Evidence for an intracellular calcium store releasable by surface stimuli ifibroblasts (L cells)

ER-plasma membrane junctions: Why and how do we study them?

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are membrane microdomains important for communication between the ER and the PM. ER-PM junctions were first reported in muscle cells in 1957, but mostly ignored in non-excitable cells due to their scarcity and lack of functional significance. In 2005, the discovery of stromal interaction molecule 1 (STIM1) mediating a universal Ca²⁺ feedback mechanism at ER-PM junctions in mammalian cells led to a resurgence of research interests toward ER-PM junctions. In the past decade, several major advancements have been made in this emerging topic in cell biology, including the generation of tools for labeling ER-PM junctions and the unraveling of mechanisms underlying regulation and functions of ER-PM junctions. This review summarizes early studies, recently developed tools, and current advances in the characterization and understanding of ER-PM junctions. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Slow oscillations of free intracellular calcium ion concentration in human fibroblasts responding to mechanical stretch

Calcium transients in single, human gingival fibroblasts were studied after mechanical stretching of flexible culture substrates. A model system was developed to reproducibly stretch and rapidly (<1 sec) refocus cells in the same focal plane so that changes in the concentration of free intracellular calcium ions ([Ca2+]i) were monitored without delay. Attached cells were grown on flexible bottom Petriperm dishes, loaded with fura-2/AM, and stretched by 1% or 2.8% of substrate area. The stretch caused no significant cell detachment or membrane lesions. A 1% stretch induce no calcium response, but a 2.8% stretch stimulated an initial calcium transient and the subsequent generation of [Ca2+]i oscillations of up to 2,000 sec. At 1% stretch, there was no calcium response. Cell shape and plating time were important determinants in the calcium response to mechanical stimulation: the responder cells were small and round without long processes. Major calcium transients were inhibited completely by 5 mM EGTA or by 10 microM gadolinium ions, by 50 microM nifedipine, or 250 microM verapamil, suggesting an influx of calcium through stretch-activated (SA) channels and L-type calcium channels. Depolarization by high KCl (144 mM) in the extracellular medium enhanced the amplitude of calcium transients by 54%. Calcium oscillations were not inhibited by preincubation with thapsigargin, caffeine, cholera toxin, staurosporine or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), indicating that IP3 sensitive pools, IP3 insensitive pools, GS alpha subunits, and protein kinase C, respectively, were not involved in the generation of calcium oscillations. Pretreatment with genistein, a specific tyrosine kinase inhibitor or cytochalasin D, an inhibitor of actin polymerization, or pertussis toxin, an inhibitor of Gi alpha and G(o) alpha subunits, completely abolished calcium transients and oscillations. These results indicate that Ca2+ flux due to mechanical stretching is likely mediated through SA ion channels and is dependent on tyrosine kinases, pertussis toxin-sensitive subunits of G-proteins, and actin filaments.

The increase in the intracellular Ca2+ concentration induced by mechanical stimulation is propagated via release of pyrophosphorylated nucleotides in mammary epithelial cells

Mechanical stimulation of one mammary tumor cell in culture induced an increase in its intracellular calcium concentration which spread to surrounding cells. The increase in calcium can also be induced by addition of a solution in which cultured mammary tumor cells were stimulated by repeated pipetting (solution after pipetting cells, SAPC). The activity of the SAPC was completely abolished by treatment with snake venom phosphodiesterase or pyrophosphatase. Uridine triphosphate (UTP), uridine diphosphate (UDP) and ATP (1 microM each) were detected in the SAPC, whereas 5'-UMP and 5'-AMP were produced by phosphodiesterase digestion. A mixture of UTP, UDP and ATP (1 microM each) elicited a calcium response which was comparable to that induced by SAPC, while UTP, UDP or ATP alone at 1 microM elicited a small increase in calcium concentration in mammary tumor cells. Suramin, a competitive antagonist of P2 purinoceptors, diminished the spreading of the calcium wave induced by mechanical stimulation. It also blocked the responses to SAPC, UTP, UDP and ATP. These findings suggest that the mechanical stimulation results in the release of UTP, UDP and ATP into the extracellular space which mediates induction of the spreading calcium response via P2U-type purinoceptors.

Spontaneous calcium oscillations and mechanically and chemically induced calcium responses in mammary epithelial cells

Changes of intracellular calcium activity (Ca2+i) in mouse mammary epithelial cells in primary culture (normal) and in an established cell line (MMT060562, cancerous) were investigated by microfluorometry and image analysis of fura-2 fluorescence. In both types of cells, some populations exhibited occasional Ca2+i oscillations with a period of 50-160 s. Slight mechanical stimulation of a cell with a fine glass pipette induced a Ca2+i increase, which spread from the stimulated cell to the surrounding cells with a speed of 7-12 microns/s. ATP (> 1 mumol/l) and ADP, but not AMP induced a Ca2+i increase in both cell types. Bradykinin was highly effective (> 10 nmol/l) only in the cancerous mammary epithelial cells. In Ca(2+)-free solution, all these Ca2+i responses remained unchanged at the first application, and decreased abruptly at the second trial. La3+ (> 0.5 mmol/l) suppressed the response to ATP but not the response to bradykinin. Addition of extracellular Mn2+ rapidly quenched the fura-2 fluorescence in the cell even in a non-stimulated state. Influx of Mn2+ did not increase during Ca2+i responses. These results indicate that the sources of Ca2+i responses in mammary epithelial cells are intracellular stores, which exchange Ca2+ with the extracellular medium.

