The permeant molecule urea stimulates prolactin secretion in GH4C1 cells by inducing Ca2+ influx through dihydropyridine-sensitive Ca2+ channels

Final report of the safety assessment of Urea

Although Urea is officially described as a buffering agent, humectant, and skin-conditioning agent-humectant for use in cosmetic products, there is a report stating that Urea also is used in cosmetics for its desquamating and antimicrobial action. In 2001, the Food and Drug Administration (FDA) reported that Urea was used in 239 formulations. Concentrations of use for Urea ranged from 0.01% to 10%. Urea is generally recognized as safe by FDA for the following uses: side-seam cements for food contact; an inhibitor or stabilizer in pesticide formulations and formulations applied to animals; internal sizing for paper and paperboard and surface sizing and coating of paper and paper board that contact water-in-oil dairy emulsions, low-moisture fats and oils, moist bakery products, dry solids with surface containing no free fats or oil, and dry solids with the surface of fat or oil; and to facilitate fermentation of wine. Urea is the end product of mammalian protein metabolism and the chief nitrogenous compound of urine. Urea concentrations in muscle, liver, and fetuses of rats increased after a subcutaneous injection of Urea. Urea diffused readily through the placenta and into other maternal and fetal organs. The half-life of Urea injected into rabbits was on the order of several hours, and the reutilization rate was 32.2% to 88.8%. Urea given to rats by a bolus injection or continuous infusion resulted in distribution to the following brain regions: frontal lobe, caudate nucleus, hippocampus, thalamus plus hypothalamus, pons and white matter (corpus callosum). The permeability constant after treatment with Urea of whole skin and the dermis of rabbits was 2.37 +/- 0.13 (x 10(6)) and 1.20 +/- 0.09 (x10(3)) cm/min, respectively. The absorption of Urea across normal and abraded human skin was 9.5% +/- 2.3% and 67.9% +/- 5.6%, respectively. Urea increased the skin penetration of other compounds, including hydrocortisone. No toxicity was observed for Urea at levels as high as 2000 mg/kg in acute oral studies using female rats or mice. No signs of toxicity were observed in male piglets dosed orally with up to 4 g/kg Urea for 5 days. Dogs dosed orally with 5 to 30 g/L Urea for 4 to 10 days had signs of toxicity, including weakness, anorexia, vomiting and retching, diarrhea and a decreased body temperature, which led to a deep torpor or coma. No significant microscopic changes were observed in the skin of male nude mice dermally exposed to 100% Urea for 24 h. No observable effect on fetal development was seen in rats and mice dosed orally with an aqueous solution of Urea (2000 mg/kg) on days 10 and 12 of gestation. The mean number of implants, live fetuses, percent fetal resorptions, mean fetal weight, and percent fetuses malformed were comparable to control group. A detergent containing 15% Urea was injected into pregnant ICR-JCl mice and dams and fetuses had no significant differences when compared to control animals. Urea given orally did not enhance the developmental toxicity of N-nitrosomethylurea. Female Sprague-Dawley rats injected in the uterine horn with 0.05 ml Urea on day 3 (preimplantation) or on day 7 (post implantation) exhibited no maternal mortality or morbidity; a dose-dependent reduction in embryo survival was seen with preimplantation treatment. Urea injected intra-amniotically induces mid-trimester abortions in humans. Urea was not genotoxic in several bacterial and mammalian assays; although in assays where Urea was used at a high concentration, genotoxicity was found, many in in vitro assays. Urea is commonly used in studies of DNA because it causes uncoiling of DNA molecules. Urea was not carcinogenic in Fisher 344 rats or C57B1/6 mice fed diets containing up to 4.5% Urea. Exposure of normal human skin to 60% Urea produced no significant irritation in one study, but 5% Urea was slightly irritating and 20% Urea was irritating in other reports. Burning sensations are the most frequently reported effect of Urea used alone or with other agents in treatment of diseased skin. Overall, there are few reports of sensitization among the many clinical studies that report use of Urea in treatment of diseased skin. The Cosmetic Ingredient Review (CIR) Expert Panel determined the data provided in this report to be sufficient to assess the safety of Urea. The Panel did note that Urea can cause uncoiling of DNA, a property used in many DNA studies, but concluded that this in vitro activity is not linked to any in vivo genotoxic activity. Although noting that formulators should be aware that Urea can increase the percutaneous absorption of other chemicals, the CIR Expert Panel concluded that Urea is safe as used in cosmetic products.

Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity ([Os]e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (IL) and T-type (IT) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]e) and strong (31-32% decrease in [Os]e). Exposure to moderate hyposmotic stimuli resulted in three response types in IL (a decrease, a biphasic effect, and an increase in IL) and in increase in IT. Exposure to strong hyposmotic stimuli resulted only in increases in both IL and IT. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both IL and IT. Thus it appears that the main effect of decrease in [Os]e is increase in calcium channel currents. This increase was differential (IL were more sensitive than IT) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of IL and IT may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed.

Signal transduction mechanisms mediating rapid, nongenomic effects of cortisol on prolactin release

While the mechanisms governing genomically mediated glucocorticoid actions are becoming increasingly understood, relatively little is known with regard to the cell signaling pathways that transduce rapid glucocorticoid actions. Studies of the cultured tilapia rostral pars distalis (RPD), a naturally segregated region of the fish pituitary gland that contains a 95-99% pure population of prolactin (PRL) cells and is easily dissected and maintained in a completely defined, serum-free media, indicate that physiological concentrations of cortisol rapidly inhibit PRL release. The attenuative action of cortisol on PRL release occurs within 10-20 min, is insensitive to the protein synthesis inhibitor, cycloheximide, and mimicked by its membrane impermeable analog, cortisol-21 hemisuccinate-conjugated bovine serum albumin (BSA). Cortisol and somatostatin, a peptide known to work through membrane receptors to inhibit PRL release, rapidly and reversibly reduces intracellular free Ca(2+) (Ca(i)(2+)), and inhibits 45Ca(2+) influx and BAYK-8644 induced PRL release. Preliminary investigations show cortisol, but not somatostatin, suppresses phospholipase C (PLC) activity in PRL cell membrane preparations. In addition, cortisol and somatostatin reduce intracellular cAMP and membrane adenylyl cyclase activity. These findings indicate that the acute inhibitory effects of cortisol on PRL release occur through a nongenomic mechanism involving interactions with the plasma membrane and inhibition of both the Ca(2+) and cAMP signal transduction pathways. Cortisol may reduce Ca(i)(2+) by inhibiting influx through L-type voltage-gated channels and possibly release through a PLC/inositol triphosphate sensitive intracellular Ca(2+) pool. In addition, it is also likely the steroid inhibits adenylyl cyclase activity in events leading to reduced cAMP production and the subsequent release of PRL.

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.

Isotonic ethanol inhibits the generation of superoxide anion in neutrophils by inducing cell expansion

The effects of ethanol on the production of oxygen-derived free radicals by neutrophils are controversial. Osmolarity-mediated alteration of cell volume appears to be an important mechanism for regulating neutrophil activity. We investigated in neutrophils from healthy volunteers the effect of isotonic/hypertonic ethanol on both chemiluminescence amplified by a Cypridina luciferin analog in response to N-formyl-Met-Lue-Phe and cell volume measured with a Coulter counter. Both isotonic and hypertonic ethanol significantly decreased chemiluminescence in a dose-dependent manner. Isotonic ethanol produced a greater magnitude of inhibition than hypertonic ethanol (P<0.01). Another permeable molecule, urea, and hypotonic solution had the same effects on chemiluminescence. Isotonic and hypertonic ethanol caused a prompt cell expansion and shrinking, respectively. On the other hand, isotonic sucrose, an impermeable molecule, was ineffective in both chemiluminescence and cell volume changes. These data suggest that isotonic ethanol inhibits the superoxide anion production by inducing cell expansion probably due to increased intracellular osmotic pressure caused by rapid ethanol permeation through the plasmalemma. This impaired neutrophil function may, in some part, contribute to the susceptibility to infection in alcoholics.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Tetraethylammonium blockade of K+ channels in GH4C1 cells regulates prolactin secretion by inducing Ca2+ influx

Tetraethylammonium (TEA), a K+ channel blocker, induced PRL secretion and an increase in cytosol Ca2+ concentration [Ca2+]i in a dose-dependent manner between 5-20 mM in GH4C1 rat pituitary tumor-derived cells. Removal of medium Ca2+ or the addition of 1 microM nifedipine abolished both the induced [Ca2+]i increment and PRL secretion. TEA augmented the TRH-induced rise in [Ca2+]i and inhibited the rise in [Ca2+]i induced by 30 mM K+. The dynamics of TEA-, TRH- and K(+)-induced PRL secretion were different, with the TEA-induced secretory peak occurring at about 10 min compared to 2-3 min for TRH and K+. Tolbutamide, which blocks ATP-sensitive K+ channels, induced PRL secretion without causing a rise in [Ca2+]i. The results suggest that: (a) K+ channels have a complex interaction with the PRL secretory process in GH4C1 cells; (b) TEA induces PRL secretion by causing Ca2+ influx through dihydropyridine-sensitive Ca2+ channels; and (c) K+ channels play a different role in the [Ca2+]i rise induced by TRH than in that induced by depolarizing K+.

