The IP3-sensitive calcium store of HIT cells is located in a surface-derived vesicle fraction

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

Cellular target of weak magnetic fields: ionic conduction along actin filaments of microvilli

The interaction of weak electromagnetic fields (EMF) with living cells is a most important but still unresolved biophysical problem. For this interaction, thermal and other types of noise appear to cause severe restrictions in the action of weak signals on relevant components of the cell. A recently presented general concept of regulation of ion and substrate pathways through microvilli provides a possible theoretical basis for the comprehension of physiological effects of even extremely low magnetic fields. The actin-based core of microfilaments in microvilli is proposed to represent a cellular interaction site for magnetic fields. Both the central role of F-actin in Ca2+ signaling and its polyelectrolyte nature eliciting specific ion conduction properties render the microvillar actin filament bundle an ideal interaction site for magnetic and electric fields. Ion channels at the tip of microvilli are connected with the cytoplasm by a bundle of microfilaments forming a diffusion barrier system. Because of its polyelectrolyte nature, the microfilament core of microvilli allows Ca2+ entry into the cytoplasm via nonlinear cable-like cation conduction through arrays of condensed ion clouds. The interaction of ion clouds with periodically applied EMFs and field-induced cation pumping through the cascade of potential barriers on the F-actin polyelectrolyte follows well-known physical principles of ion-magnetic field (MF) interaction and signal discrimination as described by the stochastic resonance and Brownian motor hypotheses. The proposed interaction mechanism is in accord with our present knowledge about Ca2+ signaling as the biological main target of MFs and the postulated extreme sensitivity for coherent excitation by very low field energies within specific amplitude and frequency windows. Microvillar F-actin bundles shielded by a lipid membrane appear to function like electronic integration devices for signal-to-noise enhancement; the influence of coherent signals on cation transduction is amplified, whereas that of random noise is reduced.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Microvillar cell surface as a natural defense system against xenobiotics: a new interpretation of multidrug resistance

The phenomenon of multidrug resistance (MDR) is reinterpreted on the basis of the recently proposed concept of microvillar signaling. According to this notion, substrate and ion fluxes across the surface of differentiated cells occur via transporters and ion channels that reside in membrane domains at the tips of microvilli (MV). The flux rates are regulated by the actin-based cytoskeletal core structure of MV, acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. The expression of this diffusion barrier system is a novel aspect of cell differentiation and represents a functional component of the natural defense system of epithelial cells against environmental hazardous ions and lipophilic compounds. Because of the specific organization of epithelial Ca(2+) signaling and the secretion, lipophilic compounds associated with the plasma membrane are transferred from the basal to the apical cell surface by a lipid flow mechanism. Drug release from the apical pole occurs by either direct secretion from the cell surface or metabolization by the microvillar cytochrome P-450 system and efflux of the metabolites and conjugation products through the large multifunctional anion channels localized in apical MV. The natural microvillar defense system also provides a mechanistic basis of acquired MDR in tumor cells. The microvillar surface organization is lost in rapidly growing cells such as tumor or embryonic cells but is restored during exposure of tumor cells to cytotoxins by induction of a prolonged G(0)/G(1) resting phase.

Inositol 1,4,5-trisphosphate receptor isoform expression in mouse pancreatic islets: effects of carbachol

The inositol 1,4,5-trisphosphate receptors (IP3Rs) are ligand-gated Ca2+ channels that regulate intracellular Ca2+ mobilization. Among the IP3R mRNA isoforms I, II, and III, IP3R-I mRNA was expressed in mouse islets and the beta-cell line betaTC3, and was quantitatively the most abundant isoform as determined by reverse transcriptase-polymerase chain reaction. IP3R-II and -III mRNAs were expressed at similar levels in mouse islets, but neither isoform was detected in betaTC3 cells. Culture of mouse islets for 30 min and 2 hr at 20 mM glucose, or for 7 days at 11 mM glucose did not affect IP3R-I mRNA expression compared with islets cultured in 5.5 mM glucose. Culture of islets or betaTC3 cells with carbachol (0.5 mM) reduced IP3R-I mRNA expression levels below control. Mouse islet alpha- and beta-cells expressed IP3R-I and -III proteins, but IP3R-II protein was not detected by immunoblot or double-label immunohistochemistry. Culture of islets for up to 6 hr with carbachol reduced IP3R-I and -III protein expression in a time-dependent manner with a half-maximal effect on type I at 1 hr. Glucose (20 mM) stimulation for 2 hr did not affect IP3R-1 levels. The carbachol-induced decrease in IP3R-I and -III protein expression was reversed by carbobenzoxyl-leucinyl-leucinyl-leucinyl-H (MG-132), a proteasome inhibitor. Thus, glucose failed to regulate mouse islet IP3R mRNA expression, whereas carbachol stimulation down-regulated IP3R mRNA and protein. A proteasomal protein degradative pathway appeared to mediate the muscarinic receptor-induced effects on IP3R-I and -III.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

