Regulation of hormone secretion by acute cell volume changes: Ca(2+)-independent hormone secretion

Pituitary somatolactotropes evade an oncogenic response to Ras

Distinct cell types have been shown to respond to activated Ras signaling in a cell-specific manner. In contrast to its pro-tumorigenic role in some human epithelial cancers, oncogenic Ras triggers differentiation of pheochromocytoma cells and medullary thyroid carcinoma cells. Furthermore, we have previously demonstrated that in pituitary somatolactotropes, activated Ras promotes differentiation and is not sufficient to drive tumorigenesis. These findings demonstrate that lactotrope cells have the ability to evade the tumorigenic fate that is often associated with persistent activation of Ras/ERK signaling, and suggest that there may be differential expression of inhibitory signaling molecules or negative cell cycle regulators that act as a brake to prevent the tumorigenic effects of sustained Ras signaling. Here we aim to gain further insight into the mechanisms that allow GH4T2 cells to evade an oncogenic response to Ras. We show that Ral, but likely not menin, plays a key role in directing Ras-mediated differentiation of somatolactotropes, which may allow these cells to escape the tumorigenic fate that is often associated with activated Ras signaling. We also show that dominant negative Ras expression results in reduced GH4T2 cell proliferation and transformation, but does not influence differentiation. Taken together, the data presented here begin to shed light on the mechanisms by which pituitary somatolactotropes evade an oncogenic response to persistently activated Ras signaling and suggest that the architecture of the Ras signaling cascade in some endocrine cell types may be distinct from that of cells that respond to Ras in an oncogenic manner.

Calcium-dependent ultrasound stimulation of secretory events from pancreatic beta cells

BACKGROUND

Our previous studies have indicated that ultrasound can stimulate the release of insulin from pancreatic beta cells, providing a potential novel treatment for type 2 diabetes. The purpose of this study was to explore the temporal dynamics and Ca2+-dependency of ultrasound-stimulated secretory events from dopamine-loaded pancreatic beta cells in an in vitro setup.

METHODS

Carbon fiber amperometry was used to detect secretion from INS-1832/13 beta cells in real time. The levels of released insulin were also measured in response to ultrasound treatment using insulin-specific ELISA kit. Beta cells were exposed to continuous wave 800 kHz ultrasound at intensities of 0.1 W/cm2, 0.5 W/cm2 and 1 W/cm2 for several seconds. Cell viability tests were done with trypan blue dye exclusion test and MTT analysis.

RESULTS

Carbon fiber amperometry experiments showed that application of 800 kHz ultrasound at intensities of 0.5 and 1 W/cm2 was capable of stimulating secretory events for durations lasting as long as the duration of the stimulus. Furthermore, the amplitude of the detected peaks was reduced by 64% (p < 0.01) when extracellular Ca2+ was chelated with 10 mM EGTA in cells exposed to ultrasound intensity of 0.5 W/cm2. Measurements of released insulin in response to ultrasound stimulation showed complete inhibition of insulin secretion by chelating extracellular Ca2+ with 10 mM EGTA (p < 0.01). Viability studies showed that 800 kHz, 0.5 W/cm2 ultrasound did not cause any significant effects on viability and metabolic activity in cells exposed to ultrasound as compared to sham-treated cells.

CONCLUSIONS

Our results demonstrated that application of ultrasound was capable of stimulating the release of insulin from pancreatic beta cells in a safe, controlled and Ca2+-dependent manner.

Roles of cell volume in molecular and cellular biology

Extracellular tonicity and volume regulation control a great number of molecular and cellular functions including: cell proliferation, apoptosis, migration, hormone and neuromediator release, gene expression, ion channel and transporter activity and metabolism. The aim of this review is to describe these effects and to determine if they are direct or are secondarily the result of the activity of second messengers.

Angiotensin-(1-7) attenuates hyposmolarity-induced ANP secretion via the Na+-K+ pump

