Potassium channels in human and avian fibroblasts

Mechanics rules cell biology

Cells in the musculoskeletal system are subjected to various mechanical forces in vivo. Years of research have shown that these mechanical forces, including tension and compression, greatly influence various cellular functions such as gene expression, cell proliferation and differentiation, and secretion of matrix proteins. Cells also use mechanotransduction mechanisms to convert mechanical signals into a cascade of cellular and molecular events. This mini-review provides an overview of cell mechanobiology to highlight the notion that mechanics, mainly in the form of mechanical forces, dictates cell behaviors in terms of both cellular mechanobiological responses and mechanotransduction.

Nitric oxide stimulates a large-conductance Ca-activated K+ channel in human skin fibroblasts through protein kinase G pathway

In order to investigate the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel and determine the effects of nitric oxide (NO) on the channel in human skin fibroblasts, we performed electrophysiological patch clamp recordings on 5th-passage cells of human genital skin cultures. The whole-cell outward K(+) current was increased with depolarization, and proved to be sensitive to NS1619 (a selective BK(Ca) channel activator) and iberiotoxin (a specific BK(Ca )channel inhibitor). The single-channel currents showed 226 pS of mean conductance in symmetrical K(+). Sodium nitroprusside (SNP; an NO donor) significantly increased the K(+) current amplitude in the whole-cell mode, and open probability of the channel (NPo) in the cell-attached mode, but not in the inside-out mode. S-nitroso-N-acetylpenicillamine (an NO donor) and 8-Br-cGMP (a membrane-permeant cGMP analogue) also increased the BK(Ca )channel activity. The stimulatory effect of SNP on BK(Ca) channels was inhibited by pretreatment with 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (a soluble guanylyl cyclase inhibitor), or KT5823 [a specific protein kinase G (PKG) inhibitor]. Cytoplasmic PKG also increased the channel activity in inside-out patches. In conclusion, the present data indicate that BK(Ca) channels constitute a significant fraction of K(+) current in human skin fibroblasts, and that NO increases NPo of BK(Ca) channels, which are mediated via the cGMP/PKG pathway, without direct effects on the channel.

Mechanically induced potentials in atrial fibroblasts from rat hearts are sensitive to hypoxia/reoxygenation

Membrane potential changes of atrial fibroblasts in response to mechanical stress have been considered to modulate the rhythmic electrical activity of healthy hearts. Our recent findings suggest that cardiac arrhythmia after infarction is related to enhanced susceptibility of the fibroblasts to physical stretch. In this study, we analysed the effect of hypoxia/reoxygenation, which are major components of tissue ischemia/reperfusion, on the membrane potential of atrial fibroblasts. Intracellular microelectrode recordings were performed together with isometric force measurements on isometrically contracting right atrial tissue preparations from adult rats. Lowering the oxygen tension in the perfusate from 80 kPa to 3.5 kPa reduced active force development and decreased the resting membrane potential of the cardiac fibroblasts from -23+/-5 mV to -5+/-2 mV ( n=35). Application of gadolinium (40 microM) to inhibit non-selective cation channels prevented hypoxia-induced membrane depolarization of the fibroblasts. Reoxygenation of the myocardial tissue resulted in a transient increase of the resting membrane potential to maximally -60+/-8 mV. These findings indicate that transmembrane currents in atrial fibroblasts are sensitive to changes in tissue oxygenation. In conclusion, altered electro-mechanical function of the ischemic heart may possibly involve changes of the membrane potential of the cardiac fibroblasts.

The effect of mechanical deformation on the distribution of ions in fibroblasts

The extracellular and intracellular sodium, potassium and chloride concentrations were determined in fibroblast cells located in the rat calvarium. The ionic values were determined by fluorescence microscopy after incubation with the fluorescent probes, sodium-binding benzofuran isophthalate (SBFI), potassium-binding benzofuran isophthalate (PBFI) and 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) (Dyes were supplied by CALBIOCHEM, Nottingham, England). After determination of the resting membrane potential, the calvaria were placed under tension by retraction of a micromanipulator. The fluorescence was measured again. A statistically significant difference was found in the calculated potassium ion concentration (Mann-Whitney; p < 0.05). This affected the resting cell membrane potential by an average of 5.2 mV. This effect was blocked by the addition of a potassium channel blocker, tetraethylammonium (TEA).

