A Method to Simultaneously Detect Changes in Intracellular Ca<Superscript>2+</Superscript> Concentration and Cell Volume

A method to simultaneously assess the changes in intracellular calcium concentration and cell volume in single cells was developed using the Ca²⁺-sensitive fluorescent probe Fura-2 and a three-dimensional image-surface reconstruction technique, respectively. Studies with this method showed that Fura-2 loading had no significant effect on the kinetics of A549 human epithelial cell swelling in a hypotonic solution, as well as the volume restoration kinetics. Significant changes in intracellular Ca²⁺ concentration were not observed in the examined volume modulation range. The results suggest that Ca²⁺-mediated signaling pathways are not involved in the autoregulation of the cell volume in A549 cells exposed to hypotonic conditions.     

Cytoplasmic free calcium in single cells of centric diatoms. The use of fura-2

Summary Cytoplasmic free calcium concentration has been measured in centric diatoms using Fura-2. Cells hydrolysed Fura-2 acetoxymethyl ester and accumulated free Fura-2 in the cytoplasm. The [Ca²⁺] distribution in two species (Coccinodiscus concinnus andGuinardia flaccida) has been mapped using digital image analysis.Abbreviations [Ca _cyt ²⁺ ] Cytoplasmic free calcium concentration - Quin-2/AM Quin-2 acetoxymethyl ester - Fura-2/AM Fura-2 acetoxymethyl ester DMSO dimethyl sulphoxide - CCTV closed circuit television     

Recruitment of the intracellular Ca by ultrashort electric stimuli: The impact of pulse duration

Nanosecond-duration electric stimuli are distinguished by the ability to permeabilize intracellular membranes and recruit Ca²⁺ from intracellular stores. We quantified this effect in non-excitable cells (CHO) using ratiometric Ca²⁺ imaging with Fura-2. In a Ca²⁺-free medium, 10-, 60-, and 300-ns stimuli evoked Ca²⁺ transients by mobilization of Ca²⁺ from the endoplasmic reticulum. With 2 mM external Ca²⁺, the transients included both extra- and intracellular components. The recruitment of intracellular Ca²⁺ increased as the stimulus duration decreased. At the threshold of 200–300 nM, the transients were amplified by calcium-induced calcium release. We conclude that nanosecond stimuli mimic Ca²⁺ signaling while bypassing the usual receptor- and channels-mediated cascades. The recruitment of the intracellular Ca²⁺ can be controlled by the duration of the stimulus.     

Changes in cytosolic CA2+ are not involved in DDT-induced loss of gap junctional communication in WB-F344 cells

Recent studies have demonstrated that the insecticide DDT is a tumor promoting agent. Similar to many other tumor promoting agents, DDT has been shown to inhibit gap junctional intercellular communication (GJIC) between cells in culture, and it has been suggested that DDT-induced loss of communication between adjacent cells may depend on changes in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). In the present study, the role of[Ca²⁺]_i in DDT-induced loss of GJIC was investigated in WB-F344 rat liver cells using the scrape-loading/dye transfer assay (SLDT) and the Ca²⁺ fourescent indicator, furà-2. Our results show that DDT at non-cytotoxic concentrations caused a reversible loss of GJIC. Inhibition of GJIC was not associated with detectable increases in [Ca²⁺]_i, and was not prevented by loading cells with the intracellular Ca²⁺ chelator, BAPTA. In addition, the hydroquinone, tBuBHQ, which caused a 2+3 fold sustained increase in [Ca²⁺]_i, did not inhibit GJIC. Conversely, when untreated cells were loaded with increasing BAPTA concentrations, GJIC were lost. These results indicate that increases in [Ca²⁺]_i are not responsible for DDT-induced loss of communication and that, in general an increase in [Ca²⁺]_i, within physiological levels is not sufficient to abolish GJIC. However, Ca²⁺-dependent processes that are active at normal resting [Ca²⁺ _i appear to be required for the maintenance of GJIC.Abbreviations [Ca²⁺] cytosolic free Ca²⁺ concentration - GJIC gap junctional intercellular communication - SLDT scrape-loading/dye transfer assay - DDT 1,1,1-trichloro-2,2-di-(4-chlorophenyl)ethane - tBuBHQ 2,5-di(tert-butyl)-1,4-benzohydroquinone - LDH lactate dehydrogenase - ER endoplasmic reticulum - Fura-2 1-[2-(5carboxyoxazol-2-yl)-6-aminobenzofuran-5-oxyl]-2-(2amino-5-methylphenoxy)-ethane-N,N,N,N-tetraacetic acid - BAPTA bis-(o-aminophenoxy)-ethane-N,N,N,N-tetraaceticacid - Fura-2/AM and BAPTA/AM are the cell permeant acetoxymethyl ester forms of fura-2 and BAPTA, respectively     