Mechanically induced electrical and intracellular calcium responses in normal and cancerous mammary cells

Mechanically induced channel activities and increase of intracellular calcium ([Ca2+]i) in normal and cancerous murine mammary cells (MMT 060562) were investigated using the patch clamp technique and Fura-2 fluorescence. Both cell types showed similar properties. Upon mechanical stimulation, activation of the Ca(2+)-dependent K+ channel or outward membrane current was recorded in cells which were several cells distant from the stimulated cell. Mechanical stimulation also induced an increase of [Ca2+]i in the touched cell, and this increase of [Ca2+]i spread to the surrounding cells. The [Ca2+]i signal travelled a distance of 100-200 microns within 20-40 s and then diminished. The presence of cell-to-cell communication between adjacent mammary cells through gap junction was indicated by injection of lucifer yellow and measurements of electrical coupling (coupling constant = 0.2-0.3). The mechanically induced increase of the [Ca2+]i signal spread to adjacent cells even when the stimulated cell had no physical contact with them. In the absence of fluid movement, the pattern of the spread of the [Ca2+]i signal was a concentric circle. However, in the presence of fluid movement, the pattern changed to elongate to the direction of the flow. These findings suggested that a certain factor was released from the mechanically stimulated cell to the extracellular space, and this factor induces the increase of [Ca2+]i in surrounding cells.

Bradykinin induces inositol 1,4,5-trisphosphate-dependent hyperpolarization in K+ M-current-deficient hybrid NL308 cells: comparison with NG108-15 neuroblastoma x glioma hybrid cells

External application of bradykinin (BK) to mouse neuroblastoma X mouse fibroblast hybrid NL308 cells and mouse neuroblastoma X rat glioma hybrid NG108-15 cells produced a transient outward (hyperpolarizing) current. In NG108-15 cells, BK also induced an inward (depolarizing) current associated with a decrease in input membrane conductance, which results from the inhibition of a voltage-sensitive potassium current, the M-current. However, in NL308 cells, either no depolarization was elicited by BK or, even if the BK-induced depolarization was evoked, it was associated with an increased conductance. To explain the above difference, the intracellular second messenger system of NL308 cells was examined in detail. BK induced the rapid accumulation (three- to fivefold higher than the control level) of inositol 1,4,5-trisphosphate (InsP3) in NL308 cells. The cytosolic Ca2+ concentration was also elevated to 540 nM from 180 nM at a basal level. This seems to be enough to activate a voltage-independent and Ca2(+)-sensitive K+ current, resulting in the hyperpolarization. Intracellular injection of InsP3 replicated the hyperpolarization. NL308 cells possess protein kinase C (C-kinase), with specific activities of C-kinase in cytosolic and membrane fractions being 233 and 24 pmol/min/mg protein, respectively. The activity associated with particulates became higher after phorbol dibutyrate (PDBu) treatment. But NL308 cells did not show the characteristic inward relaxation by step hyperpolarizations and the outward rectification in the current-voltage relationship, indicating that the M current is deficient in NL308 cells. Therefore, application of PDBu failed to mimic the inward current. The results suggest the role of InsP3 and C-kinase in controlling two K+ currents.

Pentylenetetrazole-induced seizure activity produces an increased release of calcium from endoplasmic reticulum by mediating cyclic AMP-dependent protein phosphorylation in rat cerebral cortex

1. To determine the involvement of the convulsant agent pentylenetetrazole (PTZ) in intracellular calcium release in neurons, its effect on stored calcium in the endoplasmic reticulum of rat cortical neurons was tested. 2. Intraperitoneal injection of PTZ caused marked release of calcium from the endoplasmic reticulum which was similar to that observed when cortical slices were incubated with this convulsant. 3. Superfusion of dibutyryl cAMP and isobutylmethylxanthine to the cortical slices mimicked PTZ-induced calcium release from this reservoir. A similar effect was observed under depolarizing conditions brought about by either an elevation of extracellular K+ concentration or addition of veratridine. 4. Isoquinolinesulfonamide, a protein kinase inhibitor, reduced PTZ-stimulatory effect of calcium release and blocked the cAMP-induced calcium release. 5. Intracellular cAMP level was enhanced at about 3-fold by both intraperitoneal injection of PTZ and its superfusion. 6. These findings are taken to suggest that PTZ may release stored calcium in the endoplasmic reticulum by mediating a cAMP-dependent protein phosphorylation in cortical neurons.

GTP- and inositol 1,4,5-trisphosphate-induced release of 45Ca2+ from a membrane store co-localized with pancreatic-islet-cell plasma membrane

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], arising from hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], is proposed as the link between membrane-receptor activation and mobilization of Ca2+ from intracellular sites in hormone-secreting cells. The location of Ins(1,4,5)P3-sensitive membranes was investigated in cultured neonatal beta-cells. Membranes were obtained after lysis of cells attached to positively charged Sephadex. After lysis the presence of the enzyme markers 5'-nucleotidase, glucose-6-phosphatase, NADH-cytochrome c reductase, UDP-galactosyltransferase and succinate dehydrogenase indicated the mixed nature of the preparation. After sonication, however, UDP-galactosyltransferase and succinate dehydrogenase activities were undetectable, but 4.8% of total cellular glucose-6-phosphatase and 3.4% of total cellular NADH-cytochrome c reductase remained with 5'-nucleotidase in the preparation, indicating endoplasmic-reticulum association. ATP-dependent 45Ca2+ accumulation was shown in this preparation (410 +/- 24 pmol/mg of protein at 150 nM free Ca2+) and was inhibited by vanadate (100 microM). Ca2+ release was effected by Ins(1,4,5)P3, with half-maximal release at 0.5 +/- 0.14 microM-Ins(1,4,5)P3, t1/2 11.2 +/- 1.1 s. GTP- and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG)-promoted release of 45Ca2+ was demonstrated in this preparation, but the kinetics of release (half-maximal Ca2+ release at 5.4 +/- 0.7 microM, with t1/2 77.3 +/- 6.9 s, and at 51.1 +/- 4.2 microM, with t1/2 19.0 +/- 2.2 s, for GTP and p[NH]ppG respectively), and the ability of neomycin sulphate to block p[NH]ppG-induced release only, are indicative of separate release mechanisms after treatment with these agents. A close association between plasma membrane and elements of the endoplasmic reticulum is indicated in this model, providing a possible mechanism for local alterations in free Ca2+ in the sub-plasma-membrane region.

