Elementary currents through Ca2+ channels in Guinea pig myocytes

Elementary Ca2+ and Ba2+ currents were recorded from cell-attached membrane patches of ventricular myocytes from adult guinea pig hearts using the improved patch-clamp technique (Hamill et al. 1981). High concentrations of Ba2+ or Ca2+ (50 or 90 mM) were used in the pipettes to increase the signal-to-noise ratio. All data were derived from elementary current analyses in patches containing only one channel. 1) In response to voltage steps, channel openings occurred singly or in bursts of closely spaced unitary current pulses separated by wider shut intervals. During depolarizations of small amplitude from the resting potential, channel openings occurred almost randomly, whereas during larger depolarizations the events were grouped preferentially at the beginning. 2) Channel openings became more probable with increased depolarization; simultaneously, unitary current amplitudes declined in an ohmic manner. Elementary current amplitudes were slightly larger, when 50 mM Ba2+ replaced 50 mM Ca2+ in the pipettes (slope conductances 9 and 10 pS, respectively), but more than doubled, when Ba2+ was increased to 90 mM (slope conductance 18 pS). Clear outward currents through Ca2+ channels were not observed under these conditions. 3) Peak amplitudes of reconstructed mean currents doubled when 50 mM Ba2+ replaced 50 mM Ca2+ and were larger still when 90 mM Ba2+ was used in the pipettes. The current-voltage relations of the reconstructed mean currents showed a positive shift along the voltage axis as Ba2+ was increased or substituted equimolarly by Ca2+. correspondingly, the open state probability-voltage relations (activation curves) showed a parallel shift as Ba2+ was increased, which was less pronounced when Ba2+ was replaced equimolarly by Ca2+. 4) Determination of Ca2+ channel inactivation using 90 mM Ba2+ in the pipettes indicated an overlap with channel activation in a limited voltage range, resulting in a steady-state "window" current. Inactivation can occur without divalent cation influx. 5) Formation of an inside-out patch resulted in a fast rundown of elementary Ca2+ channel currents. 6) Channel openings were often grouped in bursts. The lifetimes of the open state, the bursts, and the closed states were estimated for Ba2+ and Ca2+ as permeating ions. At least two exponentials were needed to fit the histogram of the lifetimes of all closed states. The lifetimes of the individual openings and bursts were mono-exponentially distributed. The kinetics of the Ca+ channel depended on the voltage and the permeating ion. During +30 mV depolarizations, no significant effect on the permeating ion on channel gating could be detected.(ABSTRACT TRUNCATED AT 400 WORDS)     

Single-channel analysis of fast transient outward K+ currents in frog skeletal muscle

 The patch-clamp method was employed to examine the voltage-dependent gating mechanism of A-type K⁺ channels, which generate the transient outward K⁺ currents described previously in a study of vesicles derived from the sarcolemma of frog skeletal muscle. Channels were activated by depolarizing pulses. There is evidence for non-random grouping of records with channel openings and blank records when depolarizations were repeated at brief intervals, suggesting a slow process similar to slow inactivation. Binomial analysis was consistent with independent behaviour of the channels. Ensemble average currents obtained from multichannel patches had kinetics similar to those of the macroscopic A-type K⁺ current, I _A. The rate of activation, fitted to n ⁴ kinetics, was fast and voltage dependent. The rate of inactivation had an exponential time course with a voltage-independent time constant. The mean open time and the probability of a channel being open increased with depolarization. The histograms of latency to first opening revealed the presence of more than two voltage-dependent closed states. Channel openings occurred in bursts and the closed-time histograms could be fitted by the sum of two or three exponentials. These results suggest a gating scheme with at least three closed states, probably two open states, and two inactivated states. Received: 4 November 1997 / Received after revision: 9 January 1998 / Accepted: 12 January 1998     