Staphylococcal serine proteinase increases intracellular free calcium concentration in human polymorphonuclear leukocytes

Staphylococcal serine proteinase (SSP) can influence various functions of human polymorphonuclear leukocytes (PMNL) including chemotaxis and phagocytosis. Since the rise in intracellular free calcium concentration is an important step in signal transduction leading to phagocyte activation, we tested the ability of SSP to increase the intracellular free calcium concentration in human PMNL using the fluorescent calcium indicator Fura-2AM. PMNL isolated from healthy donors responded to SSP in the concentration range of 10 to 100 µg/ml. The highest Ca²⁺ rise (104 ± 47 nM) was observed for 10 µg/ml SSP. It was mainly dependent (81 ± 11%) on extracellular calcium influx, however, SSP mobilized 68 ± 7% of Ca²⁺ from intracellular calcium stores. Boiling of SSP or preincubation with phenylmethylsulphonylfluoride (an serine proteinase inhibitor) did not change its ability to increase intracellular free calcium concentration in PMNL. It suggests that active center of SSP is not responsible for Ca²⁺ mobilization. Finally, PMNL responded to each of three consecutive stimulations with SSP independently of the presence of high or low extracellular Ca² concentration. This may be an additional mechanism responsible for activation of human PMNL and degradation of alveolar walls during the staphylococcal infection in the lower airways.     

The Role of Calcium in the Volume Regulation of Rat Lacrimal Acinar Cells

Earlier studies have suggested a role for Ca²⁺ in regulatory volume decrease (RVD) in response to hypotonic stress through the activation of Ca²⁺-dependent ion channels (Kotera & Brown, 1993; Park et al., 1994). The involvement of Ca²⁺ in regulating cell volume in rat lacrimal acinar cells was therefore examined using a video-imaging technique to measure cell volume. The trivalent cation Gd³⁺ inhibited RVD, suggesting that Ca²⁺ entry is important and may be via stretch-activated cation channels. However, Fura-2 loaded cells did not show an increase in [Ca²⁺] _i during exposure to hypotonic solutions. The absence of any changes in [Ca²⁺] _i resulted from the buffering of cytosolic Ca²⁺ by Fura-2 during hypotonic shock and therefore inhibition of RVD. The intracellular Ca²⁺ chelator, BAPTA, also inhibited the RVD response to hypotonic shock. An increase in [Ca²⁺] _i induced by either acetylcholine or ionomycin, was found to decrease cell volume under isotonic conditions in lacrimal acinar cells. Cell shrinkage was inhibited by tetraethylammonium ion, an inhibitor of Ca²⁺-activated K⁺ channels. On the basis of the presented data, we suggest an involvement of intracellular Ca²⁺ in controlling cell volume in lacrimal acinar cells. Received: 20 February 1998/Revised: 1 May 1998     

Product fermented by <Emphasis Type="Italic">Lactobacilli</Emphasis> induces changes in intracellular calcium dynamics in rat brain neurons

