Effects of membrane potential and muscarine on potassium M-channel kinetics in rat sympathetic neurones

1. Using cell-attached patch pipettes, sustained activity of single potassium M-channels was recorded from dissociated rat superior cervical ganglion neurones. Previous results indicated that this activity, consisting of three main levels of open-channel conductance (congruent to 7, congruent to 12 and congruent to 19 pS) was activated by membrane depolarization and inhibited by muscarine added outside the patch. Consequently, a kinetic analysis was undertaken in order to identify M-channel states sensitive to muscarine and membrane potential. 2. Channel activity recorded at 30 mV positive to the resting membrane potential level (congruent to -60 mV) showed three shut and two open times. Mean shut times were: tau s1 = 8.0 +/- 2.2 ms; tau s2 = 71.3 +/- 8.6 ms and tau s3 = 740 +/- 220 ms. Mean open times were: tau o1 = 10.6 +/- 1.9 ms and tau o2 = 59.3 +/- 8.7 ms. When bursts of channel openings were determined as those including tau s1, two exponential components were evident in burst duration distributions (tau b1 = 11.0 +/- 0.9 ms and tau b2 = 80.4 +/- 11.0 ms). 3. Membrane hyperpolarization significantly lengthened all three shut times and shortened both open times. It also slightly enhanced the relative contribution of high-conductance channels and decreased the relative contribution of low-conductance channels to overall activity. 4. All three shut times of the M-channels were lengthened by 10 microM muscarine without significantly affecting their open times. 5. It is concluded that both open and shut states of the M-channel are voltage sensitive while only shut states are sensitive to muscarine.

Intracellular calcium changes associated with cholinergic nicotinic receptor activation in cultured myenteric plexus neurones

Intracellular free calcium concentration ([Ca2+]i) was measured in cultured explants of myenteric plexus neurones by using the fluorescent calcium indicator Indol in combination with patch-clamp techniques. The basal [Ca2+]i was 94 nM and spontaneous oscillations in the internal free calcium concentration were recorded. These oscillations were associated with bursts of action potentials triggered by spontaneous nicotinic excitatory synaptic potentials. Under voltage clamp conditions, application of the selective nicotinic agonist m-hydroxyphenylpropyl-trimethylammonium iodide (10 microM) induced an inward current and increased the intracellular free calcium concentration. We conclude that cholinergic synaptic excitatory activity provide a regular calcium entry in myenteric neurone and suggest that the nicotinic channel might be significantly permeable to calcium.

On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells

1. Acetylcholine (ACh) produces two membrane current changes when applied to NG108-15 mouse neuroblastoma x rat glioma hybrid cells transformed (by DNA transfection) to express m1 muscarinic receptors: it activates a Ca(2+)-dependent K+ conductance, producing an outward current, and it inhibits a voltage-dependent K+ conductance (the M conductance), thus diminishing the M-type voltage-dependent K+ current (IK(M)) and producing an inward current. The present experiments were undertaken to find out how far inhibition of IK(M) might be secondary to stimulation of phospholipase C, by recording membrane currents and intracellular Ca2+ changes with indo-1 using whole-cell patch-clamp methods. 2. Bath application of 100 microM ACh reversibly inhibited IK(M) by 47.3 +/- 3.2% (n = 23). Following pressure-application of 1 mM ACh, the mean latency to inhibition was 420 ms at 35 degrees C and 1.79 s at 23 degrees C. Latencies to inhibition by Ba2+ ions were 148 ms at 35 degrees C and 92 ms at 23 degrees C. 3. The involvement of a G-protein was tested by adding 0.5 mM GTP-gamma-S or 10 mM potassium fluoride to the pipette solution. These slowly reduced IK(M), with half-times of about 30 and 20 min respectively, and rendered the effect of superimposed ACh irreversible. Effects of ACh were not significantly changed after pretreatment for 24 h with 500 ng ml-1 pertussis toxin or on adding up to 10 mM GDP-beta-S to the pipette solution. 4. The role of phospholipase C and its products was tested using neomycin (to inhibit phospholipase C), inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4), heparin, and phorbol dibutyrate (PDBu) and staurosporin (to activate and inhibit protein kinase C respectively). Both neomycin (1 mM external) and InsP3 (100 microM intrapipette) inhibited the ACh-induced outward current and/or intracellular Ca2+ transient but did not block ACh-induced inhibition of IK(M). Intrapipette heparin (1 mM) blocked activation of IK(Ca) and reduced Ach-induced inhibitions of IK(M), but also reduced inhibition of ICa via endogeneous m4 receptors. PDBu (with or without intrapipette ATP) and staurosporin had no significant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium entry through nicotinic receptor channels and calcium channels in cultured rat superior cervical ganglion cells

