Cerebral arteriovenous malformation in pregnancy: presentation and neurologic, obstetric, and ethical significance

Cerebral arteriovenous malformations infrequently complicate pregnancy. We sought to determine the neurologic, obstetric, and ethical significance of such malformations. We present the clinical course of 2 pregnant women with arteriovenous malformations who experienced cerebral hemorrhage and a loss of capacity for decision making. We also review the neurologic and obstetric significance of arteriovenous malformations in pregnancy. Various treatment options with concern for pregnancy and the prognosis for arteriovenous malformations are outlined. The ethical issues involved for pregnant patients whose decisional capacity is compromised as a result of cerebral injury are explored. A review of persistent vegetative state and brain death (death by neurologic criteria) occurring in pregnancy allows us to explore many issues that are applicable to decisionally incapacitated but physiologically functioning pregnant women. We outline a document, the purpose of which is to obtain advance directives from pregnant women regarding end-of-life decisions and to appoint a surrogate decision maker. We believe that evaluation and treatment of the arteriovenous malformation may be undertaken without regard for the pregnancy and that the pregnancy should progress without concern for the arteriovenous malformation.

Muscimol increases acetylcholine release by directly stimulating adult striatal cholinergic interneurons

Because GabaA ligands increase acetylcholine (ACh) release from adult striatal slices, we hypothesized that activation of GabaA receptors on striatal cholinergic interneurons directly stimulates ACh secretion. Fractional [3H]ACh release was recorded during perifusion of acutely dissociated, [3H]choline-labeled, adult male rat striata. The GabaA agonist, muscimol, immediately stimulated release maximally approximately 300% with EC50 = approximately 1 microM. This action was enhanced by the allosteric GabaA receptor modulators, diazepam and secobarbital, and inhibited by the GabaA antagonist, bicuculline, by ligands for D2 or muscarinic cholinergic receptors or by low calcium buffer, tetrodotoxin or vesamicol. Membrane depolarization inversely regulated muscimol-stimulated secretion. Release of endogenous and newly synthesized ACh was stimulated in parallel by muscimol without changing choline release. Muscimol pretreatment inhibited release evoked by K+ depolarization or by receptor-mediated stimulation with glutamate. Thus, GabaA receptors on adult striatal cholinergic interneurons directly stimulate voltage- and calcium-dependent exocytosis of ACh stored in vesamicol-sensitive synaptic vesicles. The action depends on the state of membrane polarization and apparently depolarizes the membrane in turn. This functional assay demonstrates that excitatory GabaA actions are not limited to neonatal tissues. GabaA-stimulated ACh release may be prevented in situ by normal tonic dopaminergic and muscarinic input to cholinergic neurons.

Pyridostigmine treatment selectively amplifies the mass of GH secreted per burst without altering GH burst frequency, half-life, basal GH secretion or the orderliness of GH release

