Mitogens and oncogenes can block the induction of specific voltage-gated ion channels

Myocyte Response to Stimulation of Receptors and Ion Channels

Ventricular myocytes dissociated from adult rat heart and cultured chick embryo ventricular cells were utilized to examine mechanisms by which neurotransmitters, hormones, and ontogeny modulate expression and function of β-adrenergic receptors and L-type calcium channels. Either freshly dissociated cells or cultured cells were studied by an optical-video system to characterize contractility and, in some instances, by a microspectrofluorimeter to determine [Ca2+]i as reported by fura 2. Ligand binding studies in intact cells and membranes were conducted with receptor and ion channel antagonists and agonists. Exposure of intact cells to isoproterenol produced contractile de-sensitization, loss of high affinity receptors from the sarcolemma and closely coupled decline in hormone-sensitive adenylate cyclase activity. Desensitization was by a microfilament-dependent process. Down-regulation depended upon microtubular function. During development of the chick heart, there was an increase in number of dihydropyridine binding sites, taken as a measure of number of L-type calcium channels, at a time when sensitivity to [Ca2+]o and to Bay k 8644 declined. Thyroid hormone was capable of up-regulating L-type calcium channels. Prolonged exposure to a β-adrenergic agonist produced coordinate down-regulation of β-receptors and calcium channels. Down-regulation was a cAMP-dependent process. Thus, the β-adrenergic receptor and a distal component of the effector-response coupling system, the L-type calcium channel, can be regulated independently and in concert by physiologically and pathophysiologically important mechanisms.

Identification of acid-sensing ion channels in adenoid cystic carcinomas

Tissue acidosis is an important feature of tumor. The response of adenoid cystic carcinoma (ACC) cells to acidic solution was studied using whole-cell patch-clamp recording in the current study. An inward, amiloride-sensitive Na(+) current was identified in cultured ACC-2 cells while not in normal human salivary gland epithelial cells. Electrophysiological and pharmacological properties of the currents suggest that heteromeric acid-sensing ion channels (ASICs) containing 2a and 3 may be responsible for the proton-induced currents in the majority of ACC-2 cells. Consistent with it, analyses of RT-PCR and Western blotting demonstrated the presences of ASIC2a and 3 in ACC-2 cells. Furthermore, we observed the enhanced expression of ASIC2a and 3 in the sample of ACC tissues. These results indicate that the functional expression of ASICs is characteristic feature of ACC cells.

Predominant expression of Kv1.3 voltage-gated K+ channel subunit in rat prostate cancer cell lines: electrophysiological, pharmacological and molecular characterisation

Voltage-gated K+ currents expressed in two rat prostate cancer ("Dunning") cell lines of markedly different metastatic ability were characterised using electrophysiological, pharmacological and molecular approaches. Whole-cell patch-clamp recordings showed that both strongly metastatic MAT-LyLu and weakly metastatic AT-2 cell lines possessed outward (delayed-rectifier type) K+ currents, which activated at around -40 mV. From the parameters measured, several characteristics of the two cell lines were similar. However, a number of statistically significant differences were noted for MAT-LyLu versus the AT-2 cells as follows: (1) current densities were smaller; (2) the slope factor for channel activation was smaller; (3) the voltage at which current was half-inactivated, and the slope factor for channel inactivation were greater; (4) the time constants for current decay at -20 and 0 mV were smaller; and (5) the residual peak current was larger following 60 s of repetitive voltage pulses for stimulation frequencies in the range 0.05-0.2 Hz. On the other hand, the K+ currents in both cell lines showed similar pharmacological profiles. Thus, the currents were blocked by 4-aminopyridine, tetraethylammonium, verapamil, margatoxin, and charybdotoxin, with highly similar IC(50)s for given blockers. The electrophysiological and pharmacological data taken together suggested expression of voltage-gated K+ channels of the Kv1 family, expression of the Kv1.3 subunit being predominant. Western blot and RT-PCR tests both confirmed that the cells indeed expressed Kv1.3 and to a lesser extent Kv1.4 and Kv1.6 channel alpha-subunits. In view of the similarity of channel expression in the two cell lines, voltage-gated K+ channel activity may not be a primary determinant of metastatic potential in the rat model of prostate cancer, but the possible contribution of K+ channel activity to the metastatic process is discussed.

Hyperpolarization, but not depolarization, increases intracellular Ca(2+) level in cultured chick myoblasts

Ca(2+) influx appears to be important for triggering myoblast fusion. It remains, however, unclear how Ca(2+) influx rises prior to myoblast fusion. The present study examines a possible involvement of the voltage-dependent Ca(2+) influx pathways. Treatment with the L-type Ca(2+) channel blockers, diltiazem, and nifedipine did not alter cytosolic Ca(2+) levels. Depolarization with high K(+) solution and activation of Ca(2+) channel with Bay K 8644, and agonist of voltage dependent Ca(2+) channels, failed to elicit increases intracellular Ca(2+) level, indicating the absence of depolarization-operated mechanisms. In contrast, phloretin, an agonist of Ca(2+)-activated potassium (K(Ca)) channels, was able to hyperpolarize membrane potential and promoted Ca(2+) influx. These effects were completely abolished by treatment of charybdotoxin, a specific inhibitor of K(Ca) channels. In addition, gadolinium, a potent stretch-activated channel (SAC) blocker, prevented the phloretin-mediated Ca(2+) increase, indicating the involvement of SACs in Ca(2+) influx. Furthermore, phloretin stimulated precocious myoblast fusion and this effect was blocked with gadolinium or charybdotoxin. Taken together, these results suggest that induced hyperpolarization, but not depolarization increases Ca(2+) influx through stretch-activated channels, and in turn triggers myoblast fusion.

Effects of voltage-gated ion channel modulators on rat prostatic cancer cell proliferation: comparison of strongly and weakly metastatic cell lines

BACKGROUND

The strongly metastatic MAT-LyLu and the weakly metastatic AT-2 rat prostatic cancer cell lines have been shown to express voltage-gated ion channels differentially. In the present study, the possible contribution of voltage-gated ion channel activity to the proliferation of these cell lines was investigated, in a comparative approach.

METHODS

Several voltage-gated ion channel modulators were tested for their effects on proliferation over 54 hr, using an in vitro assay. The modes of action of the chemicals were monitored by electrophysiological (patch-clamp) recording.

RESULTS

The voltage-gated K(+) channel blockers 4-aminopyridine (4-AP; 2 mM), margatoxin (5 nM), charybdotoxin (4.5 nM), and verapamil (50 microM) inhibited the K(+) channels of both cell lines by between 38-65% and reduced the proliferation of the AT-2 cell line, in a dose-dependent manner, by 8-51%. However, only 4-AP reduced proliferation of the MAT-LyLu cell line. Tetrodotoxin (6 microM) blocked completely the voltage-gated Na(+) channel expressed selectively in the MAT-LyLu cell line, but had no effect on the proliferation of either cell line. On the other hand, the presumed Na(+) channel "opener" veratridine (10-50 microM) reduced significantly, in a dose-dependent manner, the proliferation of both cell lines by up to approximately 30%.

