Characterization of K+ currents in rat malignant lymphocytes (Nb2 cells)

Modulation of Outward K(+) Conductance Is a Post-Activational Event in Rat T Lymphocytes Responsible for the Adoptive Transfer of Experimental Allergic Encephalomyelitis

Experimental allergic encephalomyelitis (EAE) is an accepted animal model for the human demyelinating disease multiple sclerosis. The continuously propagated line of Lewis rat T helper lymphocytes (GP1 T cells), specific for the encephalitogenic 68-86 sequence of guinea pig myelin basic protein (GPMBP), mediates the adoptive transfer of EAE into normal syngeneic Lewis rats. Because mitogenic activation of T cells can increase K(+) conductance, this study investigated changes in the outwardly rectifying K(+) conductance in GP1 T cells following activation with the encephalitogen, GPMBP. Using the gigohm.seal whole-cell variation of the patch clamp technique, GP1 T cells were studied during a 3-day culture with GPMBP and throughout the subsequent 10 days, as cells progressed through both GPMBP-induced activation (EAE transfer activity) and proliferation responses, finally reverting to the resting state. Resting GP1 T cells exhibited peak K(+) conductances around 2 nS, while GPMBP-induced activation resulted in 5- to 10-fold increases in peak K(+) conductance, which temporally coincided with the optimal period for EAE transfer activity. During and immediately after the optimal period for EAE transfer, 20-mV depolarizing shifts in the voltage dependence of both activation and inactivation developed, abruptly reversing to resting values as cells reverted to the resting state. Accompanying the depolarizing shifts were a slowing of the K(+) current activation kinetics and an acceleration of the deactivation kinetics. These results indicate that the K(+) conductance in GP1 rat T helper cells is modulated over the full time course of GPMBP-induced cellular responses and that K(+) channels should be optimally available during the period of adoptive EAE transfer, preceding disease manifestation. Copyright 1997 S. Karger AG, Basel

Inactivation of the cloned potassium channel mouse Kv1.1 by the human Kv3.4 'ball' peptide and its chemical modification

1. This study used the whole-cell patch clamp technique to investigate the action of a 28-mer 'inactivation peptide' based on part of the N-terminal sequence of the human Kv3.4 K+ channel (hKv3.4 peptide) on the cloned mouse brain K+ channel mKv1.1 expressed in Chinese hamster ovary (CHO) cells, and compared this with the inactivation produced by Shaker B inactivation peptide (ShB peptide). 2. Inclusion of the hKv3.4 peptide in the patch electrode (320 microM) transformed non-inactivating mKv1.1 into a rapidly inactivating current. The voltage dependence of time constants of decay and steady-state inactivation induced by hKv3.4 peptide were characteristic of an 'A-type' K+ current. 3. The hKv3.4 peptide had no effect on the voltage dependence of activation of mKv1.1, with a mid-point of activation of -8 mV, and a slope factor of 15 mV. Steady-state inactivation curves had a mid-point of inactivation of -36 mV and a slope factor of -7 mV; the time constant of recovery from inactivation at -90 mV was 1.3 s. 4. The chemical modification reagents N-bromoacetamide (NBA, 100 microM) and chloramine-T (CL-T, 500 microM) had no effect on the fast inactivation of mKv1.1 induced by ShB peptide. In contrast, the inactivation caused by hKv3.4 peptide was removed by brief exposure to NBA and CL-T. 5. Chemical modification resulted in a hyperpolarizing shift of -8 mV (CL-T) and -11 mV (NBA) in the voltage dependence of activation of mKv1.1 in the presence of hKv3.4 peptide. 6. Chemical modification was critically dependent on the presence of a cysteine residue at position 6, and not position 24, of hKv3.4 peptide. 7. NBA and CL-T caused only a slight inhibition of unmodified mKv1.1 current with no significant effect on the voltage dependence of mKv1.1 activation, and also had no effect on channel deactivation at -90 mV. 8. Chemical modification experiments were consistent with a selective action on the hKv3.4 peptide itself, specifically at the cysteine residue at position 6.

Diacylglycerol-induced activation of protein kinase C attenuates Na+ currents by enhancing inactivation from the closed state

The causes of attenuation of Na+ currents by diacylglycerol (DAG)-induced protein kinase C (PKC) activation in mouse neuroblastoma N1E-115 cells were investigated using the cell-attached patch, and the perforated-patch (nystatin based) whole-cell voltage-clamp techniques. Activation of PKC by DAG attenuated Na+ currents. Attenuation occurred in the absence of significant changes in the time-course of Na+ currents. However, the steady-state inactivation curve of these currents shifted to more negative voltages by approximately 20 mV. Here we demonstrate that the time-course of inactivation is accelerated by treatment with DAG-like substances in a voltage-dependent manner (time constant of inactivation decreased by 2- and 3.6-fold at -60, and -30 mV, respectively). In cell-attached patches, treatment with DAG compounds increased the percentage of current traces showing no single Na+ channel openings in response to depolarizing voltage-clamp pulses. Moreover, the average of current traces containing single Na+ channel openings was essentially the same in control conditions and after treatment with DAG compounds. Removal of Na+ channel inactivation by the alkaloid batrachotoxin prevented the attenuation of Na+ currents by PKC activation via DAGs. Taken together, these data strongly suggest that PKC-induced attenuation of Na+ currents is linked to an enhancement of Na+ channel inactivation. This attenuation is caused by an increase in the number of Na+ channels inactivating directly from the closed state(s). This inactivation pathway represents a simple and efficient physiological mechanism by which PKC activation might modulate the electrical activity of excitable cells.