Growth hormone (GH) release from the anterior pituitary gland is predominantly regulated by the two antagonistic hypothalamic peptides, growth hormone-releasing hormone (GHRH) and somatostatin. Appraising endogenous GHRH action is thus made difficult by the confounding effects of (variable) hypothalamic somatostatin inhibitory tone. Accordingly, to evaluate endogenous GHRH actions, we used a clinical model of presumptively acute endogenous somatostatin withdrawal with concomitant GHRH release. To this end, we administered in randomized order placebo or the indirect cholinergic agonist, pyridostigmine, for 48 h to 13 healthy men of varying ages (29-77 years) and body mass indices (21-47 kg/m2). We sampled blood at 10-min intervals for 48 h during both placebo and pyridostigmine (60 mg orally every 6 h) administration, and used an ultrasensitive GH chemiluminescence assay (sensitivity 0.0002-0.005 microgram/l) to capture GH pulse profiles. Multiparameter deconvolution analysis was applied to quantitate the number, amplitude, mass, and duration of significant underlying GH secretory bursts, and simultaneously estimate the GH half-life and concurrent basal GH secretion. Approximate entropy was utilized as a novel regularity statistic to quantify the relative orderliness of the hormone release process. All measures of GH secretion/half-life and orderliness were statistically invariant across the two consecutive 24-h placebo sessions. In contrast, pyridostigmine treatment significantly increased the mean serum GH concentration from 0.23 +/- 0.054 microgram/l during placebo to 0.45 +/- 0.072 microgram/l during the first day of treatment (P < 0.01). There was also a significant rise in the calculated 24-h pulsatile GH production rate from 8.9 +/- 1.7 micrograms/l/day on placebo to 27 +/- 5.6 micrograms/l/day during active drug treatment (P < 0.01). Pyridostigmine significantly and selectively amplified GH secretory burst mass to 1.5 +/- 0.35 micrograms/l compared with 0.74 +/- 0.19 microgram/l on placebo (P < 0.01). This was attributable to stimulation of GH secretory burst amplitude (maximal rate of GH secretion attained within the release episode) with no prolongation of estimated burst duration. Basal GH secretion and approximate entropy were not altered by pyridostigmine. However, age was strongly related to more disorderly GH release during both days of pyridostigmine treatment (r = +0.79, P = 0.0013). During the second 24-h of continued pyridostigmine treatment, most GH secretory parameters decreased by 15-50%, but in several instances remained significantly elevated above placebo. Body mass index, but not age, was a significantly negative correlate of the pyridostigmine-stimulated increase in GH secretion (r = -0.65, P = 0.017). In summary, assuming that somatostatin is withdrawn and (rebound) GHRH release is stimulated via pyridostigmine administration, we infer that relatively unopposed GHRH action principally controls GH secretory burst mass and amplitude, rather than apparent GH secretory pulse duration, the basal GH secretion rate, or the serial regularity/orderliness of the GH release process in the human. Moreover, we infer that increasing age is accompanied by greater disorderliness of somatostatin-withdrawn GHRH, and hence rebound GH, release. The strongly negative correlation between pyridostigmine-stimulated GH secretion and body mass index (but not age) further indicates that increased relative adiposity may result in decreased effective (somatostatin-withdrawn) endogenous GHRH stimulus strength.

Carbachol inhibits basal and forskolin-evoked adult rat striatal acetylcholine release

Acutely dissociated adult rat striatal cholinergic neurons labeled with [3H]choline were used in a perifusion system to study muscarinic regulation of basal and forskolin-stimulated fractional [3H]acetylcholine ([3H]-ACh) efflux in the absence of synaptic modulation. Carbachol inhibited basal (40% maximal inhibition; IC50 approximately 0.7 microM) and forskolin-evoked release (75% inhibition; IC50 approximately 0.05 microM) in a concentration-dependent manner, and both carbachol actions were abolished with atropine. Thus, activation of striatal muscarinic cholinergic autoreceptors potently inhibits basal and adenylate cyclase-stimulated ACh release. Tonic inhibitory control of cholinergic activity by functional striatal circuitry apparently prevents detection of these important physiological interactions in slices or in situ.

D2 dopamine receptor activation inhibits basal and forskolin-evoked acetylcholine release from dissociated striatal cholinergic interneurons

We tested whether D2 ligands inhibit basal and forskolin-stimulated [3H]ACh release from dissociated striata, as opposed to striatal slices. Quinpirole inhibited both basal (40% maximal inhibition; IC50 approximately 50 nM) and 10 microM forskolin-stimulated release (80% inhibition; IC50 approximately 25 nM quinpirole) and both actions were blocked by a D2 antagonist. Vesamicol prevented the quinpirole and forskolin actions. The ability of D2 agonists to inhibit basal and cyclase-stimulated acetylcholine release emanating from vesamicol-sensitive vesicles appears to be tonically suppressed by inhibitory elements within striatal circuitry.

