The small GTPase RhoA regulates the expression and function of the sodium channel Nav1.5 in breast cancer cells

Voltage-gated Na+ channels (VGSCs) are highly expressed in several types of carcinomas including breast, prostate and lung cancers as well as in mesothelioma and cervical cancers. Although the VGSCs activity is considered crucial for the potentiation of cancer cell migration and invasion, the mechanisms responsible for their functional expression and regulation in cancer cells remain unclear. In the present study, the role of the small GTPase RhoA in the regulation of expression and function of the Nav1.5 channel in the breast cancer cell lines MDA-MB 231 and MCF-7 was investigated. RhoA silencing significantly reduced both Nav1.5 channel expression and sodium current indicating that RhoA exerts a stimulatory effect on the synthesis of an active form of Nav1.5 channel in cancer cells. The inhibition of Nav1.5 expression dramatically reduced both cell invasion and proliferation. In addition, a decrease of RhoA protein levels induced by Nav1.5 silencing was observed. Altogether, these findings revealed: i) the key role of the small GTPase RhoA in upregulation of Nav1.5 channel expression and tumor aggressiveness, and ii) the existence of a positive feedback of Nav1.5 channels on RhoA protein levels.

PACAP inhibits delayed rectifier potassium current via a cAMP/PKA transduction pathway: evidence for the involvement of I k in the anti-apoptotic action of PACAP

Abstract Activation of potassium (K(+)) currents plays a critical role in the control of programmed cell death. Because pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to inhibit the apoptotic cascade in the cerebellar cortex during development, we have investigated the effect of PACAP on K(+) currents in cultured cerebellar granule cells using the patch-clamp technique in the whole-cell configuration. Two types of outward K(+) currents, a transient K(+) current (I(A)) and a delayed rectifier K(+) current (I(K)) were characterized using two different voltage protocols and specific inhibitors of K(+) channels. Application of PACAP induced a reversible reduction of the I(K) amplitude, but did not affect I(A), while the PACAP-related peptide vasoactive intestinal polypeptide had no effect on either types of K(+) currents. Repeated applications of PACAP induced gradual attenuation of the electrophysiological response. In the presence of guanosine 5'-[gammathio]triphosphate (GTPgammaS), PACAP provoked a marked and irreversible I(K) depression, whereas cell dialysis with guanosine 5'-[betathio]diphosphate GDPbetaS totally abolished the effect of PACAP. Pre-treatment of the cells with pertussis toxin did not modify the effect of PACAP on I(K). In contrast, cholera toxin suppressed the PACAP-induced inhibition of I(K). Exposure of granule cells to dibutyryl cyclic adenosine monophosphate (dbcAMP) mimicked the inhibitory effect of PACAP on I(K). Addition of the specific protein kinase A inhibitor H89 in the patch pipette solution prevented the reduction of I(K) induced by both PACAP and dbcAMP. PACAP provoked a sustained increase of the resting membrane potential in cerebellar granule cells cultured either in high or low KCl-containing medium, and this long-term depolarizing effect of PACAP was mimicked by the I(K) specific blocker tetraethylammonium chloride (TEA). In addition, pre-incubation of granule cells with TEA suppressed the effect of PACAP on resting membrane potential. TEA mimicked the neuroprotective effect of PACAP against ethanol-induced apoptotic cell death, and the increase of caspase-3 activity observed after exposure of granule cells to ethanol was also significantly inhibited by TEA. Taken together, the present results demonstrate that, in rat cerebellar granule cells, PACAP reduces the delayed outward rectifier K(+) current by activating a type 1 PACAP (PAC1) receptor coupled to the adenylyl cyclase/protein kinase A pathway through a cholera toxin-sensitive Gs protein. Our data also show that PACAP and TEA induce long-term depolarization of the resting membrane potential, promote cell survival and inhibit caspase-3 activity, suggesting that PACAP-evoked inhibition of I(K) contributes to the anti-apoptotic effect of the peptide on cerebellar granule cells.

Compartmentalized and binary behavior of terminal dendrites in hippocampal pyramidal neurons

The dendritic arbor of pyramidal neurons is not a monolithic structure. We show here that the excitability of terminal apical dendrites differs from that of the apical trunk. In response to fluorescence-guided focal photolysis of caged glutamate, individual terminal apical dendrites generated cadmium-sensitive all-or-none responses that were subthreshold for somatic action potentials. Calcium transients produced by all-or-none responses were not restricted to the sites of photolysis, but occurred throughout individual distal dendritic compartments, indicating that electrogenesis is mediated primarily by voltage-gated calcium channels. Compartmentalized and binary behavior of parallel-connected terminal dendrites can greatly expand the computational power of a single neuron.