In this work we investigated the effect of concentrated metabolic products of lactobacilli (PP) on the dynamics of intracellular calcium concentration ([Ca²⁺]_i) in rat brain neurons. [Ca²⁺]_i was recorded using a fluorescent probe Fura-2 and a ratiometric Ca²⁺ imaging. It was found that PP increased [Ca²⁺]_i, stimulating the intracellular signaling mechanisms. In these processes the activation of ryanodine receptors and protein kinase C are involved at least partially. Continuous application of PP stimulated a sustained release of Ca²⁺ from the endoplasmic reticulum and subsequent entry of Ca²⁺ into the cell. Given that PP is able to stimulate circulation and neurogenesis and is involved in calcium homeostasis in nerve cells in the brain, PP can be regarded as a product for the improvement of psychological parameters and cognitive functions of the brain.     

Redox modulation of calcium entry and release of intracellular calcium by thimerosal in GH4C1 pituitary cells

In the present work we have investigated the actions of the oxidizing sulfhydryl reagent thimerosal on different mechanisms which regulate intracellular free Ca²⁺ concentration ([Ca²⁺]_i) in GH₄C₁ pituitary cells. In intact Fura-2 loaded cells, low concentrations of thimerosal potentiated the spike phase of the TRH-induced (thyrotropin-releasing hormone) rise in [Ca²⁺]_i, whereas high thimerosal concentrations inhibited it. The effect of thimerosal on the plateau phase was always inhibitory.The effect of thimerosal on the IP₃-induced calcium release (IICR) was studied in permeabilized cells using the Ca²⁺ indicator Fluo-3. A low concentration of thimerosal (10 μM) stimulated IICR: the Ca²⁺ release induced by 300 nM inositol-1,4,5-trisphosphate (IP₃) was enhanced in cells treated with thimerosal for 1 or 6 min (67 ± 11 nM and 34 ± 5 nM, respectively) as compared to control cells (17 ± 2 nM). On the other hand, a high concentration of thimerosal (100 μ inhibited IICR: when IP₃ (10 μM) was added after a 5 min preincubation with thimerosal, the IP₃-induced rise in [Ca²⁺]_i (46 ± 14 nM) was 57% smaller as compared with that seen in control cells (106 ± 10 nM).The effect of thimerosal on the voltage-operated Ca ²⁺ channels (VOCCs) was studied by depolarizing intact Fura-2 loaded cells by addition of 20 mM K⁺ to the cuvette. The depolarization-evoked increase in [Ca²⁺]_i was inhibited in a dose-dependent manner by thimerosal. Direct evidence for an inhibitory effect of thimerosal on VOCCs was obtained by using the whole-cell configuration of the patch-clamp technique: thimerosal (100 μM) potently inhibited the Ba²⁺ currents through VOCCs.In addition, our results indicated that thimerosal inhibited the caffeine-induced increase in [Ca²⁺]_i, and activated a capacitative Ca²⁺ entry pathway. The actions of thimerosal were apparently due to its oxidizing activity because the effects were mostly reversed by the thiol-reducing agent dithiothreitol (DTT).We conclude that, in GH₄C₁ pituitary cells, the mobilization of intracellular calcium and the different Ca²⁺ entry pathways are sensitive to redox modulation.     

The effect of hyposmotic and isosmotic cell swelling on the intracellular [Ca2+] in lactating rat mammary acinar cells

The effect of hyposmotic and isosmotic cell swelling on the free intracellular calcium concentration ([Ca²⁺]_i) in rat mammary acinar cells has been examined using the fura-2 dye technique. A hyposmotic shock (40% reduction) increased the [Ca²⁺]_i in rat mammary acinar cells in a fashion which was transient; the [Ca²⁺]_i returned to a value similar to that found under isomotic conditions within 180 sec. The increase in the [Ca²⁺]_i was dependent upon the extent of the osmotic shock. The hyposmotically-activated increase in the [Ca²⁺]_i could not be attributed to a reduction in extracellular Na⁺ or a change in the ionic strength of the incubation medium. Thapsigargin (1 M) enhanced the hyposmotically-activated increase in the [Ca²⁺]_i. Isosmotic swelling of rat mammary acinar cells, using urea, had no significant effect on the [Ca²⁺]_i. Similarly, a hyperosmotic shock did not affect the [Ca²⁺]_i in rat mammary acinar cells. It appears that the effect of cell swelling on the [Ca²⁺]_i in rat mammary acinar cells depends on how the cells are swollen (hyposmotic vs. isosmotic). This finding may have important physiological implications given that it is predicted that mammary cell volume will change in vivo under isomotic conditions.     

