Phagocytic activity and hyperpolarizing responses in L-strain mouse fibroblasts

Characterization of ion channels seen in subconfluent human dermal fibroblasts

1. Ion channels expressed in human dermal fibroblasts are characterized using the patch-clamp technique. 2. A number of different ion channels were found but their expression occurred at various frequencies. The most commonly found phenotype was the expression of voltage-gated K+ current. This 'typical' K+ current was seen in about 60% of the cells recorded. 3. Subtypes of voltage-gated K+ channels could be discerned by differences in gating kinetics. One has fast inactivation and resembles the 'A' K+ current. Additional subtypes were sometimes discerned based on activation kinetics. 4. The large-conductance Ca(2+)-activated K+ channel (maxi-K+) could be found in nearly every cell but required large depolarizations to activate using the standard Ca(2+)-buffered pipette solution (10(-8) M [Ca2+]i). 5. Inward rectifier K+ channels were seen in a low percentage of cells. The inward rectifier K+ current was sensitive to 'wash-out' if guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was included in the pipette solution dialysing the cell. 6. Tetrodotoxin (TTX)-sensitive voltage-gated Na+ channels were seen but in a lower number of cells recorded, about 20%. Evidence for subtypes of Na+ channels were sometimes seen based on differences in gating kinetics. 7. An ATP-dependent osmotically activated Cl- current was also found. This current showed some outward rectification but was otherwise voltage independent. 8. In addition, a cell-to-cell contact-associated K+ current was described. This current was linear over the voltage ranges used and whose gating correlated with the existence of gap junctions. 9. These currents were characterized to determine the baseline behaviour of unstimulated cells and to compare to bradykinin-stimulated cells described in the following paper. As unexcitable cells, human dermal fibroblasts are capable of expressing a surprising diversity of ion channel phenotypes and of ion channel modulations.

Studies on the alignment of fibroblasts in uniform applied electrical fields

Uniform electrical fields have been applied to human gingival fibroblasts by means of uniform ionic currents passed through a thin chamber. Cells were observed to align in fields between 0.1 and 1.5 V/mm but did not display directed motion toward the anode or the cathode of the chamber. Statistical analysis of directional data was used to distinguish threshold levels of orientation at low field intensities, to quantify the dependence of alignment on time and field intensity, and to analyze differences between alignment of cells treated with the Ca2+ transport modifiers A23187, verapamil, and lanthanum. Alignment occurred at a steady rate and was dependent in a saturating fashion on field strength. The Ca2+ ionophore A23187 had a significant inhibitory effect on cell alignment in applied electrical fields; however, the Ca2+ channel blockers lanthanum and verapamil did not have a significant effect on alignment.

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.

Ca2+-activated K+ conductance causes membrane hyperpolarizations in a monkey kidney cell line (JTC-12)

We have previously reported hyperpolarizing membrane potential changes in a monkey kidney cell line (JTC-12) which has characteristics resembling proximal tubular cells. These hyperpolarizations could be observed spontaneously or evoked by mechanically touching adjacent cells. In this report, we have shown further evidence that these hyperpolarizations are elicited by an increase in membrane conductance to K+ which is caused by an increase in cytosolic Ca2+ concentration. In addition, we have found another type of hyperpolarization which is evoked by applying flow of extracellular fluid to the cell. Intracellular injection of Ca2+ and Sr2+ evoked hyperpolarizations, while intracellular injection of Mn2+ and Ba2+ did not. Intracellular injection of EGTA suppressed both spontaneous and mechanically evoked hyperpolarizations. In Ca2+-free medium, both spontaneous and flow-evoked hyperpolarizations were not observed, while mechanical stimuli consistently evoked hyperpolarization. In Na+-free medium, the incidence of cells showing the spontaneous or flow-evoked hyperpolarization increased, and the amplitude and the duration of the mechanically evoked hyperpolarization became greater. Quinidine inhibited all types of hyperpolarization. These data suggest that hyperpolarizations in JTC-12 cells are due to an increase in Ca2+-activated K+ conductance.

