Dissociation of the porcine anterior pituitary: the kinetics of luteinizing hormone release in response to luteinizing hormone-releasing hormone

Bovine lactotroph cultures for the study of prolactin synthesis functions

The aim of this study was to establish a bovine anterior pituitary-derived lactotroph (BAPDL) line that expresses prolactin (PRL) in vitro to study the mechanisms of bovine PRL synthesis and secretion. Immunohistochemistry assay of PRL in the newborn calves' anterior pituitary glands showed that most lactotrophs were located within the superior border of the lateral wings of the anterior pituitary. Tissues of the superior border of the lateral wings of the anterior pituitary were dispersed and cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS). The limiting dilution method was used to establish BAPDL from single cell clone. BAPDL cells constantly expressed mRNAs for PRL and pituitary-specific transcription factor 1 (Pit-1) gene and grew steadily and rapidly in the DMEM supplemented with 10% FBS. PRL immunoreactivity was present in BAPDL at passage 20. The concentration of bovine PRL in BAPDL at passage 20 culture supernatant was decreased to below 35% compared with that in BAPDL at passage 1. The effects of human epidermal growth factor (hEGF) and dopamine (DA) on the expression and secretion of PRL in BAPDL at passage 4 were also investigated. The results are consistent with those of previous studies. Thus, it can be used successfully for studying the mechanisms of stimuli regulating PRL synthesis and release.

Differences in the cyclic nucleotide mediation of luteinizing hormone-releasing hormone action on the rat and hamster anterior pituitary gland

A continuous flow superfusion system which was previously developed in our laboratory was utilized to study the modulation of LH and FSH release by cyclic nucleotides and LHRH from anterior pituitary glands (APG) obtained from rats or hamsters. There was a transient increase in LH and FSH secretion from superfused rat APG in response to superfusion with 1 X 10(-3) M 1-methyl-3-isobutyl-xanthine (MIX), while 1 X 10(-4) MIX M had no effect. Furthermore, a dose of 5 X 10(-5) M MIX did not potentiate the gonadotrophin-releasing effect of 1 X 10(-10) M LHRH. Neither 1 X 10(-3) M 8-Br-cAMP nor 1 X 10(-3) M 8-Br-cGMP mimicked the gonadotrophin-releasing effects of 1 X 10(-9) M LHRH. In the experiments utilizing hamster APG, FSH release gradually increased during superfusion with 1 X 10(-3) M MIX or 1 X 10(-4) M MIX, while LH release was transient but significantly increased in response to superfusion with both doses of MIX. A dose of 5 X 10(-5) M MIX potentiated the effect of a low dose of LHRH (1 X 10(-10) M) upon both LH and FSH secretion. 1 X 10(-3) M 8-Br-cAMP mimicked the effect of LHRH upon LH and FSH released from superfused hamster APG, while 1 X 10(-3) M 8-Br-cGMP was inhibitory. These results suggest that cyclic nucleotides are involved in the mediation of the LHRH-induced release of gonadotropins from the anterior pituitary gland of the hamster, but do not mediate the LHRH-induced release of gonadotropins from the rat anterior pituitary gland.

LH release from dispersed bovine pituitary cells in culture: in vitro effects of estradiol and procedural variables

Procedures for cell dissociation and in vitro culture were validated to investigate secretion of luteinizing hormone (LH) from bovine anterior lobe (AL) pituitary cells. The concentration of trypsin used for dissociation affected cell yield, cell loss during preincubation, LH secretion, and response to gonadotropin-releasing hormone (GnRH). Optimum results were obtained with trypsin concentrations of 8-16 micrograms/mg fresh tissue. Duration of preincubation and of experimental culture markedly affected LH secretion and response to GnRH. Immediately after dissociation, cells contained relatively low quantities of LH, but they were able to release a substantial proportion of this LH. Basal release of LH and GnRH-induced release of LH were highly correlated with total quantities of LH, and all three parameters increased with time of preincubation until 24 hr. Experimental treatments of 2 hr duration were optimal for investigating GnRH stimulation of LH release, whereas longer treatments may be required to investigate effects of agents that inhibit the release of LH. Preincubation of dissociated AL cells with physiological concentrations of estradiol increased all three LH parameters. Progesterone had no effect either alone or in combination with estradiol. In conclusion, the procedures described for cell dissociation and culture of suspended cells provide a useful tool for studying release of LH from the bovine AL cell.

5,6-Epoxyeicosatrienoic acid mobilizes Ca2+ in anterior pituitary cells

Luteinizing hormone releasing hormone stimulates the concomitant release of luteinizing hormone and 45Ca2+ from prelabeled anterior pituitary cells. Indomethacin (10 microM) and nordihydroguaiaretic acid (10 microM) had no effect on the luteinizing hormone releasing hormone-stimulated release of either luteinizing hormone or 45Ca2+. Eicosatetraynoic acid (10 microM) blocked both luteinizing hormone releasing hormone-stimulated luteinizing hormone secretion and luteinizing hormone releasing hormone-stimulated 45Ca2+ efflux. 5,6-Epoxyeicosatrienoic acid stimulated both luteinizing hormone secretion and 45Ca2+ efflux from anterior pituitary cells. Additionally, 5,6-epoxyeicosatrienoic acid closely mimics the ability of luteinizing hormone releasing hormone to increase intracellular free calcium. These results are consistent with the hypothesis that 5,6-EET alters calcium homeostasis in a manner similar to that observed during luteinizing hormone releasing hormone stimulation of luteinizing hormone release.