Cyclic adenosine monophosphate is not part of the transduction chain by which cell-swelling induces secretion in either normal or tumor-derived GH4C1 pituitary cells

We have evaluated whether cyclic adenosine monophosphate (cAMP) generation plays a role in the burst of hormone secretion caused by osmotic cell-swelling in tumor-derived and normal pituitary cells. Up to 45% hyposmolarity induced a dose-dependent increase in prolactin (PRL) secretion, but had no effect on cAMP generation. However, hyposmolarity inhibited forskolin-induced cAMP accumulation in a dose-dependent manner, but had no effect on 3-isobutyl-1-methyl-xanthine (IBMX)-induced cAMP accumulation. Ca2+ depletion of the medium partially blocked the inhibitory effect of hyposmolarity on forskolin-induced cAMP generation and completely blocked stimulation of PRL secretion by hyposmolarity. High levels of K+ in medium inhibited forskolin-induced cAMP generation, while thyrotropin-releasing hormone (TRH) enhanced it. Our results indicate that in both GH4C1 and normal pituitary cells, cAMP is not a regulatory factor for hormone secretion induced by cell-swelling.

Hypertonic glucose inhibits the production of oxygen-derived free radicals by rat neutrophils

We studied the influence of graded degrees of hypertonic glucose or sucrose on the generation of oxygen-derived free radicals by rat neutrophils. Hypertonic glucose and sucrose exerted dose- and time-dependent inhibition of chemiluminescence amplified by luciferin analog (CLA-DCL) and luminol (L-DCL) in response to fMLP. Hypertonic glucose was more effective to this chemiluminescence inhibition than hypertonic sucrose was. This inhibition of hypertonicity was more effective in CLA-DCL than in L-DCL. Although the production of superoxide anion measured by the reduction of ferricytochrome c was more inhibited by hypertonic glucose than by hypertonic sucrose, the myeloperoxidase activity was not affected by either glucose or sucrose hyperosmolarity. These data suggest that hyperosmotic state by itself and an additional direct glucose-toxicity may contribute to the impaired neutrophil function in the diabetic state.

Lidocaine inhibits prolactin secretion in GH4C1 cells by blocking calcium influx

The mechanism of the inhibitory effect of local anesthetics on hormone secretion was studied in the GH4C1 line of rat pituitary tumor-derived cells. Lidocaine between 0.1 and 5 mM exerted significant dose-dependent inhibition on the increment in cytosol Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion induced by 30 mM K+. For both effects the IC50 was 0.25 mM and maximal inhibition occurred at 5 mM. A normal response returned within 20 min after removal of lidocaine from the incubation medium. 1 microM tetrodotoxin had no effect on the 30 mM K+ induced [Ca2+]i transient or PRL secretion, indicating that Na+ channels are not involved in the inhibitory effect of lidocaine. Lidocaine similarly inhibited the [Ca2+]i increment and PRL secretion induced by 30% medium hyposmolarity and 1 microM Bay K 8644. Lidocaine was much less effective in inhibiting secretion induced by 1 microM phorbol 12-myristate 13-acetate (TPA) or 5 microM forskolin. 5 mM procaine produced effects similar to those of lidocaine. Our data suggest that in GH4C1 cells local anesthetics depress secretagogue-induced PRL secretion primarily by blocking Ca2+ influx, probably through L-type Ca2+ channels.