Voltage-sensitive calcium channels in normal and transformed 3T3 fibroblasts

Patch clamp recordings of whole-cell and single channel currents revealed the presence of two voltage-sensitive calcium channel types in the membrane of 3T3 fibroblasts. The two calcium channel types were identified by their unitary properties and pharmacological sensitivities. Both calcium channel types were present in all control 3T3 cells, but one type was selectively suppressed in 3T3 cells that had been transformed by activated c-H-ras, EJ-ras, v-fms, or polyoma middle T oncogenes. The presence of voltage-sensitive calcium channels in these nonexcitable cells and the control of their functional expression by transforming oncogenes raises questions about their role in the control of calcium-sensitive processes such as cell motility, cytoskeletal organization, and cell growth.

The calcium-mobilizing action of low concentrations of sodium fluoride in single fibroblasts

The sensitivity of fibroblasts (L cells) to low concentrations of sodium fluoride (NaF) was examined using the same cell during a series of stimuli. NaF with increasing concentrations up to 1 mM elevated cytosolic free Ca2+ concentrations ([Ca2+]i) in single cells. [Ca2+]i increased within 15 sec of addition of NaF and lowered to basal [Ca2+]i levels quickly. This elevation was observed both in the presence and absence of external Ca2+ and was enhanced by 1 microM Al3+. These results suggest that low concentrations (below 1 mM) of NaF induce Ca2+ release from intracellular Ca2+ stores in single L cells through guanine nucleotide-binding protein (G protein).

Effects of caffeine and tetracaine on outer hair cell shortening suggest intracellular calcium involvement

Outer hair cell (OHC) shortening has previously been induced in vitro by the application of solutions containing high potassium (a depolarizing agent), acetylcholine (a suggested efferent transmitter) and cationized ferritin (a positively charged macromolecule), as well as by electrical current. The application of caffeine, which causes contractures in skeletal and smooth muscle by releasing calcium from intracellular stores to activate actin and myosin interaction, also causes shortening of OHCs. Tetracaine, which interferes with calcium movement in muscle and non-muscle cells, blocks potassium-induced and caffeine-induced shortening of OHCs, but does not block electrically-induced shortening. Sodium dantrolene which is an inhibitor of intracellular calcium release in skeletal muscle does not block potassium-induced OHC shortening. Immunocytochemical studies using antibodies to muscle-like contractile and regulatory proteins on unfixed, freeze-dried OHCs demonstrate the co-localization of calmodulin with actin throughout the OHC cytoplasm. These results support the ideas that in OHCs, intracellular calcium release is involved in the activation of shortening and that an actin-mediated cell shape change may be regulated by calmodulin in a manner similar to that which occurs in contraction of smooth muscle.

Membrane conductance oscillations in astrocytes induced by phorbol ester

Glial cells in the central nervous systems (CNS) have complex functions which are difficult to decipher because of the intimate intertwining of glial cells with neurons. We have therefore developed an essentially neuron-free preparation of CNS astrocytes in the kainic acid lesioned hippocampal slice. With this preparation we have examined the effect of activating protein kinase C in astrocytes with a phorbol ester, TPA (12-O-tetradecanoyl-phorbol-13-acetate). In most cells, TPA induced rhythmic oscillations (0.1-3.0 Hz) of membrane potential which were typically 5-10 mV in amplitude and were associated with increases of up to eightfold in input resistance during the depolarizing phase. These large changes in membrane conductance are the first reported observations of endogenously generated conductance changes in astrocytes of the mammalian CNS and they could influence excitability of surrounding neurons, possibly by altering extracellular ion concentrations.

Mechanisms of movement in outer hair cells and a possible structural basis

Isolated outer hair cells were found to slowly shorten when subjected to a solution that would induce contraction in a muscle fibre. Two possible mechanisms underlying this behaviour emerge from ultrastructural and immunocytochemical investigations. Antibody labelling at the electron microscopic level demonstrates that actin is present not only in the stereocilia and in the cuticular plate but also along the wall of outer hair cells, between the plasma membrane and the subsurface fenestrated cisternae. The latter are interconnected by regularly spaced pillars, resembling those seen between the T-tubules and sarcoplasmic reticulum in muscle fibres. Contraction also results from the application of positively charged macromolecules to the bathing solution. This implies sensitivity of the membrane-associated complex (the cortex system) to an electrical current. A second contractile system may reside in the cytoplasm, where calmodulin is present in contracted hair cells. This protein is a calcium-binding control protein for contraction-like events in smooth muscle and non-muscle cells. The unique presence of the cortex system in outer hair cells, and its absence in inner hair cells, indicates a functional significance that relates to a motor function of outer hair cells in hearing.