Regulation of cell volume via microvillar ion channels

A novel mechanism of cellular volume regulation is presented, which ensues from the recently introduced concept of transport and ion channel regulation via microvillar structures (Lange K, 1999, J Cell Physiol 180:19-35). According to this notion, the activity of ion channels and transporter proteins located on microvilli of differentiated cells is regulated by changes in the structural organization of the bundle of actin filaments in the microvillar shaft region. Cells with microvillar surfaces represent two-compartment systems consisting of the cytoplasm on the one side and the sum of the microvillar tip (or, entrance) compartments on the other side. The two compartments are separated by the microvillar actin filament bundle acting as diffusion barrier ions and other solutes. The specific organization of ion and water channels on the surface of microvillar cell types enables this two-compartment system to respond to hypo- and hyperosmotic conditions by activation of ionic fluxes along electrochemical gradients. Hypotonic exposure results in swelling of the cytoplasmic compartment accompanied by a corresponding reduction in the length of the microvillar diffusion barrier, allowing osmolyte efflux and regulatory volume decrease (RVD). Hypertonic conditions, which cause shortening of the diffusion barrier via swelling of the entrance compartment, allow osmolyte influx for regulatory volume increase (RVI). Swelling of either the cytoplasmic or the entrance compartment, by using membrane portions of the microvillar shafts for surface enlargement, activates ion fluxes between the cytoplasm and the entrance compartment by shortening of microvilli. The pool of available membrane lipids used for cell swelling, which is proportional to length and number of microvilli per cell, represents the sensor system that directly translates surface enlargements into activation of ion channels. Thus, the use of additional membrane components for osmotic swelling or other types of surface-expanding shape changes (such as the volume-invariant cell spreading or stretching) directly regulates influx and efflux activities of microvillar ion channels. The proposed mechanism of ion flux regulation also applies to the physiological main functions of epithelial cells and the auxiliary action of swelling-induced ATP release. Furthermore, the microvillar entrance compartment, as a finely dispersed ion-accessible peripheral space, represents a cellular sensor for environmental ionic/osmotic conditions able to detect concentration gradients with high lateral resolution. Volume regulation via microvillar surfaces is only one special aspect of the general property of mechanosensitivity of microvillar ionic pathways.

Regulation of inositol trisphosphate receptor isoform expression in glucose-desensitized rat pancreatic islets: role of cyclic adenosine 3',5'-monophosphate and calcium

The regulation of inositol 1,4,5-trisphosphate receptor (IP3R) messenger RNA (mRNA) and protein expression was investigated in glucose-desensitized rat isolated pancreatic islets. Islets were cultured for 4 days with glucose (11 mM; G-treated) to induce desensitization; IP3R-I mRNA levels were similar to basal (5.5 mM glucose) values, whereas IP3R-II mRNA levels were increased and IP3R-III levels were reduced compared with basal levels. Somatostatin increased the expression of IP3R-II mRNA and reduced the expression of IP3R-III mRNA compared with basal values, but did not significantly affect G-treated islet IP3R expression. When forskolin (FSK), 8-bromo-cAMP, and glucagon-like peptide 1-(7-36) amide were added to G-treated islets after 4 days of culture, IP3R-II mRNA levels were reduced, whereas IP3R-III mRNA levels increased, to levels observed in control islets, within 3 h. The levels of IP3R-I mRNA were unaffected by either somatostatin or FSK. The protein kinase A inhibitor. H-89, and actinomycin D prevented the effects of FSK. A Ca2+ ionophore mimicked the effects of FSK on IP3R mRNA expression, whereas blockade of voltage-dependent Ca2+ channels or chelation of intracellular Ca2+ inhibited the actions of FSK. cAMP also increased IP3R-III mRNA in insulinoma cells. In G-treated islets, FSK slowed IP3R-III mRNA degradation. FSK, but not glucose, stimulated protein kinase A activation in G-treated islets. Thus, cAMP mediates changes in IP3R-II and -III mRNA transcription and stability in glucose-desensitized islets. The regulated expression of IP3R-II and -III mRNA is mediated in part by intracellular Ca2+ availability.