NMDA Receptor-Mediated Calcium Influx in Cerebral Cortical Synaptosomes of the Hypoxic Guinea Pig Fetus

Calcium influx via the NMDA receptor has been proposed as a mechanism of hypoxia-induced neuronal injury. The present study tests the hypothesis that the increase of [Ca²⁺]_i observed under hypoxic conditions is the result of an NMDA-mediated Ca²⁺ influx. Changes of [Ca²⁺]_i, measured fluorometrically with Fura-2, were followed after activation of the NMDA receptor with NMDA and glutamate, in the presence of glycine, in cortical synaptosomes prepared from six normoxic and six hypoxic guinea pig fetuses. [Ca²⁺]_i was significantly higher in hypoxic vs normoxic synaptosomes, at baseline and in the presence of glycine as well as following activation of the NMDA receptor. Increase in [Ca²⁺]_i was not observed in a Ca²⁺ free medium and was significantly decreased by MK-801 and thapsigargin. These results demonstrate that hypoxia-induced modifications of the NMDA receptor ion-channel results in increased [Ca²⁺]_i in hypoxic vs normoxic synaptosomes. This increased accumulation may be due to an initial influx of Ca²⁺ via the altered NMDA receptor with subsequent release of Ca²⁺ from intracellular stores. Increase in intracellular calcium may initiate several pathways of free radical generation including cyclooxygenase, lipoxygenase, xanthine oxidase and nitric oxide synthase, and lead to membrane lipid peroxidation resulting in neuronal cell damage.     

Inhibitors of the Metabolism of Arachidonic Acid Suppress Ca<Superscript>2+</Superscript> Responses Induced by Trifluoperazine in Macrophages

The influence of the neuroleptic trifluoperazine on the intracellular concentration of Ca²⁺ in macrophages of rats was studied using a Fura-2AM fluorescent Ca²⁺ probe. It was found that trifluoperazine causes a dose-dependent increase in the intracellular Ca²⁺ concentration associated with Ca²⁺ mobilization from intracellular Ca²⁺ stores and subsequent entry of Ca²⁺ into peritoneal macrophages of rats. It was also shown that inhibitors of phospholipase A₂ (4-bromophenacyl bromide, prednisolone, and dexamethasone), cyclooxygenases (aspirin and indomethacin), and lipoxygenases (caffeic acid, zileuton, and baicalein) suppress Ca²⁺ responses induced by trifluoperazine in macrophages. The data obtained indicate the participation of enzymes and/or products of the cascade of arachidonic acid metabolism in the influence of trifluoperazine on the intracellular concentration of Ca²⁺ in peritoneal macrophages.     

Effectors of the frequency of calcium oscillations in HEK-293 cells: wavelet analysis and a computer model

Oscillations of the intracellular concentration of Ca²⁺ in cultured HEK-293 cells, which heterologously expressed the calcium-sensing receptor, were recorded with the fluorophore Fura-2 using fluorescence microscopy. HEK-293 cells are extremely sensitive to small perturbations in extracellular calcium concentrations. Resting cells were attached to cover slips and perifused with saline solution containing physiologically relevant extracellular Ca²⁺ concentrations in the range 0.5–5 mM. Acquired digitized images of the cells showed oscillatory fluctuations in the intracellular Ca²⁺ concentration over the time course, and were processed as a function of the change in Fura-2 excitation ratio and frequency at 12–37°C. Newly developed data processing techniques with wavelet analysis were used to estimate the frequency at which the rectified sinusoidal oscillations occurred; we estimated ~4 min⁻¹ under normal conditions. Temperature variations revealed an Arrhenius relationship in oscillation frequency. A critical Ca²⁺ concentration of ~2 mM was estimated, below which oscillations did not occur. These data were used to develop a kinetic model of the system that was simulated using Mathematica; kinetic parameter values were adjusted to match the experimentally observed oscillations of intracellular Ca²⁺ concentration as a function of extracellular Ca²⁺ concentration, and temperature; and from these, limit cycles were obtained and control coefficients were estimated for all parameters.     