Melatonin and its analogs potentiate the nifedipine-sensitive high-voltage-activated calcium current in the chick embryonic heart cells

Effects of melatonin and its analogs on the voltage-activated calcium current of embryonic chick ventricular cardiomyocytes were investigated. Myocytes were dissociated from 14- to 16-day-old chicks (yellow Red Rob) embryonic hearts and cultured for 2 3 days. Calcium currents were studied by the patch-clamp technique. Whole-cell current recording showed nifedipine-sensitive, high-voltage-activated L-type calcium current inactivated in 70-100 ms during the voltage step period of 200 ms. There was no evidence of low-voltage-activated T-type calcium channels. Melatonin (ejected solution: 50 micromol/L melatonin; concentration at the vicinity of recording cell: about 1-5 micromol/L melatonin) and its analogs, 2-iodomelatonin and 2-iodo-n-butanol-5-methoxytryptamine, significantly increased the amplitude of the calcium current by 42-62%. The effect of melatonin on the L-type calcium current was not desensitised by repeated melatonin treatment. Our results suggest a specific melatonin receptor-mediated action on the calcium channel of the embryonic chick myocyte. The melatonin-induced increase in high-voltage calcium current may increase myocyte contractility and enhance cardiac output. A regulatory role of melatonin on the chick cardiac function should be further considered.

Swelling-activated chloride currents in embryonic chick heart cells

AIM

To characterize a swelling-activated chloride current, I(Cl, swell), in white Leghorn chick heart cells and the effects of chlorpromazine (CPZ) effects.

METHODS

The patch-clamp technique in the whole-cell configuration was used.

RESULTS

Hyposmotic swelling elicited I(Cl, swell) in white Leghorn chick heart cells. The current amplitude increased from (452 +/- 200) pA to (849 +/- 373) pA with a reduction of osmolarity from 300 mmol.L-1 to 270 mmol.L-1, 4',4-Diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) 100 mumol.L-1 decreased I(Cl, swell) from (1196 +/- 505) pA to (830 +/- 328) pA in hyposmotic solution. In white Leghorn chick heart cells I(Cl, swell) was not induced by CPZ 30 mumol.L-1, which is different from the case of E coli spheroplast.

CONCLUSION

Swelling-activated chloride current was activated by hyposmotic swelling in white Leghorn chick heart cells. The mechanism for activating the current is different from that of mechanosensitive ion channels of E coli.

4-aminopyridine, a specific blocker of K(+) channels, inhibited inward Na(+) current in rat cerebellar granule cells

The effects of 4-aminopyridine (4-AP), a specific blocker of outward K(+) current, on voltage-activated transient outward K(+) current (I(K(A))) and inward Na(+) current (I(Na)) were investigated on cultured rat cerebellar granule cells using the whole cell voltage-clamp technique. At the concentration of 1-5 mM, 4-AP inhibited both I(K(A)) and I(Na). It reduced the amplitude of peak Na(+) current without significant alteration of the steady-state activation and inactivation properties. The inhibitory effect was not enhanced by repeated depolarizing pulses (0.5 or 0.1 Hz), suggesting that the binding affinity of 4-AP on Na(+) channels is state-independent. In contrast, the effect of 4-AP on Na(+) channels appeared to be voltage-dependent, the weaker inhibition occurred at more depolarization. Moreover, 4-AP slowed both the activation and inactivation kinetics of Na(+) current. These effects were similar to those induced by alpha-scorpion toxin and sea anemone toxins on Na(+) channels in other cell model. Our data demonstrate for the first time that 4-AP is able to block not only A-type K(+) channels, but also Na(+) channels in rat cerebellar granule cells. It is concluded that the inhibition exerted by 4-AP on Na(+) current likely differs from that provoked by local anesthetics. The possibility that the binding site of neurotoxin receptor 3 may be involved is discussed.

Characterization of outward potassium current in embryonic chick heart cells

AIM

To characterize a voltage-dependent outward K+ current in cultured heart cells of 14-16-day-old embryos of yellow chick.

METHODS

The patchclamp technique in the whole-cell configuration was used.

RESULTS

The kinetics and the pharmacology of the outward K+ current in our cell mold were different from those described in white chick. Like the calcium-activated K+ current, blocker of calcium channel, CdCl2, eliminated the current of more than 95%. Isoproterenol provoked an increase of peak amplitude (137% +/- 47%, n = 16 cells) and acceleration of activation kinetics in the outward K+ current (the time reaching a peak current reduced from 36 ms +/- 10 ms to 16 ms +/- 9 ms). This effect of isoproterenol was mimiced by cAMP. In addition, a frequency-dependent decrease in peak amplitude of the current occurred after cAMP-induced phosphorylation.