Single voltage-dependent potassium channels in cultured rat hippocampal neurons

Single-channel recordings using the gigohm seal patch-clamp technique were carried out on the somatic membranes of dissociated embryonic rat hippocampal neurons grown in cell culture. The recording medium contained tetrodotoxin to block the voltage-dependent Na+ conductance and Cd2+ to block Ca2+ and Ca2+-activated conductances. In the cell-attached configuration, depolarizing voltage steps activated outward directed single-channel currents with conductance 15-20 pS. The channel openings exhibited a moderate degree of flickering. The mean burst lifetimes ranged from 5 to 13 ms with a tendency to increase slightly at more depolarized potentials (T = 21-25 degrees C). Reversal potential measurements using excised membrane patches indicated that the channels behaved as expected of a K+-selective membrane pore. Channel opening occurred in Ca2+-free EGTA-containing solutions but was never observed in the presence of tetraethylammonium (TEA; 20 mM). The frequency of channel opening increased as the membrane was depolarized by up to 50 mV from resting potential; the fraction of time spent in the open state during the first 300 ms following a step depolarization increased e-fold for a 8-25 mV change in potential. First-latency histograms and simulations of the macroscopic current based on channel data obtained during repeated depolarizing voltage steps indicated that the probability of the channel being in the open state increases gradually with time after a step depolarization. During repeated depolarizing steps the channels appeared to randomly enter and exit a long-lived inactive state. It is concluded that these channels may underly the slowly activating, very slowly inactivating, TEA-sensitive voltage-dependent K+ current (IK) in cultured hippocampal neurons.     

Single-channel analysis of a point mutation of a conserved serine residue in the S2 ligand-binding domain of the NR2A NMDA receptor subunit

We have examined the function of a conserved serine residue (Ser670) in the S2 ligand-binding region of the NR2A N -methyl- d -aspartate (NMDA) receptor subunit, using recombinant NR1/NR2A receptors expressed in Xenopus laevis oocytes. Mutation of Ser670 to glycine (S670G) in NR2A reduced the potency of glutamate by 124-fold. Single-channel conductance and the duration of apparent open periods of NR2A(S670G) receptor mutants were, however, indistinguishable from wild-type NMDA receptors. NR1/NR2A(S670G) shut-time distributions were best described by a mixture of six exponential components, and the four shortest shut intervals of each distribution were considered to occur within a channel activation (burst). Bursts of single-channel openings were fitted with a mixture of four exponential components. The longest two components carried the majority of the charge transfer and had mean durations of 9.6 ± 0.5 and 29.6 ± 1.5 ms. The overall channel open probability during a burst was high (mean, 0.83 ± 0.06). Consistent with a shortening of NMDA receptor-channel burst lengths was the observation of an increased deactivation rate of macroscopic currents evoked by brief applications of glutamate to outside-out membrane patches. Correlations between shut times and adjacent open times were observed in all data records. Noticeably, shorter than average openings tended to occur next to long closed periods, whereas longer than average openings tended to occur next to short closings. Our single-channel data, together with modelling using a kinetic scheme to describe channel activations, support our hypothesis that the S670G point mutation reduces the dwell time of glutamate in its binding site.     

Microscopic heterogeneity in unitary N-type calcium currents in rat sympathetic neurons.