The alteration in osmolarity challenges cell volume regulation, a vital element for cell survival. Hyposmolarity causes an increase in cell volume. Recently, it has been reported that the renin-angiotensin system (RAS) plays a role in cell volume regulation. We investigated the effect of angiotensin-(1-7) [Ang-(1-7)] on hyposmolarity-induced atrial natriuretic peptide (ANP) secretion in normal and diabetic (DM) rat atria and modulation of the effect of Ang-(1-7) by the Na(+)-K(+) pump. Using isolated control rat atria, we observed that perfusion of hyposmotic solution into the atria increased ANP secretion. When Ang-(1-7) [0.1 microM or 1 microM] was perfused in a hyposmolar solution, it decreased the hyposmolarity-induced ANP secretion in a dose-dependent manner. This effect of Ang-(1-7) could be mediated by the Na(+)-K(+) pump, since ouabain, an Na(+)-K(+) pump inhibitor, significantly decreased the effect of Ang-(1-7) on hyposmolarity-induced ANP secretion. In contrast, N(omega) Nitro-l-arginine methyl ester hydrochloride (l-NAME) did not modify the effect of Ang-(1-7) on the hyposmolarity-induced ANP secretion. Interestingly, the ANP secretion was increased robustly by the perfusion of the hyposmolar solution in the DM atria, as compared to the control atria. However, the inhibitory effect of Ang-(1-7) on the hyposmolarity-induced ANP secretion was not observed in the DM atria. In the DM atria, atrial contractility was significantly increased. Taken together, we concluded that Ang-(1-7) attenuated hyposmolarity-induced ANP secretion via the Na(+)-K(+) pump and a lack of Ang-(1-7) response in DM atria may partly relate to change in Na(+)-K(+) pump activity.

Hypotonicity-induced Renin exocytosis from juxtaglomerular cells requires aquaporin-1 and cyclooxygenase-2

The mechanism by which extracellular hypotonicity stimulates release of renin from juxtaglomerular (JG) cells is unknown. We hypothesized that osmotically induced renin release depends on water movement through aquaporin-1 (AQP1) water channels and subsequent prostanoid formation. We recorded membrane capacitance (C(m)) by whole-cell patch clamp in single JG cells as an index of exocytosis. Hypotonicity increased C(m) significantly and enhanced outward current. Indomethacin, PLA(2) inhibition, and an antagonist of prostaglandin transport impaired the C(m) and current responses to hypotonicity. Hypotonicity also increased exocytosis as determined by a decrease in single JG cell quinacrine fluorescence in an indomethacin-sensitive manner. In single JG cells from COX-2(-/ -) and AQP1(-/ -) mice, hypotonicity increased neither C(m) nor outward current, but 0.1-muM PGE(2) increased both in these cells. A reduction in osmolality enhanced cAMP accumulation in JG cells but not in renin-producing As4.1 cells; only the former had detectable AQP1 expression. Inhibition of protein kinase A blocked the hypotonicity-induced C(m) and current response in JG cells. Taken together, our results show that a 5 to 7% decrease in extracellular tonicity leads to AQP1-mediated water influx in JG cells, PLA(2)/COX-2-mediated prostaglandin-dependent formation of cAMP, and activation of PKA, which promotes exocytosis of renin.

Effect of extracellular osmolality and ionic levels on pituitary prolactin release in euryhaline silver sea bream (Sparus sarba)

In many euryhaline fish, prolactin (PRL) plays a key role in freshwater adaptation. Consistent with this function, the present study showed a remarkable reduction in pituitary PRL content of silver sea bream abruptly transferred to low salinity (6ppt). This reduction in pituitary PRL content followed closely the temporal changes in serum osmolality and ion levels. Serum osmolality, Na(+) and Cl(-) levels of silver sea bream abruptly transferred to hyposmotic salinity (6ppt) were markedly reduced 2h after the transfer. The decline in pituitary PRL content lagged behind the serum changes implying that reduction in pituitary PRL content is a response to the drop in serum ion levels and osmotic pressure. Silver sea bream pituitary cells were dispersed and exposed to a medium with reduced ion levels and osmolality in vitro, and PRL released from pituitary cells was significantly elevated. In hyposmotic exposed anterior pituitary cells, cell volume exhibited a 20% increase when exposed to a medium with a 20% decrease in osmolality. The enlarged pituitary cells did not shrink until the surrounding hyposmotic medium was replaced, a phenomenon suggesting an osmosensing ability of silver sea bream PRL cells for PRL secretion in response to a change in extracellular osmotic pressure. The decrease in pituitary PRL content in vivo and stimulated pituitary PRL release in vitro under reduced osmolality together suggest hyposmotic exposure triggers PRL release from the pituitary.

Chronic ethanol intake modifies pyrrolidon carboxypeptidase activity in mouse frontal cortex synaptosomes under resting and K+ -stimulated conditions: role of calcium