1. Patch-clamp techniques in conjunction with indo-1 fluorescent measurements were used to measure increases in intracellular free calcium concentration and membrane conductance induced by the activation of nicotinic and calcium channels in cultured rat sympathetic neurons. 2. Under voltage-clamp conditions, pressure application of the nicotinic agonist DMPP (1,1-dimethyl-4-phenylpiperazinium iodide, 100 microM, 100 ms) increased [Ca2+]i by 193 +/- 26 nM at a clamp potential of -60 mV. This was accompanied by an inward current of -4.53 +/- 0.89 nA, giving a mean ratio of the delta (Ca2+]i to the total inward charge transfer of 42.7 nmoles per litre of free calcium per nanocoulomb of charge (M/q ratio). 3. The DMPP-induced current and associated delta [Ca2+]i were reduced by mecamylamine (100 nM-10 microM) but were unaffected by alpha-bungarotoxin (100 nM) or cadmium (100 microM). 4. The M/q ratio was not affected by the holding potential (from -80 to -40 mV) but was a function of the external calcium concentration. 5. The M/q ratio was reduced by increasing the intracellular calcium buffering capacity and increased by heparin but not affected by ryanodine or by depletion of the caffeine-sensitive calcium store. 6. Under the same recording conditions, we quantified the increase in [Ca2+]i associated with activation of the voltage-dependent calcium current. On average at -60 mV, the M/q ratio of this highly calcium-selective permeability was 1961 mM nC-1, which is 46 times that obtained for the nicotinic channel. 7. Assuming constant-field theory, ion-substitution experiments suggest that in 2.5 mM external calcium, the permeability sequence for the nicotinic conductance was Cs+ < Li+ < Na+ < K+ < Ca2+. 8. We conclude that the nicotinic channels in rat sympathetic neurones are significantly permeant to Ca2+ and that the influx of Ca2+ through these channels is the principal cause of the rise in [Ca2+]i seen under voltage clamp.

Zn2+ potentiates ATP-activated currents in rat sympathetic neurons

The ATP-activated inward current (IATP) in cultured rat superior cervical ganglion neurons and its modulation by extracellular Zn2+ were examined. ATP activated a non-specific cation conductance and caused a transient rise in intracellular Ca2+. The current response was specifically activated by ATP and was blocked by the P2-purinoceptor antagonist, suramin. Low concentrations of extracellular Zn2+ rapidly and reversibly potentiated both IATP and the intracellular Ca2+ rise. The potentiation by 10 microM Zn2+ was dependent on the concentration of agonist; Zn2+ increased the sensitivity of activation without potentiating the maximum response. Higher concentrations of Zn2+ reduced and prolonged the current, consistent with open-channel block. We hypothesize that there exist two sites of action for Zn2+: a positively acting allosteric site that enhances current amplitude and a site, possibly within the pore, that blocks conductance through the channel.