1. Single N-type calcium (Ca2+) channels in rat superior cervical ganglion neurons display complex patterns of activity in both inactivating and non-inactivating gating modes. Unitary currents were elicited by holding the patch at -90 mV and stepping to +30 mV for 740 ms. Barium (110 mM) was used as the charge carrier. The dihydropyridine agonist (+)-202-791 was included in the bath to ensure that single channel recordings showed no L-type Ca2+ channel mode 2 activity. Using this protocol, we characterized three additional patterns of N-type Ca2+ channel activity named: (1) LLP for large unitary current amplitude (i = -0.92 pA) and low open probability (Po = 0.26); (2) SLP for small unitary current amplitude (i = -0.77 pA) and low open probability (Po = 0.25); and (3) SHP for its small unitary current (i = -0.77 pA) and higher open probability (Po = 0.39). 2. Transitions among these patterns of activity occur more slowly than transitions between closed and open states, resulting in significant clustering of like sweeps. Thus, the complicated gating of single N-type Ca2+ channels can be dissected into multiple, independent modes, each with the same reproducible pattern of activity. 3. This heterogeneous activity is not unique to sympathetic neurons, for inactivating (4), non-inactivating (4), SLP (4) and SHP (3 patches) gating modes were also observed in cell-attached patch recordings (n = 4) of single N-type Ca2+ channels in differentiated phaeochromocytoma (PC12) cells. 4. The 1568 sweeps from four single N-type Ca2+ channel recordings that used the same voltage protocol were categorized by mode to determine the frequency of occurrence of each. Of the 54% of sweeps that showed activity, 42% were inactivating and 58% were non-inactivating. The contribution by each mode to the sustained current was estimated using the equation: I = NPoi, where N is the frequency of occurrence of each mode and Po and i are the mean values of open probability and unitary current amplitude respectively. The LLP mode contributed 18%, the SLP mode 16%, and the SHP mode 66% of the sustained whole cell N-type Ba2+ current. 5. The variability in the incidence among these modes in other cell types may resolve some of the controversy surrounding the characterization of N- and L-type whole cell Ca2+ current components in peripheral neurons. In addition, the number of different modes provides a source of plasticity that may be a target of modulation by neurotransmitters and cellular signals.     

Modal behavior of the Kv1.1 channel conferred by the Kvβ1.1 subunit and its regulation by dephosphorylation of Kv1.1

Modulation of fast-inactivating voltage-gated K+ channels can produce plastic changes in neuronal signaling. Previously, we showed that the voltage-dependent K+ channel composed of brain Kv1.1 and Kvbeta1.1 subunits (alpha(beta) channel) gives rise to a current that has a fast-inactivating and a sustained component; the proportion of the fast-inactivating component could be decreased by dephosphorylation of a basally phosphorylated Ser-446 on the alpha subunit. To account for our results we suggested a model that assumes a bimodal gating of the alpha(beta) channel. In this study, using single-channel analysis, we confirm this model. Two modes of gating were identified: (1) an inactivating mode characterized by low open probability and single openings early in the voltage step, and (2) a non-inactivating gating mode with bursts of openings. These two modes were non-randomly distributed, with spontaneous shifts between them. Each mode is characterized by a different set of open time constants (tau) and mean open times (t(0)). The non-inactivating mode is similar to the gating mode of a homomultimeric alpha channel. The phosphorylation-deficient alphaS446Abeta channel has the same two gating modes. Furthermore, alkaline phosphatase promoted the transition to the non-inactivating mode. This is the first report of modal behavior of a fast-inactivating K+ channel; furthermore, it substantiates the notion that direct phosphorylation is one mechanism that regulates the equilibrium between the two modes and thereby regulates the extent of macroscopic fast inactivation of a brain K+ channel.     

Effect of acetylcholine on single Ca2(+)-activated K+ channels in bovine chromaffin cells

The single channel current amplitudes of "maxi" Ca2(+)-activated K+ channels from bovine chromaffin cell membranes are reduced when acetylcholine is applied to the internal surface of the membrane, which can be explained by a fast channel block. The block is concentration dependent with moderate affinity. It becomes progressively greater with depolarization although the voltage dependence is not pronounced. Acetylcholine reduces the probability of the open state in the same concentration range and in an essentially voltage independent manner. The changes in the channel kinetics are complex. Whilst the long component of the open intervals is shortened (by 48%; from 9.5 to 5.1 ms), the long component of the closed intervals is prolonged (by 96%; from 45 to 89 ms). The short components (open and closed) are essentially unaffected. Short open intervals are reduced by 4% (from 1.09 to 1.05 ms), whilst short closed intervals are reduced by 5% (from 2.3 to 2.2 ms). These changes in the channel kinetics can be explained at least partly if one assumes that acetylcholine, in addition to its fast channel blocking activity, acts also as a slow blocker. If so, both binding sites are expected to be located close to the mouth of the channel pore. Alternatively, acetylcholine may be affecting the gating mechanism, presumably by interfering with the Ca2+ binding.     

