Membrane potential changes associated with pinocytosis of serum lipoproteins in L cells

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

Factors responsible for oscillations of membrane potential recorded with tight-seal-patch electrodes in mouse fibroblasts

In giant fibroblastic L cells, penetration of a conventional microelectrode brought about marked decreases in the membrane potential and input resistance measured with a patch electrode under tight-seal whole-cell configuration, and repeated hyperpolarizations were often observed upon penetration. Therefore, the question arose whether such leakage artifact is a causal factor for generation of the membrane potential oscillation even in giant L cells. During whole-cell recordings, however, regular potential oscillations were observed in the cells that had not been impaled with a conventional microelectrode, as far as the Ca2+ buffer was not strong in the pipette solution. Oscillatory changes in the intracellular potential were detected by extracellular recordings with a tight-seal patch electrode in the cell-attached configuration. Thus, the potential oscillation occurs even in the absence of penetration-induced leakage or without rupture of the patch membrane. Withdrawal of a micropipette from one cell was often found to induce marked cell damage and elicit oscillatory hyperpolarizations in a neighboring cell with a certain time lag. The longer the distance between the injured and recorded cells, the greater was the time lag. Application of the cell lysate on the cell surface also gave rise to oscillatory hyperpolarizations. After repeated applications of the lysate, the membrane became unresponsive (desensitized), suggesting the involvement of receptors for the lysate factor. The lysates of different cell species (mouse lymphoma L5178Y cells or human epithelial Intestine 407 cells) produced similar effects. The effective component was heat stable and distinct from ATP. Lysate-induced hyperpolarizations were inhibited by deprivation of extracellular Ca2+ and by application of a Ca2+ channel blocker (nifedipine) or a K+ channel blocker (quinine) in the same manner as spontaneous oscillatory hyperpolarizations. It is concluded that the mouse fibroblast exhibits membrane potential oscillations, when the cell was activated, presumably via receptor systems, by some diffusible factors released from damaged cells.

Long-lasting and rapid calcium changes during mitosis

A more complete understanding of calcium's role in cell division requires knowledge of the timing, magnitude, and duration of changes in cytoplasmic-free calcium, [Ca2+]i, associated with specific mitotic events. To define the temporal relationship of changes in [Ca2+]i to cellular and chromosomal movements, we have measured [Ca2+]i every 6-7 s in single-dividing Pt K2 cells using fura-2 and microspectrophotometry, coupling each calcium measurement with a bright-field observation. In the 12 min before discernable chromosome some separation, 90% of metaphase cells show at least one transient of increased [Ca2+]i, 72% show their last transient within 5 min, and a peak of activity is seen at 3 min before chromosome separation. The mean [Ca2+]i of the metaphase transients is 148 +/- 31 nM (61 transients in 35 cells) with an average duration of 21 +/- 14 s. The timing of these increases makes it unlikely that these transient increases in [Ca2+]i are acting directly to trigger the start of anaphase. However, it is possible that a transient rise in calcium during late metaphase is part of a more complex progression to anaphase. In addition to these transient changes, a gradual increase in [Ca2+]i was observed starting in late anaphase. Within the 2 min surrounding cytokinesis onset, 82% of cells show a transient increase in [Ca2+]i to 171 +/- 48 nM (53 transients in 32 cells). The close temporal correlation of these changes with cleavage is consistent with a more direct role for calcium in this event, possibly by activating the contractile system. To assess the specificity of these changes to the mitotic cycle, we examined calcium changes in interphase cells. Two-thirds of interphase cells show no transient increases in calcium with a mean [Ca2+]i of 100 +/- 18 nM (n = 12). However, one-third demonstrate dramatic and repeated transient increases in [Ca2+]i. The mean peak [Ca2+]i of these transients is 389 +/- 70 nM with an average duration of 77 s. The necessity of any of these transient changes in calcium for the completion of mitotic or interphase activities remains under investigation.

Stretch-activated cation channels in human fibroblasts

Nonconfluent fibroblasts are relatively depolarized when compared with confluent fibroblasts, and transient hyperpolarizations result from a range of external stimuli as well as internal cellular activities. This electrical activity ceases, along with growth and mitogenic activity, when the cells become confluent. A calcium-activated potassium conductance is thought to be responsible for these hyperpolarizations, but in human fibroblasts the large calcium-activated potassium channel is not stretch-activated. We report here the identification of single stretch-activated cation channels in human fibroblasts, using the cell-attached and inside-out patch clamp techniques. The most prominent channel had a conductance of approximately 60 pS (picoSeimens) in 140 mM potassium and was permeable to potassium and sodium. The channel showed significant adaptation of activity when stretch was maintained over a period of several seconds, but a static component persisted for much longer periods. Higher conductance channels were also observed in a few excised patches.

Serum and alpha 2-macroglobulin induce transient hyperpolarizations in the membrane potential of an osteoblastlike clone

Using microelectrode techniques, we have observed that the application of serum or alpha 2-macroglobulin (alpha 2M) induces transient hyperpolarizations in the membrane potential of a rat osteosarcoma clone (ROS 17/2). Hyperpolarizations arose from activation of Ca2+-dependent K+ channels by transient increases in the concentration of intracellular free Ca2+. Hyperpolarizing spikes were observed for several h following the addition of fetal bovine serum (FBS) to cell cultures. Application of small volumes of FBS or alpha 2M rapidly induced synchronized bursts of hyperpolarizing spikes. No response was elicited by serum-free medium, latex beads, or bovine serum albumin (BSA). Immunofluorescence labeling patterns were consistent with the receptor-mediated endocytosis of alpha 2M but not BSA. The ligand specificity and kinetics of these hyperpolarizations suggest that they are associated with a receptor-mediated event, possibly an early stage of receptor-mediated endocytosis.

