Short term kinetics of luteinizing hormone secretion studied in dissociated pituitary cells attached to manipulable substrates

Use of high concentrations of dimethyl sulfoxide for cryopreservation of HepG2 cells adhered to glass and polydimethylsiloxane matrices

Animal cells are generally cryopreserved in cryovials in a cell suspension state containing 5%-10% v/v dimethyl sulfoxide (DMSO) used as a cryoprotective agent. However, cryopreservation of cells in an attached state has not been intensively studied, and the effective freezing solution remains unknown. Here we determined the suitable DMSO concentration for the cryopreservation of human hepatoma HepG2 cells attached to glass and polydimethylsiloxane (PDMS) matrices coated with poly-l-lysine. With the use of the glass matrix, the rate of cell adhesion increased with the DMSO concentration up to 30% v/v in the freezing solution. In contrast, the cell-adhesion rate remained constant in the case of the PDMS matrix irrespective of the DMSO concentration between 10% v/v and 30% v/v. The viability of post-thawed cells attached to glass or PDMS matrix was also investigated. The viability was highest at the DMSO concentration of 20% v/v in the freezing solution. The DMSO concentration of 30% v/v, however, had a cytotoxic effect on the cell viability. Thus, the 20% v/v DMSO concentration was found to be most suitable for the cryopreservation of HepG2 cells in the attached state. This dose is high compared to the DMSO concentration used for the cryopreservation of cells in the suspended state.

Secretory activity of gonadotropin and the responsiveness of gonadotrophs to gonadotropin-releasing hormone during the annual reproductive cycle of male bats, Rhinolophus ferrumequinum: analysis by cell immunoblot assay

The purpose of this study was to examine secretory activity of gonadotropin (Gn) and the responsiveness of Gn secretion to Gn-releasing hormone (GnRH) in male horseshoe bats, Rhinolophus ferrumequinum, during the annual reproductive cycle. Anterior pituitary cells were monodispersed and subjected to cell immunoblot assay for Gn. Cell blots specific for follicle stimulating hormone (FSH) or luteinizing hormone (LH) were quantified using a microscopic image analyzer. The percentages of LH- or FSH-secreting cells detected as immunoreactive cell blots were markedly increased in the spermatogenic period (summer) and decreased in the hibernation period (winter). The mean Gn secretion from individual cells and total Gn secretion per unit area of the transfer membrane also showed similar changes. The responsiveness of Gn secretion to GnRH was greater in the spermatogenic period than in other seasons. On the other hand, although the secretory activity of Gn was markedly decreased during hibernation, a stimulatory effect of GnRH on Gn secretion was observed. These findings suggest that seasonal changes in the release of Gn required for gametogenesis and gonadal steroidogenesis varied depending on the reproductive activity and seasonal changes in Gn sensitivity to stimulatory effects of GnRH due to alterations in GnRH receptor numbers and/or in postreceptor events of gonadotrophs.

Effect of age of donor on the responsiveness of dispersed and cultured chicken anterior pituitary cells to GnRH-I

1. The aim of this study was to devise a method to prepare and culture anterior pituitary cells from juvenile and adult chickens in order to investigate mechanisms controlling gonadotrophin-releasing hormone-I (GnRH-I)-induced luteinising hormone (LH) release in vitro. 2. The optimum culture medium for maintaining gonadotroph responsiveness to GnRH-I was bicarbonate-buffered and phenol red-free Medium 199 supplemented with 10% foetal calf serum. 3. Cultured pituitary cells from juvenile chickens were more responsive to GnRH-I than cells from adult cockerels, while no LH was released in response to GnRH-I from pituitary cells from laying hens. 4. Cultured pituitary cells from adult chickens of both sexes released LH in response to 12-O-tetradecanoyl-13-phorbol acetate (TPA), an activator of an enzyme involved in intracellular signalling, protein kinase C. 5. It is concluded that freshly-dispersed and cultured gonadotrophs from adult chickens do not regain their responsiveness to GnRH-I as well as freshly-dispersed and cultured gonadotrophs from juvenile chickens. It appears that the stimulus-secretion coupling pathway between the GnRH-receptor and the activation of protein kinase C in gonadotrophs from adult chickens is more easily disrupted by dispersion and culture than in gonadotrophs from juvenile chickens.