L-selectin mediates neutrophil rolling in inflamed venules through sialyl LewisX-dependent and -independent recognition pathways

The glycoprotein (GP) L-selectin initiates adhesive interactions between leukocytes and endothelial cells (EC). It functions as a lymphocyte-lectin homing receptor recognizing carbohydrate determinants of the peripheral lymph node addressing on high endothelial venules. It also mediates neutrophil rolling, the earliest interaction of neutrophils with acutely inflamed venules. Neutrophil L-selectin presents sialyl-LewisX (sLe(X)) as a ligand to P- and E-selectin in vitro, and we have proposed that this is a major mechanism of L-selectin-mediated rolling in vivo. In contrast, the contribution of neutrophil L-selectin as a receptor protein recognizing one (or more) ligand(s) on inflamed EC is unclear. To address this question, an sLe(X)-negative murine pre-B cell line, L1-2, that can neither bind vascular selectins nor roll in inflamed rabbit venules, was transfected with human L-selectin cDNA. L-selectin expression in stable transfectants was sufficient to confer significant rolling in vivo. Rolling was unaffected by neuraminidase treatment but completely blocked by anti-L-selectin monoclonal antibody (MoAb) DREG-56. Thus, L-selectin can initiate leukocyte interactions with EC determinants potentially through recognition of endothelial carbohydrates. In contrast, when human neutrophils were tested, rolling was reduced, but not abolished, by MoAb DREG-56. Likewise, treatment with neuraminidase or anti-sLe(X) MoAbs decreased, but did not abrogate, neutrophil rolling, consistent with residual EC recognition via L-selectin. Combination of MoAb DREG-56 and neuraminidase resulted in almost complete loss of rolling, as did removal of glycosylated L-selectin by chymotrypsin. Together with the demonstrable rolling of L-selectin transfectants, our results support the concept of a bidirectional interaction between L-selectin bearing sLe(X) on neutrophils and activated EC in vivo. These findings also suggest that L-selectin may mediate rolling of lymphocytes that lack carbohydrate ligands for E- or P-selectin, although probably less efficiently than through bidirectional recognition.

M2 muscarinic receptor-mediated inhibition of the Ca2+ current in rat magnocellular cholinergic basal forebrain neurones

1. The actions of muscarinic agonists and antagonists upon the Ca2+ current (ICa) in acutely dissociated magnocellular cholinergic basal forebrain neurones from 11 to 14-day-old postnatal rats were studied using the whole-cell patch-clamp technique. 2. In all cells studied, muscarinic agonists inhibited a transient component of high-voltage-activated (HVA) current, but had no effect upon the low-voltage-activated (LVA) current. The mean IC50 values for ACh and oxotremorine methiodide (oxo-M), obtained from non-cumulative dose-response curves, were 204 and 363 nM respectively. Superfusion with the K+ channel blocker, tetraethylammonium chloride (TEA; 30 mM) shifted the ACh dose-response curve to the right giving an IC50 value of 22:9 microM. 3. Pirenzepine (0.1-1 microM) and methoctramine (0.03-0.3 microM) produced parallel shifts to the right in the agonist dose-response curves. Schild analysis of the agonist dose ratios yielded pA2 (negative log of the apparent dissociation constant KB) values for pirenzepine and methoctramine of 6.8 and 8.2 respectively, indicating the involvement of an M2 receptor subtype. 4. In the presence of GTP-gamma-S (10-500 microM) in the patch pipette, the agonist-induced inhibition of ICa became irreversible. Dialysis with GDP-beta-S (1 mM) abolished all agonist-induced inhibition of the Ca2+ current. The agonist-induced inhibition of ICa was totally blocked by pretreatment with pertussis toxin (500 ng ml-1) but unaffected by preincubation with cholera toxin (500 ng ml-1). Superfusion with 8-bromo cAMP (0.5-1 mM) did not mimic or prevent the effect of agonist application. 5. Inhibition of the Ca2+ current by muscarinic agonists was only partially blocked by omega-conotoxin GVIA (omega-CgTX GVIA), with approximately 46% of the agonist-sensitive current being resistant to omega-CgTX GVIA. Both the agonist- and omega-CgTX GVIA-sensitive components of the current were abolished following maximal inhibition of ICa by GTP-gamma-S. 6. These results indicate that inhibition of the Ca2+ current by muscarinic agonists is mediated via an M2 muscarinic receptor coupled to a pertussis toxin-sensitive G-protein. The possible modulation of multiple HVA Ca2+ channels by muscarinic agonists and the role that these receptors may play in presynaptic modulation of neurotransmitter release are discussed.

