The dopamine agonist, PHNO ((+)-4-propyl-9-hydroxynaphthoxazine), inhibits cyclic adenosine 3',5'-monophosphate formation and prolactin release from anterior pituitary cells

In cultured rat anterior pituitary cells incubated for 30-180 min, PHNO caused a concentration-dependent inhibition of prolactin release (IC50 approximately 0.5 nM) with maximal suppression at 5 nM. Forskolin increased cyclic adenosine 3',5'-monophosphate (cAMP) generation by stimulating adenylate cyclase and PHNO inhibited this effect with the same concentration profile as for inhibition of prolactin release. Inhibitory effects of 0.5 nM PHNO on prolactin release and cAMP generation were abolished by coincubation with 10 nM haloperidol, a D2 dopamine receptor antagonist. Within 30 min, 0.5 nM PHNO blunted the stimulation of prolactin release due to 10 nM thyrotropin-releasing hormone (TRH) or angiotensin II (AII). Thus, PHNO appears to activate the D2 dopamine receptor to inhibit the formation of cAMP and the secretion of prolactin.

Characterization of the MMQ cell, a prolactin-secreting clonal cell line that is responsive to dopamine

Although dopamine inhibits PRL release from the normal anterior pituitary lactotroph, a conclusive demonstration of the mechanisms involved in this response has been impeded by the presence of other cell types in the anterior pituitary. To circumvent this problem, we have isolated a clonal cell line, designated MMQ, from the 7315a rat pituitary tumor. The MMQ cell is an exemplary model for our use because it only secretes PRL. Our studies show that dopamine inhibits secretagogue-induced PRL release from these cells. In addition, dopamine decreases the intracellular cAMP concentration in MMQ cells that have been exposed to forskolin, cholera toxin, or vasoactive intestinal polypeptide, each a stimulator of cAMP generation. This inhibition is, in turn, reversed by the dopamine antagonist haloperidol and by pertussis toxin, an inactivator of the GTP-binding coupling protein. Dopamine also decreases the uptake and fractional efflux of 45Ca2+ by MMQ cells that have been exposed to the calcium channel activator maitotoxin. It seems, therefore, that dopamine decreases PRL release from MMQ cells at least in part by decreasing intracellular cAMP levels and calcium uptake. In additional experiments, we have found that MMQ cells are responsive to somatostatin, estrogen, progesterone, and acetylcholine, but not to TRH, angiotensin II, neurotensin, or bombesin. Furthermore, these cells possess a functional protein kinase-C system, as evidenced by the increase in PRL release and decrease in stimulated intracellular cAMP levels that occur in response to treatment with phorbol diesters. We suggest that the MMQ cell line will prove a useful model system for study of the biochemical effects of dopamine and other factors that modify PRL release.

Dopaminergic reduction of intracellular calcium: the role of calcium influx

The effects of dopamine (DA) on 45Ca2+ ion movement and prolactin release in dispersed female rat anterior pituitary cells were studied to elucidate the mechanism for DA reduction of intracellular calcium levels. In 45Ca2+ prelabeled cells, DA inhibited fractional calcium efflux and prolactin release simultaneously and continuously in a concentration-dependent manner (IC50 20 nM DA). We then studied unidirectional calcium influx and observed haloperidol-reversible, concentration-dependent DA suppression of calcium influx into unlabeled cells. These data complement and extend reported fluorescent dye studies and suggest that dopamine primarily inhibits calcium influx, thereby reducing intracellular calcium levels, which leads to suppression of prolactin release and is manifest secondarily as a reduction in fractional 45Ca2+ efflux.

Dopamine inhibits calcium flux in the 7315a prolactin-secreting pituitary tumour

Cells of the 7315a prolactin-secreting tumour express biochemically normal cell-surface receptors for dopamine. However, dopamine inhibits prolactin release from these cells only when the basal rate of prolactin release is augmented by increasing the intracellular and/or extracellular calcium concentration of the tumour cells. This suggests that dopaminergic modulation of calcium ion flux could have a central physiological role in these neoplastic cells. In 7315a cells we examined the ability of dopamine to regulate 45Ca2+ influx and fractional 45Ca2+ efflux under conditions of enhanced calcium flux using the calcium channel activator, maitotoxin. It was observed that unidirectional calcium influx stimulated by maitotoxin was significantly inhibited by dopamine. Maitotoxin stimulated fractional efflux and prolactin release from the tumour cells and dopamine simultaneously inhibited both processes by a haloperidol-reversible mechanism. Therefore, in 7315a cells dopamine receptor activation is coupled to inhibition of calcium flux as at least one component in the regulation of prolactin release. These cells may provide further opportunity to study intracellular signalling mechanisms that are modulated by dopamine receptor activity.

