Divalent cations activate small- (SK) and large-conductance (BK) channels in mouse neuroblastoma cells: selective activation of SK channels by cadmium

Rapid and Direct Action of Lipopolysaccharide (LPS) on Skeletal Muscle of Larval Drosophila

The endotoxin lipopolysaccharide (LPS) from Gram-negative bacteria exerts a direct and rapid effect on tissues. While most attention is given to the downstream actions of the immune system in response to LPS, this study focuses on the direct actions of LPS on skeletal muscle in Drosophila melanogaster. It was noted in earlier studies that the membrane potential rapidly hyperpolarizes in a dose-dependent manner with exposure to LPS from Pseudomonas aeruginosa and Serratia marcescens. The response is transitory while exposed to LPS, and the effect does not appear to be due to calcium-activated potassium channels, activated nitric oxide synthase (NOS), or the opening of Cl- channels. The purpose of this study was to further investigate the mechanism of the hyperpolarization of the larval Drosophila muscle due to exposure of LPS using several different experimental paradigms. It appears this response is unlikely related to activation of the Na-K pump or Ca2+ influx. The unknown activation of a K+ efflux could be responsible. This will be an important factor to consider in treatments of bacterial septicemia and cellular energy demands.

Fe2+-Mediated Activation of BKCa Channels by Rapid Photolysis of CORM-S1 Releasing CO and Fe2

Heme catabolism by heme oxygenase (HO) with a decrease in intracellular heme concentration and a concomitant local release of CO and Fe²⁺ has the potential to regulate BK_Ca channels. Here, we show that the iron-based photolabile CO-releasing molecule CORM-S1 [dicarbonyl-bis(cysteamine)iron(II)] coreleases CO and Fe²⁺, making it a suitable light-triggered source of these downstream products of HO activity. To investigate the impact of CO, iron, and cysteamine on BK_Ca channel activation, human Slo1 (hSlo1) was expressed in HEK293T cells and studied with electrophysiological methods. Whereas hSlo1 channels are activated by CO and even more strongly by Fe²⁺, Fe³⁺ and cysteamine possess only marginal activating potency. Investigation of hSlo1 mutants revealed that Fe²⁺ modulates the channels mainly through the Mg²⁺-dependent activation mechanism. Flash photolysis of CORM-S1 suits for rapid and precise delivery of Fe²⁺ and CO in biological settings.

Estimation of nonuniform quantal parameters with multiple-probability fluctuation analysis: theory, application and limitations

Synapses are a key determinant of information processing in the central nervous system. Investigation of the mechanisms underlying synaptic transmission at central synapses is complicated by the inaccessibility of synaptic contacts and the fact that their temporal dynamics are governed by multiple parameters. Multiple-probability fluctuation analysis (MPFA) is a recently developed method for estimating quantal parameters from the variance and mean amplitude of evoked steady-state synaptic responses recorded under a range of release probability conditions. This article describes the theoretical basis and the underlying assumptions of MPFA, illustrating how a simplified multinomial model can be used to estimate mean quantal parameters at synapses where quantal size and release probability are nonuniform. Interpretations of the quantal parameter estimates are discussed in relation to uniquantal and multiquantal models of transmission. Practical aspects of this method are illustrated including a new method for estimating quantal size and variability, approaches for optimising data collection, error analysis and a method for identifying multivesicular release. The advantages and limitations of investigating synaptic function with MPFA are explored and contrasted with those for traditional quantal analysis and more recent optical quantal analysis methods.

Short-term plasticity of cyclic adenosine 3',5'-monophosphate signaling in anterior pituitary corticotrope cells: the role of adenylyl cyclase isotypes

Anterior pituitary corticotropes show a wide repertory of responses to hypothalamic neuropeptides and adrenal corticosteroids. The hypothesis that plasticity of the cAMP signaling system underlies this adaptive versatility was investigated. In dispersed rat anterior pituitary cells, depletion of intracellular Ca2+ stores with thapsigargin combined with ryanodine or caffeine enhanced the corticotropin releasing-factor (CRF)-evoked cAMP response by 4-fold, whereas reduction of Ca2+ entry alone had no effect. CRF-induced cAMP was amplified 15-fold by arginine-vasopressin (AVP) or phorbol-dibutyrate ester. In the presence of inhibitors of cyclic nucleotide phosphodiesterases and phorbol-dibutyrate ester, the depletion of Ca2+ stores had no further effect on CRF-induced cAMP accumulation. Adenohypophysial expression of mRNAs for the Ca2+-inhibited adenylyl cyclases (ACs) VI and IX, and the protein kinase C-stimulated ACs II and VII was demonstrated. ACIX was detected in corticotropes by immunocytochemistry, whereas ACII and ACVI were not present. The data show negative feedback regulation of CRF-induced cAMP levels by Ca2+ derived from ryanodine receptor-operated intracellular stores. Stimulation of protein kinase C by AVP enhances Ca2+-independent cAMP synthesis, thus changing the characteristics of intracellular Ca2+ feedback. It is proposed that the modulation of intracellular Ca2+ feedback in corticotropes by AVP is an important element of physiological control.