Dependency of phorbol ester-induced gonadotropin secretion on estradiol

Phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC) was used to investigate the estradiol dependency of PKC-stimulated luteinizing hormone (LH) secretion from perifused anterior pituitaries. Infusions of PMA stimulated LH secretion from diestrous II, ovariectomized + estradiol-treated, and orchidectomized + estradiol-treated quartered pituitaries, by protein synthesis-dependent mechanisms. In contrast, pituitaries from intact, orchidectomized males, or ovariectomized females were unresponsive to PMA. Interestingly, dispersed male pituitary cells differed from male pituitary tissue blocks, in that the dispersed cells responded to PMA with increased LH secretion. These results indicate that PKC's ability to directly stimulate LH secretion is dependent on de novo protein synthesis and estradiol. Moreover, the effects of estradiol on PKC-stimulated secretion form at least one basis for the estradiol-induced increased responsiveness of gonadotrophs to GnRH. Additionally, it appears that dispersed pituitary cells may not respond to activators of PKC in a physiological manner.

Ba2+ stimulation of luteinizing-hormone release demonstrates two mechanisms of Ca2+ entry in gonadotrope cells

Kinetic studies on gonadotropin-releasing-hormone (gonadoliberin, GnRH)-stimulated luteinizing-hormone (lutropin, LH) release in the cultured rat gonadotrope demonstrated a biphasic pattern of LH release. The first rapid phase of release was unaffected by the voltage-gated Ca2+-channel blockers methoxyverapamil (D600) and nifedipine [a dihydropyridine (DHP)], whereas the later second phase was partially inhibited by both drugs. These results suggested that the initial phase of LH release is independent of Ca2+ entry through dihydropyridine (DHP)-sensitive Ca2+ channels and might depend on entry of extracellular Ca2+ by another mechanism. These mechanisms were further studied by utilizing Ba2+ as a Ca2+ substitute. Ba2+, which freely permeates DHP-sensitive Ca2+ channels in the absence of GnRH, induced LH release which was sensitive to blockade by D600 and nifedipine. However, in the presence of the channel blockers, Ba2+-induced LH release could be elicited when GnRH was added to the system. This indicates that GnRH stimulates LH release by initially activating a DHP-insensitive Ca2+-entry mechanism and then a DHP-sensitive mechanism. The DHP-sensitive mechanism freely allows Ba2+ entry in the absence of GnRH-receptor occupancy, whereas the DHP-insensitive mechanism requires GnRH-receptor activation for Ba2+ entry.

Superfused pituitary cell cultures: effects of culture conditions on apparent responsiveness to LHRH stimulation administered as short duration pulses

We wished to study estrous cycle related differences in LH and FSH responsiveness to pulsatile LHRH. Such studies are very difficult to perform in vivo under controlled conditions; therefore, an in vitro superfused anterior pituitary cell culture system was evaluated for its capacity to support differences in estrous stage associated LHRH responsiveness. Three vital culture system parameters were evaluated; these parameters were (1) culture medium composition, (2) duration allowed for cell attachment to microcarrier beads and (3) superfusion flow rate utilized during pulsatile LHRH stimulation. It was found that a culture system which utilized 10% Nu Serum in DMEM (final protein concentration of 1.8 mg/ml; final serum concentration of 2.5%), an attachment time of 48 hrs and a flow rate of 0.125 ml/min most successfully maximized LH responsiveness at the lowest serum concentration. These studies indicated that although one may be able to observe LHRH responsiveness under a wide range of culture conditions, responsiveness may nonetheless be maximized by judicious adjustment of culture conditions.