Thymosin fraction 5 stimulates prolactin and growth hormone release from anterior pituitary cells in vitro

Thymosin fraction 5 (TF5) is a partially purified extract of bovine thymus containing 40-60 peptides. In addition to its well documented immunopotentiating effects, TF5 reportedly modulates the secretion of some hypothalamic peptides and pituitary hormones. In this study, TF5 (10-100 micrograms/ml) stimulated PRL release from normal, MtTW15, and 7315a cells and GH release from normal and MtTW15 cells, but had no apparent effect on LH release. No changes in intracellular cAMP or cGMP levels could be correlated with these responses. Stimulation of PRL release from perifused normal anterior pituitary cells was rapid, sustained, and concentration related. Although it had no apparent effect on normal prelabeled anterior pituitary cells with respect to 45Ca2+ efflux, the calcium channel blocker D-600 inhibited TF5-mediated hormone release from these cells. Additive increases in TRH-stimulated PRL release and GRF-stimulated GH release by TF5 suggested independent mechanisms of action. Dopamine (500 nM) blocked TF5-stimulated PRL release, but somatostatin (10-100 nM) had no effect on TF5-stimulated PRL or GH release. TF5 failed to affect either basal or TRH-induced polyphosphoinositide hydrolysis. Perifused normal anterior pituitary cells prelabeled with [3H]arachidonate responded to TF5 treatment with a liberation of radioactive arachidonate and/or its metabolites. BW755c, an inhibitor of all known catabolic pathways of arachidonic acid, blocked the ability of TF5 to stimulate PRL and GH release. Reversed phase HPLC separation of TF5 into five fractions resulted in two fractions that exhibited hormone-releasing activity. These data suggest that TF5 stimulates pituitary hormone release through a mechanism different from that ascribed to TRH or GRF. The stimulus-secretion coupling mechanism involves neither polyphosphoinositide hydrolysis nor cAMP generation, but appears to be dependent on the generation of arachidonate metabolites.

Immunoglobulins of Lambert-Eaton myasthenic syndrome inhibit rat pituitary hormone release

We speculated that IgG from patients with Lambert-Eaton myasthenic syndrome (LES) would inhibit calcium-dependent hormone secretion in rat anterior pituitary (AP) cells. Primary cultures of normal AP cells were continuously exposed to crude IgG from an LES patient or from healthy control subjects, and the incubation media were assayed for prolactin (PRL) or growth hormone (GH). The LES IgG caused a time- and concentration-dependent reduction of PRL and GH release compared with control IgG over 0.5 to 48 hours using concentrations of 0.01 to 4.0 mg/ml. The calcium-channel activator maitotoxin stimulated 45Ca2+ uptake and PRL release from AP cells under control conditions, and the LES IgG significantly reduced both actions of maitotoxin. Thus LES IgG appears to modulate an AP antigenic site, perhaps representing an integral component of voltage-gated calcium channels that may share properties with similar presynaptic elements at the neuromuscular junction.

Impaired calcium mobilisation in the 7315a prolactin-secreting pituitary tumour

The 7315a tumour secretes prolactin, but is refractory to enhancement of prolactin release by thyrotrophin-releasing hormone (TRH). In order to investigate further this refractoriness of the 7315a tumour cell, we compared cells from the tumour and from the normal pituitary with regard to TRH-enhanced fractional 45Ca2+ efflux and inositol phosphate production. TRH caused a large efflux of calcium from normal pituitary cells, but only mildly enhanced calcium efflux from the tumour cells. In contrast, TRH enhanced total inositol phosphate generation in both groups of cells to a similar degree. We therefore conclude that prolactin release from 7315a tumour cells is refractory to TRH due, at least in part, to impaired mobilisation of intracellular calcium by inositol phosphates.