Pyrrolidon carboxypeptidase (Pcp) is an omega peptidase that removes pyroglutamyl N-terminal residues of peptides such as thyrotrophin-releasing hormone (TRH), which is one of the neuropeptides that has been localized into many areas of the brain and acts as an endogenous neuromodulator of several parameters related to ethanol (EtOH) consumption. In this study, we analysed the effects of chronic EtOH intake on Pcp activity on mouse frontal cortex synaptosomes and their corresponding supernatant under basal and K+ -stimulated conditions, in presence and absence of calcium (Ca2+) to know the regulation of Pcp on TRH. In basal conditions, chronic EtOH intake significantly decreased synaptosomes Pcp activity but only in absence of Ca2+. However, supernatant Pcp activity is also decreased in presence and absence of calcium. Under K+-stimulated conditions, chronic EtOH intake decreased synaptosomes Pcp activity but only in absence of Ca2+, whereas supernatant Pcp activity was significantly decreased only in presence of Ca2+. The general inhibitory effect of chronic EtOH intake on Pcp activity suggests an inhibition of TRH metabolism and an enhancement of TRH neurotransmitter/neuromodulator functions, which could be related to putative processes of tolerance to EtOH in which TRH has been involved. Our data may also indicate that active peptides and their degrading peptidases are released together to the synaptic cleft to regulate the neurotransmitter/neuromodulator functions of these peptides, through a Ca2+ -dependent mechanism.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Cell swelling-induced signaling for insulin secretion bypasses steps involving G proteins and PLA2 and is N-ethylmaleimide insensitive

BACKGROUND

This study was undertaken to examine putative mechanisms of calcium independent signal transduction pathway of cell swelling-induced insulin secretion.

METHODS

The role of phospholipase A(2), G proteins, and soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) in insulin secretion induced by 30% hypotonic medium was studied using isolated rat pancreatic islets.

RESULTS

In contrast to glucose stimulation, osmotically induced insulin secretion from pancreatic islets was not inhibited by 10 micromol/l bromoenol lactone, an iPLA(2) (Ca(2+) independent phospholipase) inhibitor. Similarly, preincubation of islets for 20 hours with 25 microg/ml mycophenolic acid to inhibit GTP synthesis fully abolished glucose-induced insulin secretion but was without effect on hypotonicity stimulated insulin release. Glucose-induced insulin secretion was prevented by preincubation with 20 nmol/l tetanus toxin (TeTx), a metalloprotease inactivating soluble SNARE. Cell swelling-induced insulin secretion was inhibited by TeTx in the presence of calcium ions but not in calcium depleted medium. The presence of N-ethylmaleimide (NEM, 5 mmol/l, another inhibitor of SNARE proteins) in the medium resulted in high basal insulin secretion and lacking response to glucose stimulation. In contrast, high basal insulin secretion from NEM treated islets further increased after hypotonic stimulation.

CONCLUSION

G proteins and iPLA(2) - putative mediators of Ca(2+) independent signaling pathway participate in glucose but not in hypotonicity-induced insulin secretion. Hypotonicity-induced insulin secretion is sensitive to clostridial neurotoxin TeTx but is resistant to NEM.

Differential dependence of regulatory volume decrease behavior in rabbit corneal epithelial cells on MAPK superfamily activation

We characterized the dependence of hypotonicity-induced regulatory volume decrease (RVD) responses on mitogen-activated protein kinase (MAPK) pathway signaling in SV40-immortalized rabbit corneal epithelial cells (RCEC). Following calcein-AM loading, RVD was monitored using a microplate fluorescence reader. Western blot analysis determined MAPK activation. After 30 min, the RVD response restored the relative cell volume to nearly isotonic values, whereas it was inhibited when cells were bathed either in a Cl- -free solution or with the Cl- -channel inhibitors: 5-nitro-2-(3-phenylpropylamino)benzoic acid or niflumic acid. Similar declines occurred with either a high-K+ (20 mM) supplemented solution or the K+ channel inhibitor 4-aminopyridine. Activation of extracellular signal-regulated kinase (ERK), p38, and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) was time and tonicity-dependent. Stimulation of ERK and SAPK/JNK was maximized earlier than that of p38. Activation of ERK and SAPK/JNK was insensitive to Cl- and K+ channel inhibitors, whereas inhibition with either PD98059 or SP600125, respectively, blocked RVD. However, inhibition of p38 with SB203580had no effect on RVD. Suppression of RVD instead blocked p38 activation. Differences in the dependence of RVD activation on Erk1/2 and p38 signaling were validated in dominant negative (d/n)-Erk1 and d/n-p38 cells. Volume-sensitive Cl- and K+ channel activation contributes, in concert, to RVD in RCEC. Therefore, swelling-induced ERK and SAPK/JNK stimulation precedes Cl- and K+ channel activation, whereas p38 activation occurs as a consequence of RVD.

