Influence of cell cycle phases on the electrical activity and hormone release in a transformed line of anterior pituitary cells

Electrophysiological experiments have shown that about 50% of cultured GH3 cells (tumoral cell line, from the anterior pituitary gland) are inexcitable i.e. they do not display action potentials either spontaneously or when depolarized by a current pulse. We report here this inexcitability may be related to cellular kinetics. Thus we have studied the relationship between the various phases of the cell cycle, the electrophysiological properties of GH3/B6 cells and spontaneous or induced Prolactin and Growth Hormone (GH) release rates. Asynchronous populations of viable cells were stained with Hoechst 33 342 DNA fluorescent dye, and sorted using a flow cytometer into G1 and S phases. After selection intracellular potentials were recorded using a single glass micro-electrode; the basal or TRH stimulated rates of PRL and GH secretions were determined by RIA. Electrical properties of the cells i.e. resting potentials, input membrane resistance and excitability, reached a maximum for cells in G2+M phases. Only cells in G2+M displayed action potentials and TRH increased their secretion by 5 times for GH and by 6 times for PRL. In G1 and S phases the cells were electrically inactive and secretion rates remained at their basal levels. These findings demonstrate that the mechanism of stimulus secretion coupling is dependent upon the phases of the cell cycle.

Vasopressin receptors influencing the secretion of ACTH by the rat adenohypophysis

The effects of selective agonists and antagonists of type 1 (V1) and type 2 (V2) vasopressin receptors on the secretion of ACTH in vitro by segments of adenohypophysial tissue and in vivo in rats pretreated with pentobarbitone and chlorpromazine were studied in the presence and absence of the 41 amino acid-containing peptide, corticotrophin-releasing factor-41 (CRF-41). The non-selective vasopressin receptor agonist, arginine vasopressin (AVP) and the V1-receptor agonist, felypressin caused dose-related increases in ACTH release in vivo and in vitro but the V2-receptor agonist, desmopressin was only weakly active in this respect. Their actions in vitro were antagonized competitively by the V1-receptor antagonist, d(C2H5)2-AVP, but were unaffected by the V2-receptor antagonist, d(CH2)5-D-Iso2-Thr4-AVP. Arginine vasopressin, felypressin and desmopressins in concentrations considerably lower than those necessary to elicit directly the release of ACTH, potentiated, in a dose-related manner, the activity of CRF-41 in vitro. The potentiating effects were not antagonized by the V2-receptor antagonist or by low concentrations of the V1-receptor antagonist. At a higher concentration, the V1-receptor antagonist reduced, but did not abolish, the potentiating effects of AVP and its analogues. However, at this concentration, it also exhibited weak intrinsic activity and, like the agonists, potentiated the response to CRF-41. The results suggest that the direct effect of AVP on ACTH release is mediated by V1-like receptors. The vasopressin receptors involved in the potentiation of CRF-41 activity appear to be different.

Receptor-mediated interrelationships between progesterone and estradiol action on the anterior pituitary-hypothalamic axis of the ovariectomized immature rat

The ovariectomized immature rat was used as a model for analysis of action of progesterone as a modulator of receptor-mediated functional responsiveness in the anterior pituitary and hypothalamus. In response to estrogen exposure, cytosolic progesterone receptors appear rapidly, rise in concentration to a peak at 12 h, then fall to a plateau level well above control, which is maintained for at least an additional 20 h. Progesterone administration at the peak 12-h interval induces maximal nuclear accumulation of its own receptor within 1-2 h, with apparent extensive processing occurring thereafter. To this point, no differences were seen between anterior pituitary and hypothalamic responses. If animals were administered progesterone (0.8 mg/kg BW) at the 12-h peak interval, subsequent nuclear accumulation of anterior pituitary estrogen receptor by an injection of estradiol was suppressed if, and only if, the interval between progesterone and estradiol injection did not exceed 2 h; at no time interval did progesterone have an effect in the hypothalamus. In both tissues, estradiol readministration at 12 h after an initial injection stimulates a second wave of progesterone receptor activity, again peaking 12 h later. A single injection of progesterone 1 h before the second estradiol administration blocks the second peak of progesterone receptor in the anterior pituitary, but not in the hypothalamus. If the interval between the progesterone and second estradiol injections is extended to 4 h, the second progesterone receptor peak appears as though no progesterone had been introduced. The results indicate a critical temporal reliance of the inhibitory effects of progesterone on estrogen receptor activity and estrogen function in a well defined animal model. The effect is progesterone receptor-mediated and is manifested in the anterior pituitary, but not in the hypothalamus, even though the kinetics of estrogen-induced progesterone receptor activity are indistinguishable between the two tissues.

Investigation of anterior pituitary function in elderly in-patients over the age of 75

Dynamic pituitary function tests were carried out on 50 randomly selected in-patients, (30 women and 20 men) over the age of 75 during early convalescence. Twenty-five subjects also had CT scans of the pituitary fossa. One male patient had hypogonadotrophic hypogonadism; another had a large prolactinoma and hypothyroidism; these were excluded from the analysis. Of the remaining 48 subjects, 14 (29 per cent) had reduced TSH responses to TRH and 28 (64 per cent) decreased GH responses to L-dopa compared with our laboratory's reference ranges. Fifteen women (51 per cent) had low basal gonadotrophin levels of which 10 (34 per cent) gave a poor LH response to LHRH and eight (27 per cent) a poor FSH response. Forty-three subjects (93 per cent) gave a good ACTH response to metyrapone. Only six subjects (24 per cent) had an apparently normal pituitary fossa; 16 (64 per cent) had partially or completely empty fossas. There was no significant correlation between the diminished pituitary responses. Only one patient had responses which suggested panhypopituitarism. There was also no significant correlation between the appearance on CT scan of the pituitary fossa and the results of the dynamic tests. It is suggested that laboratory reference range for pituitary function which have been derived from young ambulatory subjects are not appropriate for hospital in-patients over the age of 75. A series of reference ranges for such subjects is proposed.

Involvement of postnatal gonads in the maturation of dopaminergic regulation of prolactin secretion in female rats

Permanent effects of postnatal gonads on dopamine (DA)-mediated regulation of PRL secretion were examined in adult female rats of the Wistar-Imamichi strain. Experiments were uniformly done at the age of about 10 weeks. First, rats were bilaterally ovariectomized on varying postnatal days, i.e. 24 h after birth [neonatal castration (NC)] or at 1, 2, 3, 4, or 6 weeks of age. Intact diestrous females were also examined. All groups were injected ip with saline or sulpiride (1 mg/kg BW), and blood samples were collected by rapid decapitation 20 min later. Also, in saline control groups, the anterior pituitary (AP) PRL content was determined. In NC females, significant PRL release was not observed by either sulpiride or saline. However, stimulation of PRL release by sulpiride was observed consistently in rats ovariectomized on or after 1 week of age. Although there was not a complete correlation between the plasma PRL level attained after sulpiride treatment and the AP PRL content, the general trend was that the AP PRL content was increased as the postnatal age of the ovariectomy was delayed. Values in intact adult females exceeded those in all of the castrated groups in both parameters. Second, daily sc injection for 7 days with estradiol benzoate (E2; 2.5 micrograms/kg BW) and/or progesterone (P; 5 mg/kg BW) was given after NC to determine which ovarian secretion(s) is involved in the aforementioned findings. Sulpiride stimulated PRL release in NC females that had been treated neonatally (postnatal days 3-9) with either E2 or P. This effect was accompanied by an increase in the AP PRL content. An enhancement of both parameters was also observed when the same dose of E2 or P was given in the fourth week (postnatal days 22-28). However, neither parameter was further augmented by concurrent administration of E2 and P at either postnatal time. Characteristics of AP DA receptors were subsequently analyzed in some of the foregoing groups using [3H]spiperone. The number of binding sites, when expressed as femtomoles per pituitary, did not differ among all groups examined. However, a significant difference in the dissociation constant (Kd) was observed between NC females and females castrated at 6 weeks of age. The Kd in NC females was not altered by subsequent neonatal treatment with E2 and/or P. Nonetheless, the E2 effect on Kd was dependent on when E2 was given after NC.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of postnatal gonads in the maturation of dopaminergic regulation of prolactin secretion in male rats

Permanent effects of postnatal gonads on dopamine (DA)-mediated regulation of PRL secretion were examined in adult male rats of the Wistar-Imamichi strain. Rats were bilaterally orchidectomized at varying postnatal days, i.e. 8 h after birth [neonatal castration (NC)] or at 1, 2, 3, 4, or 6 weeks of age, and subjected to experiments at the age of about 10 weeks. Ten-week-old intact adults were also examined. All groups were injected ip with saline or sulpiride (1 mg/kg BW), and blood samples were collected by rapid decapitation 20 min later. Anterior pituitaries (AP) thus obtained were subjected to determination of PRL content. The circulating level of testosterone was similar among all castrated groups. Rats castrated on or after 3 weeks of age showed a significantly smaller PRL response to sulpiride than intact adults. In turn, sulpiride-induced PRL release in rats castrated on or before 2 weeks of age was similar to that in intact adults. However, the AP PRL content of saline control was uniformly lower in castrated males than in intact adults regardless of age at castration (P less than 0.01). The value in males castrated at 1 week was 2.0-fold higher than that in NC males, but a decreasing trend was observed in groups castrated at 2 or 3 weeks. That such a stimulation of AP PRL content by the neonatal testis is not mediated by an action of estradiol converted from testosterone has been suggested by the failure of daily neonatal tamoxifen (estrogen antagonist) treatment of intact males, before castration at 1 week, to decrease AP PRL content during adulthood. Sulpiride-induced reduction of the AP PRL content was significant only in males castrated at 6 weeks and intact adults. Characteristics of AP DA receptors labeled by [3H]spiperone were not different between males castrated at 6 weeks and intact adults. However, a heterogeneity was observed between NC males and a set of males castrated at 6 weeks and intact adults. The Kd was significantly (P less than 0.05) greater in NC males (mean +/- SE, 1.19 +/- 0.22 nM) than in males castrated at 6 weeks (0.52 +/- 0.08 nM), and the number of binding sites was significantly (P less than 0.05) larger in NC males (20.4 +/- 1.6 fmol/pituitary) than in both males castrated at 6 weeks and intact adults (12.6 +/- 2.0 and 13.1 +/- 1.9 fmol/pituitary, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Nimodipine block of calcium channels in rat anterior pituitary cells

1. Ca channels were studied in the GH4C1 clonal cell line derived from rat anterior pituitary cells. The whole-cell variation of the patch-electrode voltage-clamp technique was used. 2. Two types of Ca channels were found. One type ('slowly inactivating' channels) is insensitive to changes in holding potential, does not inactivate during test pulses lasting several seconds, and deactivates very quickly upon repolarization. For holding potentials less than -40 mV, a second type of Ca channel is available for opening. This population ('transient' channels) differs from the first type in that it activates at more negative potentials, inactivates rapidly with either Ca or Ba as the charge carrier, deactivates about 10 times more slowly upon repolarization, and is less selective for Ba over Cs. 3. Nimodipine preferentially blocks the slowly inactivating channels. Block of these channels is time- and voltage-dependent, such that block is maximized by long depolarizations. 4. A comparison of the voltage dependence of steady-state nimodipine block with the voltage dependence of channel activation indicates that channel block is directly proportional to the number of open channels. The results are accounted for by a model that postulates 1:1 high-affinity drug binding to open Ca channels. The apparent dissociation constant for binding to open channels is 517 pM. Similar binding constants were previously reported for the inhibition of high-K-induced hormone secretion and high-affinity ligand binding of [3H]nimodipine to isolated plasma membranes. 5. The rate of onset of nimodipine block increases with the test potential, in quantitative agreement with the model of open-channel block. The apparent association rate is about 9.6 X 10(7) M-1 s-1; the dissociation rate is about 0.050 s-1. At therapeutic concentrations (less than 10 nM) nimodipine block takes many seconds to reach equilibrium. 6. Nimodipine should have little effect on stimulus-secretion coupling in healthy pituitary cells in vivo because: (a) the drug binds very weakly to the transient channels that are open at normal resting potentials, and (b) negligible high-affinity binding occurs during spontaneous activity because the onset of block is very slow.

Involvement of sodium channels and two types of calcium channels in the regulation of adrenocorticotropin release

Recent electrophysiological studies from this laboratory demonstrated that anterior lobe corticotropes exhibited a tetrodotoxin-sensitive sodium current and two types of voltage-dependent calcium currents, consisting of low threshold (transient) and high threshold (long lasting) components. The present report describes cytophysiological and cytochemical studies that used specific blockers of each of these currents to assess their role in the regulation of CRF binding and ACTH secretion and storage. Two dihydropyridines, nimodipine and the pure antagonist enantiomer (-)R202-791, which block high threshold Ca2+ channels, decreased 1 h basal release by 54-74% and CRF-mediated (5 min or 3 h) release completely. Percentages of CRF-bound cells were reduced as much as 74%; however, the inhibitory effect on percentages of CRF-bound cells could be reversed by adding 10 nM Bay K 8644, (a pure dihydropyridine agonist) with the antagonists. CdCl2, which blocks both high and low threshold calcium currents, inhibited basal and CRF-stimulated ACTH release, but only the highest concentration (0.1 mM) reduced percentages of CRF-bound cells. Involvement of the low threshold Ca2+ channels could not be proved by adding dihydropyridine antagonists with 0.1 mM CdCl2. Basal and CRF-mediated ACTH release were blocked by the potent sodium channel blocker tetrodotoxin, and the highest concentration (3 microM) reduced percentages of CRF-bound cells. Basal (1 h) and CRF-stimulated (5 min) ACTH release were also inhibited in medium containing 1 mM EGTA and no Ca2+; however, percentages of CRF-bound cells were within the normal range. Densitometric analysis of stains for ACTH showed an increase in the concentration of stain per cell after a 1-h exposure to the highest concentrations of the inhibitors or to no Ca2+ and 1 mM EGTA coupled with a significant (10%) decrease in corticotrope cell area. Finally, in the last series of tests, the Bay K 8644 agonist or arginine vasopressin were used to study mechanisms of augmentation of basal or CRF-mediated ACTH release. Bay K 8644 augmented basal release in a concentration of 1 microM and CRF-mediated release in a concentration of 100 nM or 1 microM. After pretreatment with either Bay K 8644 or arginine vasopressin (10 nM) there was a significant (30%) increase in the percentage of CRF-bound cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Hollow fiber-encapsulated pituitary cells for the study of adenohypophyseal regulation of growth in poultry: 2. Recipient growth responses

A technique for the encapsulation and transplantation of viable, functionally active adenohypophyseal cells to pituitary-intact poultry by means of hollow fiber units has been developed. Endogenous pituitary glands of broiler pullets that received high density capsules (1.2 X 10(6) cells) were observed 30 days after fiber implantation at 2 weeks of age. Glands were reduced in size and immunoreactive growth hormone (I-GH) concentration in comparison to sham and low density (.3 X 10(6) cells) groups. Although these changes were not significant, the combined reduction in mass and I-GH concentration equates to a 25 to 35% reduction in total pituitary I-GH content for the high density group, in comparison with sham and low density group birds, respectively. These changes suggest a feedback effect of high density cell implants on endogenous pituitary function. Relative gains of each group reflect apparent differences in pituitary I-GH content: high density group birds gained less than shams (P greater than .05) and significantly less than low density group birds. In a second study of older birds, cell implants (.73 X 10(6) cells) resulted in a significantly greater average relative gain (52%) in comparison with shams (33%) over the 6 week implantation period. Mixed adenohypophyseal cell populations transplanted to the hypophysiotropic region of pituitary-intact broiler pullets resulted in changes in endogenous pituitary function and growth of recipient birds. These studies suggest that dosage administered (e.g., cell density) may interact with age at application in determining the ultimate response of birds to exogenous manipulation of endocrine regulation of growth.

Hollow fiber-encapsulated pituitary cells for the study of adenohypophyseal regulation of growth in poultry: 1. Preparation and use

A technique for transplantation of viable, functionally active pituitary cells was adapted for use in poultry. Enzymatically dispersed adenohypophyseal cells were prepared from broiler strain donor chicks and loaded into hollow fiber capsules; ultrafiltration membranes consisting of an open-celled, spongy outer layer of polymer material with an internal lumen which is surrounded by a thin skin of tightly controlled pore size. Cell-filled capsules were surgically implanted intracranially into pituitary-intact, recipient birds for extended time periods. Immunoreactive growth hormone (I-GH)-containing cells persisted in hollow fiber capsules for up to 30 days of implantation. Final capsule I-GH reflected starting cell densities. Capsules implanted intracranially became highly vascularized, with capillary endings imbedded in the spongy, open-celled outer surface of the capsule. This provides a means for hormones secreted by cells contained within the capsule to reach the peripheral circulation. Mean plasma I-GH concentration of birds implanted at 10 weeks of age with capsules containing donor cells from 2 to 3-week-old chicks was significantly higher than sham-implanted controls by 2 weeks postimplantation. Thus, hollow fiber-encapsulated pituitary cells remain viable for extended time periods when implanted intracranially in pituitary-intact chickens. These capsules provide a means by which naturally secreted pituitary hormones may be administered in vivo and provide a viable model system for studies of adenohypophyseal hormone action.

Effects of hypophysectomy on plasma levels of pancreatic polypeptide (APP) and insulin in adult chickens

Adenohypophysectomy (hypox) was carried out in adult chickens in an attempt to assess what role, if any, the anterior pituitary gland plays in maintaining basal levels of plasma insulin (IRI) and avian pancreatic polypeptide (APP) both before and immediately after a fast-refeed regimen. Each bird was tube-fed 61 gms twice daily, body weights were taken daily, and blood samples drawn daily just before the second feeding. All birds were fasted for 24 hr on days 4-5, another blood sample taken, and then refed the usual gruel. Blood samples were taken at 5,15,30,90 and 180 min after refeeding. Hypox caused an immediate and sustained decrease in plasma IRI and a significant but transient increase in plasma APP which lasted 3-4 days before returning to normal; plasma glucose was marginally decreased. Refeeding resulted in a trend of less response (increase) in all three parameters studied in the hypox group. It is suggested that in chickens, hypox may lead to a release phenomenon from a normal inhibitory role which the anterior pituitary gland plays on APP release.

Adenohypophysial-thyroid activity of the tiger salamander, Ambystoma tigrinum, as a function of metamorphosis and captivity

The aim of this study was to determine the timing of adenohypophysial activation during metamorphosis of the tiger salamander, Ambystoma tigrinum. It consisted of two parts: 1) determination of plasma thyroid hormone concentrations and analysis of thyroid gland histology as a function of metamorphic stage and 2) analysis of the time-course of uptake of 125I by the thyroids during metamorphosis as an indicator of endogenous thyrotropin (TSH) levels. Significant increases in both triiodothyronine (T3) and thyroxine (T4) first were evident at the onset of metamorphic climax (stage II). Maximum levels of both hormones were not observed, however, until the completion of gill resorption (stage VII). No changes in thyroid histology were observed that could be unambiguously related to metamorphic transformation. The thyroids accumulated 125I in a slow but linear fashion in premetamorphic larvae (stage I). However, uptake exhibited a rapid peak during early climax (stage II), before maximum concentrations of thyroid hormones were observed. In addition, uptake was maintained above premetamorphic levels at stage VII, in conjunction with maximum levels of T4 and T3. Captivity alone produced a small but significant increase in plasma concentrations of T3. It produced no significant effect on either thyroid histology or uptake of 125I. These results indicate that adenohypophysial activation occurs rapidly and is maximal at the onset of metamorphic climax.

Prolactin influence on cytosol and nuclear androgen receptors in the ventral, dorsal, and lateral lobes of the rat prostate

PRL augments testosterone-mediated growth of the prostate in a permissive manner. To elucidate the mechanism of this hormonal interaction, the present study examined the effect of PRL on cytosol and nuclear androgen receptors in the three prostate lobes. Adult male Sprague-Dawley rats were castrated, given 5-mm Silastic implants of testosterone, and either grafted with two anterior pituitary glands under the kidney capsule or sham operated. Three weeks later, animals were killed, serum was collected for PRL and testosterone RIA, and the ventral, dorsal, and lateral prostate lobes were processed for either cytosol or nuclear androgen receptor quantitation. Pituitary grafts significantly elevated the serum PRL concentration and increased the weight and the content of protein and DNA of the lateral prostate lobe compared to control values. There was no effect on these parameters in the ventral or dorsal lobes. Androgen receptor levels and apparent distributions were different in the ventral, dorsal, and lateral lobes of control animals. Unoccupied and total cytosolic androgen receptors in the three separate prostate lobes were not significantly affected by the presence of the grafts. However, an elevated PRL concentration was associated with an increase (P less than 0.005) in nuclear androgen receptor content in the lateral lobe exclusively. The binding affinity was not altered by pituitary grafts in any of the lobes. These findings suggest that PRL promotes lateral prostatic growth by increasing nuclear androgen receptor levels in that tissue and, thus, optimizes its response to circulating testosterone.

Two distinct calcium-activated potassium currents in a rat anterior pituitary cell line

1. The single 'giga-seal' patch-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was used to record whole-cell currents in the GH3 rat anterior pituitary cell line. 2. GH3 cells have a rapidly inactivating, voltage-dependent K+ current that is selectively inhibited by 4-aminopyridine (4-AP) but not by tetraethylammonium chloride (TEA). 3. The majority of the Ca2+-activated K+ current in these cells is blocked by TEA with an inhibitory concentration that is half-maximal at 1 mM. An additional Ca2+-activated K+ current is also present that is relatively resistant to TEA and is blocked by the polypeptide apamin. The apamin-sensitive component represents less than 18% of the total Ca2+-activated K+ current at 0 mV. 4. The time course of the slowly declining components of the Ca2+-activated K+ tail currents measured at the -50 mV holding potential was usually biexponential with time constants of 0.21 +/- 0.02 and 1.75 +/- 0.23 s (mean +/- S.E. of mean, n = 14). Both of the two slowly decaying components contribute to the TEA- and apamin-sensitive currents. 5. It is concluded that GH3 cells have at least two pharmacologically distinct Ca2+-activated K+ currents and a 4-AP-sensitive voltage-dependent K+ current.

Abnormal daily periodicity of serum thyrotropin (TSH) and evidence for defective TSH suppression in a case of non-neoplastic syndrome of inappropriate TSH secretion

A non-neoplastic syndrome of inappropriate secretion of TSH (ITSHS) was diagnosed in a hemithyroidectomized and clinically euthyroid 44-yr-old man, who also exhibited limping (Perthes' disease), genu valgum, pes supinatus and lateral nystagmus. Computed tomography demonstrated an enlarged sella turcica due to empty sella. Baseline serum T3, T4, free T3, free T4 and TSH fluctuated between 179 and 274 ng/dl, 6.0 and 13.2 micrograms/dl, 4.2 and 6.0 pg/ml, 7.6 and 15.3 pg/ml, and 4.3 and 33.0 microU/ml, respectively. Serum alpha-TSH subunit was repeatedly normal (0.36-0.69 ng/ml) over the follow-up period (greater than 3 yr). No changes in serum liver enzymes and lipids were observed after thyroid hormone administration, whereas red blood cell glucose-6-phosphate dehydrogenase (G-6-PD) and urinary OH-proline were slightly enhanced during 120 micrograms/day L-T3 regimen. This also resulted in an inappropriately normal glucagon-stimulated cAMP levels. Tachycardia was experienced only during L-T3 and very high L-T4 dose treatments. Therefore, the patient showed some evidence for thyroid hormone peripheral refractoriness. Patient's TSH was physiologically responsive to agents (thyrotropin releasing hormone, methimazole, the dopamine antagonists domperidone and sulpiride) known to elicit its release into circulation, while it responded paradoxically to those which normally inhibit TSH secretion. In fact, the infusion of somatostatin (320 micrograms/h) or dopamine (4 micrograms/Kg/min), and the oral administration of bromocriptine or nomifensine (two dopamine agonists) or corticosteroids (dexamethasone) provoked an unexpected elevation of both unstimulated and TRH-stimulated TSH levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone stimulates a calcium-activated potassium current in a rat anterior pituitary cell line

1. The 'giga-seal' patch-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was used for constant current and voltage-clamp recordings in the GH3 rat anterior pituitary cell line. 2. Thyrotropin-releasing hormone (TRH) causes a membrane hyperpolarization that is mediated by a selective increase in K+ permeability. The hyperpolarization cannot be evoked when the cell is internally perfused with a Ca2+ chelator but persists when the external solution that bathes the cell is Ca2+-free or contains a Ca2+-channel blocker. 3. Under voltage clamp the TRH-induced current is approximately linear at negative potentials (-90 to -30 mV) but markedly enhanced at voltages above -30 mV). Thus, the affected conductance has a voltage-dependent component. 4. The TRH-induced increase in K+ permeability is sensitive to inhibition by 30 mM-TEA and 200 nM-apamin, inhibitors of two distinct Ca2+-activated K+ permeabilities in GH3 cells. 5. The time course of the TRH-induced K+ current is similar to the time course of a TRH-induced transient peak elevation of cytosolic Ca2+ that is due to mobilization of Ca2+ from intracellular stores. 6. The effects of TRH on the K+ current and the rise in cytosolic Ca2+ are half-maximal at 7 nM and 1.7 nM, respectively. 7. It is concluded that the TRH-induced hyperpolarization is mediated by two distinct Ca2+-activated K+ conductances that are activated by release of Ca2+ from an intracellular site.

Membrane currents of identified isolated rat corticotropes and gonadotropes

Membrane currents of identified, isolated corticotropes and gonadotropes from mammalian anterior pituitary gland have been evaluated. Pituitary gonadotropes and corticotropes were isolated enzymatically and stained in the living state using biotinylated gonadotropin-releasing hormone (Bio-GnRH) or biotinylated corticotropin-releasing hormone (Bio-CRF) followed by avidin fluorescein. Electrophysiological recordings were made with patch-clamp electrodes in the whole-cell clamp configuration. Tetrodotoxin (TTX)-sensitive sodium currents were larger in corticotropes than in gonadotropes. Corticotropes showed two components of calcium currents, a transient low-threshold component and a longer lasting high-threshold component. Small TTX-resistant inward currents were present also in gonadotropes, and both cell types had transient and steady potassium currents.

Inhibin--a non-steroidal regulator of pituitary follicle stimulating hormone

Inhibin is a gonadal glycoprotein produced by the granulosa and Sertoli cell under the influence of FSH and acts to specifically suppress pituitary FSH secretion. Recently, ovarian inhibin has been purified from several species and its amino acid sequence deduced using cloning techniques. Inhibin consists of two disulphide-linked heterologous subunits of which the smaller may exist in two different forms accounting for two different forms of inhibin in humans and pigs. Heterogeneity of inhibin also exists as a result of proteolytic processing of the molecule during its passage into the circulation. Significant homology exists between the subunits of inhibin and the dimeric peptides TGF-beta and Mullerian inhibitory substance (MIS), suggesting they are all derived from a common ancestral gene. Furthermore, dimers of the smaller subunit of inhibin (FSH-releasing protein (FRP) or activin) have now been found in follicular fluid (FF) and, along with TGF-beta, shown to be potent and specific stimulators of FSH secretion. These proteins may be involved in controlling FSH by another as yet unknown pathway and may prove to be the FSH-releasing factor, analogous to LHRH, which has been postulated to exist for some years. Inhibin can no longer be simply considered as an isolated FSH-suppressing protein. The physiological significance and relationship between inhibin and its related proteins represent one of the most challenging and interesting areas in reproductive endocrinology. Further studies, particularly with the development and use of sensitive assays for both the FSH releasing hormone and inhibin will clarify their role in reproduction and their usefulness in monitoring or treating fertility.

Release of luteinizing hormone-releasing hormone (LHRH) and neuroactive substances in unanesthetized animals as estimated with push-pull cannulae (PPC)

In this report, we have reviewed recent information gathered by probing with a push-pull cannula (PPC) the in vivo activity of the suprachiasmatic nucleus (SCN), hypothalamus, and anterior pituitary gland of freely moving animals. In male and female rats, probing of the SCN with the PPC revealed distinct oscillatory patterns of 5-hydroxy indole-acetic acid (5-HIAA) output very much dependent on the position of the cannula. In males, it was also possible to demonstrate, for the first time, in vivo output of immunoreactive vasopressin (VP) most likely from the SCN. Interestingly, the output of VP was stimulated by local activation of probable 5-hydroxytryptamine (5-HT) terminals with 5-hydroxytryptophan (5-HTP), a precursor of 5-HT synthesis. Probing the hypothalamus of rats and rabbits revealed that the in vivo release of luteinizing hormone-releasing hormone (LHRH) (frequency and amplitude of the LHRH signal) can be altered by administration of estrogen to ovariectomized rats; in both species, progesterone stimulated the amplitude of the LHRH signal, but only when this steroid was infused in pulses--the physiological mode of circulating progesterone in the rat. Further, in male rabbits, pulses of progesterone did not stimulate LHRH release. Last, probing the anterior pituitary with the PPC revealed that a series of push-pull perfusions could be performed in the same animal under different experimental conditions for nearly 60 days of experimentation. It also resolved the apparent paradox that after castration, decreased instead of increased activity of the neural LHRH apparatus was noticed when the PPC was positioned in the hypothalamus. Moving the PPC to the anterior pituitary revealed that castration was accompanied by an increase in the amplitude and frequency of the LHRH signals arriving in the anterior pituitary of castrated male rats. This mode of operation of the LHRH pulse generator is clearly compatible with the mode of luteinizing hormone (LH) release in gonadectomized animals. Finally, based on these results, a hypothetical model of the operation of the LHRH pulse generator has been proposed.

Evidence for functional communication between folliculo-stellate cells and hormone-secreting cells in perifused anterior pituitary cell aggregates

Dispersed anterior pituitary cells from adult female rats were separated by gradient sedimentation at unit gravity. The small-sized cell population on top of the gradient consisted of 65.6 +/- (SE) 4.2% (n = 8) cells immunoreactive to antiserum against S-100 protein, a marker of folliculo-stellate (FS) cells in rat pituitary. The corresponding fraction derived from adult male or immature female rats were also enriched in S-100 positive cells but to a lower extent. Only small numbers of S-100 positive cells were found in medium- and large-sized cell populations. Coaggregating the S-100 cell-enriched populations from adult females with other pituitary cell populations resulted in a clear-cut inhibition of the GH response to rat GH-releasing factor and beta-adrenergic agents, of the PRL response to TRH and angiotensin II (AII) and the LH response to LHRH. The magnitude of inhibition increased with the number of FS cells put into the coaggregates. In perifused aggregates prepared from different gradient fractions from immature females, there was a negative correlation between the occurrence of FS cells and the magnitude of the PRL response to AII. The low responsiveness to AII in FS cell enriched aggregates was not abolished when these aggregates were redissociated into single cells. It is suggested that FS cells constitute an intercellular messenger system for local inhibitory control of pituitary hormone secretion which is not based on direct and intimate contact between the interacting cells.