Microvillar Ca++ signaling: a new view of an old problem

Proceeding from the recent finding that the main components of the Ca++ signal pathway are located in small membrane protrusions on the surface of differentiated cells, called microvilli, a novel concept of cellular Ca++ signaling was developed. The main features of this concept can be summarized as follows: Microvilli are formed on the cell surface of differentiating or resting cells from exocytic membrane domains, growing out from the cell surface by elongation of an internal bundle of actin filaments. The microvillar tip membranes contain all functional important proteins synthesized such as ion channels and transporters for energy-providing substrates and structural components, which are, in rapidly growing undifferentiated cells, distributed over the whole cell surface by lateral diffusion. The microvillar shaft structure, a bundle of actin filaments, forms a dense cytoskeletal matrix tightly covered by the microvillar lipid membrane and represents an effective diffusion barrier separating the microvillar tip compartment (entrance compartment) from the cytoplasm. This diffusion barrier prevents the passage of low molecular components such as Ca++ glucose and other relevant substrates from the entrance compartment into the cytoplasm. The effectiveness of the actin-based diffusion barrier is modulated by various signal pathways and effectors, most importantly, by the actin-depolymerizing/reorganizing activity of the phospholipase C (PLC)-coupled Ca++ signaling. Moreover, the microvillar bundle of actin filaments plays a dual role in Ca++ signaling. It combines the function of a diffusion barrier, preventing Ca++ influx into the resting cell, with that of a high-affinity, ATP-dependent, and IP3-sensitive Ca++ store. Activation of Ca++ signaling via PLC-coupled receptors simultaneously empties Ca++ stores and activates the influx of external Ca++. The presented concept of Ca++ signaling is compatible with all established data on Ca++ signaling. Properties of Ca++ signaling, that could not be reconciled with the basic principles of the current hypothesis, are intrinsic properties of the new concept. Quantal Ca++ release, Ca(++)-induced Ca++ release (CICR), the coupling phenomen between the filling state of the Ca++ store and the activity of the Ca++ influx pathway, as well as the various yet unexplained complex kinetics of Ca++ uptake and release can be explained on a common mechanistic basis.

Action of insulin on the surface morphology of hepatocytes: role of phosphatidylinositol 3-kinase in insulin-induced shape change of microvilli

In previous studies we have shown that the insulin-responding glucose transporter isoform of 3T3-L1 adipocytes, GluT4, is almost completely located on microvilli. Furthermore, insulin caused the integration of these microvilli into the plasma membrane, suggesting that insulin-induced stimulation of glucose uptake may be due to the destruction of the cytoskeletal diffusion barrier formed by the actin filament bundle of the microvillar shaft regions [Lange et al. (1990) FEBS Lett. 261, 459-463; Lange et al. (1990) FEBS Lett. 276, 39-41]. Similar shape changes in microvilli were observed when the transport rates of adipocytes were modulated by glucose feeding or starvation. Here we demonstrate that the action of insulin on the surface morphology of hepatocytes is identical to that on 3T3L1 adipocytes; small and narrow microvilli on the surface of unstimulated hepatocytes were rapidly shortened and dilated on top of large domed surface areas. The aspect and mechanism of this effect are closely related to "membrane ruffling" induced by insulin and other growth factors. Pretreatment of hepatocytes with the PI 3-kinase inhibitor wortmannin (100 nM), which completely prevents transport stimulation by insulin in adipocytes and other cell types, also inhibited insulin-induced shape changes in microvilli on the hepatocyte surface. In contrast, vasopressin-induced microvillar shape changes in hepatocytes [Lange et al. (1997) Exp. Cell Res. 234, 486-497] were insensitive to wortmannin pretreatment. These findings indicate that PI 3-kinase products are necessary for stimulation of submembrane microfilament dynamics and that cytoskeletal reorganization is critically involved in insulin stimulation of transport processes. The mechanism of the insulin-induced cytoskeletal reorganization can be explained on the basis of the recent finding of Lu et al. [Biochemistry 35(1996) 14027-14034] that PI 3-kinase products exhibit much higher affinity for the profilin-actin complex than the primary products, PIP and PIP2. Thus, activated PI 3-kinase may direct a flux of profilin-actin complexes to the membrane locations of activated insulin receptors, where, due to the release of actin monomers after binding of profilactin to PI(3,4)P2 and PI(3,4,5)P3, massive actin polymerization is initiated. As a consequence, PI 3-kinase activation initiates a vectorial reorganization of the cellular actin system to membrane sites neighboring activated insulin receptors, giving rise to local membrane stress as visualized by extensive surface deformations and shortening of microvilli. In addition, extensive high-affinity binding of F-actin-barbed endcapping proteins enhances the cytoplasmic concentration of rapidly polymerizing filament ends. Consequently, the actin monomer concentration is lowered and the (cytoplasmic) pointed ends of the microvillar shaft bundle depolymerize and become shorter. The observations presented strengthen the previously postulated diffusion-barrier concept of glucose- and ion-uptake regulation and provide a mechanistic basis for explaining the action of insulin and other growth factors on transport processes across the plasma membrane.