Modal gating transitions in cardiac ryanodine receptors during increases of Ca2+ concentration produced by photolysis of caged Ca2+

Channel adaptation is a basic property of the sarcoplasmic reticulum Ca²⁺-release channels/ryanodine receptors (RyRs). It allows channel activity to decay during sustained increases in the concentration of activating Ca²⁺. Despite the potential physiological importance of this self-confining process, its molecular mechanism is not well understood. To define the mechanism of adaptation we studied the dynamics of cardiac Ca²⁺-release channel (RyR) gating using the planar lipid bilayer technique in combination with photolysis of caged Ca²⁺ (DM-nitrophen). Channels activated by rapid and sustained increases in Ca²⁺ concentration (from 0.1 to 0.5 μmol/l) displayed three distinct gating modes, manifested as current records with frequent and long openings (H-mode), with rare and short openings (L-mode), and with no openings (I-mode). H-mode channel activity occurred primarily at early times while L- and I-modes predominated at late times after the rapid Ca²⁺ concentration increase. The decrease in probability of H-mode, mirrored by an increase in the probability of the I-mode, proceeded with a time constant similar to that observed for spontaneous decay in channel activity (i.e., adaptation) in ensemble average records. These results indicate that RyR adaptation transpires by a shift of channel gating from a high open probability mode to low open probability and inactivated modes of the channel. Received: 1 March 1999 / Received after revision: 16 April 1999 / Accepted: 19 April 1999     

Different conductance states of the bursting Na channel in guinea-pig ventricular myocytes

Single channel currents flowing through Na channels were studied in cell-attached patches from guinea-pig ventricular myocytes after removal of the fast component of inactivation by the S enantiomer of DPI 201-106. In addition to openings with a single channel conductance of 15 pS, two other types of events occur. A low conductance state has a single channel conductance of 5 pS, while a medium conductance state can sometimes be seen at 8 pS. These different conductance states can coexist in the same patch. The low conductance state appears to have the same kinetic properties as the high conductance state. However it is less sensitive to block by TTX. Averaged currents obtained from exclusively low conductance openings also show a decay (time constant at –30 mV of 0.8±0.4s) which is not significantly different from the decay of the high conductance channel. The average open and closed times are also similar to the values for the high conductance channel. The medium conductance state has a single channel conductance of 8 pS and a reversal potential which is similar to the high conductance channel. Closings of the high conductance state to the medium conductance state can be observed as well as openings from the medium to the high conductance state. The average current of sweeps showing only the medium conductance state has the same time course and shows a similar TTX-sensitivity as the average current of the high conductance state. Also the voltage range for steady-state activation and inactivation for the medium conductance channel matches that of the high conductance channel. Furthermore the open and closed time distributions of the medium channel are described by two exponentials. The mean open and closed tunes are not significantly different from the ones of the high conductance state. We argue that the low conductance channel is likely to be a second type of Na channel, whereas the medium conductance channel appears to be a low conductance substate of the normal bursting Na channel.     

Protein kinase C activation inhibits α1D L-type Ca channel: A single-channel analysis