Analysis of uroporphyrinogen decarboxylase complementary DNAs in sporadic porphyria cutanea tarda

BACKGROUND

Sporadic porphyria cutanea tarda (S-PCT) has been considered an acquired disease because of the generation of liver-specific inhibitors of uroporphyrinogen decarboxylase (URO-D) activity. Several families have been described with S-PCT in multiple generations, raising the possibility of an inherited basis for the disease. To determine if S-PCT is associated with mutant URO-Ds that might be sensitive to liver-specific inhibitors, a molecular analysis of genomic and hepatocellular URO-Ds was undertaken.

METHODS

Total RNA from lymphoid cell lines from three unrelated patients with S-PCT and poly A+ RNA from liver biopsy samples from two additional patients was used as a template for single-stranded cDNA synthesis, and URO-D sequences were amplified and sequenced. DNA prepared from peripheral blood leukocytes was used as a template to polymerase chain reaction (PCR) amplify the promoter region of the URO-D gene. Sequencing of PCR products was performed completely in both directions by the chain termination method using a variety of custom oligonucleotide primers.

RESULTS

Ten URO-D alleles were sequenced, and no mutations were found. The promoter region of the URO-D gene was also normal.

CONCLUSIONS

It is concluded that S-PCT is not due to mutations at the URO-D locus. If inherited factors are involved, other loci must be affected.

HIV induces thymus depletion in vivo

Human immunodeficiency virus (HIV) disease is typified by declining CD4+ T lymphocyte counts in the peripheral circulation, a loss which may be secondary to accelerated destruction, to suppressed differentiation, and/or to sequestration of circulating cells into tissue spaces. As it is hard to distinguish between these possibilities in human subjects, the pathogenic mechanisms associated with HIV infection are unclear. In particular, little is known about the events that occur within infected lymphoid organs in which most CD4 T lymphocytes mature and function. To obtain a better description of HIV pathogenesis in vivo, we have implanted human haematolymphoid organs into the immunodeficient SCID mouse to create the SCID-hu mouse. We have previously shown that these organ systems promote long-term multilineage human haematopoiesis and are permissive for infection with HIV. Here we report that human thymopoiesis is suppressed by HIV infection, thereby precluding regeneration of the peripheral T-cell compartment.

Characterization of a calcium-dependent current generating a slow afterdepolarization of CA3 pyramidal cells in rat hippocampal slice cultures