The initial phase of GnRH-stimulated LH release from pituitary cells is independent of calcium entry through voltage-gated channels

Kinetic studies on gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone (LH) release were undertaken using rat and chicken pituitary cell cultures. In response to continuous GnRH stimulation, a biphasic pattern of LH release was demonstrated. The two phases showed different susceptibility to the voltage-gated Ca2+ channel blockers D600 and nifedipine. The first (transient) phase of LH release was unaffected by the Ca2+ channel blockers whereas the second (sustained) phase was inhibited by both drugs. These results indicate that the initial phase of LH release is independent of Ca2+ entry through voltage-gated Ca2+ channels and may depend on mobilisation of intracellular Ca2+ or entry of extracellular Ca2+ through another mechanism.

Characterization of luteinizing hormone-releasing hormone receptor binding in rat pituitary cell monolayer cultures; influence of intercellular communication

Receptors for the luteinizing hormone-releasing hormone (LHRH) were characterized in rat pituitary cells cultured for 3 days as monolayers on coverslips using 125I-[D-Ala6-Pro9-LHRH-NEt] as the labeled ligand. The monolayers were left intact during the binding assay. Specific binding displayed the various characteristics of binding to the physiological LHRH receptor. Various kinetic data corresponded to those reported previously. However, in these cultured cells, in which binding was tested in a physiological medium, the dose response of competition for binding by LHRH agonists ranged over a smaller concentration range (less than 2 orders of magnitude) than that by LHRH antagonists. In a cation-free buffer competition curves of agonists and antagonists were parallel but the apparent dissociation constant was lower than in the physiological medium. In cultures of pituitary cell populations separated by unit gravity sedimentation, the specific binding increased with the proportional number of gonadotrophs in the various populations. However, when the gonadotroph-richest population (approximately equal to 70% gonadotrophs) was cultured after recombination with gonadotroph-poor populations, binding capacity significantly increased. Microscopic examinations suggested that this phenomenon was the consequence of disrupting cellular contacts among gonadotrophs. It is concluded that certain characteristics of LHRH receptors tested on cells in a tissue-like organization and in a physiological environment are different from those reported previously in disrupted cells or monodispersed cell suspensions and that intercellular communication is an important factor controlling LHRH receptors.

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.

Luteinizing hormone releasing hormone of fixed pulse frequency and duration. A simplified system for studying the effect of varying pulse concentration on LH release from cytodex I attached anterior pituitary cells

The literature indicates agreement concerning basic differences in the behavior of the pituitary toward pulsatile and continuous luteinizing hormone releasing hormone (LHRH); however, conflicting results seem to exist concerning pituitary behavior toward pulsatile LHRH (Hopkins, 1977; Smith and Vale, 1981). Most superfusion studies have utilized pulses of 15-30 minutes during which the cells were exposed to pharmacological quantities of LHRH. Differences in results may have arisen because of the varying methodologies utilized to administer pulse frequency, pulse duration, and pulse concentration; therefore, the present studies utilized standardized methodology in which the LHRH pulse frequency and pulse duration were maintained constant while the pulse concentration was varied. Pulsatile LHRH of fixed concentration was associated with a relatively rapid loss of responsiveness, while small increases in each subsequent pulse served to prolong the period of responsiveness. The results indicated that seemingly small changes in the methological pattern of LHRH stimulation are capable of exerting an influence on the response to subsequent LHRH stimulation. Caution should therefore be exerted in comparing the results from different experiments utilizing different methodological designs for applying LHRH stimulation. In practical terms, these studies indicate that results must be interpreted carefully from experiments in which a fixed pool of pituitary cells has been repeatedly stimulated by LHRH. This is especially true with dose-response curves generated by this method and with experiments designed to study LHRH self-priming and desensitization.