Adenylate cyclase in striatal cholinergic interneurons regulates acetylcholine release

Fractional [3H]ACH efflux from dissociated rat striata tested whether tonic inhibition prevents stimulation of acetylcholine (ACH) release by adenylate cyclase. Forskolin stimulated release from the dissociated cells (threshold at 300 nM; EC50 > or = 1 MicroM). Release was also stimulated by 3-isobutyl-1-methylxanthine and was additive with forskolin. The 1,9-dideoxy forskolin analog that lacks cyclase-stimulating activity was ineffective. Thus, stimulation of adenylate cyclase within striatal cholinergic interneurons increases ACH secretion but is tonically inhibited by endogenous striatal transmitters. Disinhibition of the excitatory cyclase by denervation of striatal cholinergic interneurons in situ could contribute to supersensitivity without receptor upregulation.

A D1 dopamine agonist stimulates acetylcholine release from dissociated striatal cholinergic neurons

We tested the hypothesis that a D1 dopamine agonist could stimulate acetylcholine release directly from striatal cholinergic neurons. A suspension of viable dissociated striatal cells was made enzymatically and mechanically from normal adult male rats. The heterogeneous suspension was incubated in [3H]choline to allow synthesis of [3H]acetylcholine selectively by cholinergic neurons. Fractional [3H]acetylcholine release from the cholinergic cells in the suspension was recorded during continuous dynamic perifusion. The D1 agonist, 50 microM (+/-) SKF 38393, increased the basal rate of release from the cholinergic cells by 50% and the action was inhibited by the D1 antagonist, SKF 83566. Stimulation of [3H]acetylcholine secretion was recorded as low as 500 nM SKF 38393. The (S, -) SKF 38393 stereoisomer was significantly less effective than the (R, +) isomer in stimulating release. The D1-mediated stimulation of acetylcholine secretion was abolished in a low-calcium environment that also inhibited basal release. The data suggest that striatal cholinergic cells express D1 receptors functionally coupled to the regulation of acetylcholine release. These D1 actions in the absence of synaptic circuitry imply that such circuitry is not required in situ. In vivo however, indirectly mediated D1 actions and those of other transmitters may modify the manifestations of this direct cholinergic stimulation.

Acetylcholine release from dissociated striatal cells

To study the regulation of striatal acetylcholine (ACH) release, adult male rat striata were dissociated and incubated with 3H-choline to synthesize 3H-ACH. Fractional 3H-ACH efflux per min during continuous perifusion was: (1) tightly regulated; (2) dependent on calcium influx; (3) stimulated by 10 mM K+ and 1 mM glutamate; and (4) comparable to ACH release detected by HPLC. Thus, acutely dissociated striata exhibit calcium-sensitive, voltage-dependent secretion of 3H-ACH and direct receptor-mediated stimulation of release through the glutamate receptor family. This new approach toward cholinergic secretory physiology will help clarify complex striatal circuitry.

Acute dopamine depletion potentiates independent stimulatory and inhibitory D1 DA receptor-mediated control of striatal acetylcholine release in vitro

Fractional release of [3H]ACh was evaluated under basal and evoked conditions in striatal slices from normal and acutely dopamine-depleted adult rats for the influence of D1- and D2-DA receptor agonists. The D1 ligand had no effect on normal slices but DA depletion unmasked two independent but simultaneous supersensitive responses: augmentation of K(+)-evoked and inhibition of glutamate-evoked release. The D2 ligand inhibited evoked release in normal slices and this effect was not potentiated. This is a new cholinergic model of acute D1 receptor supersensitivity.