CONCLUSIONS

We conclude that the mechanism(s) controlling the proliferation of the weakly metastatic AT-2 cells involves voltage-gated K(+) channels. In contrast, the proliferation of strongly metastatic MAT-LyLu cells is much less dependent upon voltage-gated K(+) channel activity.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Role of hyperpolarization attained by linoleic acid in chick myoblast fusion

Our previous report has suggested that hyperpolarization generated by reciprocal activation of calcium-activated potassium (K(Ca)) channels and stretch-activated channels induces calcium influx that triggers myoblast fusion. Here we show that linoleic acid is involved in the process of generating hyperpolarization in cultured chick myoblasts and hence in promotion of the cell fusion. Linoleic acid dramatically hyperpolarized the membrane potential from -14 +/- 3 to -58 +/- 5 mV within 10 min. This effect was partially blocked by 1 mM tetraethylammonium (TEA) or 30 nM charybdotoxin, a selective K(Ca) channel inhibitor, and completely abolished by 10 mM TEA. Single-channel recordings revealed that linoleic acid activates TEA-resistant potassium channels as well as K(Ca) channels. Furthermore, linoleic acid induced calcium influx from extracellular solution, and this effect was partially blocked by 1 mM TEA and completely prevented at 10 mM, similar to the effect of TEA on linoleic acid-mediated hyperpolarization. Since the valinomycin-mediated hyperpolarization promoted calcium influx, hyperpolarization itself appears capable of inducing calcium influx. In addition, gadolinium prevented the valinomycin-mediated increase in intracellular calcium level under hypotonic conditions, revealing the involvement of stretch-activated channels in calcium influx. Furthermore, linoleic acid stimulated myoblast fusion, and this stimulatory effect could completely be prevented by 10 mM TEA. These results suggest that linoleic acid induces hyperpolarization of membrane potential by activation of potassium channels, which induces calcium influx through stretch-activated channels, and thereby triggers myoblast fusion.

Malignant human gliomas express an amiloride-sensitive Na+ conductance

Human astrocytoma cells were studied using whole cell patch-clamp recording. An inward, amiloride-sensitive Na+ current was identified in four continuous cell lines originally derived from human glioblastoma cells (CH235, CRT, SKMG-1, and U251-MG) and in three primary cultures of cells obtained from glioblastoma multiforme tumors (up to 4 passages). In addition, cells freshly isolated from a resected medulloblastoma tumor displayed this same characteristic inward current. In contrast, amiloride-sensitive currents were not observed in normal human astrocytes, low-grade astrocytomas, or juvenile pilocytic astrocytomas. The only amiloride-sensitive Na+ channels thus far molecularly identified in brain are the brain Na+ channels (BNaCs). RT-PCR analyses demonstrated the presence of mRNA for either BNaC1 or BNaC2 in these tumors and in normal astrocytes. These results indicate that the functional expression of amiloride-sensitive Na+ currents is a characteristic feature of malignant brain tumor cells and that this pathway may be a potentially useful target for therapeutic intervention.

1,25 (OH)2D3 enhances PTH-induced Ca2+ transients in preosteoblasts by activating L-type Ca2+ channels

We previously demonstrated electrophysiologically that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] shifts the activation threshold of L-type Ca2+ channels in osteoblasts toward the resting potential and prolongs mean open time. Presently, we used single-cell Ca2+ imaging to study the combined effects of 1,25(OH)2D3 and parathyroid hormone (PTH) during generation of Ca2+ transients in fura 2-loaded MC3T3-E1 cells. Pretreatment with 1,25(OH)2D3 concentrations, which alone did not produce Ca2+ transients, consistently enhanced Ca2+ responses to PTH. Enhancement was dose dependent over the range of 1 to 10 nM and was blocked by pretreatment with 5 microM nitrendipine during pretreatment. A 1,25(OH)2D3 analog that activates L-type channels and shifts their activation threshold also enhanced PTH responses. In contrast, an analog devoid of membrane Ca2+ effects did not enhance PTH-induced Ca2+ transients. The PTH-induced Ca2+ transient involved activation of a dihydropyridine-insensitive cation channel that was inhibited by Gd3+. Together, these data suggest that 1,25(OH)2D3 increases osteoblast responsiveness to PTH through rapid modification of L-type Ca2+ channel gating properties, whose activation enhances Ca2+ entry through other channels such as the PTH-responsive, Gd(3+)-sensitive cation channel.

Coordinated incorporation of skeletal muscle dihydropyridine receptors and ryanodine receptors in peripheral couplings of BC3H1 cells

Rapid release of calcium from the sarcoplasmic reticulum (SR) of skeletal muscle fibers during excitation-contraction (e-c) coupling is initiated by the interaction of surface membrane calcium channels (dihydropyridine receptors; DHPRs) with the calcium release channels of the SR (ryanodine receptors; RyRs, or feet). We studied the early differentiation of calcium release units, which mediate this interaction, in BC3H1 cells. Immunofluorescence labelings of differentiating myocytes with antibodies against alpha1 and alpha2 subunits of DHPRs, RyRs, and triadin show that the skeletal isoforms of all four proteins are abundantly expressed upon differentiation, they appear concomitantly, and they are colocalized. The transverse tubular system is poorly organized, and thus clusters of e-c coupling proteins are predominantly located at the cell periphery. Freeze fracture analysis of the surface membrane reveals tetrads of large intramembrane particles, arranged in orderly arrays. These appear concomitantly with arrays of feet (RyRs) and with the appearance of DHPR/RyS clusters, confirming that the four components of the tetrads correspond to skeletal muscle DHPRs. The arrangement of tetrads and feet in developing junctions indicates that incorporation of DHPRs in junctional domains of the surface membrane proceeds gradually and is highly coordinated with the formation of RyR arrays. Within the arrays, tetrads are positioned at a spacing of twice the distance between the feet. The incorporation of individual DHPRs into tetrads occurs exclusively at positions corresponding to alternate feet, suggesting that the assembly of RyR arrays not only guides the assembly of tetrads but also determines their characteristic spacing in the junction.

Hypotonicity and thrombin activate taurine efflux in BC3H1 and C2C12 myoblasts that is down regulated during differentiation

The efflux of organic osmolytes such as taurine is an important mechanism by which cells regulate their volume. The effects of hypotonicity and thrombin on taurine efflux were studied in BC3H1 and C2C12 cells, two mouse myoblastic cell lines that can be induced to differentiate with serum deprivation. In proliferating cultures of both cell types preloaded with [3H]taurine, exposure to 27% hypotonicity activated a 10- to 20-fold increase in [3H]taurine efflux (Jtau). This effect was blocked by the C1- channel inhibitors NPPB and flufenamic acid. Thrombin and the thrombin receptor agonist SFLLRN also activated Jtau that was abolished by NPPB and flufenamic acid. Together, hypotonicity and thrombin synergistically activated Jtau. In differentiated myocytes, the effect of thrombin was abolished, while that of hypotonicity was significantly reduced. These results suggest that (i) hypotonicity and thrombin activate taurine-permeable anion channels in BC3H1 and C2C12 cells, and (ii) these anion channels may be involved in cell proliferation.