The recently reported α_1D Ca channel in the heart is known to be regulated by protein kinase C (PKC) at the whole cell level and has been implicated in atrial fibrillation. The biophysical basis of this regulation at the single-channel level is not known. Therefore, the effect of PKC activation was studied on α_1D Ca channel expressed in tsA201 cells using cell-attached configuration. Unitary currents were recorded in the presence of 70 mM Ba²⁺ as the charge carrier at room temperature. Under basal condition, channel activity was rare and infrequent; however, Bay K 8644 (1 μM) induced channel openings with a conductance of 22.3 pS. Single channel analysis of open and closed time distributions were best fitted with a single exponential. PKC activation by 4α-phorbol 12-myristate 13-acetate (PMA; 10 nM), a phorbol ester derivative, resulted in a decrease in open probability and increase in closed-time without any significant effect on the conductance of the α_1D Ca channel. This is consistent with a decreased entry of α_1D Ca channel into open states in the presence of PMA. PMA effects could not be reproduced by 4-α Phorbol, an inactive PMA analogue. These data show, for the first time, (1) the α_1D Ca channel activity at the single-channel level and (2) the biophysical basis by which PKC activation inhibits the α_1D Ca channel. The shortening of the open-time and the lengthening of the closed-time constants and the increase in blank sweeps may explain the inhibition of the previously reported whole-cell α_1D Ca current. Altogether, these data are essential for understanding the complex role of α_1D Ca channel not only in physiological settings but also in pathological settings such as atrial fibrillation.     

Ca2+ and voltage dependence of cardiac ryanodine receptor channel block by sphingosylphosphorylcholine

The effect of sphingosylphosphorylcholine (SPC) on the cytoplasmic Ca(2+) and voltage dependence of channel gating by cardiac ryanodine receptors (RyR) was examined in lipid bilayer experiments. Micromolar concentrations of the lysosphingolipid SPC added to cis solutions rapidly and reversibly decreased the single-channel open probability (P(o)) of reconstituted RyR channels. The SPC-induced decrease in P(o) was marked by an increase in mean closed time and burst-like channel gating. Gating kinetics during intraburst periods were unchanged from those observed in the absence of the sphingolipid, although SPC induced a long-lived closed state that appeared to explain the observed decrease in channel P(o). SPC effects were observed over a broad range of cis [Ca(2+)] but were not competitive with Ca(2+). Interestingly, the sphingolipid-induced, long-lived closed state displayed voltage-dependent kinetics, even though other channel gating kinetics were not sensitive to voltage. Assuming SPC effects represent channel blockade, these results suggest that the blocking rate is independent of voltage whereas the unblocking rate is voltage dependent. Together, these results suggest that SPC binds directly to the cytoplasmic side of the RyR protein in a location in or near the membrane dielectric, but distinct from cytoplasmic Ca(2+) binding sites on the protein.     

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle.

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.     

A human congenital myasthenia-causing mutation (ɛL78P) of the muscle nicotinic acetylcholine receptor with unusual single channel properties

A mutation in the epsilon subunit of the human nicotinic acetylcholine receptor (ɛL78P) is known to cause a congenital slow channel myasthenic syndrome. We have investigated the changes in receptor function that result in the mutant receptor producing prolonged endplate currents, and consequent muscle damage. The rate constants for channel gating and for the binding and dissociation of acetylcholine were investigated by analysis of single ion channel recordings. A conventional mechanism with two non-equivalent binding sites, and variations upon this mechanism, were fitted to data using a maximum likelihood method that uses the Hawkes-Jalali-Colquhoun (HJC) treatment of missed brief events. The mutant receptor produced prolonged activations, bursts of openings that cause a slow decay of simulated synaptic currents. The main reason for the longer bursts of openings seen with mutant receptor was a decrease in the rate of ACh dissociation from diliganded receptors, though the lifetime of individual openings was somewhat increased too. As well as producing long bursts, the mutant receptor also produced many very short openings, though these carry little current. The burst structure for the mutant receptor at low ACh concentration is unusual in that most long bursts appear to start in a very brief monoliganded open state that then usually binds another ACh molecule to produce a long diliganded activation. The first opening is so short that it will usually be missed (together with the shut time that follows it), so the true burst length is likely to be underestimated.     

Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels

Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.     