Prolactin secretion by cultured anterior pituitary cells: influence of culture conditions and endocrine status of the pituitary donor

The characteristics of prolactin (PRL) secretion by cultured anterior pituitary cells in the presence and absence of catecholamines were studied. PRL secretion was markedly influenced by culture conditions such as cell density, culture duration and length of short-term incubation. Dopamine (DA) inhibited PRL release in a dose-dependent manner within a physiological range, and this inhibition was reversed by the stereospecific DA receptor antagonist (+)-butaclamol. In contrast, the inhibition of PRL secretion by norepinephrine (NE) required much higher doses and lacked specificity. Cells obtained from male donors had the lowest basal PRL secretion and were the least responsive to DA inhibition, whereas those obtained from females in late pregnancy had the highest basal PRL secretion and were the most sensitive to DA.

[Calcium, modulator of the expression of gonadoliberin at the intracellular level]

GnRH has been entrapped in liposomes. Chromatographic studies and enzymatic peptidase treatments, show the efficiency of the encapsulation. A purification method on G75 Sephadex of the entrapped GnRH is described. This method prevents any dilution of the liposome fraction. A free GnRH contamination, lower than 0.4 per cent, has been observed. Superfused hypophyses respond to the message of the internalized GnRH only when calcium is present in the extracellular medium. The intensity of the answer depends on the duration of the entrapped GnRH infusion. The decrease observed in the response intensity after a long stay of the GnRH in the cytoplasm allows us to say that GnRH controls its own expression: The binding of GnRH to the membrane receptor during the early phase induces a calcium uptake necessary to the expression of the internalized GnRH, this being the late phase in LH release. A too low calcium concentration does not allow GnRH expression. As a consequence, GnRH is enzymatically degradated by the cytoplasmic peptidases. The LH release during the late phase is the result of a combined action of calcium and cytoplasmic peptidases. To support this idea we show: 1- that an extracellular calcium concentration around 0.5 or 0.6 mM is the best condition for the expression of the internalized GnRH. 2- that a GnRH agonist (D-Ala6-GnRH) known to be peptidase resistent induces a higher LH release in our experimental conditions.

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.

Quantitative immunocytochemical studies on the gonadotrophs isolated from the pituitary of the male rat

Quantitative studies on the population of the gonadotropic cells in the pituitary of adult male rats were performed after trypsic dissociation of the pituitary glands and immunoenzymatic staining with anti-beta-LH or anti-beta-FSH antisera. Number, area and extinction of labelled cells were measured by use of an image analyser and a cytophotometer. The gonadotrophs represent approximately 14% of the pituitary cells. The mean area of gonadotrophs is significantly larger after staining with anti-beta-LH serum than after staining with anti-beta-FSH serum. Planimetric measurement of the gonadotrophs reveals their variability in size ranging between 30 and 160 microns2. Moreover, the size distribution depends on the staining serum used: more numerous small-sized cells (less than 75 microns2) are stained with anti-beta-FSH serum than with anti-beta-LH serum, which conversely stains more numerous large-sized cells. Cytophotometric measurements indicate that immunostaining varies greatly among cells of the same size class and that the staining intensity appears to increase according to the cell size. These results emphasize the morphofunctional heterogeneity of the gonadotropic cell population.

Effect of neonatal androgenization on the LHRH response of dispersed pituitary cell cultures

To determine if neonatal androgenization affects pituitary luteinizing hormone releasing hormone (LHRH) response, female rats were androgenized 24-48 hr following birth. Intact male and female litter mates were maintained for comparison. At 83 days of age, randomly selected males and androgenized females were gonadectomized. At 90 days of age, pituitary cell cultures were begun. On day 3 the cells were challenged with LHRH. Gonadotropin response was determined by radioimmunoassay (RIA). Basal gonadotropin release from androgenized female cells was similar to male cells, while the pattern and magnitude of the LHRH response was similar to intact female cells. After castration, the pattern and magnitude of the LHRH response of the androgenized female pituitary was most like the male. Androgenized female cells exhibited mixed male and female LHRH response.