Ion channels in rabbit cultured fibroblasts

Large outward currents are recorded with the whole-cell patch-clamp technique on depolarization of rabbit cultured fibroblasts. Our findings suggest that these outward currents consist of two voltage-dependent components, one of which also depends on cytoplasmic calcium concentration. Total replacement of external Cl- by the large anion ascorbate does not affect the amplitude of the currents, indicating that both components must be carried by K+. Consistent with these findings with whole-cell currents, in single channel recordings from fibroblasts we found that most patches contain high-conductance potassium-selective channels whose activation depends on both membrane potential and the calcium concentration at the cytoplasmic surface of the membrane. In a smaller number of patches, a second population of high-conductance calcium-independent potassium channels is observed having different voltage-dependence. The calcium- and voltage-dependence suggest that these two channels correspond with the two components of outward current seen in the whole-cell recordings. The single channel conductance of both channels in symmetrical KCl (150 mM) is 260-270 pS. Both channels are highly selective for K+ over both Na+ and Cl-. The conductance of the channels when outward current is carried by Rb+ is considerably smaller than when it is carried by K+. Some evidence is adduced to support the hypothesis that these potassium channel populations may be involved in the control of cell proliferation.

Oscillations of cytoplasmic concentrations of Ca2+ and K+ in fused L cells

Using Ca2+- and K+-selective microelectrodes, the cytosolic free Ca2+ and K+ concentrations were measured in mouse fibroblastic L cells. When the extracellular Ca2+ concentration exceeded several micromoles, spontaneous oscillations of the intracellular free Ca2+ concentration were observed in the submicromolar ranges. During the Ca2+ oscillations, the membrane potential was found to oscillate concomitantly. The peak of cyclic increases in the free Ca2+ level coincided in time with the peak of periodic hyperpolarizations. Both oscillations were abolished by reducing the extracellular Ca2+ concentration down to 10(-7) M or by applying a Ca2+ channel blocker, nifedipine (50 microM). In the presence of 0.5 mM quinine, an inhibitor of Ca2+-activated K+ channel, sizable Ca2+ oscillations still persisted, while the potential oscillations were markedly suppressed. Oscillations of the intracellular K+ concentration between about 145 and 140 mM were often associated with the potential oscillations. The minimum phase of the K+ concentration was always 5 to 6 sec behind the peak hyperpolarization. Thus, it is concluded that the oscillation of membrane potential results from oscillatory increases in the intracellular Ca2+ level, which, in turn, periodically stimulate Ca2+-activated K+ channels.

Increases in cytosolic free Ca2+ induced by ATP, complement and beta-lipoprotein in mouse L fibroblasts

By means of Ca2+- and K+-selective microelectrodes, the changes in intracellular free Ca2+ and K+ were measured during the hyperpolarizing responses induced by ATP, complement and beta-lipoprotein in mouse fibroblastic L cells. The cytoplasmic Ca2+ concentration [( Ca]i) was about 0.4 microM in the resting state. The hyperpolarizing responses always coincided with a phasic increase in [Ca]i. ATP or beta-lipoprotein induced about a 2-fold rise in [Ca]i, and complement did up to 3-fold. Both the hyperpolarizing responses and [Ca]i increases were prevented by removal of external Ca2+ or by application of a Ca-channel blocker, nifedipine. Quinine, a Ca-activated K-channel inhibitor, suppressed the hyperpolarizing responses but not the [Ca]i increases. During the hyperpolarizing response, the intracellular free K+ concentration gradually decreased from about 120 to 110 mM. Thus, it is concluded that ATP, complement and beta-lipoprotein caused a transient elevation of cytoplasmic free Ca2+ due to Ca2+ influxes, thereby inducing electrical membrane responses through activation of Ca-dependent K-channels in the fibroblasts.

Membrane voltage, resistance, and channel switching in isolated mouse fibroblasts (L cells): a patch-electrode analysis

The whole-cell patch-electrode technique of Fenwick, Marty & Neher (1982) has been applied to single suspension-cultured mouse fibroblasts. Seals in the range of 10-50 G omega were obtained without special cleaning of the cell membranes. Rupture of the membrane patch inside the electrode was accompanied by a shift of measured potential into the range -10 to -25 mV, but in most cases with little change in the recorded resistance. The latter fact implied that the absolute resistance of the cell membrane must be in the same range as the seal resistance and the recorded potential is a poor measure of actual cell membrane potential. Steady-state current-voltage curves (range -160 mV to +80 mV) were generated before and after rupture of the membrane patch, and the difference between these gave (zero-current) membrane potentials of -50 to -75 mV, which represents a leak-corrected estimate of the true cell-membrane potential. The associated slope conductivity of the cell membrane was 5-15 microS/cm2 (assumed smooth-sphere geometry, cells 13-15 microns in diameter) and was K+-dominated. With 0.1 mM (or more) free Ca2+ filling the patch electrode, membrane potentials in the range -60 to -85 mV were observed following patch rupture, with associated slope conductivities of 200-400 microS/cm2, also K+-dominated. Similar voltages and conductivities were observed at the peak of pulse-induced 'hyperpolarizing activation' (Nelson, Peacock, & Minna, 1972), and the two phenomena probably reflect the behaviour of Ca2+-activated K+ channels. Both the pulse-induced conductance and the Ca2+-activated conductance spontaneously decayed, the latter over periods of 5-15 min following patch rupture. Sr2+, Ba2+, and Co2+ could also activate the putative K+ channels, but only Sr2+ really mimicked Ca2+. Co2+ and Ba2+ activated with a delay of several minutes following patch rupture, and deactivated quickly with a small decrease of conductance and a large decrease of membrane potential. Evidently, Co2+ and Ba2+ affect channel specificity as well as channel opening and closing kinetics.