A possible role of inwardly rectifying K+ channels in chick myoblast differentiation

We examined the developmental change of inwardly rectifying K+ channels (IRK) and its possible role in myogenesis. Northern blot analysis revealed an increase in the level of IRK mRNA during myogenesis. Accordingly, IRK current was not detectable in replicating myoblasts but first appeared in aligned myoblasts that were competent for fusion and gradually increased thereafter. The time course change of IRK activity was closely related to the increase in resting membrane potential during myogenesis. Application of 0.5 mM Ba2+ to the bath depolarized the membrane and blocked IRK currents dramatically but not outwardly rectifying K+ currents. Myoblasts devoid of IRK had low resting K+ permeability, whereas myotubes that possess IRK had high resting K+ permeability. In some aligned myoblasts, anomalous hyperpolarization was elicited by increasing extracellular K+ concentration, which may be attributable to the increased conductance of IRK. Noteworthy was the fact that maximal fusion was obtained at this range of K+ concentration. These findings imply that IRK is responsible for the change in the K+ permeability during chick myogenesis, which may provide a larger driving force for Ca2+ influx that is a prerequisite for myoblast fusion.

Downregulation of volume-activated Cl- currents during muscle differentiation

We have used the whole cell configuration of the patch-clamp technique to investigate volume-activated Cl- currents in BC3H1 and C2C12 cells, two mouse muscle cell lines that can be switched from a proliferating to a differentiated musclelike state. Reducing the extracellular osmolality by 40% evoked large Cl- currents in proliferating BC3H1 and C2C12 cells. These currents were outwardly rectifying and had an anion permeability sequence as follows: I- > Br- > Cl- >> gluconate. They were inhibited by >50% by flufenamic acid (500 microM), niflumic acid (500 microM), and 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) but were relatively insensitive to tamoxifen (100 microM). A reduction in the serum concentration in the culture medium induced growth arrest in both cell lines, and the cells started to differentiate into spindle-shaped nonfusing muscle cells (BC3H1) or myotubes (C2C12). This differentiation was accompanied by a drastic decrease in the magnitude of the volume-activated Cl- currents. The close correlation between volume-activated Cl- currents and cell proliferation suggests that these currents may be involved in cell proliferation.

Down-regulation of L-type Ca2+ channel transcript levels by 1,25-dihyroxyvitamin D3. Osteoblastic cells express L-type alpha1C Ca2+ channel isoforms

Osteoblast Ca2+ channels play a fundamental role in controlling intracellular and systemic Ca2+ homeostasis. A reverse transcription-polymerase chain reaction strategy was used to determine the molecular identity of voltage-sensitive calcium channels present in ROS 17/2.8 osteosarcoma cells. The amino acid sequences encoded by the two resultant PCR products matched the alpha1C-a and the alpha1C-d isoforms. The ability of 1, 25-dihydroxyvitamin D3 (1,25(OH)2D3) and structural analogs to modulate expression of voltage-sensitive calcium channel mRNA transcripts was then investigated. ROS 17/2.8 cells were cultured for 48 h in the presence of either 1,25(OH)2D3,1,24-dihydroxy-22-ene-24-cyclopropyl D3 (analog BT) or 25-hydroxy-16-ene-23-yne-D3 (analog AT), and the levels of mRNA encoding alpha1C were quantitated using a competitive reverse transcription-polymerase chain reaction assay. We found that 1, 25(OH)2D3 and analog BT reduced steady state levels of alpha1C mRNA. Conversely, the Ca2+-mobilizing analog AT did not alter steady state levels of voltage-sensitive calcium channel mRNA. Since analog BT, but not analog AT, binds and transcriptionally activates the nuclear receptor for 1,25(OH)2D3, these findings suggest that the down-regulation of voltage-sensitive calcium channel mRNA levels may involve the nuclear receptor.

Myoblast fusion requires cytosolic calcium elevation but not activation of voltage-dependent calcium channels

Many studies of in vitro skeletal myogenesis have demonstrated that fusion of myoblasts into multinucleated myotubes is regulated by calcium-dependent processes. Calcium ions appear to be necessary at the outer face of the membrane, and an additional internal calcium increase seems required to promote fusion of aligned myoblasts. It has been proposed that a calcium influx could take place prior to fusion and that this may be mediated by voltage-dependent calcium channels. Previously, we showed that two types of voltage-dependent calcium currents were expressed in multinucleated myotubes but not in rat myoblasts growing in primary culture before the withdrawal of the growth medium. We also showed that the previous formation of multinucleated synticia was not a prerequisite of developmental appearance of calcium currents, suggesting that the two events were time-correlated but not sequentially dependent. These features led us to investigate changes in internal calcium activity and the possible appearance of voltage-dependent calcium influx pathways just after the promotion of fusion by the change of culture medium. The results confirm that a rise in cytosolic calcium activity occurs slightly before fusion in confluent myoblasts and remained in newly formed myotubes. Reducing this elevation by internal calcium buffering lowered myoblast fusion and, reciprocally, blocking cell fusion prevented calcium increase. Treatment with the organic calcium channel blockers nifedipine (5 microM) and PN 200-110 (1 microM) did not alter cytosolic calcium changes nor cell fusion, and voltage-dependent calcium currents were never observed by the perforated patch-clamp technique in aligned fusion-competent myoblasts. Other voltage-operated mechanisms of calcium rise were not detected since depolarization with hyperpotassium solutions failed to elicit increases in intracellular calcium. On the contrary, acetylcholine was able to promote extracellular calcium-dependent calcium transients. Our results confirm the requirement of an increase in resting calcium during fusion, but do not support the hypothesis of an influx through voltage-dependent channels or other voltage-operated pathways. The elevation of internal calcium activity may result from other mechanisms, such as a cholinergic action for example.

Enhanced dihydropyridine receptor channel activity in the presence of ryanodine receptor

Excitation-contraction coupling in skeletal muscle involves a voltage sensor in the plasma membrane which, in response to depolarization, causes an intracellular calcium-release channel to open. The skeletal isoform of the ryanodine receptor (RyR-1) functions as the Ca2+-release channel and the dihydropyridine receptor (DHPR) functions as the voltage sensor and also as an L-type Ca2+ channel. Here we examine the possibility that there is a retrograde signal from RyR-1 to the DHPR, using myotubes from mice homozygous for a disrupted RyR-1 gene (dyspedic mice). As expected, we find that there is no excitation-contraction coupling in dyspedic myotubes, but we also find that they have a roughly 30-fold reduction in L-type Ca2+-current density. Injection of dyspedic myotubes with RyR-1 complementary DNA restores excitation-contraction coupling and causes the density of L-type Ca2+ current to rise towards normal. Despite the differences in Ca2+-current magnitude, measurements of charge movement indicate that the density of DHPRs is similar in dyspedic and RyR-1-expressing myotubes. Our results support the possibility of a retrograde signal by which RyR-1 enhances the function of DHPRs as Ca2+ channels.

