Reduction Of Inward Current Research Articles

Relationship between glial potassium regulation and axon excitability: a role for glial Kir4.1 channels.

Uptake of K(+) released by axons during action potential propagation is a major function of astrocytes. Here, we demonstrate the importance of glial inward rectifying potassium channels (Kir) in regulating extracellular K(+) ([K(+)](o)) and axonal electrical activity in CNS white matter of the mouse optic nerve. Increasing optic nerve stimulation frequency from 1 Hz to 10-35 Hz for 120 s resulted in a rise in [K(+)](o) and consequent decay in the compound action potential (CAP), a measure of reduced axonal activity. On cessation of high frequency stimulation, rapid K(+) clearance resulted in a poststimulus [K(+)](o) undershoot, followed by a slow recovery of [K(+)](o) and the CAP, which were more protracted with increasing stimulation frequency. Blockade of Kir (100 μM BaCl(2)) slowed poststimulus recovery of [K(+)](o) and the CAP at all stimulation frequencies, indicating a primary function of glial Kir was redistributing K(+) to the extracellular space to offset active removal by Na(+)-K(+) pumps. At higher levels of axonal activity, Kir blockade also increased [K(+)](o) accumulation, exacerbating the decline in the CAP and impeding its subsequent recovery. In the Kir4.1-/- mouse, astrocytes displayed a marked reduction of inward currents and were severely depolarized, resulting in retarded [K(+)](o) regulation and reduced CAP. The results demonstrate the importance of glial Kir in K(+) spatial buffering and sustaining axonal activity in the optic nerve. Glial Kir have increasing importance in K(+) clearance at higher levels of axonal activity, helping to maintain the physiological [K(+)](o) ceiling and ensure the fidelity of signaling between the retina and brain.

Dynamic changes of cardiac conduction during rapid pacing

Slow conduction and unidirectional conduction block (UCB) are key mechanisms of reentry. Following abrupt changes in heart rate, dynamic changes of conduction velocity (CV) and structurally determined UCB may critically influence arrhythmogenesis. Using patterned cultures of neonatal rat ventricular myocytes grown on microelectrode arrays, we investigated the dynamics of CV in linear strands and the behavior of UCB in tissue expansions following an abrupt decrease in pacing cycle length (CL). Ionic mechanisms underlying rate-dependent conduction changes were investigated using the Pandit-Clark-Giles-Demir model. In linear strands, CV gradually decreased upon a reduction of CL from 500 ms to 230-300 ms. In contrast, at very short CLs (110-220 ms), CV first decreased before increasing again. The simulations suggested that the initial conduction slowing resulted from gradually increasing action potential duration (APD), decreasing diastolic intervals, and increasing postrepolarization refractoriness, which impaired Na(+) current (I(Na)) recovery. Only at very short CLs did APD subsequently shorten again due to increasing Na(+)/K(+) pump current secondary to intracellular Na(+) accumulation, which caused recovery of CV. Across tissue expansions, the degree of UCB gradually increased at CLs of 250-390 ms, whereas at CLs of 180-240 ms, it first increased and subsequently decreased. In the simulations, reduction of inward currents caused by increasing intracellular Na(+) and Ca(2+) concentrations contributed to UCB progression, which was reversed by increasing Na(+)/K(+) pump activity. In conclusion, CV and UCB follow intricate dynamics upon an abrupt decrease in CL that are determined by the interplay among I(Na) recovery, postrepolarization refractoriness, APD changes, ion accumulation, and Na(+)/K(+) pump function.

Electrophysiological properties of rat retinal Müller (glial) cells in postnatally developing and in pathologically altered retinae.

Retinal glial Müller cells are characterized by dominant K(+) conductances. The cells may undergo changes of their membrane currents during ontogeny and gliosis as described in rabbit and man. Although the rat retina is often used in physiological experiments, the electrophysiology of rat Müller cells is less well studied. The aim of the present study was to characterize their membrane currents in postnatal development and in two models of retinal degeneration. Freshly isolated cells were subjected to whole-cell patch clamp recordings. During the first 4 weeks after birth of rats, their Müller cells displayed an increase in all membrane currents, particularly in the inward currents elicited at hyperpolarizing potentials. The decrease of the membrane resistance from more than 760 MOmega to less than 50 MOmega was accompanied by a shift of the zero current potential from about -20 mV to -80 mV, similar as earlier observed in developing rabbit Müller cells. These developmental changes were found in pigmented Brown Norway rats as well as in rats with inherited retinal dystrophy (RCS rats). Moreover, an infection of Lewis rats with the Borna disease virus caused substantial neuroretinal degeneration but did not result in a strong reduction of inward currents and of the zero current potential of the Müller cells. Thus, rat Müller cells fail to change their basic membrane properties in two different models of retinal pathology. This is in contrast to human and rabbit Müller cells, which have been shown to undergo dramatic changes of their membrane physiology in response to retinal diseases and injuries.