Single channel currents in mouse embryonal multipotential carcinoma cells

Electrical membrane properties of embryonal non-differentiated carcinoma cells which have been extensively used for the study of early mammalian embryogenesis were investigated by using patch clamp techniques. These multipotential cells were found to contain a restricted repertoire of a small number of ionic channels on the whole cell membrane. The most abundant type was a voltage- and calcium-activated potassium channel with characteristics similar to those described in fully differentiated cells.

Immobilisation of organelles and actin bundles in the cortical cytoplasm of the alga Chara corallina Klein ex. Wild

The mechanism by which sub-cortical actin bundles and membranous organelles are immobilised in the cortical cytoplasm of the alga Chara was studied by perfusing cells with a solution containing 1% Triton X-100. Light and scanning electron microscopy and the release of starch grains and chlorophyll-protein complexes indicated that the detergent extensively solubilised the chloroplasts. However, the sub-cortical actin bundles remained in situ even though they were originally separated from the plasma membrane by the chloroplasts. A fibrous layer between chloroplasts and plasma membrane became readily visible after detergent extraction of the cells and could be released by low-ionic-strength ethylenediaminetetraacetic acid, thioglycollate and trypsin. The same treatments applied to cells not subject to detergent extraction released the membrane-bound organelles and actin bundles and no fibrous meshwork was visible on subsequent extraction with Triton. It is, therefore, concluded that a detergent-insoluble cortical cytoskeleton exists and contributes to the immobility of the actin and cortical organelles in the cells.

Oscillatory hyperpolarizations and resting membrane potentials of mouse fibroblast and macrophage cell lines

L cells (a mouse fibroblast cell line) and macrophages have been reported to exhibit slow oscillatory hyperpolarizations and relatively low membrane potentials, when measured with glass micro-electrodes. This paper describes the role of micro-electrode-induced leakage in these oscillations for L cells and a mouse macrophage cell line (P388D1). Both L cells and macrophages showed fast negative-going peak-shaped potential transients upon micro-electrode entry. This shows that the micro-electrode introduces a leakage conductance across the membrane. The peak values of these fast transients were less negative for L cells (-17 mV) than for macrophages (-39 mV), although their sustained resting membrane potentials were about equal (-13 mV). This indicates that the pre-impaled membrane potential of macrophages is more negative than that of L cells. Ionophoretic injection of Ca2+ into the P388D1 macrophages showed the existence of a Ca2+ -dependent hyperpolarizing conductance presumed to be involved in the oscillatory hyperpolarizations of L cells and macrophages. Cells increased in size by X-ray irradiation to reduce membrane input resistances were still found to be susceptible to micro-electrode-induced leakage. Impalement transients upon entry of a second electrode during a hyperpolarization evoked by a first electrode, were often step-shaped instead of peak-shaped due to the high membrane conductance associated with hyperpolarization. Since peak-shaped impalement transients were always seen with the first impalement both in oscillating and non-oscillating cells, oscillatory hyperpolarizations cannot be regarded as spontaneously occurring in the unperturbed cells but are induced by micro-electrode penetration. Since the hyperpolarizing response can be evoked by ionophoretic injection of Ca2+, and oscillatory as well as single hyperpolarizing responses are absent in a Ca2+ -free medium, it is concluded that the Ca2+ needed intracellularly to activate the hyperpolarizing responses enters the cell via the leakage pathway introduced by the measuring electrode.

Ca2+-sensitive K+ channels in phagocytic cell membranes

In phagocytic cells evidence for properties of Ca2+-sensitive K+-selective channels comes mostly from electrophysiological studies. Macrophages and macrophage-like cells are compared with fibroblasts (L-cells) where the Ca+-dependent K+ conductance is better understood. This model shares a mesenchymal origin and an accessory phagocytic capacity with the professional phagocytes. In macrophages several values of transmembrane potentials have been measured by different groups, using various techniques. Microelectrode measurements have demonstrated a voltage-dependent K+ conductance involved in transition from low to high membrane potentials. Current-voltage relationships in mouse peritoneal exudate cells have revealed a region of negative slope resistance. Slow calcium spikes were found in a subpopulation of cells from human dialysis fluid that appear to be distinct from typical macrophages. Action potentials have been recorded from human monocyte-derived macrophages. Their ionic mechanism has not yet been established. Spontaneous and electrically elicited slow membrane hyperpolarizations have been described in macrophages and macrophage-like cells. Similar activity is well known in L-cells and in both cases it is possible to identify a Ca2+-sensitive K+ conductance as the underlying mechanism. Phagocytosis is a cell function that has been related to membrane hyperpolarization and to slow hyperpolarizing activity. In some cases no changes of electrical activity have been observed during the phagocytic process. Chemotactic factors induce membrane hyperpolarizations in macrophages, but the relation between electrical change and cell motility has not been established. Exocytosis, a is another Ca2+ sensitive cell function that awaits correlation with electrochemical changes. The evidences accumulated to date are compatible with several models for gating and modulation of the voltage-independent K+ conductance by Ca2+. The use of higher resolution techniques, such as patch-clamp, with well defined subpopulations of phagocytic cells may produce the missing link in the transduction of membrane signals into the specifically targeted cell functions.