31P NMR investigation of energy metabolism in perifused MMQ cells

The MMQ cell line is a unique prolactin-secreting rat pituitary cell line. MMQ cells entrapped in agarose gel threads are metabolically active, as determined by the uptake and phosphorylation of creatine and the maintenance of high energy phosphates for over 15 h. Forskolin activates the catalytic subunit of adenylyl cyclase and, in MMQ cells, elevates the level of cAMP and stimulates prolactin secretion. 31P NMR spectroscopy was used to investigate the energy metabolism of the MMQ cells during stimulation by forskolin. The ability to measure small changes in the energy status of these cells was enhanced by increasing the PCr levels in the cells. Administration of forskolin to the perifused MMQ cells resulted in acute, reversible, and dose-dependent changes in the 31P NMR spectra of the cells within 12 to 24 min of the beginning of forskolin exposure. Several lines of evidence indicate that the changes observed in the MMQ cells are the composite result of the interaction of forskolin with adenylyl cyclase and the plasma membrane glucose transporter. Also, preincubation of the MMQ cells with the dopamine agonist, bromocriptine, attenuates the forskolin-stimulated decrease in the PCr resonance by approximately 50%. This attenuation indicates that the forskolin-stimulated changes in energy metabolism are probably related to the prolactin secretion process.

Role of calcium in dopaminergic regulation of TRH- and angiotensin II-stimulated prolactin release

The contributions of intracellular and extracellular calcium to thyrotropin-releasing hormone (TRH)- and angiotensin II (ANG II)-stimulated prolactin (PRL) release and the role of calcium in dopaminergic inhibition of these events were examined because of unresolved controversies in these areas. Dispersed normal female rat anterior pituitary cells were used to evaluate radiocalcium fluxes and the intracellular calcium concentration ([Ca]i). Both peptides increased PRL release, fractional 45Ca2+ efflux, and [Ca]i in a spike and plateau pattern, and neither increased 45Ca2+ uptake. In a low-calcium buffer, TRH and ANG II stimulated less than 5% of the normal PRL response, yet efflux was at least 50% of normal and [Ca]i was 20-40% of normal. Dopamine reduced TRH-stimulated PRL release by greater than 90% and abolished the plateau, yet the calcium responses to TRH were at least 50% of normal. Although dopamine prevented the plateau component of peptide-stimulated [Ca]i, the plateau phase of efflux persisted. Thus TRH and ANG II may control at least two cell-associated calcium pools, one readily depleted and the other highly resistant to depletion, without evidence for stimulation of calcium uptake. Dopamine inhibits PRL release stimulated by these peptides, with a relatively greater influence on the plateau component, through mechanisms only minimally related to calcium flux. Dopamine may slightly increase the extrusion of calcium mobilized by these peptides and thus may limit the anticipated increase in [Ca]i.

Interactions of dopamine and neurotensin on calcium fluxes and prolactin release in normal rat pituitary cells

We studied the role of calcium in dopaminergic control of the neuroendocrine effects of neurotensin. In primary cultures of dispersed normal female rat anterior pituitary cells the interactions of dopamine and neurotensin were examined with reference to the rate of PRL release, the magnitude of 45Ca2+ uptake, the rate of fractional 45Ca2+ efflux, and the dynamic response of the intracellular calcium concentration (Cai) monitored with the fluorescent dye, Indo-1. Neurotensin stimulated calcium uptake and also mobilized a pool of intracellular calcium to increase Cai in a sustained plateau-like pattern. The response of PRL release and fractional efflux to neurotensin, however, each displayed typical spike and plateau profiles. In the presence of dopamine the stimulation of PRL release and calcium uptake due to neurotensin were abolished, and the rise in Cai was barely detectable, but neurotensin-stimulated fractional efflux persisted almost unchanged. These data suggest that dopamine may modulate Cai by inhibiting calcium uptake and possibly also by enhancing cellular calcium extrusion under stimulated conditions. Further, the increased inositol trisphosphate production reportedly stimulated by neurotensin apparently does not generate a spike-like response of intracellular calcium, and stimulated hormone release may display a spike and plateau pattern solely with a plateau Cai profile.