Ca2+ sensitivity of volume-regulatory K+ and Cl- channels in cultured human epithelial cells

1. During exposure to a hypotonic solution, cultured human epithelial cells (Intestine 407) exhibited a regulatory volume decrease (RVD) after initial osmotic swelling. 2. The volume readjustment was slowed by elevating the extracellular K+ concentration and facilitated by reducing the extracellular Cl- concentration. Not only putative K+ channel blockers, quinine and Ba2+, but also a stilbene derivative Cl- channel blocker (SITS) inhibited the RVD. 3. The volume recovery of hypoosmotically swollen cells was very much suppressed by the deprivation of extracellular Ca2+ ions or by chelation of cytosolic Ca2+ ions with Quin-2 loaded within the cells. 4. Biphasic membrane potential changes were associated with the RVD process at low extracellular K+ and Cl- concentrations. The initial hyperpolarizing response was inhibited by quinine and Ba2+, whereas the late depolarizing response was inhibited by SITS. The deprivation of extracellular Ca2+ inhibited the initial hyperpolarizing phase but not the late depolarizing phase. 5. Two-microelectrode voltage clamp studies showed that the initial hyperpolarization and late depolarization were associated with quinine-sensitive outward currents and SITS-sensitive inward currents, respectively. The reversal potentials estimated from the current-voltage curves were about -80 mV for the initial response and -27 mV for the late response. Tenfold changes in the K+ and Cl- concentrations shifted these reversal potentials by 50 mV for the initial response and by 42 mV for the late response. 6. Under whole-cell recordings, similar current changes were observed in the cells exposed to a hypotonic solution, when the intracellular Ca2+ ions were moderately buffered with 1 mM-EGTA in the dialysing solution filled in a patch pipette. When most Ca2+ ions were chelated with 10 mM-EGTA in the pipette solution, the initial outward current as well as the corresponding hyperpolarization was suppressed, but the late current associated with the depolarizing phase was preserved. 7. Intracellular Ca2+ injections induced an increase in the quinine-sensitive K+ conductance but failed to activate the Cl- conductance. 8. It is concluded that both K+ and Cl- channels are involved in the regulatory volume decrease, and that the former channel is exclusively activated by elevation of the cytosolic Ca2+ concentration in the epithelial cells.

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.

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.

Potassium channels in human and avian fibroblasts

The cell-attached and excised inside-out patch-clamp techniques were used to study single-channel characteristics of potassium channels in cultured human and avian fibroblasts. Six different potassium channels were distinguished with conductances of 235 +/- 25, 190 +/- 57, 114 +/- 27, 77 +/- 14, 40 +/- 6 and 21 +/- 4 pS in symmetric 140 mM potassium solutions. The channels were separable by their conductances, ion-selectivities, voltage-sensitivities and kinetic properties. All six channels were found in both fully differentiated human skin fibroblasts and primary cultures of 72 h chick sclerotome. The largest channel (235 pS) had a steep bimodal voltage dependence, being open only around the resting membrane potential. It was imperfectly selective for potassium, having a relative sodium:potassium permeability of 0.3. The 190 pS channel was very potassium-selective, had an S-shaped voltage sensitivity and was calcium-dependent. The two intermediate-size channels (114 and 77 pS) had open probabilities of less than 0.5 under all of the conditions we used. They were not completely selective for potassium and were not voltage-sensitive. The two smallest channels (40 and 21 pS) were not well characterized. They both had open probabilities of less than 0.2 and showed no evidence of voltage-sensitivity. The 40 pS channel seemed highly potassium-selective. A suction stimulus was used to test all observed channels for mechanosensitivity but none of the six potassium channels was mechanosensitive. Another small channel, with very clear mechanical sensitivity, was seen on a few occasions; this channel has not yet been characterized.

Cytosolic free calcium increases before and oscillates during frustrated phagocytosis in macrophages

When macrophages and neutrophils are allowed to settle onto an appropriate surface, they attach and spread in a frustrated attempt to phagocytose the substrate. Spreading is associated with extensive rearrangements of the actin cytoskeleton which resemble those occurring during phagocytosis. We have previously shown that spreading in human neutrophils is preceded by an increase in cytosolic-free calcium concentration [( Ca2+]i) (Kruskal, B. A., S. Shak, and F. R. Maxfield. 1986. Proc. Natl. Acad. Sci. USA. 83:2919-2923). To assess the generality of this signal, we measured [Ca2+]i in single thioglycollate-elicited mouse peritoneal macrophages as they spread on an immune complex-coated surface, using fura-2 microspectrofluorometry. A [Ca2+]i increase always precedes spreading. This increase can involve several (up to 8) [Ca2+]i spikes, with an average peak value of 387 +/- 227 nM (mean +/- SD, n = 92 peaks in 24 cells), before spreading is detected. Neither spreading nor the magnitude of these spikes is significantly altered by removal of extracellular calcium. Many of the spreading macrophages exhibit periodic [Ca2+]i increases before and during spreading. The proportion which does so varies among experiments from 0 to 90%, but it is frequently greater than 40%. The largest number of cells (approximately 25%) exhibited only a single peak. In 13 cells that showed more than 10 peaks, the median period was 29 s (range 19-69 s). The average peak [Ca2+]i was 385 +/- 266 nM (mean +/- SD, n = 208 peaks in 14 cells). The calcium producing these increases is derived from intracellular pools. The oscillations occur with spreading on either opsonized or nonopsonized surfaces. The function of these oscillations is not clear, but the large number of cells which exhibit them suggest that they may be important to macrophage function.