Conditional differentiation of heart- and smooth muscle-derived cells transformed by a temperature-sensitive mutant of SV40 T antigen

To create muscle cell lines that conditionally differentiate in vitro we introduced a temperature-sensitive SV40 T antigen by retroviral infection into rat aortic smooth muscle cells (SMCs) and neonatal heart-derived cells. After G418 selection cell lines isolated were characterized at permissive (33 degrees C) and non-permissive (39 degrees C) temperatures. [3H]Thymidine uptake showed tht progression through the cell cycle is greatly reduced at 39 degrees C. Cytoskeletal proteins, such as actins and vimentin did not change significantly after temperature shift, while the number of desmin-positive SMCs significantly increased when cells were switched to 39 degrees C. Heart-derived muscle cells showed sarcomeric myosin heavy chain reactivity only when grown at 39 degrees C. After thrombin stimulation intracellular calcium in both cell types increased severalfold in 39 degrees C-cells but not in 33 degrees C-cells. Whole cell patch-clamp recordings of SMCs and heart-derived cells revealed a strong increase in nicardipine-sensitive Ca2+ current when cells were switched to 39 degrees C. Nicardipine-insensitive Ca2+ current also increased in both cell types at the non-permissive temperature. Na+ current in SMCs was large at 33 degrees C and small or not detectable at 39 degrees C and absent in heart-derived cells. Using a cDNA probe specific for the alpha 1 subunit of the dihydropyridine-sensitive Ca2+ channel we demonstrate a temperature-sensitive expression of the dihydropyridine receptor mRNA in smooth muscle-derived cells but not in heart-derived H10 cells. Our results suggest that upon downregulation of SV40 T antigen these cells become quiescent and exhibit a more differentiated phenotype. These cell lines may provide a useful tool to investigate ion channel- and receptor signal transduction, as well as cell cycle control in smooth and possibly cardiac muscle cell differentiation.

The Lipidosterolic Extract fromSerenoa repens Interferes with Prolactin Receptor Signal Transduction

The lipidosterolic extract from the saw palmetto Serenoa repens (LSESr) is commonly used for medical treatment of benign prostatic hypertrophia due to its ability to inhibit 5alpha-reductase which permits the conversion of testosterone to dihydrotestosterone, the active androgen on prostate cell proliferation. However, the complete action mechanism of LSESr is still unknown. Several lines of evidence suggest that, in addition to inhibition of 5alpha-reductase, it may interfere with the action of prolactin (PRL). We therefore investigated a possible interference of this plant extract with another hormone that controls prostate gland growth, PRL. As the action mechanism of PRL is now fully documented in Chinese hamster ovary cells expressing the PRL receptor, we have conducted our experiments on these cells. In this study, using electrophysiological (whole-cell recording and single-channel recording), microspectrofluorimetric and biochemical techniques, we show that LSESr (1-30 &mgr;g/ml) reduced the basal activity of a K(+) channel and of protein kinase C (PKC) in CHO cells. In addition, pretreatment of the cells with 1-10 &mgr;g/ml LSESr for 6-36 h abolished the effects of PRL on [Ca(2+)](i), K(+) conductance and PKC. LSESr may block PRL-induced prostate growth by inhibiting several steps of PRL receptor signal transduction. LSESr may also be useful for diseases implicating PRL. Copyright 1995 S. Karger AG, Basel

Differential expression of voltage-activated Na+ currents in two prostatic tumour cell lines: contribution to invasiveness in vitro

The voltage-gated ionic currents of two rodent prostatic cancer cell lines were investigated using the whole-cell patch clamp technique. The highly metastatic Mat-Ly-Lu cells expressed a transient, inward Na+ current (blocked by 600 nM tetrodotoxin), which was not found in any of the weakly metastatic AT-2 cells. Although both cell lines expressed a sustained, outward K+ current, this occurred at a significantly higher density in the AT-2 than in the Mat-Ly-Lu cells. Incubation of the Mat-Ly-Lu cell line with 600 nM tetrodotoxin significantly reduced the invasive capacity of the cells in vitro. Under identical conditions, tetrodotoxin had no effect on the invasiveness of the AT-2 cells.

Differential expression of sodium channels and nicotinic acetylcholine receptor channels in nnr variants of the PC12 pheochromocytoma cell line

An important component of neuronal differentiation is the tightly controlled expression of a spectrum of ion channel proteins. Ion channels play a critical role in the generation and propagation of action potentials as well as in the cellular response to neurotransmitters, and thus are central in the transfer and integration of information in the nervous system. A model system amenable to analysis of ion channel expression and neuronal differentiation is the rat pheochromocytoma (PC12) cell line. Here, we have used electrophysiological and molecular biological approaches to analyze the expression of voltage-dependent sodium (Na) channels and nicotinic acetylcholine receptors (nAChR) in mutagenized variants (nnr cells) of the PC12 cell line. Our data reveal striking differences in the expression of these channels when compared to wild-type PC12 cells. Even in the absence of nerve growth factor (NGF), nnr cells express functional Na channels and Na channel mRNA at levels exceeding those in wild-type PC12 cells differentiated with NGF. In contrast, acetylcholine-induced currents were evident in only a small proportion of cells, presumably due to the altered pattern of expression of mRNAs encoding individual nAChR subunits. The altered ion channel expression in these variants provides an avenue for analyzing Na channel and nAChR channel function, as well as for identifying mechanisms governing their expression.

Single calcium channel behavior in native skeletal muscle

The purpose of this study was to use whole-cell and cell-attached patches of cultured skeletal muscle myotubes to study the macroscopic and unitary behavior of voltage-dependent calcium channels under similar conditions. With 110 mM BaCl2 as the charge carrier, two types of calcium channels with markedly different single-channel and macroscopic properties were found. One class was DHP-insensitive, had a single-channel conductance of approximately 9 pS, yielded ensembles that displayed an activation threshold near -40 mV, and activated and inactivated rapidly in a voltage-dependent manner (T current). The second class could only be well resolved in the presence of the DHP agonist Bay K 8644 (5 microM) and had a single-channel conductance of approximately 14 pS (L current). The 14-pS channel produced ensembles exhibiting a threshold of approximately -10 mV that activated slowly (tau act approximately 20 ms) and displayed little inactivation. Moreover, the DHP antagonist, (+)-PN 200-110 (10 microM), greatly increased the percentage of null sweeps seen with the 14-pS channel. The open probability versus voltage relationship of the 14-pS channel was fitted by a Boltzmann distribution with a VP0.5 = 6.2 mV and kp = 5.3 mV. L current recorded from whole-cell experiments in the presence of 110 mM BaCl2 + 5 microM Bay K 8644 displayed similar time- and voltage-dependent properties as ensembles of the 14-pS channel. Thus, these data are the first comparison under similar conditions of the single-channel and macroscopic properties of T current and L current in native skeletal muscle, and identify the 9- and 14-pS channels as the single-channel correlates of T current and L current, respectively.