CONCLUSION

There are species- and/or cell-type-specific difference in the K+ channels properties. In embryonic yellow chick heart cells, the phospholation of channel could not only modulate the activation kinetic properties of the calcium-activated potassium channel, but also change their recovery kinetics.

High-voltage-activated calcium current and its modulation by dopamine D4 and pituitary adenylate cyclase activating polypeptide receptors in cerebellar granule cells

Cerebellar granule cells were a good mold for electrophysiologic studies at the single neuron level. Two distinct types of high-voltage-activated Ca2+ channels were present in cerebellar granule cells. These calcium channels change their expression, gating, and pharmacological properties during development, suggesting that calcium channel must be related to the processes of granule cell maturation and excitability. Dopamine inhibited L-type calcium current by activating D4 receptor, and this effect might involve another signaling system with the exception of cAMP system. The functional D4 receptor discovered in cerebellum not only gave a possibility to find other antipsychotics, but also supported the existence of a dopaminergic system in the granule cell involving the D4 receptor. Pituitary adenylate cyclase activating polypeptide (PACAP) could increase intracellular Ca2+ content by activation of Ca2+ channel and mobilization of intracellular Ca2+ stores. The effects were also cAMP-independent. Activating Ca2+ currents might be an important and necessary role of PACAP as a neurotropic factor involved in the control of multiplication, differentiation, and migration of granule cells.

[The mechanism on hyperpolarization of membrane potential induced by adenosine in frog melanotrophs]

It has been previously shown that the activation of A1 adenosine receptors in frog melanotroph induced a hyperpolarization accompanied by blockage of spontaneous action potentials. In the present report, we explored mechanisms underlying the above phenomenon using the patch-clamp technique in whole-cell and cell-attached configuration. The result showed that adenosine could increase the opening of non-voltage-activated potassium channels leading to hyperpolarization, but an inward cation current activated by hyperpolarizing pulse was not involved.

Sigma ligands stimulate the electrical activity of frog pituitary melanotrope cells through a G-protein-dependent inhibition of potassium conductances

We have investigated the effects of sigma ligands [1,3-di(2-tolyl)guanidine (DTG) and (+)-pentazocine] on the electrical activity of cultured frog pituitary melanotrope cells by using the patch-clamp technique. DTG and (+)-pentazocine (10 microM each) induced a reversible depolarization associated with an increase in membrane resistance and action potential firing. In voltage-clamp experiments, DTG and (+)-pentazocine elicited inward currents whose intensity augmented with membrane depolarization. The currents vanished or reversed between -90 and -100 mV, at values close to the K+ equilibrium potential (E(K)+ = -102 mV). DTG (2-500 microM) and (+)-pentazocine (0.2-200 microM) reduced the outward delayed rectifier K+ current [IK (V)] in a dose-dependent manner with EC50 of 64 and 37 microM, respectively. In contrast, naloxone (50 microM) and pirenzepine (10 microM) did not affect the sigma ligand-induced inhibition of IK (V). Addition of guanosine-5'-O-(3-thiophosphate) in the pipette solution irreversibly sustained the DTG-induced current whereas guanosine-5'-O-(2-thiodiphosphate) virtually suppressed the response. Cholera toxin-pretreatment (1 microgram/ml; 18 hr) abolished the inward current and the inhibition of IK (V) induced by sigma ligands. In contrast, pretreatment with pertussis toxin (1 microgram/ml; 18 hr) had no effect. Taken together, these data indicate that DTG and (+)-pentazocine activate the electrical activity of cultured frog melanotrope cells by reducing both a tonic K+ current and a voltage-dependent [IK (V)] K+ conductance through the activation of a cholera toxin-sensitive G-protein.