Osmosignaling and volume regulation in intestinal epithelial cells

Most cells have to perform their physiological functions under a variable osmotic stress, which, because of the relatively high permeability of the plasma membrane for water, may result in frequent alterations in cell size. Intestinal epithelial cells are especially prone to changes in cell volume because of their high capacity of salt and water transport and the high membrane expression of various nutrient transporters. Therefore, to avoid excessive shrinkage or swelling, enterocytes, like most cell types, have developed efficient mechanisms to maintain osmotic balance. This chapter reviews selected model systems that can be used to investigate cell volume regulation in intestinal epithelial cells, with emphasis on the regulatory volume decrease, and the methods available to study the compensatory redistribution of (organic) osmolytes. In addition, a brief summary is presented of the pathways involved in osmosensing and osmosignaling in the intestine.

Glucose induces anion conductance and cytosol-to-membrane transposition of ICln in INS-1E rat insulinoma cells

The metabolic coupling of insulin secretion by pancreatic beta cells is mediated by membrane depolarization due to increased glucose-driven ATP production and closure of K(ATP) channels. Alternative pathways may involve the activation of anion channels by cell swelling upon glucose uptake. In INS-1E insulinoma cells superfusion with an isotonic solution containing 20 mM glucose or a 30% hypotonic solution leads to the activation of a chloride conductance with biophysical and pharmacological properties of anion currents activated in many other cell types during regulatory volume decrease (RVD), i.e. outward rectification, inactivation at positive membrane potentials and block by anion channel inhibitors like NPPB, DIDS, 4-hydroxytamoxifen and extracellular ATP. The current is not inhibited by tolbutamide and remains activated for at least 10 min when reducing the extracellular glucose concentration from 20 mM to 5 mM, but inactivates back to control levels when cells are exposed to a 20% hypertonic extracellular solution containing 20 mM glucose. This chloride current can likewise be induced by 20 mM 3-Omethylglucose, which is taken up but not metabolized by the cells, suggesting that cellular sugar uptake is involved in current activation. Fluorescence resonance energy transfer (FRET) experiments show that chloride current activation by 20 mM glucose and glucose-induced cell swelling are accompanied by a significant, transient redistribution of the membrane associated fraction of ICln, a multifunctional 'connector hub' protein involved in cell volume regulation and generation of RVD currents.

Hypotonicity and ethanol modulate BK channel activity and chloride currents in GH4/C1 pituitary tumour cells

AIM

Description of the effects of hypotonic cell swelling and ethanol on maxi Ca2+-activated K+ channel (BK channel) activity and Cl- channel activity in GH4/C1 pituitary tumour cells.

METHODS

Whole cell-, cell attached- and outside-out patch clamp measurements, fluorescence (fluo-3) measurements of intracellular Ca2+ concentration, cell size video monitoring.

RESULTS

GH4/C1 pituitary tumour cells respond to both hypotonicity and ethanol with cell swelling which is followed by a regulatory volume decrease (RVD). Tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) induced cell swelling per se and inhibited hypotonicity induced RVD. Ethanol-induced swelling is paralleled by an increase in the intracellular Ca2+ concentration and augmented by DIDS. BK channel activation by hypotonicity and ethanol is demonstrated in patch clamp experiments both in intact cells (cell attached configuration) and a subset of excised membrane patches (outside-out configuration). Cell swelling and addition of ionomycin under isotonic conditions leads to the activation of outwardly rectifying Cl- currents with time dependent activation at positive potentials.

CONCLUSIONS

In GH4/C1 cells both hypotonicity and ethanol lead to cell swelling, RVD and to activation of BK channels. The hypotonicity-induced BK channel activation can also be observed in cell free outside-out patches. Hypotonicity, but not ethanol leads to the activation of Cl- channels with features of Ca2+-activated Cl- currents.

Thyrotropin-releasing hormone in rat heart: effect of swelling, angiotensin II and renin gene

AIM

This study was performed to examine thyrotropin-releasing hormone (TRH) secretion and regulation in rat heart.

METHODS

Expression of prepro-TRH gene in left atrium and left ventricle was studied by RT-PCR. TRH secretion from slices of left auricle and left ventricle in response to cell swelling (induced by hypotonic medium or ethanol in isosmotic medium), angiotensin II and losartan and their combinations was studied.

RESULTS

RT-PCR revealed two times higher prepro-TRH expression in left auricle than left ventricle. In transgenic rats with extra copy of mouse renin gene a marked increase of prepro-TRH expression in the heart was noted but the relative difference between left atrium and left ventricle persisted. The swelling stimulated TRH release from both left auricle and left ventricle and this stimulation could not be inhibited by bumetanide. Angiotensin II (10 nmol L-1) added into medium significantly decreased basal secretion of TRH. The inhibiting effect of Angiotensin II was prevented by 1 micromol L-1 losartan, an angiotensin II AT1 receptor blocker. When angiotensin II and hypotonicity were applied simultaneously, swelling-induced secretion persisted.