Renal cells transformed with SV40 contain a high-conductance calcium-insensitive potassium channel

The inside-out variant of the patch-clamp technique was used to investigate the properties of a K+ channel occurring in 20% of patches from two renal cell lines transformed with the wild-type simian virus 40 (SV40) (Vandewalle et al. J. Cell. Physiol. 141: 203-221, 1989). This channel was practically absent from the primary cultures of renal cortical cells from which the cell lines were originally derived. With identical K(+)-rich solutions on both sides of the membrane patch, the channel showed an inwardly rectifying current-voltage relationship with unit conductances of 151.8 +/- 4.8 pS at negative and 86.4 +/- 5.9 pS at positive voltages (n = 18). When K+ in the bath was replaced by Na+, a mean reversal potential of 57.0 +/- 5.2 mV (n = 6) was observed from which a K(+)-to-Na+ permeability ratio of 13 was calculated. The channel was insensitive to internal Ca2+ and was blocked by internal Ba2+. No clear dependence on voltage was apparent. This channel bears no resemblance to any epithelial K+ channel and may be a novel type of K+ channel. Its occurrence in two transformed cell lines with quite distinct phenotypes, one of proximal cells (RC.SV1) and the other of thick ascending limb cells (RC.SV2), suggests that transformation by SV40 might be responsible for its appearance.

Kinetic properties of skeletal-muscle-like high-threshold calcium currents in a non-fusing muscle cell line

Macroscopic kinetics and single-channel properties of skeletal-muscle-type calcium currents were studied in the non-fusing, clonal muscle cell line, BC3H1. Slowly activating, dihydropyridine(DHP)-sensitive currents, associated with T-tubular DHP receptors and ion channels, could be isolated from rapidly activating, DHP-resistant currents. Description of macroscopic current activation kinetics required only a brief delay term (tau o <1 ms), two ascending exponential terms with voltage-dependent time constants (2 < tau 1 > 20 ms and 10 < tau 2 < 200 ms), and a single exponential decay term (0.5 < tau 3 < or = 5 s). Steady-state activation voltage dependence required description by two Boltzmann distribution terms with V 1/2 and slope factors differing by 20 mV and 3.5- to 4-fold respectively. These two distributions were correlated with the steady-state voltage dependence of the two ascending kinetic terms described by tau 1 and tau 2 respectively. Rundown of the DHP-sensitive slow current was correlated with a negative shift in the voltage dependence of current decay (tau 3). Three conductance levels (4.5 pS, 8 pS and 12 pS) were detected in single-channel records, two of which (the 8-pS and 12-pS events) were prolonged by BayK8644 and thus associated with DHP-sensitive single-channel events. Description of single-channel open time distributions required a minimum of two exponential terms (2.5 +/- 0.9 ms and 10.3 +/- 5.4 ms at -10 mV). Slow transitions among closed states results in biexponential latency-to-first-event distributions (47 +/- 12 ms and 470 +/- 123 ms at -10 mV).

Cardiac T-type calcium current: pharmacology and roles in cardiac tissues

A low threshold, voltage-gated calcium current is reported in most cardiac tissues but rarely in ventricular cells. This article reports some recently described characteristics and discusses their possible pathophysiologic implications. It also reviews the alterations induced in this current by a variety of chemical agents including several neuromediators in cardiac and other tissues.

Evidence for an external location of the dihydropyridine agonist receptor site on smooth muscle and skeletal muscle calcium channels

1. The location of the binding domain for agonist dihydropyridines (DHP) has been studied by comparing the action of (+)-202,791 and (-)-Bay K 8644 on Ba2+ currents (IBa) in whole cell patch clamp experiments. Drug effects were examined upon internal and external (extracellular) application in A7r5 smooth muscle cells and BC3H1 cells, a cell line expressing Ca channels of the skeletal muscle type. 2. Efficiency of internal drug application in the whole cell studies was demonstrated by inhibition of potassium currents and barium currents (IBa) upon internal perfusion with tetraethylammonium (TEA+) (10 mM) and the permanently charged phenylalkylamine, D 890 (100 microM) respectively. The uncharged DHP, (-)-STBODIPY-DHP (2 microM) was used to estimate the time course of internal perfusion by monitoring its fluorescence. 3. Intracellular application of (+)-202,791 and (-)-Bay K 8644 (5 microM) in patch clamp experiments was ineffective in stimulating Ca2+ channel currents in both cell lines. In contrast a 50 fold lower agonist concentration (0.1 microM (-)-Bay K 8644) applied to the external face of the membrane induced typical changes in tail currents and a current increase under conditions when up to 10 microM of the agonist was present in the intracellular perfusion solution. 4. In cell-attached patches in A7r5 cells, (-)-Bay K 8644 increased and (+)-PN 200,110 inhibited single channel activity when applied via the bath solution. This suggests partitioning and lateral diffusion of the DHPs in the lipid of the plasma membrane. 5. We conclude that the binding site for agonist DHPs on Ca2+ channels in A7r5 and BC3H1 cells is located close to the external surface of the membrane. The DHP binding domain can be reached by agonists and antagonists from the extracellular but not from the intracellular face of the membrane.

Ionic channel currents in cultured neurons from human cortex

Ionic channels in human cortical neurons have not been studied extensively. HCN-1 and HCN-1A cells, which recently were established as continuous cultures from human cortical tissue, have been shown by histochemical and immunochemical methods to exhibit a neuronal phenotype, but expression of functional ionic channels was not demonstrated. For the present study, HCN-1 and HCN-1A cells were cultured in Dulbecco's modified Eagle's medium with 15% fetal calf serum, in some cases supplemented with 10 ng/ml nerve growth factor, 10 microM forskolin, and 1 mM dibutyryl cyclic adenosine monophosphate to promote differentiation. Cells or membrane patches were voltage clamped using conventional patch clamp techniques. In HCN-1A cells, we identified a tetrodotoxin-sensitive Na+ current, two types of Ca2+ channel current, including L-type current and a second type that in some respects resembled N-type current, and four types of K+ current, including a delayed outward rectifier that showed voltage-dependent inactivation, two types of noninactivating Ca(2+)-activated K+ channels with slope conductances of 146 and 23 pS (K+i/K+o 145 mM/5 mM), and less frequently, a noninactivating, intermediate conductance channel that was not sensitive to internal Ca2+. When HCN-1A cells were examined after 3 days of exposure to differentiating agents, pronounced morphological changes were evident but no differences in ionic currents were apparent. HCN-1 cells also exhibited K+ and Ca2+ channel currents, but Na+ currents were not detected in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of voltage-gated Ca(2+)-currents in cultivated aortic myocytes

The expression of different types of Ca(2+)-channels was studied using the whole-cell patch-clamp technique in cultured rat aortic smooth-muscle myocytes. Ca(2+)-currents were identified as either low- or high voltage-activated (ICa,LVA or ICa,HVA, respectively) based on their distinct voltage-dependences of activation and inactivation, decay kinetics using Ba2+ as the charge carrier and sensitivity to dihydropyridines. The heterogeneity in the functional expression of the two types of Ca(2+)-channels in the cultured myocytes delineated four distinct phenotypes; (i), cells exhibiting only LVA currents; (ii), cells exhibiting only HVA currents; (iii), cells exhibiting both LVA and HVA currents and (iv), cells exhibiting no current. The myocytes exclusively expressed HVA currents both during the first five days in primary culture and after the cells had reached confluence (> 15 days). In contrast, LVA currents were expressed transiently between 5 and 15 days, during which time the cells were proliferating and had transient loss of contractility. Thus, both LVA and HVA Ca(2+)-current types contribute to Ca(2+)-signalling in cultured rat aortic myocytes. However, the differential expression of the two Ca2+ current types associated with differences in contractile and proliferative phenotypes suggest that they serve distinct cellular functions. Our results are consistent with the idea that LVA current expression is important for cell proliferation.