Trophic effects of somatostatin on calcium flux: dynamic analysis and correlation with pituitary hormone release

Somatostatin (SRIF) modulates many aspects of normal physiology, appears useful as a potent antineoplastic agent, and may influence the development of degenerative brain disorders such as Alzheimer's disease. Regulation of cellular calcium flux by SRIF may contribute greatly to many of these interactions, yet remains controversial. SRIF rapidly causes a sustained inhibition of fractional calcium efflux from prelabeled dispersed rat pituitary cells (IC50, approximately 40 pM) and evokes a large rebound increase in efflux after the infusion, each coinciding temporally with expected physiological effects on GH release. These data support a particular role for SRIF-regulated calcium flux in the normal pulsatile pattern of GH secretion and a more general role in the varied biological actions of the peptide.

Nafazatrom, an arachidonate metabolism inhibitor, decreases prolactin and GH release

Nafazatrom, an inhibitor of arachidonate metabolism by the lipoxygenase enzymes, decreases basal prolactin and growth hormone release in a concentration-dependent manner without significantly affecting the synthesis of either hormone. It is required that nafazatrom be incubated with pituitary cells for approximately 2 h for this effect to become evident; the blockade increases in magnitude for up to 4 h. Nafazatrom blocks the increase in prolactin release caused by the releasing factors TRH and angiotensin II and the increase in prolactin and growth hormone release due to the calcium channel activator maitotoxin. These data suggest that the lipoxygenase products of arachidonate metabolism may be important mediators in basal and secretagogue-induced release of prolactin and growth hormone.

Dopamine inhibits maitotoxin-stimulated pituitary 45Ca2+ efflux and prolactin release

We examined the hypothesis that dopaminergic inhibition of prolactin release is coupled to modulation of cellular calcium flux. Dispersed female rat pituitary cells were prelabeled in 45Ca2+ and perifused to determine simultaneously fractional calcium efflux and prolactin release, as stimulated by maitotoxin, a calcium channel activator. The integrated response of each parameter to 5 ng/ml maitotoxin was obtained in individual perifusion columns in the absence or presence of various concentrations of dopamine. Maitotoxin-stimulated calcium efflux was suppressed by dopamine concentrations of 0.01 microM and greater (P less than 0.01) and achieved a maximal effect at approximately 0.1 microM, at which calcium efflux was reduced by 50%. Maitotoxin-stimulated prolactin release was inhibited by 0.03 microM dopamine and greater concentrations (P less than 0.01), and at a concentration of approximately 10.0 microM dopamine the effect became maximal at approximately 85% suppression. Haloperidol (0.1 microM) blocked the effects of 0.1 microM dopamine on both parameters. Simultaneous suppression of maitotoxin-stimulated calcium efflux and prolactin release by concentrations of dopamine within the nanomolar range suggests that dopamine receptor activation is negatively coupled to modulation of calcium flux in the physiological regulation of prolactin secretion.

Profound cerebral vasospasm without radiological evidence of subarachnoid hemorrhage: case report

Profound clinical and radiographic arterial vasospasm in all major intracerebral vessels was identified in a patient with an intraparenchymal hemorrhage that extended into the lateral, 3rd, and 4th ventricles. By computed tomographic criteria and on early cerebrospinal fluid examination, no subarachnoid blood was detected. The temporal appearance of vasospasm was consistent with that typically seen in subarachnoid hemorrhage. Despite hypervolemia and iatrogenic hypertension, cerebral infarction occurred contralateral to the hemorrhage. We conclude that patients without significant subarachnoid hemorrhage (as determined by computed tomographic scanning) may still be at risk for developing vasospasm.

Maitotoxin, a calcium channel activator, increases prolactin release from rat pituitary tumor 7315a cells by a mechanism that may involve leukotriene production