Activation of calcium signaling in isolated rat hepatocytes is accompanied by shape changes of microvilli

Preceding studies using the hamster insulinoma cell line, HIT, and isolated rat hepatocytes have shown that two essential components of the Ca2+ signaling pathway, the ATP-dependent Ca2+ store and the store-coupled Ca2+ influx pathway, are both located in microvilli covering the surface of these cells. Microvilli-derived vesicles from both cell types exhibited anion and cation pathways which could be inhibited by anion and cation channel-specific inhibitors. These findings suggested that the microvillar tip compartment forms a space which is freely accessible for external Ca2+, ATP, and IP3. The entry of Ca2+ into the cytoplasm, however, is largely restricted by the microvillar core structure, the dense bundle of actin microfilaments acting as a diffusion barrier between the microvillar tip compartment and the cell body. Moreover, evidence has been presented that F-actin may function as ATP-dependent and IP3-sensitive Ca2+ store that can be emptied by profilin-induced depolymerization or reorganization [K. Lange and U. Brandt (1996) FEBS Lett. 395, 137-142]. Here we demonstrate the tight connection between microvillar shape changes and the activation of the Ca2+ signaling system in isolated rat hepatocytes. Using a combination of scanning electron microscopy (SEM) and fura-2 fluorescence technique, we confirmed a consequence of the "diffusion barrier" concept of Ca2+ signaling: Irrespective of the type of the applied stimulus, activation of the Ca2+ influx pathway is accompanied by changes in the structural organization of microvilli indicative of the loss of their diffusion barrier function. We further show that the cell surfaces of unstimulated hepatocytes isolated by either the collagenase or the EDTA perfusion technique are densely covered with microvilli predominantly of a short and slender type. Beside this rather uniformly shaped type of microvilli, a number of dilated surface protrusions were observed. Under these conditions the cells displayed the well known rather high basal [Ca2+]i of 200-250 nM as repeatedly demonstrated for freshly isolated hepatocytes. However, addition of the serine protease inhibitor, phenylmethanesulfonyl fluoride (PMSF), to the cell suspension immediately after its preparation reduced the basal cytoplasmic Ca2+ level to about 100 nM. Concomitantly, dilated surface protrusions disappeared, and cell surfaces exclusively displayed short, slender microvilli. Activation of the Ca2+ signaling pathway by vasopressin, as well as by the IP3-independent acting Ca2+ store inhibitor, thapsigargin, was accompanied by a conspicuous shortening and dilation of microvilli following the same time courses as the respective increases of [Ca2+]i induced by the effectors. Furthermore, the abundance of the large form of surface protrusions on isolated hepatocytes positively correlated with the size of a cellular Ca2+/Fura-2 compartment which is rapidly depleted from Ca2+ by extracellular EGTA. These findings support the postulated localization of the store-coupled Ca2+ influx pathway in microvilli of HIT cells also for hepatocytes and are in accord with the notion of a cytoskeletal diffusion barrier regulating the flux of external Ca2+ via the microvillar tip region in the cytoplasm.