A comparison of the concentration-dependent actions of thyrotropin-releasing hormone, angiotensin II, bradykinin, and Lys-bradykinin on cytosolic free calcium dynamics in rat anterior pituitary cells: selective effects of dopamine

Using Indo-1 as a fluorescent probe, we studied the dynamics and the underlying mechanisms of the response of cytosolic free calcium ([Ca2+]i) to different concentrations of four prolactin secretagogues, thyrotropin-releasing hormone, angiotensin II, bradykinin, and lys-bradykinin in rat anterior pituitary cells. Low concentrations (1-100 pM) of these peptides caused a sustained increase in [Ca2+]i, whereas high concentrations (up to 100 nM) caused a large transient elevation of [Ca2+]i that was followed by a lower sustained plateau. Experiments with protein kinase C-depleted cells suggested that phorbol diester-sensitive protein kinase C was not involved in the transition of [Ca2+]i from spike to plateau seen with high concentrations of secretagogue. Specific concentrations of secretagogue mobilized different pools of [Ca2+]i, as indicated by experiments with Ca2(+)-depleted medium. Low concentrations of secretagogue induced a Ca2+ response that was abolished by Ca2(+)-depleted medium, whereas high concentrations generated a [Ca2+]i response that was refractory to Ca2(+)-depleted medium. Dopamine (100 nM) abolished the [Ca2+]i plateau response to all four agents at low concentrations and selectively reduced the plateau component of the responses elicited at high concentrations of secretagogue. If the plateau component is represented by utilization of either extracellular Ca2+ or a cell-associated EGTA-accessible pool(s) of Ca2+, then dopamine modulates one or both of these calcium sources.

Physiological and biochemical effects of bradykinin and lys-bradykinin in pituitary cells

The presence of kallikrein activity, bradykinin (BK) and lys-bradykinin (LBK) in the pituitary gland suggests a possible physiological role of kinins therein. We demonstrated that BK and LBK increased prolactin (PRL), but not growth hormone release, from rat anterior pituitary cells cultured in vitro. Such stimulatory effect on PRL secretion appears to involve B2-type BK receptors, as suggested by the antagonizing effect of B6572 (a B2-type BK receptor antagonist) on PRL release. The BK-induced increase in PRL release is associated with an enhanced [3H]arachidonate (AA) efflux, an elevated cytosolic free calcium concentration [(Ca2+]i), and increased inositol phosphate (InsPx) production. Bradykinin and LBK stimulated [3H]AA liberation, [Ca2+]i elevation and PRL release at lower concentrations than those necessary to stimulate InsPx production. Therefore, AA release and [Ca2+]i elevation may be more important to PRL release than is InsPx production. Dopamine (DA) inhibited BK- or LBK-stimulated PRL release and slightly attenuated the stimulated [Ca2+]i response, but had no effect on stimulated [3H]AA efflux and InsPx generation. This study suggests that BK and LBK may have either an autocrine or a paracrine role in regulating PRL secretion, and are subject to modulation by DA.

Regulation of the intracellular calcium concentration in MMQ pituitary cells by dopamine and protein kinase C

The MMQ pituitary cell line, which expresses a membranal dopamine receptor, was used to examine the individual contributions of dopamine and protein kinase C (PKC) to control of the intracellular calcium concentration. The calcium concentrations, monitored with the fluorescent dye Indo-1, increased in response to elevated K+, BAY K8644, and maitotoxin, implicating the presence of voltage-dependent calcium channels. Dopamine had no detectable independent effect, but significantly inhibited the rise in intracellular calcium mediated by activation of voltage-dependent calcium channels; this dopaminergic action was prevented by haloperidol. Acute pharmacological activation of PKC for 60 s inhibited the stimulatory effects of these calcium channel activators, and this acute inhibitory action was abolished by prior depletion of PKC. In contrast, however, PKC depletion did not alter the calcium response to BAY K8644 or maitotoxin. Thus, MMQ cells appear to have voltage-dependent calcium channels which, at rest, are either at low density or in a closed state. The rise in intracellular calcium resulting from stimulation of the channels is under inhibitory control by an apparent D-2 dopamine receptor. When pharmacologically activated, phorbol diester-sensitive PKC limits the rise in the cellular calcium level associated with calcium uptake. In the absence of pharmacological activation, however, this enzyme system does not appear to play a role in the cellular calcium response to BAY K8644 or maitotoxin.