CONCLUSION

TRH secretion from heart slices has attributes of regulated secretion--depending on the stimulus it could be either stimulated or inhibited. Renin positively affects prepro-TRH expression in the heart. Angiotensin II inhibits TRH secretion from heart tissue by a mechanism involving AT1 receptors. Swelling-induced TRH secretion overrides inhibitory effect of angiotensin II. Swelling could be a useful tool when natural or pharmacological secretagogue is unknown. Peptides and proteins released by swelling could be mediators of local and remote ischaemic preconditioning protecting from subsequent ischaemia.

The Effect of Swelling on TRH and Oxytocin Secretion From Hypothalamic Structures

1. Cell swelling induces exocytosis of material stored in secretory vesicles resulting in a secretory burst of peptidic hormones or enzymes from various types of cells including endocrine cells and neurons. We have previously shown that swelling-induced exocytosis possesses limited selectivity; hypotonic medium evokes TRH but not oxytocin release from hypothalamic paraventricular nucleus (PVN) and neurohypophysis (NH). 2. It is the aim of this study to ascertain whether the swelling-induced oxytocin secretion could be unmasked by the inhibition of specific osmotic response using Ca(2+)-free medium and GdCl(3), an inhibitor of stretch activated channels. 3. Oxytocin release from the PVN was stimulated by the hypotonic medium only in the presence of 50 or 100 microM GdCl(3.) Oxytocin release from supraoptic nucleus (SON) was also stimulated by the Ca(2+)-free hypotonic medium in the presence of GdCl(3). Oxytocin secretion from the NH was not stimulated even in the presence of GdCl(3), both in Ca(2+) containing and Ca(2+)-free medium. TRH response to swelling-inducing stimulus was not affected by the presence of GdCl(3). 4. An intranuclear oxytocin secretion to hyposmotic stimulation within the PVN and the SON could be unmasked by the inhibiting specific response by GdCl(3). At these conditions general secretory response to swelling-inducing stimuli emerged. Secretion of oxytocin from the NH was not affected by any of these treatments. 5. Peptides and proteins released after cell swelling can play an important role in the pathophysiology of ischemia and could be mediators of local or remote preconditioning. Disruption of mechanosensitive gating in magnocellular neurosecretory cells could result in an inadequate secretory response (e.g. stimulation instead of inhibition and vice versa) of hormones engaged in water and salt metabolism regulation.

Different Signaling Pathways Involved in Glucose- and Cell Swelling-Induced Insulin Secretion by Rat Pancreatic Islets in Vitro

BACKGROUND

The objective was to compare signal transduction pathways exploited by glucose and cell swelling in stimulating insulin secretion.

METHODS

Isolated rat (Wistar) pancreatic islets were stimulated in vitro by 20 mmol/l glucose or 30% hypotonic medium (202 mOsm/kg) in various experimental conditions.

RESULTS

Glucose did not stimulate insulin release in calcium free medium. Cell swelling-induced insulin release in calcium free medium, even in the presence of the membrane permeable calcium chelator BAPTA/AM (10 micromol/l). Protein kinase C (PKC) inhibitor bisindolylmaleimide VIII (1 micromol/l) abolished the stimulation of insulin secretion by glucose but did not affect the swelling-induced insulin release. PKC activator phorbol 12-13-dibutyrate (1 micromol/l) stimulated insulin secretion in medium containing Ca2+ and did not potentiate insulin secretion stimulated by hypotonic extracellular fluid. Dilution of the medium (10-30%) had an additive effect on the glucose-induced insulin secretion. Noradrenaline (1 micromol/l) abolished glucose-induced insulin secretion but did not inhibit hypotonic stimulation either in presence or absence of Ca2+.

CONCLUSION

Glucose- and swelling-induce insulin secretion through separate signal transduction pathways. Hyposmotic stimulation is independent from both the extracellular and intracellular Ca2+, does not involve PKC activation, and could not be inhibited by noradrenaline. These data indicate a novel signaling pathway for stimulation of insulin secretion exploited by cell swelling.

Osmolarity-induced renin secretion from kidneys: evidence for readily releasable renin pools

Our study aimed to characterize the influence of changes in extracellular osmolarity on renin secretion from the whole kidney. For this purpose, the osmolarity of the perfusion medium of isolated rat or mouse kidneys was either decreased by lowering the NaCl concentration by 20% or was increased up to 133% by the addition of various salts or sugars. It turned out that changes in osmolarity led to instantaneous transient changes followed by a plateau of renin secretion, in that increases in osmolarity stimulated renin secretion, whereas decreases attenuated renin secretion. The peak amplitude of changes in renin secretion was related to steady-state renin secretion rates before the osmotic challenge but was independent of the maneuver used to modulate steady-state renin secretion. Osmolarity-induced changes in renin secretion were more related to relative rather than to absolute changes in osmolarity and were not dependent on the formation of nitric oxide or of prostanoids and did not require Na-K-2Cl cotransport function or swelling-activated chloride channels. Moreover, we obtained evidence that the pool of renin secretion excitable by hyperosmolarity is exhaustible and that its complete refilling takes at least 2 min. The observed behavior of renin secretion fits the concept about exocytosis proposing the existence of different pools of committed secretory vesicles, which have not yet undergone the final modification for initiation of exocytosis. Probably, a pool of readily releasable vesicles determines steady-state secretion rates from kidneys.

