Release of LH and FSH by anterior pituitary cell suspensions from female rats during the estrous cycle and from estrogen-treated ovariectomized rats

A mathematical model for the actions of activin, inhibin, and follistatin on pituitary gonadotrophs

The timed secretion of the luteinizing hormone (LH) and follicle stimulating hormone (FSH) from pituitary gonadotrophs during the estrous cycle is crucial for normal reproductive functioning. The release of LH and FSH is stimulated by gonadotropin releasing hormone (GnRH) secreted by hypothalamic GnRH neurons. It is controlled by the frequency of the GnRH signal that varies during the estrous cycle. Curiously, the secretion of LH and FSH is differentially regulated by the frequency of GnRH pulses. LH secretion increases as the frequency increases within a physiological range, and FSH secretion shows a biphasic response, with a peak at a lower frequency. There is considerable experimental evidence that one key factor in these differential responses is the autocrine/paracrine actions of the pituitary polypeptides activin and follistatin. Based on these data, we develop a mathematical model that incorporates the dynamics of these polypeptides. We show that a model that incorporates the actions of activin and follistatin is sufficient to generate the differential responses of LH and FSH secretion to changes in the frequency of GnRH pulses. In addition, it shows that the actions of these polypeptides, along with the ovarian polypeptide inhibin and the estrogen-mediated variations in the frequency of GnRH pulses, are sufficient to account for the time courses of LH and FSH plasma levels during the rat estrous cycle. That is, a single peak of LH on the afternoon of proestrus and a double peak of FSH on proestrus and early estrus. We also use the model to identify which regulation pathways are indispensable for the differential regulation of LH and FSH and their time courses during the estrous cycle. We conclude that the actions of activin, inhibin, and follistatin are consistent with LH/FSH secretion patterns, and likely complement other factors in the production of the characteristic secretion patterns in female rats.

Functional heterogeneity of gonadotrophs in the ovine fetus: analysis by reverse hemolytic plaque assay

Stimulation of sheep fetal gonadotrophs with 10(-7) M luteinizing hormone releasing hormone (LHRH) for 3 h in culture wells increased luteinizing hormone (LH) release over basal values. Using the reverse hemolytic plaque assay (RHPA), we demonstrated that this increase was due to a recruitment of LH-secreting cells. During gestation, the percentage of LH-containing cells able to release and the mean size of plaques were the highest at around 100-130 days and were usually lower in females than in males. In an attempt to delineate the involvement of protein kinase C (PKC) in LH release, cells were treated with an activator of PKC, phorbol 12-myristate 13-acetate (PMA). Stimulation of cells with 10(-6) M PMA for 3 h enhanced LH release in culture wells 2- to 3-fold more than did 10(-7) M LHRH. This increase was a consequence of an enhanced number of LH-secreting cells and, in males only, of an enhancement of the mean plaque size. The percentage of LH-secreting cells among LH-containing cells and the plaque areas were maximal between 110 and 120 days of gestation in both sexes. They were usually lower in females than in males. Stimulation of cells with LHRH plus PMA enhanced LH release in culture wells in an additive manner when compared to either factor alone in both sexes and at all fetal ages. This additive effect reflected an increase in the number of secreting cells. Under these conditions, plaque sizes were larger than the plaque sizes obtained with PMA alone in males and in females in late gestation. In conclusion, our results show that LHRH stimulated LH secretion from sheep fetal cells by recruiting secreting cells when compared to controls. Both 100-120 days of gestation and were higher in males than in females. Results following treatment of cells with PMA, either alone or in combination with LHRH, suggest that these two secretagogues act on two different subpopulations of gonadotrophs and probably through different pathways.

Actions of acetyl-L-carnitine on the hypothalamo-pituitary-gonadal system in female rats

Acetyl-L-carnitine (ALC) is known to affect several aspects of neuronal activity. To evaluate the neuroendocrine actions of this compound, several endocrinological parameters were followed in ALC-treated and control animals during recovery from dark-induced anestrus. In treated animals, serum luteinizing hormone (LH) and prolactin levels were higher than those of controls during the proestrous and estrous phases of the cycle, and serum estradiol levels were higher during estrus. No significant changes were observed in serum levels of follicle-stimulating hormone and progesterone. Uterine weight was increased in ALC-treated rats during proestrus and estrus, but not in diestrus. The basal release of gonadotropin-releasing hormone (GnRH) from perifused hypothalamic slices of ALC-treated animals was elevated at proestrus and diestrus, and GnRH release elicited by high K+ was higher during all three phases of the cycle. The basal release of LH from perifused pituitaries of treated animals was elevated in diestrus, and the LH response to GnRH was higher in estrus and diestrus I. Depolarization with K+ caused increased LH secretion during proestrus and estrus in treated animals. In contrast to these effects of ALC treatment in vivo, no direct effects of ALC were observed during short- or long-term treatment of cultured pituitary cells. These results indicate that ALC treatment influences hypothalamo-pituitary function in a cycle stage-dependent manner, and increases the secretory activity of gonadotrophs and lactotrophs. Since no effects of ALC on basal and agonist-induced secretory responses of gonadotrophs were observed in vitro, it is probable that its effects on gonadotropin release are related to enhancement of GnRH neuronal function in the hypothalamus.