The molecular mode of action of the Ca agonist (−) BAY K 8644 on the cardiac Ca channel

The primary drug action of (–) BAY K 8644 on whole-cell Ca current in atrial myocytes was measured under conditions where secondary Ca-mediated changes of Ca channel activity were minimized. The most direct action of (–) BAY K 8644 is the change of gating kinetics which results in a strictly voltage-dependent increase of the peak current in the voltage range between –40 and 0 mV. Peak currents were increased dose dependently in the concentration range from 1 to 30 nM. Analysis of peak current/voltage relations revealed a linear shift of the current activation by approximately 23 mV to more negative membrane potentials, without any change in its voltage dependence and in the current reversal potential or the maximum whole-cell conductance. Measurement of Ca current activation and deactivation time constants suggests that (–) BAY K 8644 prolongs the single-channel open time without affecting the closed time. From the shift of the open time function to more negative voltages by about 50 mV the energy transferred to the gating process is calculated to be 5.4 kJ/mol (1.3 kcal/mol). The drug-induced slow component of tail current has been used to estimate the true dose/response relation for (–) BAY K 8644. A K _D value of 4.3 nM and a Hill coefficient of 1.25 were determined. Flash-induced competition experiments with the Ca antagonist nifedipine allowed the measurement of binding kinetics of (–) BAY K 8644. The association rate constant is estimated to about 5×10⁶ mol^–1 · s^–1 and dissociation time constant is approximately 50–70 s; both are in close agreement with receptor binding studies. Results are discussed in relation to models for drug action of dihydropyridine-type compounds and to implications for the structure of the Ca channel protein.     

Effects of thrombin on single calcium channels in frog ventricular cells

Single calcium channel (Ca channel) currents were measured using the patch-clamp technique in isolated ventricular myocytes of the frog (Rana esculenta). Sodium was used as the charge carrier. After formation of cell-attached patches, the proteolytic enzyme thrombin was added to the bath solution, where it increased the amplitude of the averaged currents more than twofold, by decreasing the number of empty sweeps and reducing the time constant of the slow exponential term of the shut-time histogram. Single channel conductance was not changed by thrombin. If the activation kinetics of the Ca channels are described by the commonly usedC ₁-C ₂-O model, whereC ₁ andC ₂ indicate closed states 1 and 2 respectively andO denotes the open state, thrombin increases the open-state probability in the non-empty sweeps by increasing the rate constant (k ₁) for the transition fromC ₁ toC ₂. It is shown that thrombin acts via an H-7 blockable pathway.     

Membrane potential modulates the activation of GABA-gated channels

1. The activity of single gamma-aminobutyric acid (GABA)-gated Cl- channels (GABA = 0.5-2.0 microM) was recorded in inside-out patches of membrane from cultured chick cerebral neurons. 2. The distribution of open intervals of the GABA channel was described by the sum of two exponentials, which suggests the presence of at least two open states of the channel. The time constants of these two components were 0.39 +/- 0.1 and 2.1 +/- 0.9 ms (+/- SD, n = 9). 3. The distribution of shut intervals was described by the sum of either three (n = 5) or four (n = 3) exponentials. This suggests the presence of at least three or four shut states. 4. At all GABA concentrations examined, the activity of the GABA channel decreased over time. This decline in activity was most likely the result of desensitization of the GABA channels. 5. The distribution of open intervals was unchanged during desensitization of the GABA channel. Thus desensitization is not associated with an alteration in either the mean lifetime of the two open states or the relative number of transitions to these two states. Rather, desensitization results from a decrease in the probability of channel opening. 6. There was an e-fold increase in the probability of finding a GABA channel open for every 80 +/- 43 mV (n = 4) of depolarization. The degree of voltage dependence decreased as the GABA channels desensitized. 7. The depolarization-induced increase in open channel probability was not associated with any change in the distribution of open intervals. Thus depolarization does not affect the mean open time of the channel but rather increases the likelihood that it will open. 8. A simple model with three or four shut and two open states is considered for the gating of the GABA channel by the agonist. Possible sites for the voltage dependence within this proposed model are discussed.     