Effect of protein kinase C activation and Ca2+ mobilization on hexose transport in Swiss 3T3 cells

Down-modulation of Ca2+-activated, phospholipid-dependent protein kinase (protein binase C), which was accomplished by pretreatment with phorbol-12,13-dibutyrate for 24 h, resulted in the loss of a phorbol ester-induced stimulation of hexose transport activity in Swiss 3T3 cells. In these cells, however, platelet-derived growth factor as well as Ca2+ ionophore A23187 were still able to induce stimulation of hexose transport activity accompanied by the elevation of intracellular free Ca2+ concentration. Since chelation of extracellular Ca2+ inhibited this stimulation, inflow of extracellular Ca2+ into cytoplasm seemed to be essential for the stimulatory effect of platelet-derived growth factor and A23187 on hexose transport. Epidermal growth factor and insulin also stimulated hexose transport activity regardless of the absence of protein kinase C. However, in the case of epidermal growth factor, intracellular Ca2+, but not extracellular Ca2+, was found to be necessary for the stimulation. On the other hand, insulin stimulated the hexose transport independent of both intra- and extracellular Ca2+.

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.

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.

Electrophysiological study of single Leydig cells freshly isolated from rat testis. II. Effects of ionic replacements, inhibitors and human chorionic gonadotropin on a calcium activated potassium permeability

Changes in the membrane potential of isolated Leydig cells produced by modified ionic solutions were investigated in vitro either by a total change of the bathing medium (procedure P1) or by a flush of the solution around the impaled cell (procedure P2). In standard Earle's solution, the impalement of 198 cells by a glass microelectrode was accompanied by an hyperpolarization (MP1 = -37.6 +/- 0.7 mV) (means +/- S.E.M.) followed by a gradual depolarization to a steady state potential (MP2 = -25.1 +/- 0.6 mV) (Joffre et al. 1984). Experiments with K modified media (P1) showed that MP2, and to a greater extend MP1, were dependent on the external K. A tenfold increase of K decreased MP2 by 16 mV and MP1 by 25 mV. When the extracellular Cl was reduced (P1) by substituting Cl with a less permeant anion, MP2 was unchanged and MP1 was significantly decreased. A transient depolarization of MP2 occurred when a low Cl medium was flushed (P2). An equimolar Na replacement by choline chloride (P1) did not change MP1 or MP2, during the first 10 min. It suppressed MP1 and decreased MP2 after a 15 min exposure. MP1 reappeared and MP2 increased after the restoration of the normal Na solution (P1 and P2). The modification of external Ca from 0 to 3.6 mM (P1) increased both MP1 and MP2. MP1 was never cancelled in 0 mM Ca. In 18 mM Ca, MP1 was suppressed and MP2 decreased. Restoration of 1.8 mM Ca was rapidly accompanied by the MP1 reappearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Exogenous ATP induces electrical membrane responses in fibroblasts

Mouse fibroblastic L cells responded to exogenous ATP (greater than or equal to 0.2 mM) with a transient hyperpolarization due to increased membrane permeability to K+. By contrast, intracellular injection of ATP (up to about 3 mM) produced no noticeable effects on the membrane potential. The effects of a non-hydrolysable analogue of ATP (AMP-PNP) were similar to those of ATP. After successive applications of ATP, the cell membrane became virtually unresponsive (desensitized). Extracellular ADP was also effective, but AMP or adenosine was not. Antazoline suppressed the ATP response. Thus, exogenous ATP and ADP appear to stimulate P2- purinoceptors . Similar responses to ATP (or ADP) were also observed in human normal diploid fibroblasts (Flow 1000 line).

Effects of cytochalasin B and of deoxyglucose on phagocytosis-related changes in membrane potential in rat peritoneal macrophages

Cytochalasin B (CB) and deoxyglucose alter the electrical responses of the plasma membrane of rat peritoneal macrophages to the presence of phagocytosable latex particles. With both agents, instead of the initial hyperpolarization we previously observed, there is a depolarization. In the case of cytochalasin B (CB) this is followed by a gradual repolarization to the initial resting level, whereas with deoxyglucose the membrane eventually does hyperpolarize. One possible interpretation is that plasma membrane receptors mediate the depolarization in response to phagocytosable particles, but that normally this is masked by other changes effected here by the agents we used.

Ca2+-dependent stimulation of hexose transport by A23187, 12-O-tetradecanoylphorbol-13-acetate and epidermal growth factor in mouse fibroblasts

Ca2+ ionophore A23187 stimulated 2-deoxy-D-glucose (2DG) uptake in Swiss 3T3 mouse fibroblasts. Chelation of extracellular Ca2+ with ethylene-glycol-bis-(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) inhibited the effect of A23187. Similarly, the stimulation of 2DG uptake by a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was prevented by EGTA, whereas the epidermal growth factor (EGF)-stimulated 2DG uptake was not affected by EGTA alone, but in the presence of both EGTA and A23187 which effectively depleted cellular Ca2+ content, EGF could no longer stimulate 2DG uptake. These results suggest that Ca2+ regulates hexose transport system in Swiss 3T3 mouse fibroblasts, the activation of which by TPA and EGF differently depends on Ca2+.

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.