A depolarization-induced, slowly decaying inward current was examined in slice-cultured CA3 pyramidal cells by voltage-clamp techniques and microfluorometric measurements of cytosolic free Ca2+ concentration ([Ca2+]i). Action potentials elicited by intracellular injection of short-lasting (50-100 ms) depolarizing current pulses were followed by a slowly decaying afterhyperpolarization (AHP). After switching to voltage-clamp mode, short-lasting (50-100 ms) depolarizing voltage jumps from -60 mV to between -30 and 0 mV induced a slowly decaying outward aftercurrent (IAHP) which was depressed by bath application of muscarine (0.5 microM). In the presence of muscarine, the same depolarizations induced a slowly decaying afterdepolarization (ADP) or inward aftercurrent (IADP) in voltage-clamp mode. This current was also induced in the presence of trans(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 5 microM), an agonist of metabotropic glutamate receptors, but not in the presence of noradrenalin (5 microM), while both of these agonists depressed IAHP. IADP was depressed by reducing the external Ca2+ concentration from 3.8 to 0.5 mM, by external Co2+ (1 mM) and by external Cd2+ (10-100 microM). Combined voltage-clamp recordings and microfluorometric measurements of [Ca2+]i using the Ca2+ indicator fura-2 revealed that the amplitude of IADP was correlated with the amplitude of depolarization-induced Ca2+ influx. IADP was absent at membrane potentials < -90 mV, and reached maximal amplitudes at approximately -55 mV. Raising the extracellular K+ concentration from 2.7 to 13.5 mM increased the amplitude of IADP and resulted in a positively directed shift of the apparent reversal potential of IADP. When the external Na+ concentration was reduced from 157 to 33 or 18 mM the current reversed at more negative potentials and was reduced to 40 and 21%, respectively, of control amplitude. Lowering the external CI- concentration from 159 to 20 mM did not affect IADP. We conclude that IADP most likely represents a Ca(2+)-activated cation current, rather than a Ca2+ tail current, or an electrogenic Ca2+ extrusion current.

Identification of M-channels in outside-out patches excised from sympathetic ganglion cells

We have identified the species of K+ channel that underlies the neuronal M-current in rat sympathetic ganglion cells. The channels were kinetically and pharmacologically defined using outside-out and cell-attached patches. They exhibited multiple conductance levels, predominantly 3-9 pS. Their slow gating in response to voltage change in outside-out patches was exhibited only in the presence of AIF-4 or GTP gamma S on the inner membrane surface and when the lower conductance states were dominant. In the absence of AIF-4 or GTP gamma S, the channels exhibited rapid activation and deactivation. We conclude that M-channel gating may be controlled by an associated GTP-binding protein.

Human flexor reflex modulation during cycling

1. Human flexor reflex (HFR) responses were elicited during ergometer cycling in neurologically intact humans with the objective of understanding the influence of lower limb muscle activity on phase-dependent reflex modulation during movement. The experimental setup permitted control over background muscle activity and stimulus intensity without significantly interfering with the cycling motion. 2. All experiments involved cycling on an ergometer at a set rate and workload. A 333-Hz, 15-ms pulse train of electrical stimulation was randomly delivered to the skin over the tibial nerve at the ankle at selected lower limb positions. In the first group of experiments, subjects were stimulated at six cycling phases while pedaling with normal, phasic ankle activity (free-form cycling). The second and third group of experiments involved stimulation under static limb positioning conditions and during active pedaling while subjects were asked to maintain a consistent background level of isolated tibialis anterior (TA) or soleus (SOL) electromyographic (EMG) activity. 3. Control criteria were established to assure similar isolated muscle EMG levels and sensory stimulation intensities throughout the experiments. With the aid of the application of a lower extremity brace and visual EMG feedback, SOL and TA activity were confined by the subject to a narrow range during the task of cycling. Stimulus consistency was achieved through maintenance of flexor hallucis brevis M-waves to within an envelope encompassing the mean value +/- 5% of the maximum M-wave amplitude in all experimental conditions. 4. When the subject's limb was statically positioned, the HFR responses in the SOL muscle showed no significant changes in pattern when compared at various limb positions. During cycling with consistent SOL activity, a response waveform pattern of early-latency-long-duration depression was followed by a later-latency facilitation response in all positions except the initial power phase. The initial power phase was characterized by an additional early-latency facilitation in all but one subject. 5. In the TA muscle response, no change in onset latency (57.5 +/- 0.8 ms, mean +/- SD), waveform pattern, or response amplitude (7.9 +/- 1.1% maximal voluntary contraction, mean +/- SD) was observed during static limb positioning. Significant increases in response amplitude (P < 0.05) coupled with significant increases (9.2 ms, P < 0.05) in onset latency were seen during the transition from the recovery phase to the power phase during cycling. In addition, there was no correlation between the prestimulation baseline level and the onset latency during controlled TA cycling activity conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Whole-cell recording of neuroblastoma x glioma cells during downregulation of a major substrate, 80K/MARCKS, of protein kinase C

