min, with a return to control levels by 120 min. Pituitary LH content was reduced slightly at 15 min, but was increased significantly at 90 and 120 min post-treatment. Plasma LH levels were elevated by 5 min, reached a peak by 15 min and returned to control within 60 min. The present findings indicate that LHRH receptor activation may influence cytosolic Ca2+ transport through effects on membrane Ca2+-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Stability of rat pituitary gonadotropins during nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis

125I-(rLH) and 125I-(rFSH) were dissolved in sample buffer containing 0-0.1% (SDS) and subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions at room temperature. 125I-rFSH migrated as a single band, while 125I-rLH migrated as two bands corresponding to rLH and rLH subunits suggesting partial dissociation. The dissociation of rLH increased as the concentration of SDS in the sample buffer was increased. However, electrophoresis of 125I-rLH at 4 degrees C under nonreducing conditions caused minimal dissociation. These data suggest that substantial dissociation of rLH, but not rFSH, occurs during nonreducing SDS-PAGE at room temperature and running the gels at 4 degrees C minimizes the dissociation of rLH.

How the fixation-embedding protocol affects the specificity and efficiency of immunocytochemical stains for gonadotropin subunits

This report describes a study designed to test factors that may affect the efficiency and specificity of stains for gonadotropins. These include chemical or freeze-fixation and dehydration, heat polymerization of the plastic embedding compound, dehydration in organic solvents, and etching. Specifically, postembedding stains for LH or FSH subunits were applied to 1-micron sections of 1) Araldite-embedded pituitaries that were either chemically fixed and dehydrated or freeze-fixed and freeze-dried; 2) Aldehyde-fixed pituitaries that were dehydrated in water-soluble glycol methacrylate (GMA) and embedded in GMA at 4 degrees C; and 3) p-formaldehyde-fixed pituitaries that were embedded in paraffin. A fourth group of pituitaries was dispersed and grown in monolayers for 1-3 days. These were stained following glutaraldehyde fixation. The optimal dilution of the primary antisera varied with the protocol; however, the percentage of cells staining for beta subunits did not change. In contrast, postembedding stains showed that alpha subunit reactivity is masked or destroyed in pituitaries that are fixed in glutaraldehyde and embedded in Araldite. Alpha chain reactivity was detected (in 14% of cells) either after freeze-fixation and freeze-drying followed by Araldite embedding, or after 4% paraformaldehyde fixation and GMA embedding (in 17% of cells). Staining in paraffin-embedded pituitaries was seen in only 10% of the cells. Preembedding stains for alpha chains were strikingly sensitive, however, and immunoreactivity was seen in 18-26% of the population of monolayer cells. Thus, whereas the percentages of cells staining for beta subunits do not change following the use of most of the fixation and embedding protocols, alpha chain reactivity is destroyed by all but the mildest. These findings show that one can control or improve the specificity of the stains for LH and FSH by the fixation-embedding protocol.

Minimal quantities of FSH beta subunit are released by rat anterior pituitary cells in primary culture

FSH beta in the medium and extracts of cultured rat anterior pituitary cells was quantitated with a heterologous radioimmunoassay utilizing anti-rFSH beta and hFSH beta as the standard and tracer. After 4 days in culture, extracts were prepared from washed cells, cells incubated for 6 h or 24 h in fresh medium or cells incubated for 6 h in the presence of 10(-7) M GnRH. FSH beta/rFSH molar ratios were approximately 0.05, 0.04, 0.21 and 0.35, respectively. The elevation in FSH beta/rFSH molar ratios in 24 h basal and GnRH-treated cultures was due in part to an increase in intracellular FSH beta. Both unstimulated and GnRH-treated cultures contained non-detectable quantities of FSH beta in the medium (less than 0.063 ng/100 000 cells; FSH beta/rFSH molar ratio less than 0.024). Thus, it appears that cultured rat anterior pituitary cells contain a small amount of uncombined FSH beta but that minimal quantities are released. Furthermore, GnRH may increase intracellular quantities of FSH beta but does not induce its release.

Shifts in gonadotropin storage in cultured gonadotropes following GnRH stimulation, in vitro

Stimulation of gonadotropes following castration or ovariectomy results in a shift in the gonadotrope population to cells that are mostly multihormonal. The purpose of these studies was to test this phenomenon, in vitro with the use of doublestains for LH and FSH applied to GnRH-stimulated gonadotropes. One-3 day monolayers were stimulated for 10 min-4 hr with 0.1 nM [D-Lys6] GnRH and then fixed and stained for both gonadotropins. After 60 min of stimulation, there was a significant increase in the proportion of gonadotropes that contained both hormones (from 57% to 74%) with a corresponding decrease in the proportion of cells that contained only one gonadotropin. There was no significant increase in the overall percentage of gonadotropes in the cell population indicating that the shift had probably occurred as a result of stimulation of the monohormonal gonadotropes to produce the other hormone. In addition, some of the stimulated gonadotropes showed the development of processes, some of which stained for only one of the gonadotropins. These data suggest that most gonadotropes may have the capacity to produce and store both hormones but they may perform these functions in separate regions of the same cell.