Dopaminergic stimulation of prolactin release

Prolactin (PRL) secretion from anterior pituitary is believed to be under tonic inhibitory control of dopamine (DA) released from the tubero-infundibular dopaminergic neurones into the hypophysial portal blood. Inhibition of PRL release by DA seems to be mediated by sereospecific DA receptors located in PRL cells. Apomorphine and various ergot alkaloids such as bromocryptine mimic the inhibitory effect of DA both in vivo and in vitro, presumably by a direct agonist action on these 'inhibitory' receptors. We now report that PRL secretion in primary cultures of rat pituitary cells can be stimulated by DA when concentrations a thousand times lower than those required for inhibition are used. Secretion rates above basal release can also be induced by apomorphine and bromocryptine when the 'inhibitory' receptors are blocked with certain DA receptor antagonists.

Gonadotropin-releasing hormone receptor binding in bovine anterior pituitary

Synthetic gonadotropin-releasing hormone (GnRH) was monoiodinated at a high specific radioactivity with 125I. The iodinated hormone retained full biological activity as assessed by the release of luteinizing hormone in vitro from bovine anterior pituitary tissue slices. Specific binding of 125I-labeled gonadotropin-releasing hormone of high affinity and low capacity was obtained using dispersed bovine anterior pituitary cells. The binding had sigmoid characteristics, compatible with the presence of more than one binding site. The subcellular fraction responsible for binding was identified with the plasma membranes. However, significant binding also occurred in the secretory granules fraction. The plasma membranes were solubilized with sodium dodecyl sulfate. Using gonadotropin-releasing hormone covalently coupled to a solid phase, a protein was purified by an affinity technique from the solubilized plasma membrane preparation which possessed similar binding propperties as plasma membranes, both intact and solubilized. The protein migrated as a single component on polyacrylamide gel in sodium dodecyl sulfate and the estimated molecular weight was 60 000. The character of the gonadotropin-releasing hormone concentration dependence binding as well as association kinetics were multiphasic and suggested the presence of more than one binding site. When analyzed by the Hill plot, the Hill coefficient of all binding curves was always greater than one which is compatible with positive cooperativity. This was further supported by the dissociation studies where the dissociation rate was inversely proportionate to both the gonadotropin-releasing hormone concentration and the time interval during which the gonadotropin-releasing hormone-gonadotropin-releasing hormone receptor protein complex was formed. Using difference chromatography, aggregation of the purified gonadotropin-releasing hormone receptor protein was demonstrated to occur upon its exposure to gonadotropin-releasing hormone. The formed macromolecular complexes bound preferentially 125I-labeled gonadotropin-releasing hormone. It is concluded that a single receptor protein is responsible for gonadotropin-releasing hormone binding in the bovine anterior pituitary. It is a part of the plasma membranes. Its interaction with gonadotropin-releasing hormone provokes transitions of the protein into different allosteric forms and this may be related to the biological effect of gonadotropin-releasing hormone on gonadotropin secretion.

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+.

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

A method has been developed for disaggregating porcine anterior pituitary tissue and for providing dissociated preparations which have good, stable viability in culture. The secretory capacity for these preparations in terms of their ability to release luteinizing hormone in response to luteinizing hormone-releasing hormone has been documented in detail. In following the short-term kinetics of LH secretion a biphasic pattern of release has been demonstrated in which a sharp initial peak, maximal at 2 min, is followed by a second, more prolonged phase of release reaching a maximum between 10 and 20 min.

Discrimination of LH, FSH, TSH and ACTH in dissociated porcine anterior pituitary cells by light and electron microscope immunocytochemistry

Using the PAP unlabelled antibody method, LH, FSH, TSH and ACTH were localized at light microscope level in cultured cells dissociated from the porcine adenohypophysis. Antisera were shown to be specific for the beta-subunits of the porcine glycoprotein hormones by radioimmunoassay and absorption studies. Using these antisera, it was found that LH and FSH were contained within the same cell, with TSH in a separate cell. When absorbed with LH, anti-porcine ACTH stained a separate distinct population of ACTH cells. Adjacent ultra-thin sections stained with anti-pLH beta and anti-pFSH beta, and examined at electron microscope level, showed that the ovoid, 150--400nm secretory granules of the LH/FSH gonadotrophs contained both LH and FSH.