Effects of altered extracellular and intracellular calcium concentration on hyperpolarizing responses of the hamster egg

Upon fertilization the hamster egg shows transient, periodic hyperpolarizing responses (h.r.s) due to a Ca-activated K conductance; these are superimposed on a gradual, hyperpolarizing shift of the resting potential (h.s.) (Miyazaki & Igusa, 1981a, 1982a). The h.r.s and h.s. were further analysed by changing external divalent cations or by injection of Ca2+ into the egg, to study the mechanisms of the increase in the intracellular Ca2+ concentration ([Ca2+]i). The series of h.r.s was abolished by the removal of external Ca2+. The frequency of the h.r. was decreased by lowering the [Ca2+]o or by adding Mn2+ or Co2+, and it was increased by raising the [Ca2+]o in a time- and concentration-dependent manner. The h.r. frequency was decreased on sustained depolarization with steady current, and increased on hyperpolarization. In contrast to the h.r. frequency, the amplitude, conductance increase and reversal potential of each h.r. were little affected by [Ca2+]o, Mn2+ or Co2+. The h.s. was decreased by lowering the [Ca2+]o, by adding Mn2+ or Co2+, or by injection of EGTA. The h.s. may reflect continuous Ca influx stimulating a Ca-activated K conductance (GK). In unfertilized eggs a regenerative h.r. was induced by Ca injection with an apparent threshold. The relationship between GK and the injected Ca2+ showed a steep jump at the critical current, associated with a four-fold increase in GK. The regenerative h.r. was followed by a refractory period of 1-2 min. In inseminated eggs the periodic sperm-mediated h.r.s. (s.-h.r.s) were interrupted by interposed h.r.(s) induced by Ca injection(s): the periodicity of s.-h.r.s was reset by Ca-induced h.r. In inseminated eggs the regenerative h.r. was induced by Ca injection with a much smaller pulse than necessary in unfertilized eggs. The refractory period was shortened to 40-50 sec, comparable to the period of s.-h.r.s. In inseminated eggs periodic h.r.s similar to s.-h.r.s were produced by continuous, repetitive injections of Ca2+ with constant pulses. The frequency of these h.r.s was dependent on the injection current. It is concluded that each h.r. indicates an enhancement of the increase in [Ca2+]i, probably the result of Ca-induced Ca release from intracellular stores. A possible mechanism for periodic increase in [Ca2+]i reflected in s.-h.r.s is proposed, based on a linkage of the continuous Ca influx to Ca release.

Membrane junctions in Xenopus eggs: their distribution suggests a role in calcium regulation

We have observed the presence of membrane junctions formed between the plasma membrane and cortical endoplasmic reticulum of mature, unactivated eggs of xenopus laevis. The parallel, paired membranes of the junction are separated by a 10-mn gap within which electron-dense material is present. This material occurs in patches with an average center-to-center distance of approximately 30 nm. These junctions are rare in immature (but fully grown) oocytes (approximately 2 percent of the plasma membrane is associated with junctions) and increase dramatically during progesterone-induced maturation. Junctions in the mature, unactivated egg are two to three times more abundant in the animal hemisphere (25-30 percent of the plasma membrane associated with junction) as compared with the vegetal hemisphere (10-15 percent). Junction density decreases rapidly to values characteristic of immature oocytes in response to egg activation. The plasma membrane-ER junctions of xenopus eggs are strikingly similar in structure to membrane junctions in muscle cells thought to be essential in the triggering of intracellular calcium release from the sarcoplasmic reticulum. In addition, the junctions' distinctive, animal-vegetal polarity of distribution, their dramatic appearance during maturation, and their disapperance during activation are correlated with previously documented patterns of calcium-mediated events in anuran eggs. We discuss several lines of evidence supporting the hypothesis that these junctions in xenopus eggs are sites that transduce extracellular events into intracellular calcium release during fertilization and activation of development.

Estimation of the membrane potential of cultured macrophages from the fast potential transient upon microelectrode entry

Analysis of membrane potential recordings upon microelectrode impalement of four types of macrophages (cell lines P388D1 and PU5-1.8, cultured mouse peritoneal macrophages, and cultured human monocytes) reveals that these cells have membrane potentials at least two times more negative than sustained potential values (E(s)) frequently reported. Upon microelectrode entry into the cell (P388D1), the recorded potential drops to a peak value (E(p)) (mean -37 mV for 50 cells, range -15 to -70 mV) within 2 ms, after which it decays to a depolarized potential (E(n)) (mean -12 mV) in about 20 ms. Thereafter, the membrane develops one or a series of slow hyperpolarizations before a final sustained membrane potential (E(s)) (mean -14 mV, range -5 to -40) is established. The mean value of the peak of the first hyperpolarization (E(h)) is -30 mV (range -10 to -55 mV). The initial fast peak transient, measured upon microelectrode entry, was first described and analyzed by Lassen et al. (Lassen, U.V., A.M. T. Nielson, L. Pape, and L. O. Simonsen, 1971, J. Membr. Biol. 6:269-288 for other change in the membrane potential from its real value before impalement to a sustained depolarized value. This was shown to be true for macrophages by two-electrode impalements of single cells. Values of E(p), E(n), E(h), E(s), and membrane resistance (R(m)) measured for the other macrophages were similar to those of P388D1. From these results we conclude that E(p) is a better estimate of the true membrane potential of macrophages than E(s), and that the slow hyperpolarizations upon impalement should be regarded as transient repolarizations back to the original membrane potentials. Thus, analysis of the initial fast impalement transient can be a valuable aid in the estimation of the membrane potential of various sorts of small isolated cells by microelectrodes.