Transformation of renal tubule epithelial cells by simian virus-40 is associated with emergence of Ca(2+)-insensitive K+ channels and altered mitogenic sensitivity to K+ channel blockers

We compared the pattern of K+ channels and the mitogenic sensitivity to K+ channel blocking agents in primary cultures of rabbit proximal tubule cells (PC.RC) (Ronco et al., 1990) and two derived SV-40-transformed cell lines exhibiting specific functions of proximal (RC.SV1) and more distal (RC.SV2) tubule cells (Vandewalle et al., 1989). First, K+ channel equipment surveyed by the patch-clamp technique was modified after SV-40 transformation in both cell lines; although a high conductance Ca(2+)-activated K+ channel [K+200 (Ca2+)] remained the most frequently recorded K+ channel, the transformed state was characterized by emergence of three Ca(2+)-insensitive K+ channels (150, 50, and 30 pS), virtually absent from primary culture, contrasting with reduced frequency of two Ca(2+)-sensitive K+ channels (80 and 40 pS). Second, quinine (Q), tetraethylammonium ion (TEA) and charybdotoxin (CTX), at concentrations not affecting cell viability, all decreased 3H-TdR incorporation and cell growth in PC.RC cultures, but only TEA had similar effects in transformed cells. The latter were further characterized by paradoxical effects of Q that induced a marked increase in thymidine incorporation. Q also exerted contrasting effects on channel activity: it inhibited the [K+200 (Ca2+)] when the channel was highly active, with a Ki (0.2 mM) similar to that measured for 3H-TdR incorporation in PC.RC cells (0.3 mM), but increased the mean current through poorly active channels. TEA blocked all K+ channels with conductance greater than or equal to 50 pS, including the [K+200 (Ca2+)], in a range of concentrations that substantially affected cell proliferation. The unique effect of TEA on SV-40-transformed cells might be related to broad inhibition of K+ channels.

Direct contact between sympathetic neurons and rat cardiac myocytes in vitro increases expression of functional calcium channels

To test the hypothesis that direct contact between sympathetic neurons and myocytes regulates expression and function of cardiac Ca channels, we prepared cultures of neonatal rat ventricular myocytes with and without sympathetic ganglia. Contractile properties of myocytes were assessed by an optical-video system. Contractility-pCa curves showed a 60% greater increase in contractility for innervated myocytes compared with control cells at 6.3 mM [Ca]0 (n = 8, P less than 0.05). Cells grown in medium conditioned by growth of ganglia and myocytes were indistinguishable physiologically from control cells. [Bay K 8644]-contractility curves revealed a 60 +/- 10% enhancement of the contractility response at 10(-6) M for innervated cells compared with control cells. The increased response to Bay K 8644 was not blocked by alpha- or beta-adrenergic antagonists. Moreover, increased efficacy of Bay K 8644 was maintained for at least 24 h after denervation produced by removal of ganglia from the culture. Dihydropyridine binding sites were assessed with the L channel-specific radioligand 3[H]PN200-110. PN200-110 binding sites were increased by innervation (51 +/- 5 to 108 +/- 20 fmol/mg protein, P less than 0.01), with no change in KD. Peak current-voltage curves were determined by whole-cell voltage clamp techniques for myocytes contacted by a neuron, control myocytes, and myocytes grown in conditioned medium. Current density of L-type Ca channels was significantly higher in innervated myocytes (10.5 +/- 0.4 pA/pF, n = 5) than in control myocytes (5.9 +/- 0.3 pA/pF, n = 8, P less than 0.01) or myocytes grown in conditioned medium (6.2 +/- 0.2 pA/pF, n = 10, P less than 0.01). Thus, physical contact between a sympathetic neuron and previously uninnervated neonatal rat ventricular myocytes increases expression of functional L-type calcium channels as judged by contractile responses to Ca0 and Bay K 8644, as well as by electrophysiological and radioligand binding properties.

Growth factors, growth factor response elements, and the cardiac phenotype

Fibroblast growth factors (FGF) and type beta-1 transforming growth factor (TGF beta 1) are pleiotropic regulatory peptides which are expressed in myocardium in a precise developmental and spatial program and are up-regulated, in the adult heart, by ischemia or a hemodynamic burden. The accumulation of trophic factors after aortic banding supports the hypothesis that autocrine or paracrine pathways might function to mediate, in part, the consequences of mechanical load. Our laboratory has demonstrated that cardiac muscle cells are targets for the action of peptide growth factors and, more specifically, that modulation of the cardiac phenotype by basic FGF (bFGF) and TGF beta 1 strongly resembles the induction of fetal cardiac genes--including skeletal alpha-actin (SkA), beta-myosin heavy chain, and atrial natriuretic factor--which are characteristic of pressure-overload hypertrophy. Unexpectedly, and despite effects like those of bFGF on five other cardiac genes, acidic FGF (aFGF) was found to repress, rather than stimulate, SkA transcription in neonatal cardiac muscle cells. The proximal 200 nucleotides of a heterologous SkA promoter were sufficient for basal tissue-specific transcription, for induction by bFGF, and for inhibition by aFGF. Thus, both positive and negative regulation by peptide growth factors can be localized to the proximal SkA promoter. Full promoter activity required each of three CC[A/T]6GG motifs similar to the serum response element (SRE) for activation of the c-fos proto-oncogene, as previously shown for SkA transcription in a skeletal muscle background. The most proximal SRE, SRE1, was sufficient in the absence of other SkA promoter sequences for efficient tissue-specific expression in cardiac myocytes (versus cardiac fibroblasts), and was stimulated by bFGF to the same extent as the full-length promoter and endogenous gene. Despite its ability to repress the SkA promoter, aFGF had no significant effect on SRE1. Both FGFs up-regulated the canonical fos SRE, to a comparable degree. Thus, SRE1 can discriminate between signals generated in cardiac myocytes by bFGF and aFGF. In cardiac myocyte extracts, two predominant proteins contact SRE1: serum response factor (SRF) and a second protein, F-ACT-1. Thus, serum response factor and F-ACT-1 are candidate trans-acting factors for basal transcription of the SkA gene in cardiac muscle cells and for induction of SkA by bFGF and, potentially, other trophic signals.