Inhibition by various antipsychotic drugs of the G-protein-activated inwardly rectifying K(+) (GIRK) channels expressed in xenopus oocytes.

To investigate the effects of various chemical classes of antipsychotic drugs: haloperidol, thioridazine, pimozide and clozapine, on the G-protein-activated inwardly rectifying K(+) (GIRK) channels, we carried out Xenopus oocyte functional assays with GIRK1 and GIRK2 mRNAs or GIRK1 and GIRK4 mRNAs. In oocytes co-injected with GIRK1 and GIRK2 mRNAs, application of each of the various antipsychotic drugs immediately caused a reduction of inward currents through the basally active GIRK channels. These responses were not observed in the presence of 3 mM Ba(2+), which blocks the GIRK channels. In addition, in uninjected oocytes, none of the drugs tested produced any significant current response. These results indicate that all the antipsychotic drugs tested inhibited the brain-type GIRK1/2 heteromultimeric channels. Furthermore, similar results were obtained in oocytes co-injected with GIRK1 and GIRK4 mRNAs, indicating that the antipsychotic drugs also inhibited the cardiac-type GIRK1/4 heteromultimeric channels. All the drugs tested inhibited, in a concentration-dependent manner, both types of GIRK channels with varying degrees of potency and effectiveness at micromolar concentrations. Only pimozide caused slight inhibition of these channels at nanomolar concentrations. We conclude that the various antipsychotic drugs acted as inhibitors at the brain-type and cardiac-type GIRK channels. Our results suggest that inhibition of both types of GIRK channels by these drugs underlies some of the side effects, in particular seizures and sinus tachycardia, observed in clinical practice.

Effects of external Rb+ on inward rectifier K+ channels of bovine pulmonary artery endothelial cells.

Inward rectifier (IR) K+ channels of bovine pulmonary artery endothelial cells were studied using the whole-cell, cell-attached, and outside-out patch-clamp configurations. The effects of Rb+ on the voltage dependence and kinetics of IR gating were explored, with [Rb+]o + [K+]o = 160 mM. Partial substitution of Rb+ for K+ resulted in voltage-dependent reduction of inward currents, consistent with Rb+ being a weakly permeant blocker of the IR. In cells studied with a K(+)-free pipette solution, external Rb+ reduced inward IR currents to a similar extent at large negative potentials but block at more positive potentials was enhanced. In outside-out patches, the single-channel i-V relationship was approximately linear in symmetrical K+, but rectified strongly outwardly in high [Rb+]o due to a reduced conductance for inward current. The permeability of Rb+ based on reversal potential, Vrev, was 0.45 that of K+, whereas the Rb+ conductance was much lower, 0.034 that of K+, measured at Vrev-80 mV. The steady state voltage-dependence of IR gating was determined in Rb(+)-containing solutions by applying variable prepulses, followed by a test pulse to a potential at which outward current deactivation was observed. As [Rb+]o was increased, the half-activation potential, V1/2, changed less than Vrev. In high [K+]o solutions V1/2 was Vrev-6 mV, while in high [Rb+]o V1/2 was Vrev + 7 mV. This behavior contrasts with the classical parallel shift of V1/2 with Vrev in K+ solutions. Steady state IR gating was less steeply voltage-dependent in high [Rb+]o than in K+ solutions, with Boltzmann slope factors of 6.4 and 4.4 mV, respectively. Rb+ decreased (slowed) both activation and deactivation rate constants defined at V1/2, and decreased the steepness of the voltage dependence of the activation rate constant by 42%. Deactivation of IR channels in outside-out patches was also slowed by Rb+. In summary, Rb+ can replace K+ in setting the voltage-dependence of IR gating, but in doing so alters the kinetics.

Ion permeation, divalent ion block, and chemical modification of single sodium channels. Description by single- and double-occupancy rate- theory models [published erratum appears in J Gen Physiol 1994 May;103(5):937