Voltage-dependent activation of potassium current in Helix neurones by endogenous cellular calcium

1. The effect of endogenous Ca on potential-dependent K current IKD, was examined in identifiable neurones of Helix aspersa. The suction pipette method of internal perfusion was used along with a combined voltage-clamp method in which the membrane potential was measured by a separate glass micro-electrode and the current was passed by the suction pipette. Activation of the potential-dependent A current, IA, was prevented by using holding potentials of -40 mV where IA is inactivated and by the addition of the A-current blocker 4-aminopyridine. Activation of K currents by transmembrane Ca current, IKCa, was suppressed by Co substitution for Ca ion extracellularly. 2. Under these conditions, IKD rose to a peak value and then subsided to a steady level. The current-voltage (I-V) relationship for peak IKD had an upward bump at about +50 mV that gave it an S-shape. The I-V curve for steady IKD rose continuously. Peak and steady IKD were reduced by perfusing with EGTA or F ions intracellularly. The EGTA effect occurred at intracellular Ca activity levels below 10(-7) M. Increases in the concentration of EGTAi at constant Cai had no additional effect; however, recovery experiments do not allow us to rule out some direct action of EGTA on IKD. 3. Prolonged extracellular perfusion with Co-substituted solutions also reduced IKD and the effects occurred more quickly when the solutions were made hypertonic or caffeine was added to them. The peak transient was abolished, and the small remaining steady IKD (about 5-10% of normal peak IKD) was blocked by tetraethylammonium. IKD could be restored by the temporary reintroduction of Ca in the extracellular solution. 4. The S-shape of the peak I-V relationship for IKD may be due to Ca released from an endogenous site by membrane depolarization. The reduction of steady and peak IKD to very low values by Ca chelators or prolonged perfusion with Ca-free solutions indicates that Cai is important for activation of these K channels. 5. Three cellular structures were identified in electron micrographs of freeze-fractured neurones that could be involved in potential-dependent endogenous Ca release. These were a restricted extracellularly space, an intracellular membrane system of endoplasmic reticulum that may be fused to the internal face of the plasma membrane (the subsurface cisterns of Henkart & Nelson, 1979), and intracellular vesicles that also may be fused to the plasma membrane.

No direct correlation between binding of sex hormones and calcium pump activity by rat liver microsomal preparations

In the previous paper (J. steroid Biochem. 16 (1982) 437-446. [5]), we demonstrated that in vitro liver microsomal preparations of adult male rats possessed binding sites specific for progesterone (Prog) of high affinity (KD approximately 25.2 nM) and high capacity (*Nmax approximately 6.43pmol/mg of microsomal protein), using 1.30 mM NaCl-based incubation buffer. To explore the biological roles of liver microsomal Prog binding, we investigated the effects of such binding on liver microsomal Ca2+ pump activity. Firstly, we obtained results similar to those previously obtained concerning the characteristics of microsomal Prog binding using 100 mM KCl-based incubation buffer, usually used for experiments on microsomal Ca2+ pump activity. For microsomal 45Ca2+ uptake we also obtained results similar to those already demonstrated be several investigators. That is to say liver microsomal 45Ca2+ uptake was markedly increased by the addition of 30 mM oxalate and 5 mM ATP, and was not inhibited by the addition of 5 mM NaN3 into the incubation buffer. However, the addition of 1.0 microM Prog, as well as 17 beta-hydroxy-5 alpha-androstan-3-one (5 alpha-DHT) and estradiol-17 beta (E2 --17 beta), which should be a sufficiently saturable concentration for liver microsomal binding capacity for Prog was 5-10 times higher than that for 5 alpha-DHT and E2-17 beta. In addition, Prog (1.0 microM) had little effect on 45Ca2+ release from prelabeled microsomes. In conclusion, we suggest, therefore, that there is no direct correlation between binding of sex hormones and Ca2+ pump activity by rat liver microsomal preparations.

Diffusion rates of cell surface antigens of mouse-human heterokaryons. III. Regulation of lateral diffusion rates by calcium ions

In mouse-human heterokaryons, the lateral diffusion of major histocompatibility (MHC) antigens in the plasma membrane is enhanced by treatment of parent cells with ouabain. Ouabain treatment is ineffective if the medium lacks calcium ion, or if Verapamil, a blocker of calcium channels, is present. The divalent ionophore A23187 also enhances lateral diffusion of MHC antigens, to the same extent as ouabain, A23187 is effective only if calcium is present in the medium. Thus it appears that increased levels of cell calcium release constraints to lateral diffusion of MHC antigens.

Calcium channel and calcium pump involved in oscillatory hyperpolarizing responses of L-strain mouse fibroblasts

1. In fibroblastic L cells, spontaneously repeated hyperpolarizing responses (oscillation of membrane potential) and hyperpolarizing responses evoked by electrical stimuli were suppressed by the external application of a K(+) channel blocker, nonyltriethylammonium (C(9)). This hydrophobic TEA-analogue also inhibited the hyperpolarization induced by intracellular Ca(2+) injection.2. Quinine or quinidine, known inhibitors of the Ca(2+)-activated K(+) channel of red cells, instantaneously inhibited these hyperpolarizations. Thus, these hyperpolarizations are likely to be caused by the operation of Ca(2+)-sensitive K(+) channels.3. Azide, which is known to inhibit the mitochondrial Ca(2+) uptake in fibroblasts, and caffeine, dantrolene Na and oxalate, which affect the microsomal Ca(2+) transport, did not exert any effects upon the electrical potential profiles.4. On the other hand, Ca(2+) channel blockers (nifedipine, D 600 and Co(2+)) suppressed the hyperpolarizing responses, but not the hyperpolarizations produced by intracellular Ca(2+) injection, suggesting that the calcium ions responsible for the hyperpolarizing responses are mainly derived from outside the cell through Ca(2+) channels.5. Flavones of plant origin, which are known to inhibit Ca(2+)-ATPase, prolonged the duration of the hyperpolarizing phase of the oscillation or produced a sustained hyperpolarization.6. It is concluded that the Ca(2+) channel and the Ca(2+) pump play essential roles in the generation of the hyperpolarizing response and of the membrane potential oscillation in L cells, and that these hyperpolarizations are brought about by a transient elevation of cytosolic Ca(2+) level which, in turn, activates Ca(2+)-dependent K(+) channels.