Further analysis of spontaneous membrane potential activity and the hyperpolarizing response to parathyroid hormone in osteoblastlike cells

Whole cell voltage clamp measurements using the patch technique on well-attached and well-spread cells of an osteoblastlike line (ROS 17/2.8) show the same spontaneous membrane potential activity as measurements with inserted microelectrodes. Furthermore, membrane potential measurements during the first 80 milliseconds (ms) following microelectrode penetration of the cell membrane usually show no decay. There is also good agreement between values of cell membrane resistance obtained by the microelectrode technique, the whole cell patch clamp technique, and the single channel patch clamp technique. These results indicate that our microelectrode measurements are not dominated by leak-induced artifacts, and that the spontaneous membrane potential activity is not induced by Ca2+ leakage around the microelectrode. The spontaneous membrane potential activity is eliminated in the presence of the Ca2+ ionophore A23187, also in serum-free medium, and by K+ and Ca2+ channel blockers, but it is not affected by the hyperpolarizing responses to parathyroid hormone (PTH) and dibutyryl cAMP, which persist under all of these conditions. These results support the hypothesis that the spontaneous membrane potential activity is related to repeated fluctuations of internal [Ca2+] and that such fluctuations result from a feedback loop involving Ca2+ channels or Ca2+ pumps in the cell membrane.

Electrophysiological differences between bone cell clones: membrane potential responses to parathyroid hormone and correlation with the cAMP response

Electrophysiological measurements on three clonally derived bone cell populations showed a positive correlation between longer-term hyperpolarizing membrane potential responses to parathyroid hormone (PTH) and an intracellular cAMP response to PTH. One clone (RCJ 1.20) had no sustained electrophysiological response and no cAMP response to PTH. Another clone (ROS 17/2.8) had both a sustained hyperpolarizing response and a cAMP response to PTH. The third clone (RCB 2.2) initially had both an electrophysiological response and a cAMP response to PTH, but both responses were lost after prolonged growth in culture. Application of dibutyryl cAMP to RCJ 1.20 and ROS 17/2.8 cells produced both transient and sustained hyperpolarizing responses. Application of isobutylmethylxanthine produced a sustained hyperpolarization. These results suggest that the hyperpolarizing response to PTH is related to a cAMP-mediated increase in Ca2+ conductance, which leads to an increase in Ca2+-activated K+ conductance. The pronounced membrane potential spikes and fluctuations that occur in some of the clonal lines were shown to be unrelated to the hyperpolarizing response to PTH. This was demonstrated by the lack of correlation between the occurrence of the spikes or fluctuations and the occurrence of the hyperpolarizing response to PTH in the various cell lines, by the lack of effect of PTH on the spikes and fluctuations, and by the lack of effect on the hyperpolarizing response to PTH of verapamil and quinine, both of which significantly reduce the spikes and fluctuations.

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.

Hyperpolarizing membrane potential changes in a cloned monkey kidney cell line

Electrophysiological properties of a cloned monkey kidney cell line, JTC-12, were studied. The mean resting potential and input resistance were -15.3 mV and 78 M omega, respectively. Spontaneous hyperpolarizations with increased membrane conductance were observed. Similar hyperpolarization could be elicited by mechanical and electrical stimulations. The mean reversal potential of these hyperpolarizations was -72.7 mV. Hyperpolarization could be also elicited in a chloride-free solution. These data indicate that: JTC-12 cells exhibit spontaneous and induced hyperpolarizations, and occurrence of hyperpolarization is related to an increase in membrane permeability to potassium ions.