A new concept for risk assessment of the hazards of non-genotoxic chemicals--electronmicroscopic studies of the cell surface. Evidence for the action of lipophilic chemicals on the Ca2+ signaling system

Recently, we presented evidence for the localization of components of the cellular Ca2+ signaling pathway in microvilli. On stimulation of this pathway, microvilli undergo characteristic morphological changes which can be detected by scanning electron microscopy (SEM) of the cell surface. Here we show that both receptor-mediated (vasopressin) and unspecific stimulation of the Ca2+ signaling system by the lipophilic tumor promoters thapsigargin (TG) and phorbolmyristateacetate (PMA) are accompanied by the same type of morphological changes of the cell surface. Since stimulated cell proliferation accelerates tumor development and sustained elevation of the intracellular Ca2+ concentrations is a precondition for stimulated cell proliferation, activated Ca2+ signaling is one possible mechanism of non-genomic tumor promotion. Using isolated rat hepatocytes we show that all tested lipophilic chemicals with known tumor promoter action, caused characteristic microvillar shape changes. On the other hand, lipophilic solvents that were used as differentiating agents in cell cultures such as dimethylsulfoxide (DMSO) and dimethylformamide also, failed to change the microvillar shapes. Instead DMSO stabilized the original appearance of microvilli. The used technique provides a convenient method for the evaluation of non-genomic carcinogenicity of chemicals prior to their industrial application.

The role of voltage-dependent calcium channels in angiotensin-stimulated glomerulosa cells

The concept that voltage-dependent Ca2+ influx is essential in the aldosterone stimulating action of angiotensin II (AII) has been recently challenged by the demonstration of the dihydropyridine (DHP) insensitive 'capacitative' Ca2+ uptake mechanism. The DHP-sensitivity of AII-induced aldosterone secretion is still to be explained. In rat glomerulosa cells the lag phase of AII-induced depolarization is more than 30 s, and there is no enhanced Ca2+ influx within the first min of stimulation. Yet we observed that DHPs as well as diltiazem influenced also the peak of cytoplasmic Ca2+ signal, although the peak (approximately 12 s) is attributed to Ca2+ release alone. Nifedipine reduced the Ca2+ transient induced by AII even after complete inhibition of Ca2+ channel activity. Recalling the loose attachment of InsP3 receptors (IP3R) to the plasma membrane, and the homology between the cytosolic domain of IP3R and the Ca2+ release channel (ryanodine receptor) of skeletal muscle, we proposed that DHP-sensitive L-type Ca2+ channels (DHP receptors) influence InsP3-induced Ca2+ release rather than Ca2+ influx in AII-stimulated cells. Although the dominant isoform is the neuroendocrine (D) one, the skeletal muscle isoform of L-type voltage-dependent Ca2+ channel is also expressed in rat glomerulosa cells. This isoform may be a candidate for protein-protein interaction between DHPR and subplasmalemmal IP3R, similarly to that occurring between DHP receptors and ryanodine receptors in skeletal muscle.

Calcium storage and release properties of F-actin: evidence for the involvement of F-actin in cellular calcium signaling