Effects of Fe(2+) on ion channels: Na(+) channel, delayed rectified and transient outward K(+) channels

The effects of Fe(2+) on the properties of three types of ion channels were studied in acutely dissociated rat hippocampal pyramidal neurons from area CA1 at postnatal ages of 7-14 days using the whole cell patch clamp technique. The results indicated that: (1) in the existence of Fe(2+), the activation voltage threshold of transient outward K(+) currents (I(A)) was decreased. The normalized current-voltage curves of activation were well fitted with a single Boltzmann function, and the V(1/2) was 2.44+/-1.14 mV (n=15) in control, whereas 1.79+/-1.53 (n=15), -2.96+/-0.92 (n=14), -5.11+/-1.31 (n=13), -9.05+/-1.64 mV (n=12) in 1, 10, 100 and 1000 microM Fe(2+), respectively. Differences between two groups were significant (P<0.05, n=12-15), except for that between the control and 1 microM (P>0.05, n=15). (2) Fe(2+) caused a left shift of the current-voltage curves of steady-state inactivation of I(A) in a concentration-dependent manner. The curves were well fitted with a single Boltzmann function with similar slope (P>0.05, n=10-13). The V(1/2) were -70.71+/-1.23 (n=13), -71.14+/-1.37 (n=13), -78.21+/-1.17 (n=11), -84.61+/-1.34 (n=12), and -89.68+/-2.59 mV (n=10) in control, 1, 10, 100 and 1000 microM Fe(2+), respectively. Fe(2+) also shifted the current-voltage curves of Na(+) channel steady-state inactivation to more negative depolarization potentials in parallel, with V(1/2), -67.37+/-1.33 mV (n=12) in control, and -67.52+/-1.28 mV (n=12), -68.24+/-1.61 mV (n=10), -71.58+/-1.45 mV (n=10), -76.65+/-1.76 mV (n=9) in 1, 10, 100 and 1000 microM Fe(2+) solutions, respectively. (3) In Fe(2+) solutions, the recovery from inactivation of I(A) was slowed. (4) With application of different concentrations of Fe(2+), the voltage threshold of activation of delayed rectified outward K(+) currents (I(K)) was decreased, while Fe(2+) showed a little inhibition at more positive depolarization. Briefly, the results demonstrated that Fe(2+) is a dose- and voltage-dependent, reversible modulator of I(A), I(K) and Na(+) channels. The results will be helpful to explain the mechanism of Fe(2+) physiological function and Fe(2+) intoxication in the central nervous system.

Activation of recombinant human SK4 channels by metal cations

The effects of metal cations on the activation of recombinant human SK4 (also known as hIK1 or hKCa4) channels, expressed in HEK 293 cells, were tested using patch clamp recording. Of the nine metals tested, cobalt, iron, magnesium, and zinc did not activate the SK4 channels when applied, at concentrations up to 100 microM, to the inside of SK4 channel-expressing membrane patches. Barium, cadmium, calcium, lead, and strontium activated SK4 channels in a concentration-dependent manner. The rank order of potency was at Ca2+ > Pb2+ > Cd2+ > Sr2+ > Ba2+.

Potentiation and inhibition of subtypes of neuronal nicotinic acetylcholine receptors by Pb2+

Effects of inorganic lead (Pb2+) on defined subtypes of neuronal nicotinic acetylcholine receptors have been investigated. Voltage clamp experiments have been performed on Xenopus oocytes expressing alpha 3 beta 2, alpha 3 beta 4 and alpha 4 beta 2 neuronal nicotinic acetylcholine receptor subunit combinations. In oocytes expressing the alpha 3 beta 2 subunit combination Pb2+ enhances the peak amplitude of nicotinic acetylcholine receptor-mediated inward currents evoked by superfusion with 100 microM acetylcholine. At concentrations of 1-250 microM, Pb2+ potentiates alpha 3 beta 2 receptor-mediated inward current concentration dependently by a factor of 1.1-11.0. Inward currents evoked by low (3 microM) and high (1 mM) concentrations of acetylcholine are potentiated to a similar extent. Conversely, in oocytes expressing the alpha 3 beta 4 subunit combination Pb2+ inhibits the nicotinic receptor-mediated inward currents evoked with 100 microM acetylcholine. Inhibitory effects are observed in the concentration range of 1 nM-100 microM Pb2+ but the degree of inhibition varies between oocytes. A similar inhibition of the alpha 4 beta 2 nicotinic receptor-mediated inward current by Pb2+ indicates that alpha as well as beta subunits are involved in the potentiating and inhibitory effects. Possible reasons for the variation in the inhibitory effects of Pb2+ on alpha 3 beta 4 and alpha 4 beta 2 nicotinic receptor-mediated inward currents have been investigated and are discussed. The divalent cations Ca2+ and Mg2+ potentiate both alpha 3 beta 2 and alpha 3 beta 4 nicotinic receptor-mediated inward currents. The distinct modulation of receptor function by Pb2+ and by Ca2+ and Mg2+ and the dependence of the modulatory effect of Pb2+ on subunit composition suggest that Pb2+ interacts with multiple sites on the alpha and beta subunits of neuronal nicotinic acetylcholine receptors.