The intracellular degradation of poly(epsilon-caprolactone)

Poly(epsilon-caprolactone) [PEC], a biodegradable aliphatic polyester, undergoes a two-stage degradation process: The first lengthy phase involves nonenzymatic hydrolytic cleavage of ester groups, the second phase beginning when the polymer is more highly crystalline, and of low molecular weight. The cellular events of the second phase were examined by implanting gelatin capsules containing 25 mg of low molecular weight (Mn 3000) PEC powders, 106 to 500 micron, in rats. PEC fragments ultimately were degraded in phagosomes of macrophages and giant cells, the process requiring less than 13 days for completion at some sites. PEC was also identified within fibroblasts. These studies support the intracellular degradation of PEC as the principal pathway of degradation once the molecular weight of the aged polymer is reduced to 3000 or less.

Transient and sustained effects of hormones and calcium on membrane potential in a bone cell clone

Measurements were made of the electrophysiological and cAMP response to changes in extracellular [Ca2+] and to hormone application in a bone cell clone. Both transient and long-term electrophysiological responses were studied. An increase in extracellular [Ca2+] usually resulted in a transient hyperpolarization of about 60-sec duration. In addition, increases in extracellular [Ca2+] from 0.9 to 1.8 mM and from 1.8 to 3.6 mM resulted in long-term hyperpolarization and increased potential fluctuations. Increasing bathing [Ca2+] until the membrane potential reached the K+ equilibrium level resulted in a significant decrease in fluctuations. Addition to the bathing medium of quinine, a putative blocker of the Ca2+-dependent K+ channel, resulted in long-term depolarization of the mean membrane potential, and a long-term decrease in potential fluctuations. Addition of Mg2+, a mild antagonist of Ca2+ entry into the cell, produced transient depolarization and reduction of potential fluctuations. These effects suggest that the potential fluctuations reflect cytoplasmic [Ca2+] fluctuations via Ca2+-dependent K+ membrane channels. Under an extracellular [Ca2+] of 1.8 mM, the application of prostaglandin E2 (PGE2), isoproterenol, and parathyroid hormone produced no significant effect on mean membrane potential or on the sustained potential fluctuations, but PGE2 did significantly raise intracellular cAMP. Under an increased bathing [Ca2+], significant changes in mean potential and fluctuations did occur in response to PGE2, but not in response to the other hormones, while the PGE2 effect on cAMP was not greatly changed. Hyperpolarizing transients of about 30-sec duration occurred in response to all of the hormones, particularly at an extracellular [Ca2+] of 3.6 mM. Thus, there are both transient and long-term electrophysiological responses to hormone application, with only the long-term response correlated with the production of cAMP. These electrophysiological responses may represent separate transient and long-term calcium transport responses to hormone application.

Electrical membrane responses to secretagogues in parietal cells of the rat gastric mucosa in culture

Fragments of the gastric fundus of 6-8-day-old rats were maintained in tissue culture. From the explant, adhered to a plastic substrate, epithelial cells migrated and developed to form a monolayer colony. Histological and histochemical studies as well as indirect immunofluorescence studies using anti-parietal cell antibodies testified to the presence of parietal cells in the monolayer during the first week. These parietal cells were distinguished by their vesicular cytoplasmic structures using phase-contrast or differential interference-contrast microscopy. Acridine Orange, an optical probe of H+ accumulation, was taken up preferentially by these parietal cells, exhibiting orange fluorescence within the cells on the third day of culture, in response to stimulation with gastrin, histamine and carbachol. The resting potential of these cultured parietal cells was about -20 mV. On day 2-4 of culture, the cell membrane became hyperpolarized (up to -30 to -40 mV) in response to gastrin, carbachol or histamine in the presence of isobutylmethyl-xanthine (IMX). During hyperpolarization, the membrane resistances decreased significantly. The amplitude and the polarity of secretagogue-induced responses were found to be dependent on the extracellular concentration of K+ (but not Na+ and Cl-). The carbachol-induced responses were inhibited by atropine but not curare. The responses induced by histamine plus IMX were blocked by cimetidine but not pyrilamine. Neither atropine nor cimetidine affected the gastrin-evoked responses. It is concluded that rat parietal cells have separate receptors for acetylcholine (muscarinic), gastrin and histamine (H2), and that an increase in the membrane permeability to K+ is closely associated with the responses of these receptors under these in vitro conditions.

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)

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.