Preceding studies have shown that the bulk of the ATP-dependent, inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store of hamster insulinoma (HIT) cells is located in microvilli on the cell surface. Similar results were obtained with isolated rat hepatocytes. Moreover, in vesicles of microvillar origin, passive fluxes of Ca2+, ATP, and IP3 occur through cation and anion channels, respectively, suggesting that Ca2+ storage is due to ATP-dependent Ca2+ binding to an intravesicular component. Here we demonstrate that F-actin may be a possible candidate for this function. ATP-actin monomers bind Ca2+ with high affinity (Kd = 2-8 nM) to their divalent cation binding sites. Polymerization of actin monomers decreases the rate constant for divalent cation exchange at this binding site by more than 3 orders of magnitude rendering bound cations nearly unavailable. F-actin-bound Ca2+ can be released by depolymerization and dissociation from Ca(2+)-ADP-actin monomers (Kd = 375 nM). We now provide additional evidence for the possible involvement of actin in Ca2+ storage. (1) Preincubation of surface-derived Ca(2+)-storing vesicles from HIT cells with the F-actin stabilizer, phalloidin, strongly inhibited ATP-dependent Ca2+ uptake, reducing the IP3-sensitive Ca2+ pool by 70%. Phalloidin, when added after the loading process, affected neither the amount of stored Ca2+ nor IP3 action on the store. (2) F-actin polymerized in the presence of Mg2+ in nominally Ca(2+)-free buffer still contained about half of the high affinity sites occupied with Ca2+ (Mg/Ca-F-actin). (3) Using the fura-2 technique, we found that in the presence of ATP, Mg/Ca-F-actin incorporated free Ca2+ at a relatively low rate. Short pulses of ultrasound (3-10 s) strongly accelerated Ca2+ uptake, decreasing free Ca2+ from 500 nM to below 100 nM. (4) In the presence of physiological levels of Mg2+ (0.5 mM), sonication liberated large amounts of Ca2+ from Mg/Ca-F-actin. (5) Ca-F-actin released bound Ca2+ at a very slow rate. Short ultrasonic pulses rapidly elevated free Ca2+ from about 50 nM up to 500 nM. (6) Small amounts of profilin, an actin-binding protein, released Ca2+ both from Ca- and Mg/Ca-F-actin and also inhibited uptake of Ca2+ into Mg/Ca-F-actin. (7) Phalloidin completely inhibited Ca-uptake into Mg/Ca-F-actin even during ultrasonic treatment. These findings suggest that Ca2+ storage may occur by addition of Ca-ATP-actin monomers to reactive ends of the polymer and emptying of this store by profilin-stimulated release of Ca-ADP-actin. Thus, receptor-operated Ca2+ signaling, initiated by phospholipase C activation, may proceed via the well-known phosphatidylinositol phosphate-regulated profilin/gelsolin pathway of actin reorganization/depolymerization. The importance of the proposed microvillar Ca2+ signaling system for living cells remains to be established.

Dopamine (D2) receptor regulation of intracellular calcium and membrane capacitance changes in rat melanotrophs

1. Indo-1 microfluorimetry and patch clamp techniques were used to study the decrease in cytosolic [Ca2+] ([Ca2+]i) caused by dopamine (D2) receptor activation and the calcium dependence of membrane capacitance changes in single rat melanotrophs. 2. [Ca2+]i decreased when extracellular calcium was removed or when the calcium channel blockers nickel (2 mM) or cadmium (100 microM) were applied by bath perfusion. 3. Quinpirole, a dopamine (D2) receptor agonist, reduced [Ca2+]i by 55 +/- 9 nM and hyperpolarized membrane potential by 29 +/- 9 mV simultaneously. 4. Quinpirole-induced [Ca2+]i decrease required deactivation of voltage-dependent calcium channels. Voltage clamping the membrane potential at -25 mV prevented the quinpirole-induced [Ca2+]i decrease. Nickel (2 mM) reduced [Ca2+]i without hyperpolarization and precluded additional [Ca2+]i decrease by quinpirole. 5. Membrane capacitance measurement of secretion rates in cells dialysed with buffered calcium solutions showed that secretion began at approximately 400 nM Cai2+. 6. Melanotrophs have IP3-sensitive calcium stores, but no caffeine-sensitive calcium stores. Calcium released from IP3-sensitive calcium stores also stimulated secretion. 7. Secretion in melanotrophs is modulated by protein kinase activators. cAMP (200 microM) enhanced secretion at [Ca2+]i > 1000 nM. Phorbol myristate acetate (PMA; 200 nM) enhanced secretion at [Ca2+]i < 400 nM, but not in the absence of calcium. 8. Dopamine receptor activation can reduce secretion by reducing the calcium influx through calcium channels with hyperpolarization of the membrane potential. However downregulation of either cAMP or protein kinase C activity may also contribute to the decrease in secretion.

Distribution of the inositol trisphosphate receptor in the catfish retina

Inositol 1,4,5-trisphosphate (InsP3) mobilizes intracellular stored Ca2+ by binding to specific receptors that are similar to the ryanodine receptor of skeletal and cardiac muscle. We have immunolocalized the InsP3 receptor to the inner nuclear layer and limiting membranes of the catfish retina. Immunocytochemistry on dissociated retinal cells further localized the receptor in the horizontal, bipolar and Müller glial cells. Immunostaining of the rat retina localized the InsP3 receptor to the plexiform layers. These data show a different distribution of InsP3 receptor in the catfish retina compared to that of other vertebrates, that may be suggestive of a different functional role for this receptor in different species.