β-Adrenergic modulation of prepulse facilitation of L-type calcium channels in rabbit ventricular myocytes

It is reported that strong depolarization augments cardiac L-type Ca currents by inducing the special gating mode with long-lasting openings (mode 2) (Pietrobon and Hess Nature 346:651-655, 1990). However, a prepulse to +90 mV did not obviously facilitate the current at 0 mV in rabbit ventricular myocytes as measured in the whole-cell configuration of the patch-clamp method in the presence of 2 mM BaCl(2) in the external solution. In the presence of isoproterenol (1 microM), the inactivation during the prepulse was attenuated, and the prepulse evoked facilitation. However, the current at 0 mV whose amplitude was normalized to the extent of the inactivation at +90 mV still exhibited greater facilitation in the presence than in the absence of isoproterenol. In the cell-attached configuration with 110 mM BaCl(2) in pipettes, repolarization from +110 to +20 mV yielded mainly blank sweeps (mode 0) and only occasionally mode 2, leading to no facilitation of the average current. Isoproterenol augmented the prepulse-induced increase in mode 2, reciprocally inhibited that in mode 0, and increased the fraction of mode 2 in non-blank sweeps after the prepulse. Therefore, beta-adrenergic stimulation favors mode 2 rather than mode 0 at strongly depolarized potentials, thereby promoting the prepulse facilitation and attenuating the inactivation of cardiac L-type Ca currents.     

Temporal clustering of ion channel openings incorporating time interval omission

Ion channel gating mechanisms can be satisfactorily modelled by a time-reversible continuous-time Markov chain on a finite state space. The complete process is not observable, but rather the state space is partitioned into 'open' and 'closed' states corresponding to the receptor channel being open or closed, and it is only possible to observe whether the process is in an open or a closed state. Previous studies of locust muscle glutamate receptor channels have revealed single channel openings to be highly clustered in time. This clustering can be described by the ratio of the variance var N(t) to the mean E[N(t)] of the number of channel openings in a time interval of length t. In this paper we obtain expressions for (formula; see text) for the above aggregated Markov process. Applications of these expressions to a model for the locust muscle glutamate receptor channel show this aspect of the model to be reasonably consistent with experimental data. In practice very short sojourns in either the open or closed states will fail to be detected, a phenomenon known as time interval omission. Using a semi-Markov approach, we outline a general theoretical framework for analysing dynamic properties of aggregated Markov processes incorporating time interval omission. We illustrate the applicability of this framework by using it to find limt----infinity [[var N(t)]/E[N(t)]] theoretically, when time interval omission is incorporated. This allows us to study the robustness of limt----infinity [[var N(t)]/E[N(t)]] to time interval omission, as a measure of temporal clustering.     

Gating modulation by heat of the polycystin transient receptor potential channel PKD2L1 (TRPP3)

Polycystic kidney disease 2-like 1 (PKD2L1), previously called transient receptor potential polycystin 3 (TRPP3), forms a voltage-dependent nonselective cation channel that exhibits large tail currents triggered by repolarization after depolarization. Since it has previously been proposed that temperature sensitivity of some TRP channels is linked to the voltage-dependent gating, we here investigated heating effects on PKD2L1 currents in human embryonic kidney HEK293T cells overexpressing mouse PKD2L1. Tail PKD2L1 currents were increased by heating to 32 °C, but decreased at more than 36 °C. Voltage dependency of the PKD2L1 channel was shifted by heating in a bimodal fashion: an increase in temperature to 32 °C and to 36 °C shifted the activation curves toward the left and the right, respectively. In addition, heating accelerated deactivation of tail PKD2L1 currents. To analyze the channel gating kinetics, single-channel events of the PKD2L1 channel were recorded at hyperpolarized potentials under whole-cell configurations. A rise in temperature decreased the open probability of the channel. Dwell-time analysis showed that both open and closed dwell times during heating were shorter than those at room temperature. