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Reexamination of the hexapeptide GH-releasing peptide (GHRP-6) structure/function has lead to the development of four novel classes of compound that stimulate GH release. Each class is represented as follows: a pentapeptide, G-7039; a tetrapeptide, G-7134; a pseudotripeptide, G-7502; and a rigid cyclic heptapeptide, G-7203. The EC50 values for these compounds, determined by GH dose-response curves using primary cultures of rat pituitary cells, were 0.18, 0.34, 10.6, and 0.43 nM, respectively. To demonstrate that these compounds were acting at the putative GHRP receptor, challenges were made using combinations that included GHRP-6 and GH-releasing hormone (GHRH). All four new classes further increased GH release in combination with GHRH, but not with GHRP-6. Homologous desensitization occurred after 45 min of exposure to the new compounds while the cells remained sensitive to GHRH. Somatostatin inhibited all of these compounds. Additionally, G-7039 elevated free calcium, as occurs with GHRP-6. All four classes elicited a robust GH release, a small increase in PRL, and no change in LH, FSH, ACTH, or TSH. We conclude that these novel compounds are potent and direct stimulators of pituitary GH release, with in vitro attributes that suggest mediation via a specific GHRP-like mechanism.

The transmembrane signaling events of GH were investigated in the liver, a major target organ of GH action. Recombinant human GH when added to freshly isolated rat hepatocytes rapidly stimulated the production of sn-1,2-diacylglycerol (DAG). The generation of DAG was biphasic with the first transient peak observed at 2 min and the second peak at 15 min (1.2-fold and 1.4-fold over control, respectively). Levels of DAG continued to be elevated above those in control cells at 30 min. The response was dose-dependent with an EC50 of 0.15 nM. Both bovine GH and rat GH, which bind to the rat GH receptor but not to the PRL receptor, also stimulated DAG production. Similarly, human PRL, which binds to the PRL but not GH receptor, stimulated DAG formation to a comparable extent. These results suggest that production of DAG may be an early signaling event mediated by hormone stimulation of both the GH and PRL receptors.

The anterior pituitary of adult Macaca fascicularis monkeys was removed when they were killed and the potent dopamine (DA) antagonist [3H]spiperone was used to label DA receptors. Saturation isotherms of [3H]piperone binding indicated a dissociation constant of 0.09 nM and maximal sites of 170 fmol/mg assay protein (n = 2). Competition studies validated the hypothesis that [3H]spiperone labeled DA receptors; the order of potency of 12 compounds was appropriate for a dopaminergic interaction and was similar to the order found in the anterior pituitary of other mammals. A concentration of [3H]spiperone that would label 90% of the specific binding sites (0.76 nM) was used to estimate the total DA receptor activity in the anterior pituitary of individual monkeys. [3H]spiperone binding in pregnant monkeys was greater than the binding in ovariectomized and lactating females or intact males (P less than 0.05 or 0.01). The postpartum lactating group contained more [3H]spiperone binding activity than the cycling group (P less than 0.05). Determination of the serum concentrations of PRL, estradiol, testosterone, progesterone, and dehydroepiandrosterone confirmed the predicted hormonal status of the individual monkeys at the time of sacrifice. Because DA is a PRL inhibitory hormone, it was of interest that the serum PRL concentration was positively correlated with the degree of [3H]spiperone binding (r2 = 0.74). This observation, combined with the known hypertrophy and/or hyperplasia of the mammotroph during pregnancy, supports the hypothesis that the DA receptor is associated with the PRL cell of the anterior pituitary.

Anabolic proteins such as pituitary GH enhance the function of several immune cell types. The converse could also exist, that is communication from the immune cells to GH-producing somatotrophs. To test this hypothesis, tumor necrosis factor-alpha (TNF-alpha), a product of activated macrophages, was exposed to cultured rat pituitary cells, and GH release was monitored. TNF alpha inhibited basal and GH-releasing hormone-stimulated GH accumulation, with IC50 values of 170 U/ml (5.2 ng/ml) and 50 U/ml (1.5 ng/ml), respectively. This inhibition was first measured after 6 h of TNF alpha treatment, continued for at least 3 days, and was reversible. A number of measurements (e.g. trypan blue exclusion, chromium release, and GH cell content) yielded no signs of cytotoxicity to explain the inhibition. We conclude that TNF alpha can reduce basal and stimulated pituitary GH release in vitro.

The effects of synthetic hpGRF-40 on GH release from continuously perifused male rat anterior pituitary cells were studied. Pulses (2.5 min) of hpGRF-40 stimulated GH release in a log-linear dose response relationship: concentrations of 0.03, 0.1, 0.3, 1, 3, 10, 30 and 100 nM given in a random order elicited a GH response above baseline of 1.2 +/- 0.3, 2.4 +/- 0.4, 2.8 +/- 0.2, 4.3 +/- 0.2, 6.2 +/- 0.7, 7.0 +/- 1.0, 8.7 +/- 1.7, and 10.8 +/- 0.8 micrograms/10(7) cells (mean +/- SEM; n = 3; r = 0.93), respectively. During a 5-h hpGRF-40 infusion, GH stimulation peaked within 5 min and waned to near baseline by the end of the fifth h. The integrated GH responses to 0.03, 0.1 and 0.3 nM hpGRF-40 were 37.6 +/- 7.4, 52.9 +/- 8.5, and 66.15 +/- 8.2 micrograms/10(7) (mean +/- SEM; n = 3; r = 0.72), respectively. The interaction of TRH and hpGRF-40 in the control of GH secretion was studied to investigate the mechanism of the "paradoxical" TRH stimulation of GH release associated with GH excess states in humans. Dispersed cells were perifused with either 100 nM TRH for 0.5 h, 5 nM hpGRF-40 for 4 h, or 5 nM hpGRF-40 for 4 h, to which a 0.5 h pulse of TRH was added at 2 h. GH levels did not change significantly in the presence of TRH alone. When TRH was added to the ongoing hpGRF-40 perifusion, GH release increased from 1.4 +/- 0.06 to 4.0 +/- 1.0 micrograms/min.10(7) cells (n = 4; P = 0.03). Thus, dispersed pituitary cells are highly sensitive to very low concentrations of hpGRF-40 administered as both an acute pulse and as a tonic infusion. When the cells are exposed to a maximal concentration of hpGRF-40 (i.e. 5 nM), TRH becomes a secretagogue at the pituitary level, thus suggesting the site and mechanism of the "paradoxical" GH response to TRH observed in some acromegalics.

The nature of beta-adrenergic binding by swine corpora lutea and granulosa cells was examined with the specific beta-adrenergic radioligand, (+/-)3-[125I]iodocyanopindolol (ICYP). Saturation analyses revealed the presence of high affinity (Kd = 15.2 +/- 2.1 pM; n = 8 experiments) and low capacity (6.7 +/- 0.8 fmol/mg protein) beta-adrenergic receptors on porcine corpora lutea membranes. The properties of beta-adrenergic binding were determined by computer modeling of competition studies with a variety of compounds selective for beta-adrenergic subtypes. These studies disclosed predominantly beta 1-adrenergic receptors on pig luteal membranes. This inference from radioligand binding studies was corroborated functionally by the approximately equipotent biological effects of L-norepinephrine and L-epinephrine on cAMP production by luteal tissue (respective EC50s of 282 +/- 31 and 187 +/- 66 nM; n = 3 experiments). Physiological regulation of specific beta-adrenergic receptor content in the swine ovary was indicated by prominent (up to 9-fold) variations in receptor concentrations among corpora lutea and granulosa cells at various stages of maturity. In addition, there was differential expression of beta-adrenergic receptor subtype. Whereas the beta-adrenergic receptor subtype was predominantly beta 1 in corpora hemorrhagica and corpora lutea, granulosa cells and corpora albicantia contained principally beta 2 receptors. This difference could not be accounted for by blood cell contamination of corpora lutea, since swine blood cells contained predominantly (greater than 98%) beta 2-receptors, which were present at less than 8.6% the concentration of total beta-receptors in luteal tissue. In summary, swine corpora lutea and granulosa cells contain specific high affinity, low capacity beta-adrenergic receptors that are functionally coupled to biological responses. Moreover, total receptor content as well as beta-adrenergic subtype exhibit significant physiological variation in relation to maturational status of ovarian follicular and luteal tissue.

Pituitary GH secretion is regulated by Ca+2 and cAMP. We show that human pancreatic tumor GRF (hpGRF) stimulates anterior pituitary adenylate cyclase activity, cAMP accumulation, and GH release. The relationship between Ca+2 and the stimulating effects of the Ca+2 ionophore A23187 on cAMP accumulation and GH release in vitro was studied. To evaluate the role of the Ca+2-binding protein calmodulin in this system, we used the calmodulin antagonist W7, a naphthalene-sulfonamide derivative, and its less active analog W5. W7 inhibited hpGRF-stimulated adenylate cyclase activity, cAMP accumulation, and GH release, whereas W5 was either poorly effective or ineffective. Somatostatin (SRIF) also attenuated hpGRF stimulation of adenylate cyclase. These results suggest that the actions of Ca+2-calmodulin and cAMP are interrelated in modulating GH release. Calmodulin participates in hpGRF stimulation of adenylate cyclase, cAMP formation, and GH release. The attenuation of hpGRF-stimulated adenylate cyclase activity by SRIF may be one of the mechanisms for its GH inhibitory action.

Our studies demonstrated that beta-adrenergic agonists stimulate the release of GH from rat anterior pituitary (AP) cells in vitro. Concentration-response experiments with beta-adrenergic agonists demonstrated that beta 2-adrenergic receptors mediated this phasic GH release, while having no apparent effect on PRL or LH release. The ACTH response to beta-adrenergic agonists was equivocal. Half-maximal stimulation of GH release occurred at 14 +/- 2 (+/-SE) nM isoproterenol, 160 +/- 30 nM epinephrine, and over 1 microM l-norepinephrine (n = 4). Direct binding studies in membrane particulates of rat AP confirmed receptors of the beta 2-subtype. Iodocyanopindolol binding to beta-adrenergic receptors of rat AP yielded a dissociation constant of 4.6 +/- 0.1 pM and a maximal capacity of 1.9 +/- 0.4 fmol/mg protein (n = 3). In contrast, porcine AP contained beta 1-adrenergic receptors. These results support the hypothesis that the endogenous beta 2-adrenergic agonist l-epinephrine may be a GH-releasing factor of physiological significance in the rat.

The intermediary role of the putative inhibitory regulatory membrane protein "Ni" in the "antigonadal" activity of the neurohypophysial hormones was investigated in vitro with the use of a primary culture of rat testicular cells. To this end, use was made of the pertussis toxin (PT) probe, an exotoxin of Bordetella pertussis presumed to modify and inactivate a membrane protein related to or identical with Ni. Testicular cells were pretreated with PT (70 ng/ml) for 24 h, a duration of exposure known to result in a maximal PT effect. Thereafter, the cells were cultured for an additional 48 h in the absence or presence of hCG (10 ng/ml), with or without a maximal inhibitory dose (10(-6) M) of the neurohypophysial principle arginine vasotocin (AVT). Although concomitant treatment of control cells with AVT produced a profound (95%) inhibition of hCG-stimulated testosterone accumulation, PT pretreatment resulted in a long-lasting (greater than 4 days) abolition of this "antigonadal" effect. Furthermore, pretreatment with PT resulted in significant (P less than 0.05) augmentation of the hCG-stimulated accumulation of extracellular cAMP (1.5- to 2.0-fold) and testosterone (1.7- to 3.8-fold), presumably as a result of abolition of the "basal" tonic inhibition of adenylate cyclase activity. Taken together, these findings constitute the first demonstration of the involvement of Ni in the "antigonadal" activity of the neurohypophysial hormones and in the regulation of testicular function, thereby raising the intriguing possibility that testicular Ni may serve as the coupling unit of inhibitory receptors other than those for the neurohypophysial hormones, integrating varied incoming signals of endocrine, paracrine or autocrine nature.

In vivo and in vitro studies of beta-adrenergic influences on GH secretion have produced apparently conflicting data in which the in vivo effect seems to be inhibitory and the in vitro effect to be stimulatory. The present studies were designed to observe the in vivo effect of isoproterenol (ISO), a beta-adrenergic agonist, on 1) GH release during a brief interval after intraatrial infusion, and 2) GH release in response to GRF infused 10 min after ISO. ISO was found to stimulate GH release in both intact and hypothalamus-lesioned animals within 2 min after infusion, but GH returned to control levels within 10 min. ISO also profoundly inhibited the release of GH in response to GRF. Pretreatment of animals with somatostatin (SRIF) antiserum prevented the inhibitory action of ISO on GRF-induced GH release. No change in peripheral levels of SRIF was detected. Also, there was no suppression of GRF-induced GH release by ISO when the treatments were applied in vitro to dispersed perifused pituitary cells. These data show that beta-adrenergic systems can stimulate a rapid but brief release of GH in vivo, and that the subsequent inhibitory action on GRF-induced GH release might be by means of SRIF release.

The inhibitory regulation of PRL secretion by dopamine (DA) or the dopaminergic agonists bromergocryptine (CB-154) and apomorphine was studied in cultured GH3 cells, an established rat anterior pituitary cell line which produces both PRL and GH. The basal release of PRL from GH3 cells was unaffected when incubated for 6 h with DA concentrations as high as 10(-4) M. The inability of DA to suppress PRL secretion could not be explained by the catabolism of DA or the presence of unknown inhibitors (e.g. estradiol) in the fetal calf serum present in the incubation media. Apomorphine and CB-154 were only partially effective in suppressing PRL release at high concentrations of 10(-4) and 10(-5) M, respectively. Various concentrations of the dopaminergic antagonist d-butaclamol did not reverse the inhibitory action of 10(-5) M CB-154, while equal concentrations (10(-5) M) of both d- and l-butaclamol significantly suppressed PRL release. The greatly lowered responsiveness of GH3 cells to dopaminergic inhibition of PRL is suggestive of some impairment of DA receptors. This hypothesis is supported by radioligand binding studies in which high affinity dopaminergic binding sites are absent in the same cell line used in this study.

Both synthetic human pancreatic tumor GH-releasing factor (hpGRF) and prostaglandin E2 (PGE2) rapidly stimulate cellular cAMP accumulation in and GH release from primary cultures of rat anterior pituitary cells. SRIF inhibits both of these actins. A 1-h treatment with the protein synthesis inhibitor cycloheximide potentiates hpGRF-induced cAMP accumulation for hours and GH release for the first hour. This indicates that a rapidly turning over protein tonically mutes the degree of hpGRF-stimulated cAMP accumulation. Pretreatment of the cells with pertussis toxin amplifies hpGRF- and PGE2-stimulated cAMP levels and GH release; pertussis toxin also attenuates the ability of SRIF to affect these variables. This suggests that an inhibitory coupling protein contributes to these events. Finally, cholera toxin and forskolin are also potent stimulators of cAMP accumulation and GH release. We conclude that hpGRF-evoked GH release and the inhibitory action of SRIF are closely correlated with the cAMP-generating system.

Calcium (Ca2+) ionophore A23187 increased the intracellular cAMP content and PRL release in normal rat anterior pituitary cells. Cotreatment with dopamine reduced both control and A23187-stimulated cAMP accumulation and PRL release. The dopamine antagonist spiperone restored the response of cAMP to ionophoric stimulation after pretreatment with dopamine in the greatest concentration used. Penfluridol, a compound with Ca2+-calmodulin-blocking properties, decreased control and A23187-stimulated cAMP content and PRL release. W7, a selective calmodulin-blocking agent, reduced basal cAMP and PRL release, whereas W5, a W7 analog with only 20% of its calmodulin-blocking ability, did not affect cAMP or PRL secretion. These data indicate that the Ca2+-calmodulin and cAMP systems are interrelated in the regulation of PRL secretion. They are also consistent with the hypothesis that the inhibition of PRL release by dopamine occurs after Ca2+ is mobilized and when or before it stimulates adenylate cyclase activity.

A protein toxin synthesized by the bacterium Bordetella pertussis has the unique property of blocking a number of receptor-mediated inhibitory systems which are linked to adenylate cyclase. We found that pertussis toxin (PT) eliminates the ability of somatostatin to reduce both basal and GH-releasing factor-stimulated GH release in primary cultures of rat pituitary cells. Furthermore, the ability of somatostatin to reduce GH-releasing factor-induced cAMP accumulation in the cells is significantly attenuated after PT treatment. The PT effect, which is dose dependent and prevented by pretreatment with anti-PT antibodies, represents an alteration in somatostatin efficacy rather than potency. The modification of somatostatin responsiveness persists for at least 5 days after toxin removal. The PT actions on the somatotroph are similar to the effects on other eukaryotic cell types. The combination of available data indicates that the toxin acts on a highly conserved component(s) that is obligatory for transducing the inhibitory hormone message into the cell.

The anterior pituitary has recently been implicated as a relaxin target issue because of the cAMP elevation after relaxin treatment. We attempted to correlate this finding with an endocrine response to relaxin in rats. Anterior pituitary cells were enzymatically dispersed and subjected to the reverse hemolytic plaque assay. PRL secretion was significantly stimulated 1.31-fold by human relaxin at the lowest concentration studied (30 pM) and maximally stimulated 1.65-fold at 0.3 nM relaxin. Antibodies directed against relaxin inhibited this effect, as did the PRL inhibitory hormone, dopamine. In contrast to the response of PRL cells, there was no effect or a slight inhibition of LH release after incubation with relaxin. In conclusion, we propose that one of the pituitary cell types responsive to relaxin in culture is the PRL-secreting mammotroph.

Maitotoxin has been reported to activate calcium channels and stimulate calcium-dependent functions in several tissues, but a thorough investigation of 45Ca2+ fluxes is lacking. To characterize the influence of maitotoxin on 45Ca2+ flux in greater detail, we incubated dispersed GH3 pituitary tumor cells in 45Ca2+ with maitotoxin and other agents affecting calcium channels. Within 10 sec of exposure, maitotoxin induced a net calcium influx in cells at isotopic equilibrium. Calcium uptake was concentration dependent between 0.4 and 40 ng/ml maitotoxin and was inhibited by antagonists of voltage-dependent calcium channels but not by inhibitors of sodium channels. PRL and GH release from perifused GH3 cells was stimulated within 1 min by maitotoxin. We conclude that maitotoxin causes a rapid, concentration-dependent influx of calcium through presumed voltage-dependent endogenous calcium channels, culminating in enhanced hormone release. This potent toxin may provide a more precise understanding of the role of calcium in the stimulus-secretion coupling process.

Dopamine (DA) and DA agonists bind with high affinity to anterior pituitary receptors which mediate the inhibition of PRL release. Spiperone (SPIP), a DA antagonist, has also been successfully used to characterize pituitary DA receptors with a dissociation constant (Kd) of less than 1 nM. We studied the binding of SPIP to GH3D6 cells which secrete only PRL and GH. This clone was derived from a radiation-induced tumor of the rat anterior pituitary. Equilibrium binding of [3H]SPIP to living GH3 cells showed no high affinity receptors, but a low affinity (Kd = 0.83 microM) and saturable (0.06 fmol/cell) population of sites was observed. In addition, saturable binding with a similar affinity (Kd = 0.57 microM) was noted in broken GH3 cells. The interaction was completely reversible and temperature dependent. The concentration of various ligands required to compete for half of the [3H]SPIP binding to whole cells were: chlorpromazine, 0.17 microM; haloperidol, 0.68 microM; pimozide, 0.77 microM; d-butaclamol, 1.16 microM; 1-butaclamol, 1.30 microM; SPIP, 1.49 microM; bromergocryptine, 4.98 microM; apomorphine, 13.9 microM; and DA, 100 microM. The absence of a high affinity site in GH3 cells is consistent with the decreased effectiveness of various agonists and antagonists on PRL secretion. It is possible that the low affinity interactions observed in GH3 cells are normally present in the anterior pituitary and brain and do not simply represent an alteration of receptor affinity.