Glucose stimulates voltage- and calcium-dependent inositol trisphosphate production and intracellular calcium mobilization in insulin-secreting beta TC3 cells

The cellular processes leading to a rise in the intracellular free Ca2+ concentration ([Ca2+]i) after glucose stimulation and K+ depolarization were investigated in insulin-secreting beta TC-3 cells. Stimulation with 11.2mM glucose causes inositol 1,4,5-trisphosphate production and release of Ca2+ from intracellular stores. A strong correlation was observed between the changes in Ins(1,4,5)P3 concentration and the rise in [Ca2+]i, consistent with the former compound being responsible for release of Ca2+ from intracellular stores. The increase in Ins(1,4,5)P3 production was reduced by 68 +/- 4% when [Ca2+]i was kept low on glucose stimulation by loading cells with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-NNN'N'-tetra-acetic acid (BAPTA). The Ins(1,4,5)P3 production was prevented in cells hyperpolarized with diazoxide, an opener of ATP-sensitive K+-channels, consistent with the membrane potential controlling the rate of Ins(1,4,5)P3 synthesis. Depolarizing K+ concentrations evoked changes in [Ca2+]i and Ins(1,4,5)P3 production in both the presence and the absence of extracellular Ca2+, and from the relation between the extracellular K+ concentration and membrane potential we found a half-maximal Ins(1,4,5)P3 production by a 28mV depolarization from a resting potential of -56mV and by a rise in [Ca2+]i of 390nM. We conclude that stimulation-induced changes in membrane potential and [Ca2+]i are important in controlling Ins(1,4,5)P3 production in beta TC-3 cells and that glucose-stimulated Ca2+ mobilization from intracellular stores is due to voltage-dependent Ins(1,45)P3 production and depends on the concurrent increase in [Ca2+]i.

Isolation of subcellular agonist-sensitive calcium stores from the pancreatic acinar cell

The purpose of the present study was to develop a technique to identify, isolate and partially purify these membrane bound compartments for further characterizations of their Ca2+ transport and storage mechanisms. We 45Ca(2+)-loaded the agonist-sensitive Ca2+ stores in rat pancreatic acini. The loading was accomplished by first depleting the stores with carbachol stimulation followed by the addition of 45Ca2+ and atropine to the extracellular media. After homogenization of the 45Ca(2+)-loaded acini, subcellular fractions were resolved on sucrose and Nycodenz gradients. 45Ca2+ fluxes were minimized during these procedures by inclusion in the media of LaCl3. Five subcellular fractions were identified that specifically accumulated 45Ca2+ after carbachol stimulation. Electron microscopic observations of the fractions demonstrated that three of the fractions consisted of rough membrane vesicles; that one consisted of a mixture of rough and smooth membrane vesicles; and that one consisted of smooth membrane vesicles. All fractions were enriched in glucose-6-phosphatase. All 5 fractions demonstrated ATP dependent 45Ca2+ uptake. By Western blot analysis, all fractions contained calnexin, p58, sarcoplasmic reticulum type Ca(2+)-ATPase, and IP3 receptor. These results demonstrated that the 45Ca(2+)-loading technique can be used to isolate and characterize distinct compartments of the agonist-sensitive Ca2+ store in the pancreatic acinar cell.

Localization of inositol trisphosphate receptor subtype 3 to insulin and somatostatin secretory granules and regulation of expression in islets and insulinoma cells

Calcium ions play a central role in stimulus-secretion coupling in pancreatic beta cells, and an elevation of cytosolic Ca2+ levels is necessary for insulin secretion. Inositol 1,4,5-trisphosphate mobilizes intracellular Ca2+ stores in the beta cell by binding to specific receptors that are ligand-activated Ca2+ channels. The inositol trisphosphate receptors comprise a family of structurally related proteins with distinct but overlapping tissue distributions. Previous studies indicated that the predominant inositol trisphosphate receptor subtype expressed in rat pancreatic islets was the protein designated IP3R-3. We have confirmed the expression of IP3R-3 in pancreatic islets by immunohistocytochemistry and localized this protein to the secretory granules of insulin-secreting beta cells and somatostatin-secreting delta cells by immunogold electron microscopy. Secretory granules contain high levels of Ca2+, and the presence of IP3R-3 in the granule provides a mechanism for mobilizing granule Ca2+ stores in response to glucose and/or hormones. The release of Ca2+ from granule stores would increase the Ca2+ concentration in the surrounding cytoplasm and promote rapid exocytosis of granules, especially those granules in close proximity to the plasma membrane. The levels of IP3R-3 were increased in pancreatic islets of diabetic rats and rats that had been refed after a period of fasting. They were also increased in rat insulinoma RINm5F cells cultured in 25 mM glucose compared with cells cultured in 5 mM glucose. The localization of IP3R-3 to secretory granules of insulin-secreting beta cells and somatostatin-secreting delta cells suggests that granule Ca2+ stores actively participate in the secretory process and that their release is regulated by inositol 1,4,5-trisphosphate. The regulation of IP3R-3 levels by glucose, diabetes, and refeeding may allow the beta cell to adjust the insulin secretory response to changing physiological conditions.

Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Experimental observations on rat glomerulosa cells inspired a model which postulates that plasmalemmal dihydropyridine receptors are in juxtaposition and interaction with inositol 1,4,5-trisphosphate receptors in subplasmalemmal calciosomes. Activation of dihydropyridine receptors promotes the Ca2+ releasing effect of inositol 1,4,5-trisphosphate. The most important observations compatible with the model are the following: (1) angiotensin II does not influence Ca2+ influx during the peak phase of Ca2+ signal; (2) dihydropyridine drugs modify the initial peak of the Ca2+ signal induced by angiotensin II; (3) inhibitors of the dihydropyridine receptor reduce the initial Ca2+ signal also in the presence of 5 mM Ni2+, an inhibitor of voltage dependent Ca2+ influx; and (4) changes in extracellular K+ concentration within the physiological range also modify the cytoplasmic Ca2+ response to angiotensin II.

Rapid uptake of calcium, ATP, and inositol 1,4,5-trisphosphate via cation and anion channels into surface-derived vesicles from HIT cells containing the inositol 1,4,5-trisphosphate-sensitive calcium store

In a previous study [K. Lange and U. Brandt (1993) FEBS Lett. 320, 183-188], we showed that the bulk of the ATP-dependent IP3-sensitive Ca2+ store of the hamster insulinoma cell line, HIT-T15, resides in cell surface-derived vesicles most likely of microvillar origin. The origin and orientation of these vesicles suggested that Ca2+ storage is not due to a membrane-located Ca2+ pumping ATPase but rather to ATP-dependent Ca(2+)-binding within the vesicles. In this case, Ca2+, ATP and IP3 should have free access to the vesicle lumen. This hypothesis was tested. ATP-independent Ca2+ uptake occurred with biphasic kinetics. An initial rapid uptake, which was complete within 30 s, was followed by a slow linear uptake lasting about 10 min. The rapid component was shown by efflux experiments to have an equilibration half-time of about 4 s. This rapid Ca2+ efflux pathway was inhibited by externally applied La3+ (0.1 mM). A similar rapidly equilibrating La(3+)-sensitive Ca2+ pool was also present in vesicles which had been actively loaded with Ca2+ in the presence of ATP. The intravesicular distribution space of this labile Ca2+ pool was identical with that of the non-metabolizable hexose analogue 3-O-methyl-D-glucose, demonstrating that rapid Ca2+ uptake occurs into a true vesicular water space and is not due to binding. ATP and IP3 were also shown to enter the vesicles by an energy-independent pathway which is inhibited by the anion channel inhibitor, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 0.5 mM). Both ATP-dependent Ca2+ uptake and IP3-induced Ca2+ release from preloaded vesicles were inhibited by DIDS. These findings clearly demonstrate that (1) the vesicle membrane is permeable to ATP and IP3 via anion channels, and (2) Ca2+ uptake into as well as IP3-induced Ca2+ release from the vesicles occur by passive diffusion through a cation channel which is not regulated by IP3. Consequently, the mechanisms for Ca2+ storage and IP3-induced Ca2+ release must be located in the vesicle lumen. Moreover, the microvillar diffusion-barrier concept, originally proposed for the regulation of hexose transport may also be valid for the receptor-operated regulation of cation and anion influx pathways.(ABSTRACT TRUNCATED AT 400 WORDS)