Adenosine potentiates the delayed-rectifier potassium conductance but has no effect on the hyperpolarization-activated Ih current in frog melanotrophs

The effects of adenosine on the voltage-sensitive delayed-rectifier K+ (IK) currents and hyperpolarization-activated cationic inward current (Ih) were studied in cultured frog melanotrophs using the whole-cell configuration of the patch-clamp technique. The A1 receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA; 50 microM) reversibly increased IK. Perfusion of dibutyryl-cAMP (1 mM) in the external solution did not modify the R-PIA-induced enhancement of IK. Pretreatment of melanotrophs with pertussis toxin (1 microg/ml; 12 h) totally abolished the R-PIA-evoked response. Application of hyperpolarizing voltage pulses from -60 to -120 mV to melanotrophs induced a two-component inward current corresponding to an Ih-like conductance. This conductance was characterized by a high K+ selectivity and a low Na+ permeability and was resistant to tetrodotoxin (1 microM). R-PIA had no effect on Ih. The present study demonstrates that in frog melanotrophs adenosine inhibits the electrical activity by activating IK through an A1 receptor subtype coupled to a pertussis toxin-sensitive pathway independent of the cAMP/PKA system. This study also demonstrates the existence of a Ih conductance in frog melanotrophs which is not modulated by A1 receptors.

Patch clamp study on mechanism of adenosine-induced inhibitory effects in frog pituitary melanotrophs

Our laboratory demonstrated that adenosine inhibits the activation of adenylyl cyclase and the secretion of the alpha-melanocyte-stimulating hormone (alpha-MSH) from the intermediate lobe of the frog pituitary. This paper showed the bioelectric effects induced by adenosine, the ionic conductances modulated by adenosine, and the possible involvement of intracellular messengers, indicated the mechanism by which adenosine controls the secretion of alpha-MSH. The results show that adenosine acting on A1 adenosine receptor subtype reduced the Ca2+ influx necessary for the secretion, through 4 distinct mechanisms: 1) a hyperpolarization resulting from the activation of a voltage-insensitive K+ conductance, 2) a reduction of the duration of spontaneous action potentials due to an increase of the outward delayed rectifyer K+ current (lk), 3) a diminution of the cellular excitability by an activation of the transient outward K+ current (lA), and 4) an inhibition of the L- and N-type Ca2+ currents, with a predominant action on the N-type component. Cell dialysis with GTP gamma S rendered irreversible the effects of adenosine on the K+ conductances and Ca2+ channels, whereas PTX pretreatment totally abolished the response to adenosine, suggesting all bioelectric effects of adenosine were mediated by pertussis toxin-sensitive G proteins. Whether the implicated G proteins regulate the K+ and Ca2+ channels by tight-coupling or via a second-messenger system remains to be solved. With our results, the involvement of adenylyl cyclase can be excluded because addition of cAMP and IBMX, an inhibitor of phosphodiesterases, in the intracellular solution, or application of dibutyryl cAMP in the extracellular solution did not modify the adenosine-induced responses.

Adenosine inhibits L- and N-type calcium channels in pituitary melanotrophs. Evidence for the involvement of a G protein in calcium channel gating

It has been previously demonstrated that activation of A1 adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secretion. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents in cultured melanotrophs, using the whole-cell variant of the patch-clamp technique with barium as a charge carrier. Adenosine and the specific A1 adenosine receptor agonist R-PIA (50 microM each) produced a decrease of the amplitude of the barium current, while the selective A2 adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect of R-PIA was observed throughout the activation range of the current, with stronger responses at more positive potentials. R-PIA inhibited both the L- and N-type components of the current, the effect on the N-component being two-fold higher than on the L-component. The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S (100 microM) to the intracellular solution. Pre-treatment of the cells with pertussis toxin (1 microgram/ml; 12 h) totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high concentration of cAMP (100 microM) together with the phosphodiesterase inhibitor IBMX (100 microM) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotrophs, adenosine induces inhibition of L- and N-calcium currents and that this effect is mediated by a pertussis toxin-sensitive G protein. Our data also indicate that the inhibitory effect of adenosine on the calcium currents is not mediated by inhibition of adenylyl cyclase.

A-type potassium current modulated by A1 adenosine receptor in frog melanotrophs

1. Transient outward current was recorded in cultured frog melanotrophs with the whole-cell configuration of the patch-clamp technique. The ionic dependence, kinetics and pharmacological properties of the current were studied. The effects of the A1 adenosine receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA) on this current were also investigated. 2. In tetrodotoxin- and cobalt-containing solution, depolarization from -120 mV elicited both transient and delayed outward currents. Pulses from -60 mV activated only a sustained late current. 3. 4-Aminopyridine (4 mM) reduced the transient outward current much more than the delayed outward current. In contrast, tetraethylammonium (10-20 mM) selectively reduced the delayed current. 4. Tail current measurements showed a positive shift in the reversal potential when external K+ concentration was increased, indicating that K+ was the predominant charge carrier. 5. Steady-state inactivation was complete at potentials positive to -10 mV and removed by hyperpolarization. 6. Inactivation of the transient current was slowed and accelerated in oxidizing and reducing conditions, respectively, confirming the involvement of an inactivating 'ball and chain' peptide. 7. R-PIA increased the transient current. The steady-state inactivation curve was shifted towards more positive potentials without changing the activation kinetics. Pretreatment with pertussis toxin (1 microgram ml-1) blocked the response to R-PIA. 8. It is concluded that frog melanotrophs possess an A-type current that is likely to play an important role in excitability. This current, which is directly modulated by A1 adenosine receptors through a Gi/G(o) protein, appears to be responsible for the inhibitory effects of adenosine on electrical activity.