Comparison of the time courses of luteinizing hormone (LH) secretion rates during continuous stimulation by LH-releasing hormone (LH-RH) in vivo and in vitro

The patterns of LH secretion during constant stimulation of the pituitary glands of estradiol-treated ovariectomized rats with a maximally stimulating amount of LH-RH in vivo and in vitro correspond with each other qualitatively and quantitatively. In vitro the changes with time of the LH secretion rate are somewhat retarded, especially the occurrence of desensitization.

Studies on the kinetics of dopamine-regulated prolactin secretion

The regulation of prolactin (PRL) release by dopamine has been studied using short-term dissociated rat pituitary cell cultures. Concentrations of dopamine between 10(-12) and 10(-10) M stimulate PRL release, whereas concentrations between 10(-9) and 10(-7) M inhibit. By studying the kinetics of PRL release it was found that (1) the rate of PRL release increases only after a post-stimulus delay of 20 min, and (2) the response is phasic, stimulatory intervals being separated by intervals in which the rate of PRL release falls below the level of basal release. Inhibition of PRL release by dopamine occurs within 10 min after adding dopamine and is maintained tonically for a least 2 h. Cells treated with non-specific secretogogue (57 mM K+) during the inhibitory phases of dopamine-regulated release, are stimulated to secrete significant amounts of hormone.

Effect of LHRH on incorporation of [32P]-orthophosphate into phosphatidylinositol by dispersed anterior pituitary cells

Stimulated release of luteinizing hormone (LH) from the anterior pituitary in response to luteinizing hormone releasing hormone (LHRH), is apparently controlled by Ca2+-mediated events. In many tissues, when an involvement of Ca2+ in secretion has been found, there is an associated increased metabolic turnover of phosphatidylinositol. The objective of this investigation was to determine the effect of LHRH on the incorporation of [32P]orthophosphate into phosphatidylinositol by anterior pituitary cells maintained in vitro. When anterior pituitary cells were incubated for 5-40 min in the presence of [32P]orthophosphate, phosphatidylinositol and phosphatidylcholine were the most rapidly radiolabelled phospholipids. The addition of LHRH to the cell culture medium at concentrations previously demonstrated to release LH, increased the incorporation of 32P]orthophosphate into phosphatidylinositol in a dose-dependent manner. Incorporation of [32P]orthophosphate into other phospholipids was unaffected by LHRH at all concentrations employed. These results are consistent with the hypothesis that LHRH causes a receptor-mediated increase in turnover of phosphatidylinositol and this may be among the early metabolic events in the mechanism of LHRH-stimulated LH secretion from the anterior pituitary gland.

Use of protein A-coated colloidal gold particles for immunoelectronmicroscopic localization of ACTH on ultrathin sections

ACTH was localized in dissociated porcine adenohypophysial cells using a novel indirect EM immunocytochemical technique. Incubation of ultrathin resin sections in anti-ACTH was followed by incubation with protein A-coated colloidal gold particles. Protein A binds specifically to the Fc part of the IgG molecule, and thus the ACTH-containing secretory granules became labelled with electron-dense gold particles. With this method, the dissociated porcine ACTH cells was identified as containing numerous round or ovoid 170--300 nm secretory granules.

Calcium as a second messenger in the stimulation of luteinizing hormone secretion

In cells dissociated from porcine anterior pituitary glands and maintained in culture for 48 h the specific secretagogue luteinizing hormone-releasing hormone (LH-RH) induces a biphasic pattern of luteinizing hormone (LH) release. A biphasic pattern of release is also induced by 57 X 10(-3) M K+ and the ionophore A-23187. By reducing the availability of Ca2+, either by omission from the medium, chelation or interfering with Ca2+ transport across the plasma membrane, it is shown that LH release stimulated by LH-RH is much less dependent upon the availability of extracellular Ca2+ than that stimulated by either high K+ or A-23187. Nevertheless, by using a lanthanum displacement protocol to follow the influx of 45Ca2+ it is shown that LH-RH stimulation does induce an influx of extracellular Ca2+. Parallel experiments in which the stimulated 45Ca2+ efflux from preloaded cells is followed confirm the influx data but suggest, in addition, that when the influx of extracellular Ca2+ is inhibited, the peptide is able to mobilize Ca2+ from an intracellular location. It is thus concluded that while LH release can be initiated by an increase in the intracellular level of Ca2+, and although LH-RH stimulation does increase the permeability of the plasma membrane to Ca2+, the stimulation of LH release by LH-RH is not dependent upon extracellular Ca2+.