Dopamine enhances a voltage-dependent transient K+ current in the MMQ cell, a clonal pituitary line expressing functional D2 dopamine receptors

The influence of dopamine on voltage-dependent K+ current (IK) was studied in cultured MMQ cells using the whole-cell patch-clamp technique. IK in nearly all MMQ cells revealed a transient outward current component and inactivated during maintained depolarization lasting 60 ms. The transient component was inhibited by prepulse potentials more positive than -40 mV or by addition of 4 mM 4-aminopyridine to the bathing solution and was insensitive to the external Ca2+ concentration. Thus, this transient K+ current resembled the A-current (IA) found in other cells. Dopamine at 1 microM increased by 50% (P less than 0.001) the peak of IK evoked by a test potential to +80 mV and the response was prevented by pretreatment with 100 nM haloperidol, a D2 receptor antagonist. These data suggest that MMQ clonal pituitary cells possess a voltage-gated K+ A-current and that this current can be modulated by dopamine via D2 receptors.

Evidence that phorbol diester-sensitive protein kinase-C(s) may not be directly involved in secretagogue-stimulated prolactin release and arachidonate liberation

This report presents findings pertaining to the role of protein kinase-Cs in the release of PRL and liberation of arachidonate from PRL-secreting cells. In our experiments, protein kinase-C activators increased PRL release and arachidonate liberation from anterior pituitary cells and from the PRL-secreting cell line MMQ. In cells depleted of pituitary protein kinase-Cs by chronic exposure to protein kinase-C activators, such as phorbol dibutyrate or 4 beta-phorbol 12 beta-myristate 13 alpha-acetate, TRH, angiotensin-II, and neurotensin each increased PRL release and [3H]arachidonate liberation in a normal manner. In addition, the PRL-releasing activities of protein kinase-C activators and those of TRH appeared to be synergistic, an unexpected effect if these substances were functioning through the same intracellular pathways. It, therefore, appears that phorbol diester-sensitive protein kinase-Cs may not be involved in the increased secretion of PRL or liberation of arachidonate that is caused by TRH, angiotensin-II, or neurotensin.

Estradiol attenuates prolactin secretion and phosphoinositide hydrolysis in MMQ cells

We previously isolated a clonal cell line, designated MMQ, which only secretes prolactin (PRL) and whose secretory process is nonresponsive to thyrotropin releasing hormone (TRH) and angiotensin II (AII). In the present study, we injected MMQ cells into rats to determine whether the tumor cells would become responsive to secretagogues when subsequently propagated in vitro. We also investigated what effects in vivo administration of 17 beta-estradiol would have on secretagogue-induced PRL release and on intracellular biochemical mechanisms in these cells. MMQ cells were implanted subcutaneously in the backs of female rats. One group was injected with 100 micrograms polyestradiol phosphate (PEP) every 5 days, a second with saline. The inoculants grew into solid tumors within 3 weeks. The day after the tumors were removed and enzymatically dispersed, the cells, now designated MMQt cells, were perifused in vitro. Basal PRL released by MMQt cells was approximately 1 ng/min/10(7) cells and perifusions with 100 nM TRH or AII for 5 min significantly increased PRL release above baseline (integrated areas: 1.8 +/- 0.4 and 5.2 +/- 1.3 ng/10(7) cell, respectively; P less than 0.01). Two ng/ml maitotoxin (MTX), a calcium channel activator, increased PRL release (38.2 +/- 6.7 ng/10(7) cells; P less than 0.01). In PEP-treated perifused MMQt cells, basal in vitro PRL release was not different from that observed in the control group, but the responses to TRH, AII and MTX were greatly attenuated (TRH: 0.6 +/- 0.1, AII: 1.3 +/- 0.2 and MTX: 9.2 +/- 2.5 ng/10(7) cells).(ABSTRACT TRUNCATED AT 250 WORDS)

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.