The potassium channel opener NS1619 modulates calcium homeostasis in muscle cells by inhibiting SERCA

NS1619 (1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazole-2-one) is widely used as a large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel opener. It was previously reported that activation of BK_Ca channels by NS1619 could protect the cardiac muscle against ischaemia and reperfusion injury. This study reports the effects of NS1619 on intracellular Ca²⁺ homeostasis in H9C2 and C2C12 cells as well as its molecular mechanism of action. The effects of NS1619 on Ca²⁺ homeostasis in C2C12 and H9C2 cells were assessed using the Fura-2 fluorescence method. Ca²⁺ uptake by sarcoplasmic reticulum (SR) vesicles isolated from rat skeletal muscles and sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) activity were measured. The effect of NS1619 on the isometric force of papillary muscle contraction in the guinea pig heart was also examined. H9C2 and C2C12 cells treated with NS1619 released Ca²⁺ from internal stores in a concentration-dependent manner. Ca²⁺ accumulation by the SR vesicles was inhibited by NS1619 treatment. NS1619 also decreased the activity of SERCA derived from rat skeletal muscle. The calcium release from cell internal stores and inhibition of SERCA by NS1619 are pH dependent. Finally, NS1619 had a profound effect on the isometric force of papillary muscle contraction in the guinea pig heart. These results indicate that NS1619 is a potent modulator of the intracellular Ca²⁺ concentration in H9C2 and C1C12 cells due to its interaction with SRs. The primary target of NS1619 is SERCA, which is located in SR vesicles. The effect of NS1619-mediated SERCA inhibition on cytoprotective processes should be considered.     

Rapid Desensitization of Serotonin 5-HT2C Receptor-Stimulated Intracellular Calcium Mobilization in CHO Cells Transfected with Cloned Human 5-HT2C Receptors

Abstract: Serotonin 5-HT_2C receptor-mediated intracellular Ca²⁺ mobilization was investigated in Chinese hamster ovary (CHO) cells transfected with 5-HT_2C receptors. Fura-2 acetoxymethyl ester was used to investigate the regulation of 5-HT_2C receptor function. CHO cells, transfected with a cDNA clone for the 5-HT_2C receptor, expressed 287 fmol/mg of the receptor protein as determined by mianserin-sensitive [³H]mesulergine binding (K_D = 0.49 nM). The addition of 5-HT mobilized intracellular Ca²⁺ in a dose-dependent fashion, ranging from a basal level of 99 ± 1.8 up to 379 ± 18 nM, with an EC₅₀ value for 5-HT of 0.029 µM. Exposure to 5-HT, 1-(3-chlorophenyl)piperazine dihydrochloride (a 5-HT_2C agonist), and 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (a 5-HT_2C and 5-HT_2A agonist) resulted in increased intracellular Ca²⁺ levels. Mianserin, mesulergine, ritanserin, and ketanserin each blocked 5-HT-mediated intracellular Ca²⁺ mobilization more effectively than spiperone. The receptor was rapidly desensitized by preexposure to 5-HT in a time- and concentration-dependent manner. Mezerein and phorbol 12-myristate 13-acetate, protein kinase C activators, weakly inhibited the intracellular Ca²⁺ mobilization induced by 10 µM 5-HT. Furthermore, the protein kinase C inhibitor H-7 partially prevented the protein kinase C activator-induced inhibition of the 5-HT-mediated increase in intracellular Ca²⁺ concentration. The desensitization induced by pretreatment with 5-HT was blocked by W-7, added in conjunction with 5-HT, and partially inhibited by W-5, a nonselective inhibitor of protein kinases and weak analogue of W-7. Therefore, the 5-HT_2C receptor may be connected with protein kinase C and calcium/calmodulin turnover. These results suggest that 5-HT_2C receptor activation mobilizes Ca²⁺ in CHO cells and that the acute desensitization of the receptor may be due to calmodulin kinase-mediated feedback.     