The effects of tryptamine on transmitter release at a lobster neuromuscular junction

Electrophysiological techniques were employed to characterize the effects of tryptamine at excitatory and inhibitory neuromuscular junctions of the stretcher muscle in the carpopodite of lobster walking limbs. Tryptamine was found to have a concentration dependent, biphasic effect on excitatory junction potential (EJP) amplitude. At concentrations of 0.01-0.5 mM tryptamine increased the amplitude of evoked EJPs, but at higher concentrations (greater than 0.5 mM) the amplitude was decreased by this amine. The high concentrations also decreased the amplitude of inhibitory junction potentials (IJPs) and reduced the frequency of miniature excitatory junction potentials (MEJPs). When a preparation in which the EJPs had been depressed by tryptamine was washed with the control, artificial sea water solution, the EJPs were increased to an amplitude greater than that of the pre-tryptamine control. Current-voltage relationship studies showed that tryptamine did not affect the effective resistance of the muscle fiber membrane. Tryptamine had no effect on the amplitude of excitatory responses evoked by the iontophoretic application of glutamate. Concentrations of tryptamine ranging to 10.0 mM affected neither the conduction velocity in meropodite or that of giant central nervous system (CNS) axons. We conclude that tryptamine affects synaptic transmission pre-synaptically by influencing the transmitter release process. Mass spectrometric analysis showed tryptamine to be an endogenous substance in the lobster; but the concentrations were low in the tissues analyzed. The highest concentration (approximately 0.3 nmol/g wet tissue) was in the subesophageal ganglion. Tryptamine was not detected in the meropodite nerves or carpopodite muscles.

Electrophysiology of phagocytic membranes. III. Evidence for a calcium-dependent potassium permeability change during slow hyperpolarizations of activated macrophages

The roles of potassium and calcium in the slow hyperpolarizations of membranes of activated macrophages are investigated using standard intracellular electrical recording techniques. The amplitude of spontaneous slow hyperpolarizations decreases as a logarithmic function of the external potassium concentration in the culture medium. Similar dependence on the potassium gradient is observed when different levels of membrane potentials are imposed by constant current injection. The reversal potential for electrically evoked slow hyperpolarizations is -90 mV. A 10-fold increase in external potassium concentration causes a 60 mV shift of the reversal potential towards zero. Divalent cation ionophores (A23187 and X537A) can induce slow hyperpolarization responses in quiescent cells or permanent hyperpolarization in spontaneously active cells. The amplitude of the ionophore-induced hyperpolarizations is reduced by an increase in external potassium concentration in a manner consistent with data on slow hyperpolarization responses in the absence of ionophore. The calcium antagonist, verapamil, depresses the slow hyperpolarization responses at the concentration of 10(-5) M. It is suggested that the development of the hyperpolarizing response is due to a calcium-dependent potassium channel. The data support the assumption that spontaneous and artificially elicited slow hyperpolarization responses share a common calcium-dependent mechanism.

Phagocytic activity and hyperpolarizing responses in L-strain mouse fibroblasts

1. Fibroblastic L cells not only respond with a slow hyperpolarizing potential change to a mechanical or electrical stimulus but also show spontaneous, repetitive hyperpolarizations (i.e. membrane potential oscillation). 2. Almost all the cells can actively take up latex beads whose surfaces were treated by U.V. irradiation. 3. Non-phagocytic L cells hardly showed hyperpolarizing responses, while hyperpolarizing responses were obtained in all the phagocytic L cells. The exposure of the cell surface to beads, however, did not trigger the generation of hyperpolarizing responses. 4. Metabolic inhibitors, low temperature and cytochalasin B inhibited both the uptake of beads and the hyperpolarizing responses. 5. Increasing the external concentration of Ca2+ induced a remarkable stimulation of the phagocytosis of beads. Mg2+ and Ba2+, which inhibited hyperpolarizing responses due to competition for Ca2+ sites on the outer surface of the membrane, significantly suppressed the uptake of beads. 6. Verapamil, a Ca2+ channel blocker, inhibited not only hyperpolarizing membrane responses but also ingestion of beads. 7. It is concluded that the Ca2+ inflow on the hyperpolarizing membrane responses is closely associated with the phagocytic activity in L cells, probably through activation of the microfilament assembly.

Role of a 16S glycoprotein complex in cellular adhesion

Myogenic cells release into their culture medium a glycoprotein complex that mediates cellular adhesion. In the absence of calcium this complex has a sedimentation value of 16S; it aggregates in the presence of calcium. The 16S material both agglutinates and increases the rate of cell-substratum adhesion of a myoblast variant and inhibits the adhesion of a nerve-like cell line to culture dishes. It is also a hemagglutinin. The 16S particle is composed of glycosaminoglycans and several proteins, including fibronectin and collagen.