Molecular mechanisms of memory and the pathophysiology of Alzheimer's disease

Research on molecular and biophysical mechanisms of associative learning and memory storage identified a number of key elements that are phylogenetically conserved. In both vertebrates and invertebrates, K+ channels, PKC, Cp20, and intracellular Ca2+ regulation play a fundamental role in memory mechanisms. Because memory loss is the hallmark and perhaps the earliest sign of Alzheimer's disease, we hypothesized that these normal memory mechanisms might be altered in AD. With the use of a variety of experimental methodologies, our results revealed that one of the critical elements in memory storage, K+ channels, are dysfunctional in AD fibroblasts. Moreover, beta-amyloid induced the same K+ dysfunction in normal cells. Intracellular Ca2+ release, also associated with molecular memory mechanisms, was found altered in fibroblasts from patients with AD. The results therefore strongly suggest that biophysical and molecular mechanisms of associative learning could be altered in AD and that they may contribute to the memory loss observed early in the disease.

Potassium single-channel properties in normal and Rous sarcoma virus-transformed chicken embryo fibroblasts

Ion channels in normal and Rous sarcoma virus (RSV)-transformed chicken embryo fibroblasts (CEFs) were examined by using the patch-clamp technique. Three different types of ion channels were observed with single-channel conductances in symmetrical 140 mM KCl (with frequencies of occurrence in parentheses) of 186 pS (70%), 110 pS (10%), and 65 pS (20%), which are identical in normal and RSV-transformed CEFs. The total channel density in both cell types is about 0.13 per micron2. All three types of channels are highly selective for K+ ions, they are Ca(2+)- and voltage-dependent, and they can be completely blocked by external tetraethylammonium (10 mM) in both normal and RSV-transformed cells. Some channel properties, however, are different in normal and RSV-transformed CEFs. The K186 channel of normal CEFs is almost completely activated in the presence of about 1 nM free internal Ca2+ and is insensitive to charybdotoxin (100 nM). In contrast, the K186 channel of RSV-transformed CEFs has an EC50 value for activation by internal Ca2+ of about 100 nM and is highly sensitive to charybdotoxin (IC50 = 9 nM). In normal CEFs, the K186 channel activity starts at membrane potentials more positive than -50 mV and reaches a high open state probability of 0.94 at +50 mV. In RSV-transformed CEFs, the threshold of K186 channel activity is also -50 mV but the maximal open state probability is only 0.70 at +50 mV membrane potential. Averages of current traces of K186 channels show the typical features of the macroscopic K+ currents described previously for normal and RSV-transformed CEFs.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of calcium-dependent membrane currents in human fibroblasts by replicative senescence and forced expression of a gene sequence encoding a putative calcium-binding protein

Human diploid fibroblasts (HDFs) possess Ca(2+)-dependent membrane currents. These currents were suppressed in late-passage normal (senescent) HDFs and prematurely senescent HDFs derived from a subject with Werner syndrome (WS), compared with early-passage normal (young) HDFs. When young HDFs were microinjected with mRNA transcribed in vitro from a cDNA (WS3-10) which encodes a protein bearing a putative Ca(2+)-binding site and whose endogenous gene is overexpressed in senescent and WS HDFs, membrane currents fell to levels present in senescent and WS HDFs. Thus, both replicative senescence and forced expression of the WS3-10 gene sequence lead to suppression of Ca(2+)-dependent membrane currents, which suggests that a causal connection exists between these two processes.