Effects of cryopreservation on the intracellular calcium concentration of human spermatozoa and its response to progesterone

The intracellular free calcium concentration [Ca²⁺]_i of sperm from 23 ejaculates was measured before and after cryopreservation using the fluorescent probe Fura-2. Spermatozoa were treated with 3.18 μM progesterone so that the regulation of [Ca²⁺]_i in a dynamic situation could be studied. [Ca²⁺]_i (nM) was 290 ± 13 in fresh spermatozoa vs. 550 ± 26 in cryopreserved samples (mean ± S.E.M. P < 0.0001 paired t-test). Progesterone at a dose of 3.18 μM stimulated a large and rapid increase in [Ca²⁺]_i to a peak value > 1 μM after 10–20 seconds. [Ca²⁺]_i then declined to a slightly raised basal level over the next 30–40 seconds. This phenomenon occurred in all the fresh samples, but about half the frozen thawed samples failed to respond. The peak [Ca²⁺] attained by frozen samples which did respond after the addition of progesterone was similar to that observed with fresh sperm. The calcium channel blocker verapamil (200 μM) completely inhibited the transient rise in [Ca²⁺]_i produced by progesterone, but 100 μM verapamil had only a partial effect. We conclude that (1) cryopreservation causes a substantial elevation of the [Ca²⁺]_i in human spermatozoa and (2) damage to the plasma membrane during cryopreservation may result in the loss of the progesterone receptor. Both factors may contribute to the loss of fertility after cryopreservation. © 1994 Wiley-Liss, Inc.     

The effect of oxidized glutathione and its pharmacological analogue glutoxim on intracellular Ca<Superscript>2+</Superscript> concentration in macrophages Ca<Superscript>2+</Superscript>

Using Fura-2AM microfluorimetry, the effect of oxidized glutathione (GSSG) and its pharmacological analogue glutoxim on the intracellular Ca²⁺ concentration in rat peritoneal macrophages was investigated. It was shown that GSSG or glutoxim increase the intracellular Ca²⁺ concentration by inducing Ca²⁺ mobilization from thapsigargin-sensitive Ca²⁺ stores and subsequent Ca²⁺ entry from external medium. Dithiothreitol, which reduces S-S-bonds in proteins, completely prevents or reverses the increase of intracellular Ca²⁺ concentration induced by GSSG or glutoxim. This suggests that the increase of intracellular Ca²⁺ concentration induced by GSSG or glutoxim can be mediated by their interactions with functionally important SH-groups of proteins involved in Ca²⁺-signaling.Two structurally different tyrosine kinase inhibitors genistein and methyl-2,5-dihydroxycinnamate prevent or completely reverse the increase in the intracellular Ca²⁺ concentration induced by GSSG or glutoxim. On the contrary, tyrosine phosphatase inhibitor Na orthovanadate enhances the increase of intracellular Ca²⁺ concentration evoked by oxidizing agents. The data suggest that tyrosine kinases and tyrosine phosphatases are involved in the regulatory effect of GSSG and glutoxim on the intracellular Ca²⁺ concentration in macrophages.     