Separation and maturation of gonadotrophs from 2A8 clonal cells in vitro

2A8 clonal cells derived from the epithlium of Rathke's pouch of fetal rats were cultured in growth medium supplemented with fresh rat serum, median-eminence extract and 1-thyroxine. Then, in order to isolate gonadotrophs, cyanogen bromide-activated Sepharose whichwas conjugated with LHRH (LHRH-Sepharose) was added to the culture medium. Fourteen days after incubation of 2A8 cells with LHRH-Sepharose, agranular and granular cells were rapidly bound to LHRH-Sepharose when fresh serum had previously been added in the medium. The cytoplasm of granular cells that were bound to LHRH-Sepharose contained spherical secretory granules (200-250 nm in diameter). These cells were similar in morphology to the FSH and LH gonadotrophs described by Kurosumi (1968). Moreover, many of them appeared as hypertrophied cells that resembled "castration" cells. These results demonstrate that LHRH conjugated to Sepharose can be used to separate gonadotrophs from other 2A8 cells, and it is suggested that the hypertrophy of some cells might be due to persistent stimulation by LHRH which is likely bound to receptors on the cell membrane.

Cyclic and temporal differences in LH-RH-stimulated LH release in cultured rat pituitary cells

To establish whether the enhanced LH-RH responsiveness shown by pituitary gonadotrophs at proestrus in vivo could be maintained in vitro, rat anterior pituitary cells were investigated to determine differences in LH release in response to LH-RH through the estrous cycle and with time in primary culture. Pooled or individual anterior pituitary glands from each day of the cycle were dissociated with collagenase, hyaluronidase and Viokase and cultured for from 1 to 4 days. Four-day cultures of proestrous cells did not show differences in LH-RH responsiveness when compared to estrous, diestrous I and diestrous II cells. In addition, proestrous cells did not show differences in LH-RH responsiveness when compared to diestrous II cells after 1, 2, 3 or 4 days of culture; however, over the same 1--4 days of culture, proestrous cells contained higher amounts of LH and released greater quantities of LH into the growth medium than did diestrous II cells. It was also observed that both proestrous and diestrous II cells exhibited significantly greater LH-RH responsiveness after 3 or 4 days of culture than after 1--2 days of culture. These results suggest that the differential LH-RH responsiveness shown by pituitary gonadotrophs at proestrus in vivo is not maintained when pituitary cells are placed in primary culture.

Topographical localization of the receptors for luteinizing hormone-releasing hormone on the surface of dissociated pituitary cells

A derivative of the hypothalamic peptide luteinizing hormone-releasing hormone (LHRH) has been coupled to ferritin and the conjugate purified by gel chromatography. In its ability to stimulate the secretion of luteinizing hormone from pituitary cells in vitro, the conjugate has the same potency and specificity as the native peptide. When dissociated pituitary cells maintained in short-term culture are lightly fixed with formaldehyde and then incubated with the conjugate, examination in the electron microscope shows an even distribution of ferritin particles over the free cell surface of the gonadotrophin cells. This binding appears to be specific for the LHRH receptor since it is prevented by a 10-fold excess of native peptide. In addition to the gonadotrophin cells, some somatotrophin and thyrotrophin cells bind conjugate on their free surfaces under similar conditions. If living cells are incubated with the conjugate for 15 min, the bound conjugate becomes aggregated and then concentrated in one localized area of the cell surface. In this area, which lies immediately above the juxtanuclear Golgi complex, the plasma membrane is frequently invaginated in a manner which suggests that the bound, aggregated conjugate is internalized by endocytosis.