A role of thyrotropin-releasing hormone in insulin secretion by isolated rat pancreatic islets

Insulin-secreting pancreatic beta cells also express thyrotropin-releasing hormone (TRH). Although the physiological role of TRH in this localization is unclear, its participation in glucoregulation has been implied. To test this hypothesis, we blocked the last step of post-translational maturation of the TRH molecule by disulfiram, which is an active inhibitor of peptide alpha-amidation (PAM) within pancreatic islet cells. The treatment of male rats with 200 mg/kg/day of disulfiram during a 5-day period resulted in a low PAM activity, a high insulin content and its basal secretion from pancreatic islets, and the inability to release insulin in response to glucose (16.7 mM) or hypo-osmotic (30%) challenge in vitro. The addition of TRH (1 nM) to the medium during incubation restored the insulin content and both basal and glucose stimulated insulin secretions to control levels.

CONCLUSION

TRH plays an important role in the mechanism of insulin secretion and its response to glucose stimulation.

Hormone release is tied to changes in cell size in the osmoreceptive prolactin cell of a euryhaline teleost fish, the tilapia, Oreochromis mossambicus

Prolactin (PRL) cells from a teleost fish, the tilapia, Oreochromis mossambicus, facilitate the direct study of osmoreception. The release of two prolactins, PRL(188) and PRL(177), which act in freshwater osmoregulation in teleost fish, rises in vitro within 5 min after extracellular osmolality falls. An increase in cell size accompanied this rise. Cell size and PRL release also increased, albeit more slowly, following the partial replacement of medium NaCl (55 mOsmolal) with an equivalent concentration of urea, a membrane-permeant molecule. Similar replacement using mannitol, which is membrane-impermeant, elicits no response. These findings suggest that osmoreception is linked to changes in cell volume rather than to extracellular osmolality per se.

Effect of immobilization on in vitro thyrotropin-releasing hormone release from brain septum in wild-type and corticotropin-releasing hormone knock-out mice

There is considerable evidence linking alcohol consumption, sedation, and thyrotropin-releasing hormone (TRH) in the brain septum. We have shown that ethanol in clinically relevant concentrations can in vitro induce TRH release from the septum by a mechanism involving neuronal swelling. Corticotropin-releasing hormone-deficient (CRH-KO) mice serve as an interesting model to help us understand the role of CRH in the regulation of different neuroendocrine systems. The aim of this study was to compare TRH release activity in the brain septum at basal and stress conditions in CRH-KO mice and their wild-type (WT) littermates. Experimental mice were decapitated immediately or 3 h after single (2 h) or repeated (seven times for 2 h daily) immobilization stress. The brain septum was immediately cut out and incubated to measure basal-, ethanol-, and hyposmosis-stimulated TRH release in vitro. Ethanol in isosmotic medium or hyposmotic medium stimulated TRH release from mice septal explants from WT and CRH-KO mice. The response was disturbed immediately after immobilization and recovered 3 h later. Our results show that immobilization stress transiently affects the TRH system in brain septum. Inborn absence of CRH does not affect septal TRH and its response to ethanol before and 3 h after immobilization.

Cell swelling-induced peptide hormone secretion

Cell volume changes induced in various ways (anisosmotic environment, hormones, oxidative stress, substrate uptake) are an integral part of a signal transduction network regulating cell function. Cell swelling has received increasing attention as a stimulus for a variety of intracellular phenomena. One of the most remarkable effects of cell swelling is its powerful effect in inducing exocytosis of material in intracellular secretory vesicles. Secretion of essentially all so-packaged hormones including those from hypothalamus (thyrotropin-releasing hormone, TRH; gonadotropin-releasing hormone, GnRH), pituitary (LH, FSH, ACTH, MSH, TSH, prolactin, beta endorphin), pancreas (insulin, somatostatin, glucagon), heart (atrial natriuretic hormone) and kidney (renin) are stimulated in a concentration-related manner by medium hyposmolarity or isosmolar medium containing permeant molecules such as ethanol or urea (reviewed in Ref. 21). Cell swelling-induced exocytosis is not restricted to endocrine cells and hormones; medium hyposmolarity also induces secretion of exocrine pancreatic enzymes and myeloperoxidase from human polymorphonuclear leukocytes.