Calcium channels and GABA receptors in the early embryonic chick retina

The properties of calcium channels were studied at the period of neurogenesis in the early embryonic chick retina. The whole neural retina was isolated from embryonic day 3 (E3) chick and loaded with a Ca²⁺-sensitive fluorescent dye (Fura-2). The retinal cells were depolarized by puff application of high-K⁺ solutions. Increases in intracellular Ca²⁺ concentrations were evoked by the depolarization through calcium channels. The type of calcium channel was identified as l-type by the sensitivity to dihydropyridines. The Ca²⁺ response was completely blocked by 10 μM nifedipine, whereas it was remarkably enhanced by 5 μM Bay K 8644. Then we sought a factor to activate the calcium channel and found that GABA could activate it by membrane depolarization at the E3 chick retina. Puff application of 100 μM GABA raised intracellular Ca²⁺ concentrations, and this Ca²⁺ response to GABA was also sensitive to the two dihydropyridines. Intracellular potential recordings verified clear depolarization by bath-applied 100 μM GABA. The Ca²⁺ response to GABA was mediated by GABA_A receptors, since the GABA response was blocked by 10 μgM bicuculline or 50 μM picrotoxin, and mimicked by muscimol but not by baclofen. Neither glutamate, kainate, nor glycine evoked any Ca²⁺ response. We conclude that l-type calcium channels and GABA_A receptors are already are already expressed before differentiation of retinal cells and synapse formation in the chick retina. A possibility is proposed that GABA might act as a trophic factor by activating l-type calcium channels via GABA_A receptors during the early period of retinal neurogenesis. © 1993 John Wiley & Sons, Inc.     

Evidence that TRH controls prolactin release from rat lactotrophs by stimulating a calcium influx

Prolactin (PRL) release and intracellular free calcium concentration [Ca²⁺]_i were measured in two populations of normal rat lactotrophs (light and heavy fractions) in culture. Spontaneous PRL release of heavy fraction cells was more sensitive to dihydropyridines (DHPs; Bay K 8644 and nifedipine) when compared to the light fraction lactotrophs. The stimulatory effect of thyrotropin-releasing hormone (TRH) on PRL release from heavy fraction cells was inhibited by Cd²⁺ and mimicked by Bay K 8644. Indo-1 experiments revealed that TRH-increased [Ca²⁺]_i was reversibly inhibited by Cd²⁺. In a Ca²⁺-free EGTA-containing medium, TRH did not modify [Ca²⁺]_i.Abbreviations [Ca²⁺]_i intracellular free calcium concentration - DA dopamine - DHP dihydropyridine(s) - DMEM Dulbecco's Modified Eagle's Medium - Ins(1,4,5)P₃ inositol 1,4,5-trisphosphate - PRL prolactin - RIA radioimmunoassay - TRH thyrotropin-releasing hormone - VGCC voltage-gated calcium channel     

Calcium-induced calcium increase in secretory vesicles of permeabilized rat neurohypophysial nerve terminals

Digitonin-permeabilized isolated neurohypophysial nerve terminals are known to release their secretory vesicle content under calcium challenge. On this preparation, we monitored intra-organelle Ca²⁺ concentration using digital fluorescence microscopy of Fura-2. The superfusion of artificial intracellular solution containing 10 to 50 μM Ca²⁺ induced an intra-organelle [Ca²⁺] increase. Two major organelles are candidates for this increase: secretory vesicles and mitochondria. In an attempt to detect calcium changes in the vesicles, ruthenium red was used to impair mitochondrial calcium uptake. Part of the ruthenium red-insensitive intra-organelle [Ca²⁺] increase was abolished by raising sodium in the solution. Removing sodium boosted the intra-organelle [Ca²⁺] increase. These results taken together suggest the participation of Na/Ca exchange, known to exist in the membrane of these secretory vesicles. In addition to Na/Ca exchange, there would be at least another mechanism of vesicular calcium intake, as suggested by the partial inhibition of intra-organelle [Ca²⁺] increase obtained under acidic compartments: neutralization with NH₄Cl. This mechanism remains to be defined. The main conclusion presented here, that an intravesicular [Ca²⁺] increase takes place at the rate of secretion, was predicted by the hypothesis that intravesicular Ca²⁺ changes would be involved in stimulus-secretion coupling.