Periodic hyperpolarizing responses in hamster and mouse eggs fertilized with mouse sperm

1. The zona-free hamster egg allows multiple entries of heterologous as well as homologous sperm. The hamster egg inseminated with mouse sperm (M x H egg) showed recurring, transient hyperpolarizing responses (h.r.s) with the peak of -70 to -80 mV. They were superimposed on a hyperpolarizing shift of the resting potential (h.s.) which gradually reached -60 mV in 50 min after insemination.2. Unlike the hamster sperm, the cessation of flagellar motion of the first mouse sperm (;1-stop') failed to induce the first h.r. but produced only a small hyperpolarizing ;step' of 3-7 mV. Similar steps occurred for each of additional sperm with a one-to-one correspondence, 4-50 sec ahead of the cessation of sperm motion.3. In M x H eggs, the h.r. first appeared about 15 min after the ;1-stop'. The intervals of the h.r.s thereafter were in the range between 2-10 min, in contrast to 30-45 sec in hamster eggs inseminated with hamster sperm (H x H eggs).4. The h.r.s in M x H eggs were abolished by intracellular injection of EGTA, suggesting that they were caused by periodic increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) as in H x H eggs.5. The gradual h.s. in M x H eggs was considered to be due mainly to an increase in Ca-independent K permeability, since the resting potential beyond -60 mV at 50-70 min after insemination was changed by only 3-5 mV on the removal of Cl ions and on EGTA injection.6. Histological observations revealed that the resumption of the second meiosis, the indication of egg activation, is delayed in M x H eggs by about 15 min, compared with that in H x H eggs. There was a good correlation between the delay of activation and that of the occurrence of the first h.r.7. In M x H eggs, the probability of egg activation within 70 min was dependent on the number of sperm penetrations: 90% for more than ten sperm while 20-30% for less than five sperm. Eggs in which sperm penetration was not followed by activation showed no h.r.s.8. The mouse egg inseminated with mouse sperm showed small h.r.s (3-4 mV) superimposed on the h.s. from -35 to -55 mV in 50 min after insemination. Both h.r.s and h.s. were associated with an increase in the membrane conductance. The h.s. was considered to be due mainly to a Ca-independent increase in K permeability.9. Iontophoretic injection of Ca(2+) into the unfertilized mouse egg could not increase the K conductance with injection currents up to 4 nA. However, the h.r.s were suggested to be resulted from a periodic increase in [Ca(2+)](i), since they were abolished by injection of EGTA.

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.

Dependence of membrane potential on Ca2+ transport in cultured cytotrophoblasts of human immature placentas

The electrical membrane properties of cultured human cytotrophoblast were examined by means of a standard electrophysiological technique. The mean values of the membrane potential (Rm) and the membrane resistance in a physiological medium were around -49 mV and 12 M omega , respectively. The membrane potential was dependent, to a large extent, on the external Ca2+ concentration ([Ca2+]o). Deprivation of external Ca2+ reduced membrane potential to about -20 mV, and an increase in [Ca2+]o caused a hyperpolarization in a saturable manner. The Ca2+-dependency of membrane potential was affected remarkably by [K+]o, but not by [Na+]o or [Cl-]o. The intracellular Ca2+ injection hyperpolarized the membrane in a Ca2+-free medium. A Ca2+ channel blocker, verapamil, completely abolished the Ca2+-dependent Em. The Ca2+-dependent Em was also suppressed by cooling or by the application of metabolic inhibitors. It is suggested that the Ca2+-dependent Em in cultured human cytotrophoblast is caused by a Ca2+ influx which, in turn, increases the K+ conductance of the cell membrane, presumable due to stimulation of Ca2+-activated K+ channel.

Ca-mediated activation of a K current at fertilization of golden hamster eggs

Golden hamster eggs respond to fertilization with recurring hyperpolarizations [Miyazaki, S. & Igusa, Y. (1981) Nature (London) 290, 703-705]. We analyzed the ionic mechanism of the fertilization potential and examined whether the fertilization potential plays a role in polyspermy block. Each hyperpolarizing response (HR) during fertilization is found to be caused by an increase in the K conductance activated by an increase in the intracellular Ca2+ concentration. This conclusion is based on the following: (i) The reversal potential of the HR shifted with the Nernstian slope for K ions when the external K concentration was changed, whereas it was unaltered by the removal of Cl ions. (ii) The HR was blocked by the intracellular injection of EGTA. (iii) Injection of Ca2+ into an egg induced a hyperpolarization of the membrane similar to the HR. The Ca-activated K conductance shows an apparent outward rectification, which could be explained by an asymmetric distribution of K ions across the membrane. The HR associated with sperm entry into the egg occurred at any membrane potential between -160 and +50 mV. Therefore, a potential-dependent block of sperm entry does not occur in the hamster egg.

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.