Arachidonate and its metabolites may play an important role in the release of prolactin. In the present study, the effect of maitotoxin, a calcium channel activator, was measured on the release of arachidonate and its metabolites from the prolactin-secreting 7315a tumor. Maitotoxin increased the release of prolactin, arachidonate, prostaglandins E2 and F2 alpha (PGE2, PGF2 alpha) and leukotriene C4 (LTC4) from 7315a cells prelabeled with [3H]arachidonate. The magnitude of the increase of prolactin and arachidonate release was decreased in low-calcium medium. The release of arachidonate from cellular phospholipids is necessary for the effect of maitotoxin on prolactin release because quinacrine, an inhibitor of arachidonate hydrolysis from phospholipids, blocked the maitotoxin-induced release of prolactin. The ability of maitotoxin to induce prolactin release appears to require metabolic transformation of arachidonate to its metabolites because BW755c, an inhibitor of the conversion of arachidonate, blocked the maitotoxin-induced prolactin release. In particular, LTC4 may be an important component of the prolactin release process because nordihydroguaiaretic acid and nafazatrom, which block the production of leukotrienes and other lipoxygenase-generated products, decreased LTC4 and prolactin release without affecting arachidonate, PGE2 or PGF2 alpha production. In contrast, indomethacin, a prostaglandin synthesis inhibitor, decreased PGE2 and PGF2 alpha production without affecting LTC4 or prolactin release. These data indicate that release of LTC4 and prolactin are closely linked events in 7315a tumor cells.

Association of 45Ca2+ mobilization with stimulation of growth hormone (GH) release by GH-releasing factor in dispersed normal male rat pituitary cells

Dispersed normal male rat anterior pituitary cells were prelabeled with 45Ca2+ and perifused to study the influence of GH-releasing factor (GRF) on fractional calcium efflux and GH release. The cells were exposed for 2 min to 0, 0.03, 0.1, 0.3, 1.0, or 10.0 nM GRF in separate perifusion columns, and the response to each concentration was determined by integration of the area under the curve. Concentrations of 0.1 nM GRF and higher produced a simultaneous and significant stimulation of calcium efflux and GH release. The increase in calcium efflux was proportional to GRF concentration and was maximally responsive at 1 nM GRF. The value for the entire integrated response of GH release increased continuously with GRF concentration, but GH released rapidly (0-4 min) in response to GRF achieved a maximal response at 1 nM GRF and was significantly correlated with calcium efflux. Somatostatin (100 nM) abolished the stimulation of GH release and calcium efflux due to 10 nM GRF. We conclude that GRF receptor activation is intimately associated with calcium mobilization, although the relative dependence upon intracellular or extracellular calcium sources has yet to be defined. This interaction occurs at a GRF concentration about 10 times lower than that observed to cause a measurable increase in intracellular biochemical messengers such as cAMP, phosphatidylinositol, or arachidonate. We postulate that GRF-stimulated calcium mobilization is a rapid and very sensitive event contributing to GRF-stimulated GH release.

Maitotoxin activates quantal transmitter release at the neuromuscular junction: evidence for elevated intraterminal Ca2+ in the motor nerve terminal

Maitotoxin (MTX), applied in vitro to the mouse neuromuscular junction, briskly activates the spontaneous release of acetylcholine quanta, manifest as miniature end-plate potentials (MEPPs). This effect requires external Ca2+ and is accompanied by a steady post-junctional depolarization. After the peak activation of the spontaneous release process, the quantal discharge gradually declines with eventual abolishment of MEPPs. In contrast to the striking increase in MEPP frequency, the quantum content of the nerve-evoked end-plate potentials (EPPs) is increased only moderately by MTX. These effects are attributable to the ability of the toxin to elevate the level of free intracellular Ca2+ in the motor nerve terminal. With further characterization of its presynaptic site of action, maitotoxin may become a useful tool in studying synaptic physiology.

Dopamine decreases 7315a tumor cell prolactin release induced by calcium mobilization

The rat pituitary tumor 7315a secretes PRL and ACTH. Although dopamine has no effect on unstimulated PRL release from this tumor, dopamine decreases the adenylate cyclase activity in tumor cell homogenates in a manner similar to that in normal pituitary tissue. However, it was observed that under basal conditions, 7315a tumor cells have an abnormal calcium metabolism because 1) basal PRL release from tumor cells is not modified by the calcium channel blocker D-600 and is only moderately decreased by low calcium, treatments that markedly decrease normal pituitary PRL release; 2) D-600 had no effect on basal 7315a tumor calcium uptake, but blocked the increase in calcium uptake due to the calcium channel activator maitotoxin; 3) increasing the medium Ca+2 concentration above 5 mM increases 7315a PRL release, whereas this treatment decreases PRL release from normal pituitary cells. Maitotoxin and the calcium ionophore A23187 increased 7315a tumor cell PRL release in a manner similar to that in normal pituitary cells. Because dopamine blocks PRL release induced by maitotoxin, A23187, or elevated medium calcium concentration in 7315a tumor cells, the refractoriness of basal 7315a tumor cell PRL release to dopamine may be due to the abnormal calcium balance of the tumor cells under basal conditions.