Evaluation of single-channel gating kinetics produced after amplitude-based separation of unitary currents

Cell-attached patch-clamp recording has been established as a major technique for investigating ion channel behaviour in a physiological setting, despite the problems which arise in analysing records containing more than one type of unitary current. To circumvent these problems, single-channel amplitude-based assignment of discrete single-channel events to different channel types becomes increasingly necessary. Surprisingly, a systematic evaluation of the validity of this method in determining single-channel parameters has not been performed to date. Using computer-stimulated single-channel traces, and recordings from a biological preparation containing well-characterized ion channels (N1E-115 neuroblastoma cells), we have explored the accuracy by which amplitude-based separation recovers ion-channel parameters. Determination of gating kinetics after separation revealed that even a very small contamination in the selected population yields additional time constants in the probability density functions. Therefore, our results demonstrate that, whereas the use of amplitude-based separation is straightforward for determining slope conductance and reversal potential, it is prone to incorporate errors in establishing gating kinetics. Ways of identifying such errors are described.

Metal ion-induced permeability changes in cell membranes: a minireview

1. The paper summarizes the effects of the metal ions Cu2+, Pb2+, Ag+, Hg2+, Zn2+, and Cd2+ applied externally or internally to the surface membrane of different excitable cells. 2. Conductance changes induced by metal ions, and metal ion-activated current, are compared with respect to their ion and voltage dependence. 3. It is suggested that metal ion-induced effects can be realized through special structures of the cell membrane, the metal ion "receptors," although other mechanisms, as, for example, competition for Ca-binding sites in the channel forming proteins, cannot be excluded.

Differential effects of heavy metal ions on Ca(2+)-dependent K+ channels

1. The ability of various divalent metal ions to substitute for Ca2+ in activating distinct types of Ca(2+)-dependent K+ [K+(Ca2+)] channels has been investigated in excised, inside-out membrane patches of human erthrocytes and of clonal N1E-115 mouse neuroblastoma cells using the patch clamp technique. The effects of the various metal ions have been compared and related to the effects of Ca2+. 2. At concentrations between 1 and 100 microM Pb2+, Cd2+ and Co2+ activate intermediate conductance K+(Ca2+) channels in erythrocytes and large conductance K+(Ca2+) channels in neuroblastoma cells. Pb2+ and Co2+, but not Cd2+, activate small conductance K+(Ca2+) channels in neuroblastoma cells. Mg2+ and Fe2+ do not activate any of the K+(Ca2+) channels. 3. Rank orders of the potencies for K+(Ca2+) activation are Pb2+, Cd2+ > Ca2+, Co2+ >> Mg2+, Fe2+ for the intermediate erythrocyte K+(Ca2+) channel, and Pb2+, Cd2+ > Ca2+ > Co2+ >> Mg2+, Fe2+ for the small, and Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+ for the large K+(Ca2+) channel in neuroblastoma cells. 4. At high concentrations Pb2+, Cd2+, and Co2+ block K+(Ca2+) channels in erythrocytes by reducing the opening frequency of the channels and by reducing the single channel amplitude. The potency orders of the two blocking effects are Pb2+ > Cd2+, Co2+ >> Ca2+, and Cd2+ > Pb2+, Co2+ >> Ca2+, respectively, and are distinct from the potency orders for activation. 5. It is concluded that the different subtypes of K+(Ca2+) channels contain distinct regulatory sites involved in metal ion binding and channel opening. The K+(Ca2+) channel in erythrocytes appears to contain additional metal ion interaction sites involved in channel block.

Nicotinic acetylcholine receptors in cultured cells as targets of neurotoxic compounds

Using electrophysiological techniques, effects of neurotoxicants were studied on mammalian neuronal and endplate type nicotinic acetylcholine receptors (nAChR) in N1E-115 cells and in BC3H(1) cells, respectively, and insect nAChR in locust neurons. Neuronal nAChR are highly sensitive to inorganic lead (Pb(2+)). Between 1 nm and 3 mum-Pb(2+) the ACh-induced inward current is blocked in a concentration-dependent manner (IC(50) = 19 nm; maximal effect (E(max)) = 90%). In contrast, the serotonin 5-HT(3) receptor is far less sensitive to Pb(2+) (IC(50) = 49 mum). Surprisingly, between 10 mum and 100 mum Pb(2+) the blocking effect on the nAChR is reversed, and the kinetics of the ACh-induced inward current are delayed. Nitromethylene heterocyclic (NMH) compound constitute a new class of selective insecticides, that presumably affect insect nAChR. The effect of the NMH compound 1-(pyridin-3-yl-methyl)-2-nitromethylene-imidazolidine (PMNI) on the different subtypes of nAChR has been analysed. Distinct agonistic effects of PMNI on nAChR are observed on insect neurons only. Further, PMNI blocks nicotinic responses mediated by the different subtypes of nAChR in the following potency order: locust å neuronal type endplate type nAChR. These results demonstrate that the analysis of electrophysiological endpoints in cultured cells is a valuable approach to the investigation of target site selectivity and species specificity of neurotoxic compounds.