Differentiated neuroblastoma cells exhibit both the delayed rectifier potassium current (IK) and the M-current (IM). The present study was designed to determine the roles of protein kinase C (PKC) and of the calmodulin-binding protein 80K/MARCKS, a prominent substrate for PKC and possible regulator of these currents. Neuroblastoma x glioma (NG108-15) hybrid cells transfected with m1 muscarinic receptors were grown with 1% fetal bovine serum (FBS) without the prostaglandin E1 (PGE1) and isobutylmethylxanthine (IBMX) usually added in preparation for electrophysiological studies. Under these conditions, the usual pleomorphism was largely abolished, leaving two populations of small cells with stellate and spherically symmetrical geometries. Whole-cell patch clamping indicated that the two cell types had identical electrophysiological properties, displaying: IK, a small current through a "T-like" Ca2+ channel, and no M-current. Stimulation with carbachol shifted the distribution of cells to a more stellate morphology within 24 hr and later (after 48 hr) reduced the PKC substrate 80K/MARCKS by 22 +/- 7%. In contrast to the stimulation of IK observed with cardiac cells, PKC activation produced only a small inhibition of IK, which was independent of carbachol pretreatment. Thus, PKC and 80K/MARCKS can be dissociated from the regulation of IK in neuroblastoma cells.

Coupling of m2 and m4 muscarinic acetylcholine receptor subtypes to Ca(2+)-dependent K+ channels in transformed NL308 neuroblastoma x fibroblast hybrid cells

Muscarinic acetylcholine receptor (mAChR) subtype (m1-m4)-specific cDNAs were transfected into NL308 neuroblastoma-fibroblast hybrid cells and clones expressing each of the individual mAChR subtypes m1, m2, m3 and m4 obtained. Acetylcholine increased phosphoinositide (PI) turnover in m1- and m3-transformed cells, but did not produce detectable changes in m2- and m4-transformed cells. In cells expressing m1 and m3 subtypes, ACh produced an initial outward K+ current, followed by a cationic current. In cells expressing m2 and m4 receptors, only the initial K+ current was detected. The outward currents were associated with a rise in intracellular Ca2+ as measured with Fura-2 or Indo-1, and were inhibited by chelating intracellular Ca2+ with external BAPTA-AM, or by external charybdotoxin or Ba2+: hence they were attributed to the activation of a Ca(2+)-dependent K+ current. However, the outward current produced in m2- and m4-transformed cells was blocked by pretreatment with 5 ng ml-1 Pertussis toxin (PTX), whereas that in m1- and m3-transformed cells was not. These results suggest that m2- and m4-receptors in transformed NL308 cells coupled to PTX-sensitive G-protein which is capable of mobilizing intracellular Ca2+ and activate IK(Ca), whereas m1 and m3 receptors activate a similar process through a different, PTX-insensitive G-protein.

Closure of potassium M-channels by muscarinic acetylcholine-receptor stimulants requires a diffusible messenger

The M-current (IK(M)) is a slow voltage-gated K+ current which can be inhibited by muscarinic acetylcholine-receptor (mAChR) agonists. In the present experiments we have tested whether this inhibition results from a local (membrane-delimited) interaction between the receptor and adjacent channels, or whether channel closure is mediated by a diffusible messenger. To do this, single KM(+)-channel currents were recorded from membrane patches in dissociated rat superior cervical sympathetic neurons by using cell-attached patch electrodes. Channel activity was inhibited when muscarine was applied to the cell membrane outside the patch but persisted when channels were exposed to muscarine added to the pipette solution. We conclude that a diffusible molecule (or molecules) is (are) required to induce intrapatch channel closure following activation of extra-patch receptors.