Activation of dopamine D4 receptor inhibits an L-type calcium current in cerebellar granule cells

The functions of the D4 receptor, a newly cloned D2-like receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-like receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine. These results indicate the presence in the neonatal cerebellum of a functional D4 receptor inhibiting an L-type calcium current, an action involving a Gi/Go protein but independent from adenylate cyclase inhibition.

Inhibitory effect of adenosine on electrical activity of frog melanotrophs mediated through A1 purinergic receptors

1. The effects of adenosine were studied in cultured frog melanotrophs by the patch-clamp technique. 2. In cell-attached experiments, most cells responded to adenosine (50 microM) by a reversible inhibition of action current discharges without any apparent desensitization. 3. In whole-cell experiments, adenosine provoked a hyperpolarization accompanied by a depression of spontaneous action potentials and a decrease in membrane resistance. When adenosine was repeatedly applied, tachyphylaxis was observed. Addition of GTP (100 microM) in the intracellular solution augmented the percentage of cells hyperpolarized by adenosine, and the duration and amplitude of the hyperpolarization, and prevented the tachyphylaxis. 4. Pretreatment with pertussis toxin (1 microgram ml-1) blocked adenosine-induced inhibition. 5. In cells dialysed with the non-hydrolysable GTP analogue GTP gamma S (100 microM), adenosine caused a sustained, strong hyperpolarization and an irreversible inhibition of spikes. 6. The effect of adenosine was mimicked by the A1 receptor agonist R-PIA (R-N6-phenylisopropyl-adenosine; 50 microM) and blocked by the A1 receptor antagonist CPDPX (8-cyclopentyl-1,3-dipropylxanthine, 50 microM). The A2 receptor antagonist CGS15943 (9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c] quinazoline-5-imine; 50 microM) did not affect the adenosine-induced response. 7. The results suggest that, in frog melanotrophs, adenosine exerts a direct hyperpolarizing effect accompanied by blockage of spontaneous action potentials. The effect of adenosine is mediated through A1 receptors coupled to a Gi/o protein.

Characterization of the GABA-induced current in frog pituitary melanotrophs

The molecular mechanisms regulating GABAA receptor activity in cultured frog melanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED50 value, estimated from the sigmoidal dose-response curve was 2 x 10(-6) M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time. Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60 mV). Altogether these data reveal that frog melanotrophs possess a single population of GABAA receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABAA receptor appears to be independent of the accumulation of intracellular chloride ions.

[The changes in transmembrane potential and ionic currents induced by "ischemia" in sheep cardiac Purkinje fibers]

Ischemia-mimic solution (hypoxia, hyperkalemia, acidosis, no substrate) induced changes of transmembrane potential and ionic current were observed in 24 sheep cardiac Purkinje fibers. The sequence of changes of transmembrane potential was as follows: Two to three min after perfusion with ischemia-mimic solution, the maximum diastole potential (MDP) was depolarized slightly, and attended by a decrease of depolarization rate of phase 4 while the action potential duration (APD) was shortened or underwent a shortening-prolongation-re-shortening process. Then, the plateau of action potential disappeared gradually. As MDP was depolarized further, the action potential amplitude (APA) and the excitability of the Purkinje fibers decreased continuously until action potential could no longer be elicited. The whole process took 30 min to 160 min in different preparations. When the APD was shortened, the instantaneous outward current was always increased at all membrane potential levels. The steady-state current-voltage relationship (CVR) curve was changed from S shape to a straight line, and the inward rectification phenomenon disappeared. Under ischemic condition, the amplitude of Isi was decreased from 6.74 +/- 4.48 nA to 0.86 +/- 1.39 nA, and the CVR curve shifted more to the negative. The above results indicate that under the ischemic condition, the pacemaker function of cardiac Purkinje cell is inhibited, a large amount of K+ ions outflows and inward current Isi is decreased. These changes may be the reasons for the genesis of ischemic arrhythmia.