The use of mouse pituitary fragments to study LH secretion

This work evaluated a perifusion system for studying LH secretion from the anterior pituitary (AP) of female mice. Pituitary fragments were challenged with LHRH, and the effluents assayed for LH. In general, the tissue exhibited augmented release to repeated stimulation. In the dose-response study, the amount of LHRH required to produce maximum and half maximum responses dropped almost 10 fold by the 3rd stimulus. In response to various pulse frequencies LH release increased with the frequency of the 100 nM LHRH dose, but the tissue became refractive to constant nonpulsatile stimulation. Other preparations, subjected to high-frequency 10 nM LHRH pulses, released LH in two distinct episodes. All but the first hour of the response was blocked by cycloheximide, confirming the role of protein synthesis in the sustained release of LH. By varying both the pulse frequency and amplitude, a LHRH protocol was found that produced a proestrous-like surge. Lastly, rat and mouse tissues responded similarly to pulsatile LHRH, verifying their similar LH function during the preovulatory period. These studies demonstrate that the perifusion technique can be used for studying LH secretion in the mouse. Its application to other mouse-oriented studies is planned.

Superfused pituitary cell cultures: comparative responsiveness of cells derived from various stages of the estrous cycle to LHRH stimulation administered as short duration pulses

We have reinvestigated the question of maintenance of differential LHRH sensitivity in culture and further investigated the role of pulsatile LHRH in the in vitro release of pulsatile LH and FSH at different stages of the estrous cycle. Pituitaries were collected on each day of the 4 day cycle at 0800. In addition, pituitaries were also collected at 1500 and 1900 on proestrous. The cells were dispersed and exposed 48 hrs later to short duration 4 ng LHRH pulses; this dose was optimized for LH release and was applied at a frequency of 1 pulse/60 min. In terms of absolute magnitude of LH response, observed responsiveness was ranked in the following order: proestrous 1900 greater than estrous 0800 greater than diestrous 1 0800 greater than proestrous 1500 greater than diestrous 2 0800. Responsiveness was significantly greater at proestrous 1900 (p greater than 0.01), estrous 0800 (p greater than 0.05) and diestrous 1 0800 (p greater than 0.05) when compared to either of the other stages tested. The heightened LHRH sensitivity of proestrous was therefore maintained in cell culture indicating that the system should be valid for conducting studies on the control of gonadotropin secretion during this period. FSH did not respond in pulsatile manner to the LHRH levels employed further substantiating recent evidence that LHRH seems to function somehow less directly in FSH as compared to LH secretion.

Morphological characterization of LH secretory granule response to LHRH and calmodulin inhibitor

A quantitative electron microscopy study was carried out to characterize the luteinizing hormone (LH) contained secretory granules in response to exogenous luteinizing hormone releasing hormone (LHRH) alone or in combination with calmodulin inhibitor (W13) in orchidectomized rats pretreated with estrogen. The plasma LH concentration rose quickly 30 min after a single large LHRH injection, and then gradually increased further until 150 min. However, the rise was attenuated by supplemental administration of W13 30 min after LHRH exposure. The mean diameter of secretory granules quickly decreased inversely to the increase of plasma LH concentration after LHRH injection. However, the mean diameter increased significantly in accordance with the complete attenuation of the increase in LH release caused by administration of W13. There was an inverse correlation between the diameter of secretory granules and plasma LH concentration. Small secretory granules with low electron density frequently appeared in castration cells after LHRH injection. It is concluded that reduced size and density of secretory granules is the main morphological standard responsible for extraordinary LH secretion from gonadotrophs.

Pituitary LH responsiveness to GnRH in vitro as related to GnRH receptor number

The objective of this in vitro study was to determine whether the increase in the augmented phase of the biphasic luteinizing hormone (LH) response to gonadotrophin-releasing hormone (GnRH) and its enhancement by estradiol (E2) were associated with GnRH-stimulated increases in pituitary GnRH receptor concentration. Pituitary glands from 72 h ovariectomized (OVX), OVX + E2, or proestrous rats were perifused continuously with GnRH (12 ng/h). LH release was measured at 10-min intervals, and pituitary GnRH-binding capacity (GnRH-BC) was assessed at 0, 40, 80, 120, and 240 min after addition of GnRH. All treatment groups exhibited a biphasic pattern of LH release; initial (20-70 min) and augmented (120-240 min) mean rates of LH secretion (micrograms/h) were 1.78 and 3.92 (OVX), 6.40 and 16.67 (OVX + E2), and 2.79 and 18.64 (proestrus), respectively. Total LH release was significantly greater in the OVX + E2 and proestrous groups (44.0 and 45.8 micrograms) vs. the OVX group (12.4 micrograms). Throughout the GnRH infusion period, GnRH-BC did not change significantly in any of the treatment groups with the exception of the OVX group in which there was a transient small decrease at 80 min post-GnRH infusion. There were no significant differences between treatment groups in GnRH-BC at any time after infusion of GnRH. These results demonstrate that the acute and augmented phases of GnRH-stimulated LH release and the enhancement of this biphasic response by E2 occurs independent of any increase in GnRH-BC and suggest that these events are mediated by postreceptor mechanisms.