Effects of sulpiride and apomorphine on prolactin release in adrenalectomized animals. Role of sodium ions

The present experiments were performed to demonstrate in vivo the existence of a modulatory role of Na+ in the inhibitory control of prolactin (PRL) secretion. Groups of adult male rats were either adrenalectomized (adrenex) or sham-operated (normals) and implanted in the right atrium with a chronic cannula for the infusion of solutions or the sequential withdrawal of blood samples. Four days later, adrenex rats were infused for 1 h with either saline 0.9% or dextrose 5.6% (310 mOsm/kg H2O) at the rate of 10 ml/kg/h from Time -60 min until Time 0 min. All animals were injected at Time -45 min with sulpiride 0.05 mg/kg. At Time 0 min, some animals received apomorphine 5 mg/kg while others received a 0.05 mg/kg dose of the drug. Blood samples were obtained at -45, -30, -15, 0, 5, 10, 20 and 30 min for PRL assay. Plasma Na+ concentration was measured at -45, 0 and 30 min and plasma osmolarity at 0 min in adrenex rats. In normal controls plasma Na+ values were measured at the end of the experiment. Binding values (KD and Bmax) of pituitary D2-receptors were measured in adrenex and sham-operated rats using [3H]spiroperidol as ligand and D-butaclamol to define non-specific binding. Plasma Na+ values remained constant in saline-infused adrenex rats and similar to those of normal controls, while in dextrose-infused adrenex rats they fell progressively during the experimental period and were always lower than in the two other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Pituitary-induced lactation in mammary gland explants from the pregnant tammar (Macropus eugenii): a negative role for cyclic AMP

1. alpha-Lactalbumin and casein have been isolated from tammar milk. 2. alpha-Lactalbumin was induced in mammary explants by culture with anterior pituitary. 3. Casein was induced maximally in the presence of a physiological concentration of prolactin alone. 4. Progesterone did not inhibit the prolactin-induced synthesis of casein, alpha-lactalbumin, galactosyltransferase or fatty acids. 5. Both dibutyryl cAMP and a combination of cholera toxin and IBMX did significantly inhibit the induction of casein and alpha-lactalbumin. 6. Progesterone withdrawal is not a component of the lactogenic trigger in this marsupial but cAMP may be a common intracellular signal for negative control of lactogenesis in both marsupials and eutherians.

Calcium-independent and calcium-dependent mechanisms regulate corticotropin-releasing factor-stimulated proopiomelanocortin peptide secretion and messenger ribonucleic acid production

We have evaluated the role of calcium in basal and secretagogue-stimulated release of beta-endorphin and PRL and the levels of their respective mRNAs in primary cultured rat anterior pituitary cells. Treatment of anterior pituitary cells with the calcium channel blocker methoxyverapamil (D600; 10 microM) or with calcium-free medium for 1 h did not alter the basal release of beta-endorphin and only partially blocked CRF (10 nM)-stimulated beta-endorphin release. In contrast to these effects of D600 or calcium-free medium on corticotrophs, both of these test conditions decreased basal secretion of PRL from lactotrophs by 50-70% and completely blocked forskolin (10 microM)-stimulated PRL secretion. Although omission of calcium from the culture medium caused a 50% decrease in basal levels of both proopiomelanocortin (POMC) and PRL mRNA, treatment of cells with D600 did not significantly alter the basal levels of POMC or PRL mRNA. Treatment of cells with D600 partially blocked CRF-stimulated elevation of POMC mRNA and forskolin-stimulated elevation of PRL mRNA. The calcium agonist barium (1 mM) produced a 2-fold increase in both beta-endorphin and PRL release, which was blocked by D600. Treatment of cells with barium had no effect on POMC mRNA levels, but increased PRL mRNA levels. D600 treatment of cells partially blocked barium-stimulated PRL mRNA levels. These findings demonstrate a calcium-dependent as well as a calcium-independent component of CRF-stimulated beta-endorphin secretion and CRF-stimulated POMC mRNA elevation. In contrast, PRL secretion and biosynthesis appear to be totally calcium-dependent processes.