Potassium channel dysfunction in fibroblasts identifies patients with Alzheimer disease

Since memory loss is characteristic of Alzheimer disease (AD), and since K+ channels change during acquisition of memory in both molluscs and mammals, we investigated K+ channel function as a possible site of AD pathology and, therefore, as a possible diagnostic index as well. A 113-pS tetraethylammonium (TEA)-sensitive K+ channel was consistently absent from AD fibroblasts, while it was often present in young and aged control fibroblasts. A second (166-pS) K+ channel was present in all three groups. Elevated external potassium raised intracellular Ca2+ in all cases. TEA depolarized and caused intracellular Ca2+ elevation in young and aged control fibroblasts but not AD fibroblasts. The invariable absence of a 113-pS TEA-sensitive K+ channel and TEA-induced Ca2+ signal indicate K+ channel dysfunction in AD fibroblasts. These results suggest the possibility of a laboratory method that would diagnostically distinguish AD patients, with or without a family history of AD, from normal age-matched controls and also from patients with non-AD neurological and psychiatric disorders.

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.

A potassium channel in cultured chondrocytes

Chondrocytes, obtained from preosseous cartilage, were studied by patch clamp technique in cell-attached recording configuration, and single potassium channels were characterized at different stages of culture. After 3 days, outward currents were present, with an open probability increasing with depolarization, and the K+ channels showing a mean slope conductance of 82 pS in asymmetric and 168 pS in symmetric potassium solution. Tetraethylammonium (TEA) and quinidine blocked the channels. Cells at confluence showed similar channel activity, with conductances of 121 and 252 pS, respectively. We suggest that culture time and/or conditions may modify K+ channels or induce the expression of a new type of channels.

Using fractals to understand the opening and closing of ion channels

Looking at an old problem from a new perspective can sometimes lead to new ways of analyzing experimental data which may help in understanding the mechanisms that underlie the phenomena. We show how the application of fractals to analyze the patch clamp recordings of the sequence of open and closed times of cell membrane ion channels has led to a new description of ion channel kinetics. This new information has led to new models that imply: (a) ion channel proteins have many conformational states of nearly equal energy minima and many pathways connecting one conformational state to another, and (b) that these many states are not independent but are linked by physical mechanisms that result in the observed fractal scaling. The first result is consistent with many experiments, simulations, and theories of globular proteins developed over the last decade. The second result has stimulated the suggestion of several different physical mechanisms that could cause the fractal scalings observed.

A class of non-selective cation channels in human fibroblasts

Non-selective cation channels were detected in membrane patches of cultured human fibroblasts. The channels had a unitary conductance which ranged from 14 to 25 pS in symmetrical 130 mM NaCl and were permeable to both sodium and potassium ions. Open channel probability was dependent either on the membrane potential and the Ca2+ concentration on the intracellular side of the membrane. High Ca2+ concentrations in the millimolar range were needed to keep the channel active.

Serum induces the immediate opening of Ca2+-activated channels in quiescent human fibroblasts

Application of fetal calf serum to quiescent human fibroblasts produces an immediate (3-20 s delay) increase in membrane conductance which lasts about 20-30 s. This conductance is strongly outwardly-rectifying and has a reversal potential between -45 and -10 mV. The conductance increase may also be induced by application of the Ca2+ ionophore A23187 while it does not occur when intracellular K+ is replaced by Cs+. It is concluded that this early effect of serum is due to the opening of Ca2+-activated channels. This permeability change will alter the membrane potential and thus possibly interact with other voltage-sensitive processes induced by serum growth factors.

Low Ca2+-sensitive maxi-K+ channels in human cultured fibroblasts

The patch clamp technique was used to reveal single channel activity in the membrane of human cultured fibroblasts. The most frequently detected ion channel type was a Ca2+-dependent K+ channel with a conductance of 287 +/- 38 pS in symmetrical 130 mM KCl. The channel showed a peculiar low Ca2+-sensitivity compared to that of similar channels in other preparations. In fact micromolar values of internal Ca2+ were not effective in the channel activation, except at high depolarizing membrane potentials. The activity was highly increased only when the channel was exposed to relatively high internal Ca2+ concentrations (0.2-2.0 mM).

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.