Potassium channels and regulation of proliferation of human melanoma cells

1. Ion channels and their possible relation to cell proliferation have been studied in a human melanoma cell line (IGR 1). Membrane currents were recorded by the patch-clamp technique using the cell-attached, cell-free and whole-cell mode. Cell growth was monitored by counting the number of cells at different days after seeding and [3H]thymidine incorporation. 2. A voltage-dependent 10 pS non-inactivating potassium channel (delayed rectifier) is the most commonly observed ion channel in this type of human cell. The channel is active at the normal resting potential and can be blocked by tetraethylammonium chloride (TEA) and also by a membrane-permeable cyclic adenosine monophosphate (8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, cyclic AMP). A second type of potassium channel shows properties similar to voltage-dependent A-type potassium channels with complete inactivation. 3. A voltage-independent, non-selective cation channel with a single-channel conductance of approximately 20 pS could be seen in only 8% of the patches. Its properties of modulation are still unknown. 4. The incidence of the 10 pS, non-inactivated potassium channel was maximal at the fourth day after seeding (in 89% of the patches) and was significantly reduced at the seventh day (in 35% of the patches). 5. [3H]thymidine incorporation is maximal at the third day after seeding and is reduced when cells are grown in the presence of TEA or cyclic AMP. This peak of maximal [3H]thymidine incorporation correlated with the incidence of non-inactivated potassium channels. 6. In the presence of TEA or cyclic AMP, growth of the cells is inhibited. We suppose that due to block of potassium channels, most of the melanoma cells are not able to enter the S-phase in the cell division cycle. 7. It is concluded that delayed rectifier potassium channels are involved in the control of melanoma cell proliferation. A similar finding has been reported for K+ channels in T-lymphocytes and human breast carcinoma cells. It is suggested that potassium channels may be involved in controlling the driving force for a calcium influx thereby interacting with Ca(2+)-dependent cell cycle control proteins.

Ryanodine receptor protein is expressed during differentiation in the muscle cell lines BC3H1 and C2C12

BC3H1 and C2C12, murine cell lines, were assessed as model systems for the expression of ryanodine receptor protein during myogenesis. The ryanodine receptor is a calcium release channel of the sarcoplasmic reticulum and a component of the triad junction, a structure which is essential to excitation-contraction coupling in mature striated muscle. BC3H1 and C2C12 cells do not express the ryanodine receptor at detectable levels in a proliferative, nondifferentiated state. The ryanodine receptor protein is expressed during differentiation in BC3H1 and C2C12 cells, becoming detectable within 24 hr of the onset of differentiation. In both cell lines the ryanodine receptor is assembled in oligomeric form and binds [3H]ryanodine with high affinity. Fusion is not required for expression of the ryanodine receptor in either BC3H1 or nonfusing C2C12 cells. The level of expression of the ryanodine receptor protein is modulated by incubation with the growth factors TGF-beta and bFGF in a manner similar to that of other muscle-specific proteins. These initial observations suggest that the BC3H1 and C2C12 cell lines provide a model system for further investigations of the expression and function of the ryanodine receptor during myogenic differentiation.

Tissue-specific expression of high-voltage-activated dihydropyridine-sensitive L-type calcium channels

The cloning of the cDNA for the alpha 1 subunit of L-type calcium channels revealed that at least two genes (CaCh1 and CaCh2) exist which give rise to several splice variants. The expression of mRNA for these alpha 1 subunits and the skeletal muscle alpha 2/delta, beta and gamma subunits was studied in rabbit tissues and BC3H1 cells. Nucleic-acid-hybridization studies showed that the mRNA of all subunits are expressed in skeletal muscle, brain, heart and aorta. However, the alpha 1-, beta- and gamma-specific transcripts had different sizes in these tissues. Smooth muscle and heart contain different splice variants of the CaCh2 gene. The alpha 1, beta and gamma mRNA are expressed together in differentiated but not in proliferating BC3H1 cells. A probe specific for the skeletal muscle alpha 2/delta subunit did not hybridize to poly(A)-rich RNA from BC3H1 cells. These results suggest that different splice variants of the genes for the alpha 1, beta and gamma subunits exist in tissues containing L-type calcium channels, and that their expression is regulated in a coordinate manner.

Acceleration of activation and inactivation by the beta subunit of the skeletal muscle calcium channel

The L-type voltage-dependent calcium channel is an important link in excitation-contraction coupling of muscle cells (reviewed in refs 2 and 3). The channel has two functional characteristics: calcium permeation and receptor sites for calcium antagonists. In skeletal muscle the channel is a complex of five subunits, alpha 1, alpha 2, beta, gamma and delta. Complementary DNAs to these subunits have been cloned and their amino-acid sequences deduced. The skeletal muscle alpha 1 subunit cDNA expressed in L cells manifests as specific calcium-ion permeation, as well as sensitivity to the three classes of organic calcium-channel blockers. We report here that coexpression of the alpha 1 subunit with other subunits results in significant changes in dihydropyridine binding and gating properties. The available number of drug receptor sites increases 10-fold with an alpha 1 beta combination, whereas the affinity of the dihydropyridine binding site remains unchanged. Also, the presence of the beta subunit accelerates activation and inactivation kinetics of the calcium-channel current.

The ryanodine receptor/junctional channel complex is regulated by growth factors in a myogenic cell line

The ryanodine receptor/junctional channel complex (JCC) forms the calcium release channel and foot structures of the sarcoplasmic reticulum. The JCC and the dihydropyridine (DHP) receptor in the transverse tubule are two of the major components involved in excitation-contraction (E-C) coupling in skeletal muscle. The DHP receptor is believed to serve as the voltage sensor in E-C coupling. Both the JCC and DHP receptor, as well as many skeletal muscle-specific contractile protein genes, are expressed in the BC3H1 muscle cell line. In the present study, we find that during differentiation of BC3H1 cells, induced by mitogen withdrawal, induction of the JCC and DHP receptor mRNAs is temporally similar to that of the skeletal muscle contractile protein genes alpha-tropomyosin and alpha-actin. Our data suggest that there is coordinate regulation of both the contractile protein genes (which have been studied in detail previously) and the genes encoding the calcium channels involved in E-C coupling. Induction of both calcium channels is accompanied by profound changes in BC3H1 cell morphology including the development of many components of mature skeletal muscle cells, despite lack of myoblast fusion. Visualized by electron microscopy, the JCC appears as "foot structures" located in the dyad junction between the plasmalemma and the sarcoplasmic reticulum of the BC3H1 cells. Development of foot structures is concomitant with JCC mRNA expression. Expression of the JCC and DHP receptor mRNAs and formation of the foot structures are inhibited specifically by fibroblast growth factor.

The functional psychoses--are oncogenes involved in their pathogenesis? Some speculations

Oncogenes are genes whose expression has been associated with malignant transformation of cells in tissue culture and with neoplastic changein vivo(Bishop, 1987). Much of the current understanding of their nature and action has stemmed from work, over the past 20 years, on tumour viruses (Temin, 1971; Rapp, 1983). One group of tumour viruses, the retroviruses, are unique in possessing an enzyme, reverse transcriptase, which transcribes to the cell DNA a copy of the viral RNA genome (Marks, 1987). After the discovery of viral oncogenes, such DNA copies were used as probes in hybridisation studies (Stehelinet al, 1976; Frankel & Fischinger, 1976). These probes, capable of annealing to complementary DNA sequences, revealed the existence of the latter in normal, unaffected cells (Willecke & Schäfer, 1984). These sequences, called cellular or proto-oncogenes, exist in a wide range of eukaryotic organisms, from yeast to man.