Role of cell volume in K+-induced Ca2+ signaling by rat adrenal glomerulosa cells

The involvement of cell volume in the K+-evoked Ca2+ signaling was studied in cultured rat glomerulosa cells. Previously we reported that hyposmosis (250 mOsm) increased the amplitude of T-type Ca2+ current and, accordingly, enhanced the Ca2+ response of cultured rat glomerulosa cells to K+. In the present study we found that this enhancement is not influenced by the cytoskeleton-disrupting drugs cytochalasin-D (20 microM) and colchicine (100 microM). Elevation of extracellular potassium concentration ([K+]e) from 3.6 to 4.6-8.6 mM induced cell swelling, which had slower kinetics than the Ca2+ signal. Cytoplasmic Ca2+ signal measured in single glomerulosa cells in response to stimulation with 5 mm K+ for 2 min showed two phases: after a rapid rise reaching a plateau within 20-30 sec, [Ca2+]c increased further slowly by approximately one third. When 5 mM K+ was coapplied with elevation of extracellular osmolarity from 290 to 320 mOsm, the second phase was prevented. These results indicate that cell swelling evoked by physiological elevation of [K+]e may contribute to the generation of sustained Ca2+ signals by enhancing voltage-activated Ca2+ influx.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: II. Secretory membrane activity

The secretory membrane activities of two rat prostate cancer cell lines of markedly different metastatic potential, and corresponding electrophysiological characteristics, were studied in a comparative approach. In particular, voltage-gated Na(+) channels (VGSCs) were expressed in the strongly metastatic MAT-LyLu but not in the closely related, but weakly metastatic, AT-2 cells. Uptake and release of the non-cytotoxic marker horseradish peroxidase (HRP) were used as indices of general endocytotic and exocytotic membrane activity, respectively. The amount of tracer present in a given experimental condition was quantified by light microscopic digital imaging. The uptake of HRP was an active process, abolished completely by incubating the cells at low temperature (5 degrees C) and suppressed by disrupting the cytoskeleton. Interestingly, the extent of HRP uptake into the strongly metastatic MAT-LyLu cells was almost twice that into the weakly metastatic AT-2 cells. Vesicular uptake of HRP occurred in a fast followed by a slow phase; these appeared to correspond to cytoplasmic and perinuclear pools, respectively. Importantly, the overall quantitative difference in the uptake disappeared in the presence of 1 microM tetrodotoxin which significantly reduced the uptake of HRP into the MAT-LyLu cells. There was no effect on the AT-2 cells, consistent with functional VGSC expression occurring selectively in the former. A similar effect was observed in Na(+)-free medium. The uptake was partially dependent upon extracellular Ca(2+) but was not affected by raising the extracellular K(+) concentration. We suggest that functional VGSC expression could potentiate prostate cancer cells' metastatic ability by enhancing their secretory membrane activity.

Cell volume increase and extracellular Ca2+ are needed for hyposmotically induced prolactin release in tilapia

In the tilapia (Oreochromis mossambicus), as in many euryhaline teleost fish, prolactin (PRL) plays a central role in freshwater adaptation, acting on osmoregulatory surfaces to reduce ion and water permeability and increase solute retention. Consistent with these actions, PRL release is stimulated as extracellular osmolality is reduced both in vivo and in vitro. In the current experiments, a perfusion system utilizing dispersed PRL cells was developed for permitting the simultaneous measurement of cell volume and PRL release. Intracellular Ca(2+) was monitored using fura 2-loaded cells under the same conditions. When PRL cells were exposed to hyposmotic medium, an increase in PRL cell volume preceded the increase in PRL release. Cell volume increased in proportion to decreases of 15 and 30% in osmolality. However, regulatory volume decrease was clearly seen only after a 30% reduction. The hyposmotically induced PRL release was sharply reduced in Ca(2+)-deleted hyposmotic medium, although cell volume changes were identical to those observed in normal hyposmotic medium. In most cells, a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) during hyposmotic stimulation was dependent on the availability of extracellular Ca(2+), although small transient increases in [Ca(2+)](i) were sometimes observed upon introduction of Ca(2+)-deleted media of the same or reduced osmolality. These results indicate that an increase in cell size is a critical step in the transduction of an osmotic signal into PRL release and that the hyposmotically induced increase in PRL release is greatly dependent on extracellular Ca(2+).