Encephalomyeloneuritis simulating Guillain-Barré syndrome

A patient with encephalomyeloneuritis (EMN) had clinical and laboratory features consistent with severe acute inflammatory polyneuropathy (Guillain-Barré syndrome). CNS involvement was suggested clinically only by transient downbeat nystagmus and by contraction of tensor fascia lata on plantar stimulation. Postmortem examination revealed pathologic changes typical of EMN without systemic neoplasm. Clinical manifestations of the profound central pathology were largely obscured by severe radiculoneuropathy. Pathologic verification of clinically diagnosed inflammatory polyneuropathy is unusual, and CNS disease, therefore, may be more frequent than appreciated, especially in clinical "variants."

The effects of maitotoxin on 45Ca2+ flux and hormone release in GH3 rat pituitary cells

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.

Reserpine is a calcium channel antagonist in normal and GH3 rat pituitary cells

Reserpine exerts direct effects on several tissues, including inhibition of hormone release from rat anterior pituitary cells. To test the hypothesis that reserpine may be acting as a calcium channel antagonist, normal or GH3 rat anterior pituitary cells were preincubated in reserpine or the conventional calcium channel blocker, D-600, followed by exposure to 45Ca2+ together with stimulants of calcium uptake: maitotoxin, a potent calcium channel activator; A23187, a calcium ionophore; or 50 mMK+. After incubation, the cells were harvested by vacuum filtration and cell-associated radioactivity determined. In normal cells, reserpine blocked both basal and K+-stimulated calcium uptake. Reserpine selectively blocked maitotoxin but not A23187-induced calcium uptake. In GH3 cells 9 microM reserpine and 30 microM D-600 were equally effective in blocking maitotoxin-stimulated calcium uptake. Reserpine appears to block voltage-dependent calcium channels in pituitary cells in a concentration-dependent manner but not calcium uptake caused nonspecifically by A23187.

Maitotoxin stimulates hormonal release and calcium flux in rat anterior pituitary cells in vitro

The marine dinoflagellate toxin maitotoxin (MTX), an activator of calcium channels, stimulates the release of prolactin (PRL), growth hormone (GH), thyroid-stimulating hormone (TSH), and luteinizing hormone (LH) from monolayers of anterior pituitary cells in a dose-dependent manner. Maitotoxin significantly increased PRL, GH, and LH release within 1.5 min and TSH release within 3.5 min, and the stimulation continued for at least 1 h (P less than 0.01). MTX-stimulated hormonal release was blocked by the calcium channel blocker manganese (P less than 0.01). In freshly dispersed perifused pituitary cells in columns, exposure to MTX for 10 min markedly increased PRL, GH, TSH, and LH release for at least 1 h after withdrawal of the toxin. In other experiments, MTX significantly stimulated 45Ca2+ exchange by dispersed pituitary cells within 30 s, continuing for at least 30 min. We conclude that MTX increases anterior pituitary hormonal release, possibly by activating calcium channels, thereby increasing cellular calcium influx. Thus MTX may be a useful agent for investigating the involvement of Ca2+ in hormonal secretory processes.

Zinc acutely, selectively and reversibly inhibits pituitary prolactin secretion

Perifusion of dispersed female rat pituitary cells with medium containing 50 microM zinc acetate caused an acute, sustained and rapidly reversible inhibition of prolactin (Prl) secretion. This treatment had no influence on basal release of thyrotropin stimulating hormone (TSH), luteinizing hormone (LH) or growth hormone (GH). 50 microM Zn2+ also reversibly inhibited prolactin secretion stimulated by either 50 mM K+ or 10 nM TRH, but the secretion of GH, TSH and LH which was stimulated by 50 mM K+ or stimulation of TSH by 10 nM TRH was not inhibited. Thus zinc acts in a dynamic manner to selectively influence pituitary prolactin secretion.