Calcium ions, applied internally, externally, or symmetrically, have been used in conjunction with rate-theory modeling to explore the energy profile of the ion-conducting pore of sodium channels. The block, by extracellular and/or intracellular calcium, of sodium ion conduction through single, batrachotoxin-activated sodium channels from rat brain was studied in planar lipid bilayers. Extracellular calcium caused a reduction of inward current that was enhanced by hyperpolarization and a weaker block of outward current. Intracellular calcium reduced both outward and inward sodium current, with the block being weakly dependent on voltage and enhanced by depolarization. These results, together with the dependence of single-channel conductance on sodium concentration, and the effects of symmetrically applied calcium, were described using single- or double-occupancy, three-barrier, two-site (3B2S), or single-occupancy, 4B3S rate-theory models. There appear to be distinct outer and inner regions of the channel, easily accessed by external or internal calcium respectively, separated by a rate-limiting barrier to calcium permeation. Most of the data could be well fit by each of the models. Reducing the ion interaction energies sufficiently to allow a small but significant probability of two-ion occupancy in the 3B2S model yielded better overall fits than for either 3B2S or 4B3S models constrained to single occupancy. The outer ion-binding site of the model may represent a section of the pore in which sodium, calcium, and guanidinium toxins, such as saxitoxin or tetrodotoxin, compete. Under physiological conditions, with millimolar calcium externally, and high potassium internally, the model channels are occupied by calcium or potassium much of the time, causing a significant reduction in single-channel conductance from the value measured with sodium as the only cation species present. Sodium conductance and degree of block by external calcium are reduced by modification of single channels with the carboxyl reagent, trimethyloxonium (TMO) (Worley et al., 1986) Journal of General Physiology. 87:327-349). Elevations of only the outermost parts of the energy profiles for sodium and calcium were sufficient to account for the reductions in conductance and in efficacy of calcium block produced by TMO modification.

The mechanism by which procaine inhibits catecholamine secretion from bovine chromaffin cells.

1. We have investigated the action of procaine on stimulus-secretion coupling in bovine adrenal chromaffin cells. 2. Procaine inhibited the catecholamine secretion evoked by 500 microM carbachol (CCh) with an IC50 of 35 microM and the associated calcium influx (IC50 60 microM). It inhibited the catecholamine secretion evoked by depolarization with high potassium by less than 20% even at the highest concentrations tested (3.2 mM). 3. The secretion evoked by CCh was associated with an increase in sodium influx. This evoked influx was also inhibited by procaine (IC50 80 microM). 4. This selective action of procaine on the CCh-evoked catecholamine secretion was investigated further by patch-clamp techniques. 5. In agreement with the ion flux studies, procaine inhibited the inward current evoked by CCh. Procaine also altered the spectral characteristics of the noise associated with the agonist-induced current by adding an additional high frequency component. The amplitude of this component showed an e-fold increase for a 55 mV membrane hyperpolarization. 6. Data from cell-attached patches showed that increasing concentrations of procaine produced a progressive fall in the mean channel open time and an increase in mean blocked time. This combination led to a decrease in mean burst length. In addition, Popen was reduced by 50 microM procaine. These changes in channel conducting time were sufficient to account for the reduction in inward current. A limited study of the action of procaine on nicotinic channels in outside-out patches gave similar results. 7. The data were considered in relation to various schemes of anaesthetic-channel interactions. The data did not fit the sequential blocking model or the extended channel block model but could be fitted to a modified sequential blocking model in which the rate constant for channel reopening after block was itself subject to modulation by the anaesthetic and the blocked channel could close without passing through the open state.

Effects of histamine on dentate granule cells in vitro

Hippocampal slices from rat brain were exposed to histamine and related substances in a perfusion chamber. Granule cells of the dentate gyrus were studied with conventional extra- and intracellular recording and a single electrode voltage clamp. Histamine caused, through activation of H 2-receptors, a small depolarization, an increase in the number of synaptic and action potentials, a block of the long lasting (but not the early) component of spike afterhyperpolarizations and a reduction of the accommodation of action potential firing. These effects were mimicked by forskolin (suggests activation of adenylate cyclase). In voltage clamp, histamine blocked a long lasting calcium-dependent outward tail current without any reduction of inward current. Thus histamine selectively blocks the late calcium-dependent potassium current in dentate granule cells which receive histaminergic input from the posterior hypothalamus. Histamine also reduces the field excitatory postsynaptic potential evoked by perforant path stimulation. These actions allow for a powerful modulation of excitatory signals and an effective regulation of hippocampal excitability.

Burst-firing inhibition of cell R 15 in Aplysia californica: Pharmacological studies of the effects of tyramine, β-phenethylamine and D-amphetamine

1. Current-voltage relationship studies have shown that both synaptic and dopamine (DA)-induced burst-firing inhibition of cell R 15 in Aplysia californica are due to a reduction of the depolarization-activated inward current which underlies bursting. Bath application of tyramine (TYR), β-phenethylamine (PEA), and D-amphetamine (AMP) produced a similar reduction of this inward current. Kinetics experiments suggest that these three agents do not share a common mechanism with DA. 2. High doses of TYR, PEA and AMP induced synaptic activity through the activation of an interneuron. One form of this activity was inhibition of long duration following excitation (ILD-E). 3. Dihydroergotamine (DHET) blocked the effect of DA and the loss of inward current produced by TYR, PEA and AMP. The ILD-E, however, was not blocked by DHET. This suggests that these agents induce the generation of an ILD-E that is mediated by a transmitter other than DA, and that two receptors mediating burst-firing inhibition are present on cell R 15: a DA-sensitive receptor, and a DHET-insensitive receptor.