Neurotransmitter modulation of calcium channels is dependent on the charge carrier used in the recording of currents

Currents through calcium channels were recorded using calcium, barium and strontium as charge carriers in NG108-15 cells. The mean normalised peak current amplitude at 0 mV was not significantly different between the charge carriers; however, the sustained component (measured at the end of the 500 ms command step) was ca. 3 times larger in barium and strontium. Further, the inhibition by acetylcholine or noradrenaline, although the same at the peak of the current envelope, was significantly greater on the sustained portion of the current for barium and strontium. Increasing internal calcium-buffering (to reduce calcium-dependent inactivation with calcium as the charge carrier) did not increase the amount of inhibition of the sustained portion of current. These results suggest a cautious approach to analysis of neurotransmitter modulation of calcium currents using other charge carriers than calcium.

The whole-cell calcium current in acutely dissociated magnocellular cholinergic basal forebrain neurones of the rat

1. The electrophysiological and pharmacological characteristics of the calcium current (ICa) in acutely dissociated magnocellular cholinergic basal forebrain neurones from 11- to 14-day-old post-natal rats were studied using the whole-cell patch-clamp technique. 2. All cells exhibited a small transient low-voltage-activated (LVA) current with half-activation and half-inactivation potentials of -40.2 and -49.3 mV and slope factors for activation and inactivation of 4.82 and 3.85 mV per e-fold change in membrane potential (Vm) respectively. Activation and inactivation rates for the LVA current were highly voltage dependent. For test potential changes from -50 to -20 mV, the time-to-peak of the current decreased from 39.1 to 6.4 ms, and the time constant of current decay decreased from 81.7 to 15.5 ms. 3. A high-voltage-activated (HVA) component of ICa could be elicited at threshold voltages between -46 and -30 mV from a holding potential (VH) of -80 mV. The HVA current peaked around 0 mV; a 10-fold increase in [Ca2+]o produced a 13 mV positive shift in the peak, whilst the amplitude of the current showed an approximately hyperbolic relationship to [Ca2+]o with half-saturation at 2.5 mM. The transient phase of the HVA current could be described by two exponential functions with time constants tau fast and tau slow of 16.2 and 301 ms. Steady-state inactivation of the transient and extrapolated true sustained (pedestal) components of HVA current were described by Boltzmann equations, with half-inactivation potentials (slope factors) of -47.3 mV, (9.04) and -29.2 mV (11.8) respectively. 4. omega-Conotoxin (omega-CgTX; 100 nM) irreversibly inhibited a kinetically distinct component of HVA current but had no effect upon the transient LVA current. The omega-CgTX-sensitive current could not be distinguished from the control HVA current by the voltage dependence of its activation or inactivation rates. 5. Low concentrations of amiloride (< or = 300 microM) or Ni2+ (< or = 5 microM) selectively inhibited the transient LVA current, with IC50 values of 97 and 5 microM respectively. Cd2+ (< or = 1 microM) selectively blocked a component of HVA current. At higher concentrations, Cd2+ and Ni2+ were non-selective and totally blocked all components of ICa. 6. The lanthanide ions Gd3+ and La3+ produced saturable incomplete block of the HVA current. Maximally effective concentrations of Gd3+ (100 microM) or La3+ (30 microM) inhibited 76.5 and 41.2% respectively of the sustained component of HVA current with corresponding IC50 values of 2.2 and 1.1 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary and secondary chick heart fibroblasts: fast and slow-moving cells show no significant difference in microtubule dynamics

Highly motile chick heart fibroblasts in primary culture (1 degree CHFs) gradually convert into much slower-moving secondary (2 degrees) cells. The polarized movement of the latter, but not the former, cell type has been found to be dependent on an intact microtubule (MT) network [Middleton et al., 1989, J. Cell Sci. 94:25-32]. To investigate the comparative stability of the MT networks of 1 degree s and 2 degrees s, turnover was investigated by microinjection of biotin-labeled brain tubulin to act as a reporter. MTs in both cell types were found to be very dynamic, with the MT networks effectively disassembled by about 30 min in 1 degree CHFs and 60 min in 2 degrees CHFs, with mainly MT fragments remaining beyond these times. All MTs and fragments were found to have turned over by 1 h in 1 degree CHFs and 80 min in 2 degrees s. Because 2 degrees CHFs were found to be on average six times larger than 1 degree s, the difference in MT turnover time was considered largely due to the size difference. For both 1 degree and 2 degrees cells, the more slowly turning over MTs were generally curly and perinuclear in distribution, resembling stable MTs in other systems, but they appeared significantly earlier in CHFs. However, no discrete subpopulations of slower turning over MTs were found to be associated with either the leading edges or the processes of either cell type. In addition, no major differences were identified in the patterns of modified alpha-tubulin along the MTs or of MT cold or drug stability. It is concluded that MTs do not have a direct structural or skeletal function in maintaining a polarized 2 degrees CHF cell shape, but rather play an ancillary role.

Ablation of transplanted HTLV-I Tax-transformed tumors in mice by antisense inhibition of NF-kappa B

Mice transgenic for the human T cell leukemia virus (HTLV-I) Tax gene develop fibroblastic tumors that express NF-kappa B-inducible early genes. In vitro inhibition of NF-kappa B expression by antisense oligodeoxynucleotides (ODNs) inhibited growth of these culture-adapted Tax-transformed fibroblasts as well as an HTLV-I-transformed human lymphocyte line. In contrast, antisense inhibition of Tax itself had no apparent effect on cell growth. Mice treated with antisense to NF-kappa B ODNs showed rapid regression of transplanted fibrosarcomas. This suggests that NF-kappa B expression may be necessary for the maintenance of the malignant phenotype and provides a therapeutic approach for HTLV-I-associated disease.

Interactions between GPI-anchored proteins and membrane lipids

Proteins anchored in membranes by glycosylphosphatidylinositol (GPI) are widely distributed, but the function of this unusual anchor is a puzzle. Recent evidence shows that these proteins can associate with membrane lipids in special ways. One function of GPI anchorage may be to allow proteins to interact with specialized membrane domains.

Characterization of nuclear protein binding to the interferon-gamma promoter in quiescent and activated human T cells

Nuclear protein binding to the human interferon-gamma (IFN-gamma) promoter was investigated to determine the structural basis for the control of gene expression during T cell activation. DNase I footprinting of gel-shift complexes demonstrated that proteins bind to two downstream (-124 to -114 and -36 to -30) and one upstream (-534 to -486) element in the IFN-gamma gene promoter. Treatment of human peripheral blood lymphocytes or continuous T cell tumors with phorbol 12-myristate 13-acetate (PMA) plus phytohemagglutinin or calcium ionophore results in a pattern of response that is similar when using either the upstream or downstream elements. Upon induction of T cells, the lower mobility gel-shift band disappears. Yet the equivalent band which is also present in non-T cells is unperturbed after PMA + calcium ionophore treatment. The higher mobility band which is modified upon induction is restricted to the T cell lineage. Upstream and downstream elements share similar protein-binding motifs as indicated by the homology of footprinted sequences, the similarity of protein-binding patterns, and the ability of these elements to compete against each other in gel-shift protein-binding assays. Protein binding to the downstream elements appears to be interactive, since both sites are required for complex formation. When either of the two downstream elements is disrupted by site-directed mutagenesis, the higher mobility gel-shift band is diminished by an amount that is consistent with the reduction in reporter (chloramphenicol acetyltransferase) gene expression. Therefore, proteins in the ubiquitous gel-shift band appear to be associated with the inactive state of IFN-gamma, while the modified band is closely associated with the positive regulation of IFN-gamma gene expression.