Osmomechanical stress selectively regulates translocation of protein kinase C isoforms

Osmomechanical stress, resulting in cell swelling and activation/regulation of numerous cellular processes, may play a critical role in cell signaling by selectively regulating translocation of protein kinase C (PKC) isoforms from cytosol to membrane compartments. Western blotting of renal epithelial cell fractions demonstrated the expression of five PKC isoforms. Three of these isoforms (PKCalpha, PKCepsilon, PKCzeta) translocated to the membrane fraction upon exposure of cells to osmomechanical stress (hypotonic medium). Immunohistochemical staining of cells using isoform-specific antibodies further demonstrated translocation of the phorbol ester-sensitive isoforms, PKCalpha and PKCepsilon, to both the plasma membrane and perinuclear sites, reflecting potential initial steps in regulation of specific effector pathways. Indeed, selective inhibition of PKCs indicates a potential role for PKCalpha in modulating a calcium influx channel. It is concluded that osmomechanical stress induces selective translocation of specific PKC isoforms, demonstrating a key role of osmomechanical stress in selectively regulating PKC-dependent signaling pathways.

Hyposmotic shock stimulates insulin secretion by two distinct mechanisms. Studies with the betaHC9 cell

Exposure of betaHC9 cells to a Krebs-Ringer bicarbonate-HEPES buffer (KRBH) made hypotonic by a reduction of 25 mM NaCl resulted in a prompt stimulation of insulin release. The stimulation was transient, and release rates returned to basal levels after 10 min. The response resembles that of the first phase of glucose-stimulated insulin release. The response did not occur if the reduction in NaCl was compensated for by the addition of an equivalent osmolar amount of sorbitol, so the stimulation of release was due to the osmolarity change and not the reduction in NaCl. The hyposmotic shock released insulin in KRBH with or without Ca(2+). The L-type Ca(2+) channel blocker nitrendipine inhibited the response in normal KRBH but had no effect in KRBH without Ca(2+) despite the latter response being larger than in the presence of extracellular Ca(2+). Similar data were obtained with calciseptine, which also blocks L-type channels. The T-type Ca(2+) channel blocker flunarizine was without effect, as was the chloride channel blocker DIDS. In parallel studies, the readily releasable pool of insulin-containing granules was monitored. Immunoprecipitation of the target-SNARE protein syntaxin and co-immunoprecipitation of the vesicle-SNARE VAMP-2 was used as an indicator of the readily releasable granule pool. After hypotonic shock in the presence of extracellular Ca(2+), the amount of VAMP-2 coimmunoprecipitated by antibodies against syntaxin was much reduced compared with controls. Therefore, under these conditions, hypotonic shock stimulates exocytosis of the readily releasable pool of insulin-containing granules. No such reduction was seen in the absence of extracellular Ca(2+). In conclusion, after reexamination of the effect of hyposmotic shock on insulin secretion in the presence and absence of Ca(2+) (with EGTA in the medium), it is clear that two different mechanisms are operative under these conditions. Moreover, these two mechanisms may be associated with the release of two distinct pools of insulin-containing granules.

Cell swelling induced secretion of TRH by posterior pituitary, hypothalamic paraventricular nucleus and pancreatic islets: effect of L-canavanine

The aims of this study were to test if ethanol induces thyrotropin-releasing hormone (TRH) secretion in vitro from the posterior pituitary and hypothalamic explants by a mechanism involving cell swelling, and to characterize the pathway of stimulated secretion. Ethanol, at a concentration of 80 mM, stimulated the release of TRH from the posterior pituitary, the hypothalamic paraventricular nucleus, the median eminence, and the brain septum, when administered only in isosmolar but not in hyperosmolar medium. This indicates the involvement of a cell swelling-inducing mechanism. L-canavanine in a concentration of 3 mM, increased the basal and hyposmosis-induced TRH secretion from the posterior pituitary and the paraventricular nucleus, and both basal and ethanol-induced TRH secretion from isolated pancreatic islets. This indicates the presence of both constitutive and regulatory secretory pathways. Our results suggest that cell swelling induces exocytosis from clathrin coated granules.

Osmosensitive release of neurotransmitter amino acids: relevance and mechanisms

Hyposmolarity activates amino acid efflux as part of the corrective volume process in a variety of cells. This review discusses the mechanism of amino acid release in brain cells preparations. Results present evidence of substantial differences between the efflux of taurine and that of GABA and glutamate, which besides a possible role as osmolytes, have a main function as synaptic transmitters. The differences found concern the efflux time course, the sensitivity to C1- channel blockers, the modulation by tyrosine kinases, the influence of PKC and the effect of cytoskeleton disruptive agents. While taurine efflux features fit well with the mechanisms so far described in most cell types, the efflux of GABA and glutamate does not. Alternate mechanisms for the release of these two amino acids are discussed, including a PKC-modulated, actin-dependent exocytosis.