Protein kinase C modulates cell swelling-induced Ca2+ influx and prolactin secretion in GH4C1 cells

In GH4C1 rat pituitary cells, cell swelling stimulates prolactin (PRL) secretion by increasing Ca2+ influx through nifedipine-sensitive Ca2+ channels; however, the mechanism by which cell swelling opens Ca2+ channels is still unclear. To evaluate the role of protein kinase C (PKC) in this phenomenon, we studied the effect of down-regulating PKC by 12-h pretreatment with phorbol ester or by treatment with H-7, a protein kinase C inhibitor. Cell swelling induced by either 27% medium hyposmolarity or 80 mM isotonic urea caused a prompt rise in both [Ca2+]i and PRL secretion in otherwise untreated control GH4C1 cells. Removal of medium Ca2+ enhanced the osmotically induced cell swelling but prevented the increase in [Ca2+]i and PRL secretion. Both PKC down-regulation and H-7 suppressed the cell swelling-induced increases in [Ca2+]i concentration and PRL secretion, although they enhanced the induced cell volume expansion. Our data indicate that in GH4C1 cells PKC plays an important positive modulating role in the osmotic opening of plasmalemma Ca2+ channels, a critical component of the early transduction chain by which cell swelling causes PRL secretion in tumor-derived clonal pituitary cells.

An acute release of Ca2+ from sequestered intracellular pools is not the primary transduction mechanism causing the initial burst of PRL and TSH secretion induced by TRH in normal rat pituitary cells

With 1.5 mM [Ca2+]e, 10 nM TRH induced a prompt high-amplitude burst of hormone secretion and an initial high-amplitude [Ca2+]i burst (first phase) followed by a sustained low-amplitude [Ca2+]i increment (second phase) in both tumor-derived GH4C1 and normal adenohypophyseal (AP) cells. With less than 2 microM [Ca2+]e, in both cell types the TRH-induced first phase rise in [Ca2+]i was suppressed 30% while the second phase rise was completely abolished; however, hormone secretion was inhibited only 20-30% in GH4C1 but greater than 80% in AP cells. Thapsigargin induced a first-phase rise in [Ca2+]i in AP cells equal to that induced by 10 nM TRH but only 20% as much first-phase hormone secretion. Blocking Ca2+ channels with nifedipine inhibited TRH-induced secretion in AP cells significantly more than in GH4C1 cells. Our data indicate that the TRH-induced first-phase spike in [Ca2+]i from intracellular Ca2+ stores may play a major transduction role in hormone secretion in GH4C1 cells but not in normal AP cells. Transduction mechanisms coupled to Ca2+ influx through Ca2+ channels in the plasmalemma are apparently a much more important component of TRH-induced secretion in normal than in tumor-derived pituitary cells.

Medium hyperosmolarity inhibits prolactin secretion induced by depolarizing K+ in GH4C1 cells by blocking Ca2+ influx

Medium hyperosmolarity between 300 (normal medium osmolarity) and 600 mOsm inhibited in a concentration-correlated fashion (r greater than 0.97, p less than 0.001) the rise in intracellular Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion induced in GH4C1 cells by depolarizing 30 mM K+. [Ca2+]i concentration and PRL secretion were tightly related between 300 and 600 mOsm (r = 0.976, p less than 0.001); 50% inhibition of both occurred at 450 mOsm. Medium hyperosmolarity slowed the rate of Ca2+ influx. At 600 mOsm the rise in both [Ca2+]i and PRL secretion was abolished but PRL secretion induced by 1 microM phorbol 12-myristate 13-acetate was not significantly reduced. Our data suggest that inhibition of Ca2+ influx may be the primary mechanism by which extracellular hyperosmolarity inhibits PRL secretion induced by high medium K+ in GH4C1 cells. Depression of the Ca2+ intracellular transduction system may play a pathophysiological role in vivo in conditions such as dehydration and hypertonic coma.

Dopamine inhibits cell swelling-induced prolactin secretion in MMQ cells by blocking Ca2+ influx

To evaluate the role of Ca2+ influx on hormone secretion induced by cell swelling, we have utilized a prolactin (PRL)-secreting rat tumor cell line, MMQ, which has plasmalemma dopamine receptors. Medium hyposmolarity or osmotically equivalent isotonic urea caused prompt cell swelling and a rise in both [Ca2+]i and PRL secretion in a dose-dependent manner. Dopamine inhibited the induced increase in both [Ca2+]i and PRL secretion in a dose-dependent manner but the maximum inhibition was only 50%. This effect of dopamine was prevented by haloperidol. Depletion of medium Ca2+ or blocking Ca2+ influx with nifedipine completely abolished the osmotically induced rise in both [Ca2+]i and PRL secretion. These data indicate that Ca2+ influx through nifedipine-sensitive Ca2+ channels is an essential component of PRL secretion induced by osmotic cell swelling in MMQ cells and that a dopaminergic receptor-linked mechanism influences the opening of these channels.

Hormone secretion stimulated by ethanol-induced cell swelling in normal rat adenohypophysial cells

Ethanol has been reported to affect endocrine functions, but its mechanism of action is unclear. To evaluate the hypothesis that cell swelling induced by ethanol permeation through the plasmalemma triggers hormone secretion, we studied the effect of ethanol on both hormone secretion and cell volume in acutely dispersed rat adenohypophysial cells under isotonic and hypertonic conditions. Isotonic ethanol caused a prompt cell swelling and an explosive secretory burst of prolactin and thyrotropin, which were proportional to the concentration of ethanol between 10 and 120 mM. The lowest effective dose of isotonic ethanol was 10 mM, which is below the plasma levels of legal intoxication (16 mM). Removal of medium Ca2+ enhanced the isotonic ethanol-induced increases in both cell volume and secretion. Hypertonic ethanol was ineffective in these effects. These data indicate that, in normal rat adenohypophysial cells, cell swelling caused by the rapid passage of ethanol through the plasmalemma is a potent mechanism for stimulating hormone secretion and this induced secretion is negatively modulated by extracellular Ca2+.

Medium hyperosmolarity depresses thyrotropin-releasing hormone-induced Ca2+ influx and prolactin secretion in GH4C1 cells

We studied the influence of graded degrees of hyperosmolarity on the dynamics of the thyrotropin-releasing hormone (TRH)-induced rise in cytosol Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells. TRH caused two phases of increase in [Ca2+]i that were differentially altered by hyperosmolarity: 100% hyperosmolarity (600 mOsm) depressed only 20% of an initial high-amplitude [Ca2+]i burst (first phase) dependent on Ca2+ mobilized from intracellular pools, but it abolished a sustained low-amplitude second phase dependent on extracellular Ca2+ influx. Low degrees of hyperosmolarity suppressed PRL secretion due to Ca2+ influx while high degrees suppressed secretion due to mobilized Ca2+. These data suggest that in GH4C1 cells hypertonic inhibition of secretion may result from both blocking Ca2+ influx and mechanisms unrelated to [Ca2+]i.

Dual effect of osmotic cell swelling on prolactin secretion by acutely dispersed adenohypophyseal cells

Cell swelling induced by acute exposure to the permeant molecule urea or by medium hyposmolarity evoked a prompt PRL secretory burst from dispersed rat anterior pituitary cells. However, during continuous exposure greater than or equal to 10 min to these conditions inhibition of basal and TRH-induced PRL secretion occurred and there was an "off" burst of PRL secretion following return to basal conditions. Compared with continuous TRH stimulation which causes biphasic PRL secretion with a rapid high amplitude first phase secretory burst followed by a sustained low level second phase of secretion, cell swelling induced only "first phase" secretion. Removing Ca2+ from the medium or adding 50 microM verapamil markedly depressed the "off" secretory burst following return to basal conditions but had no effect on the initial high amplitude burst. Our data suggest that the effect of cell swelling on PRL secretion is complex and that there are at least two mechanisms for PRL secretion in normal anterior pituitary cells; these are differently affected by cell swelling and Ca2+ influx.

Lidocaine and procaine inhibit the increase in cytosol Ca2+ induced by thyrotropin-releasing hormone or K+ depolarization in GH4C1 cells

Lidocaine at greater than or equal to 1 mM and procaine at greater than or equal to 2.5 mM exerted dose-dependent inhibition of the increment in [Ca2+]i induced by 100 nM thyrotropin-releasing hormone (TRH) or 30 mM K+ in GH4C1 cells. The rise in [Ca2+]i induced by K+ was more sensitive to this inhibition than that induced by TRH. Lidocaine was more potent than procaine in inhibiting the [Ca2+]i increment induced by secretagogues. Maximal lidocaine inhibition of the TRH-induced [Ca2+]i increment occurred within 15-20 min and a normal response to secretagogues returned within 20 min after removal of lidocaine from the incubation medium. Our data suggest that in GH4C1 cells local anesthetics depress secretagogue-induced intracellular Ca2+ mobilization, depolarization of the cell membrane, and the opening of voltage-dependent Ca2+ channels. This may explain the depression of secretagogue-stimulated hormone secretion induced by these agents.