Changes in the mechanism of Ca2(+) mobilization during the differentiation of BC3H1 muscle cells

Ca2+ sequestration and release in BC3H1 muscle cells is strongly dependent on the stage of differentiation. In proliferating cells, more than 90% of the sequestered Ca2+ was Ins(1,4,5)P3-sensitive and 25% was caffeine-sensitive. In differentiated cells, the Ca2+ accumulation was 5-fold higher and was InsP3-insensitive, but about 60% of the sequestered Ca2+ was caffeine-sensitive. These changes were reversible upon addition of growth stimuli. Similarly, by measuring the intracellular Ca2+ concentration in single intact BC3H1 cells, it was found that the number of histamine-responsive cells decreased and the number of caffeine-responsive cells increased during muscle cell differentiation. These data indicate that the development of the muscle phenotype in BC3H1 myoblasts induces a major rearrangement of the mechanisms for Ca2+ mobilization.

A Ca2(+)-activated K+ current in ras-transformed fibroblasts is absent from nontransformed cells

Biochemical similarities between ras proteins and the GTP-binding proteins and correlation of ras-induced cell transformation with altered transmembrane cation fluxes indicate that ras proteins may act to modulate ion channel activity. To test this idea, whole cell, tight-seal, patch-clamp recording was used to compare macroscopic currents of ras-transformed fibroblasts with currents of their nontransformed counterparts. A prominent calcium-activated, voltage-independent potassium current was observed in 83-100% of cells from three separate fibroblast lines transformed by two different oncogenic ras alleles, whereas the same current was present at much smaller amplitudes in only 0-15% of nontransformed cells. The calcium-activated potassium current is blocked by charybdotoxin and by concentrations of tetraethylammonium above 1 mM, but it is insensitive to apamin. Both normal and ras-transformed cells have another calcium-activated current that is not potassium selective, and, consistent with other studies, normal cells display a voltage-activated calcium conductance. These results suggest that the mechanisms by which ras triggers or maintains cell transformation may involve alterations in the number or activity of certain ion channels, in particular, a type of calcium-activated potassium channel.

Early enhancement of calcium currents by H-ras oncoproteins injected into Hermissenda neurons

Influx of calcium through membrane channels is an important initial step in signal transduction of growth signals. Therefore, the effects of Ras protein injection on calcium currents across the soma membrane of an identified neuron of the snail Hermissenda were examined. With the use of these post-mitotic cells, a voltage-sensitive, inward calcium current was increased 10 to 20 minutes after Harvey-ras oncoproteins were injected. The effects of oncogenic Harvey ras p21 protein (v-Ras) occurred quickly and were sustained, whereas the effects of proto-oncogenic ras protein (c-Ras) were transient. This relative potency is consistent with the activities of these oncoproteins in stimulating cell proliferation. Thus, this calcium channel may be a target for Ras action.

Properties of a potassium-selective ion channel in human melanoma cells

Currents through ion channels were measured from cells of a human melanin-producing melanoma cell line (IRG 1) with the patch clamp technique. In these cells the most frequently observed channel is a potassium channel. The channel activates slowly at depolarizing voltage steps but does not inactivate. Single channel potassium currents can be measured in cell-attached patches at the resting potential of melanoma cells. The channel has a conductance of approximately 10 pS. As measured from the reversal potentials of single channel currents, the permeability ratio for sodium and potassium, PNa/PK, is between 0.03 and 0.04. Open probability is increased at positive potentials. Mean open times are prolonged at voltage steps to more positive potentials. Closed time histograms are fitted by two exponentials. The slow shut time is decreased at positive potentials. In whole cell measurements, cell conductance measured between -20 and + 70 mV was reduced by 10 mM tetraethylammonium chloride from 6.4 +/- 1.2 nS (n = 4) to 0.8 +/- 0.3 nS (n = 3). Application of isoproterenol decreases the probability of the channel being open without any change in the single channel conductance. A possible role of the 10 pS potassium channel in the growth of melanoma cells is discussed.

Differential regulation of skeletal alpha-actin transcription in cardiac muscle by two fibroblast growth factors

In cardiac muscle, acidic and basic fibroblast growth factors (aFGF and bFGF) regulate at least five genes in common (including alpha and beta myosin heavy chains, atrial natriuretic factor, and the sarcoplasmic reticulum calcium ATPase), provoking a generalized "fetal" phenotype similar to events in pressure-overload hypertrophy; however, aFGF and bFGF differentially control the striated alpha-actins. bFGF stimulates and aFGF inhibits skeletal alpha-actin transcripts associated with the embryonic heart, whereas cardiac alpha-actin mRNA is inhibited by aFGF but not bFGF. To elucidate mechanisms for these selective and discordant actions of aFGF and bFGF on cardiac muscle, chicken skeletal and cardiac alpha-actin promoter-driven reporter genes were introduced into neonatal rat cardiac myocytes by electroporation. Skeletal alpha-actin transcription was selectively stimulated by bFGF, whereas the cardiac alpha-actin promoter was unaffected. In contrast, aFGF suppressed both transfected alpha-actin genes. The differential regulation of skeletal alpha-actin transcription was equivalent with either purified or recombinant FGFs and was observed with 5' flanking sequences from either nucleotide -202 or -2000 to nucleotide -11. Positive and negative modulation of alpha-actin transcription by growth factors corresponded accurately to the endogenous genes in all permutations studied. These investigations provide a model for reciprocal control of gene transcription by aFGF vs. bFGF.

Mechanism of gating of T-type calcium channels

We have analyzed the gating kinetics of T-type Ca channels in 3T3 fibroblasts. Our results show that channel closing, inactivation, and recovery from inactivation each include a voltage-independent step which becomes rate limiting at extreme potentials. The data require a cyclic model with a minimum of two closed, one open, and two inactivated states. Such a model can produce good fits to our data even if the transitions between closed states are the only voltage-dependent steps in the activating pathway leading from closed to inactivated states. Our analysis suggests that the channel inactivation step, as well as the direct opening and closing transitions, are not intrinsically voltage sensitive. Single-channel recordings are consistent with this scheme. As expected, each channel produces a single burst per opening and then inactivates. Comparison of the kinetics of T-type Ca current in fibroblasts and neuronal cells reveals significant differences which suggest that different subtypes of T-type Ca channels are expressed differentially in a tissue specific manner.

Early regulation of membrane excitability by ras oncogene proteins

Two electrode voltage clamp conditions were used to study the early effects on ionic membrane channels of the intracellularly injected proto-oncogenic form of c-Ha-ras (c-ras) and its oncogenic counterpart v-Ha-ras (v-ras). These experiments were conducted on isolated somata of identified fully differentiated neurons of the sea snail Hermissenda. 20 min after c-ras, and 10 min after v-ras intracellular injections into type B medial photoreceptors of Hermissenda, the peak amplitude of two outward potassium currents (IA and IC), across the isolated Type B soma membrane begin to decrease. These two currents have been previously isolated by differences in activation and inactivation kinetics and their response to pharmacological blockers. c- or v-ras injections did not have any effect on a voltage-dependent inward calcium current. Reduction of IA preceded that of IC. Current reductions due to c-ras, but not to v-ras injection reversed spontaneously after 40 min. The voltage dependence of the steady state inactivation of IA shifted toward more negative potentials with ras injections. Ras-mediated cell transformations therefore, could involve, perhaps as initial events, prolonged modification of membrane currents.