State-dependent inactivation of the Kv3 potassium channel

Inactivation of Kv3 (Kv1.3) delayed rectifier potassium channels was studied in the Xenopus oocyte expression system. These channels inactivate slowly during a long depolarizing pulse. In addition, inactivation accumulates in response to a series of short depolarizing pulses (cumulative inactivation), although no significant inactivation occurs within each short pulse. The extent of cumulative inactivation does not depend on the voltage during the depolarizing pulse, but it does vary in a biphasic manner as a function of the interpulse duration. Furthermore, the rate of cumulative inactivation is influenced by changing the rate of deactivation. These data are consistent with a model in which Kv3 channel inactivation is a state-dependent and voltage-independent process. Macroscopic and single channel experiments indicate that inactivation can occur from a closed (silent) state before channel opening. That is, channels need not open to inactivate. The transition that leads to the inactivated state from the silent state is, in fact, severalfold faster then the observed inactivation of current during long depolarizing pulses. Long pulse-induced inactivation appears to be slow, because its rate is limited by the probability that channels are in the open state, rather than in the silent state from which they can inactivate. External potassium and external calcium ions alter the rates of cumulative and long pulse-induced inactivation, suggesting that antagonistic potassium and calcium binding steps are involved in the normal gating of the channel.

Multiple effects of protein kinase C activators on Na+ currents in mouse neuroblastoma cells

The effects of externally applied different protein kinase C (PKC) activators on Na+ currents in mouse neuroblastoma cells were studied using the perforated-patch (nystatin-based) whole cell voltage clamp technique. Two diacylglycerol-like compounds, OAG (1-oleoyl-2-acetyl-sn-glycerol), and DOG (1-2-dioctanoyl-rac-glycerol) attenuated Na+ currents without affecting the time course of activation or inactivation. The reduction in Na+ current amplitude caused by OAG or DOG was dependent on membrane potential, being more intense at positive voltages. The steady-state activation curve was also unaffected by these substances. However, both OAG and DOG shifted the steady-state inactivation curve of Na+ currents to more hyperpolarized voltages. Surprisingly, phorbol esters did not affect Na+ currents. Cis-unsaturated fatty acids (linoleic, linolenic, and arachidonic) attenuated Na+ currents without modifying the steady-state activation. As with DOG and OAG, cis-unsaturated fatty acids also shifted the steady-state inactivation curve to more negative voltages. Interestingly, inward currents were more effectively attenuated by cis-fatty acids than outward currents. Oleic acid, also a cis-unsaturated fatty acid, enhanced Na+ currents. This enhancement was not accompanied by changes in kinetic or steady-state properties of currents. Enhancement of Na+ currents caused by oleate was voltage dependent, being stronger at negative voltages. The inhibitory or stimulatory effects caused by all PKC activators on Na+ currents were completely prevented by pretreating cells with PKC inhibitors (calphostin C, H7, staurosporine or polymyxin B). By themselves, PKC inhibitors did not affect membrane currents. Trans-unsaturated or saturated fatty acids, which do not activate PKC's, did not modify Na+ currents. Taken together, the experimental results suggest that PKC activation modulates the behavior of Na+ channels by at least three distinct mechanisms. Because qualitatively different results were obtained with different PKC activators, it is not clear how Na+ currents would respond to activation of PKC under physiological conditions.

Divalent cations selectively alter the voltage dependence of inactivation of A-currents in chick autonomic neurons

A-currents were studied in acutely dissociated chick autonomic neurons. Switching from salines containing 4 mM Mg2+ to salines containing 4 mM Ca2+ caused a large positive shift in the voltage dependence of steady-state inactivation, but had no effect on the voltage dependence or kinetics of activation, deactivation, the rate at which channels became inactivated, or the rate at which channels recovered from inactivation. This effect saturated with increasing concentrations of Ca2+. Other group IIA divalent cations were less effective than Ca2+, in the order Ca2+ > Ba2+ > Mg2+ = Sr2+. Application of 4 mM Mg2+ partially antagonized the effects of 4 mM Ca2+. The results suggest that divalent cations selectively alter the voltage dependence of steady-state inactivation by acting at sites accessible from the outside of the cell.

Involvement of K+ channels in the quercetin-induced inhibition of neuroblastoma cell growth

The effects of the flavonoid quercetin on cell proliferation and voltage-dependent K+ current were studied on mouse neuroblastoma x glioma hybrid cells. In the presence of 1% fetal calf serum, quercetin inhibited both cell proliferation and K+ current with effective doses inducing half-maximum effects of 10 microM and 70 microM respectively. Valinomycin (1 nM) antagonized 80% of the growth-inhibitory effects of 10 microM quercetin. The results suggest that at least a part of the anti-proliferative effect of quercetin is mediated by a K+ channel blockade. They further confirm a role for K+ channels in mitogenesis and cell proliferation.

K-current mediation of prolactin-induced proliferation of malignant (Nb2) lymphocytes

The effects of different concentrations of various K-current blockers on prolactin-induced proliferation and membrane K-currents in malignant lymphocytes (Nb2 cells) were investigated. Membrane currents were measured with the whole cell patch-clamp technique, and lymphocyte density was quantified by both spectrophotometric and conventional methods. K-current blockers tested (quinidine, 4-aminopyridine, barium, and tetraethylammonium) exhibited similar rank order potency for K-current block and inhibition of prolactin-induced proliferation of malignant lymphocytes. Because Nb2 cells proliferate independently of a transmembrane Ca-influx, these results suggest that K-currents per se rather than K-current modulation of Ca-influx is an essential event for lymphocyte proliferation.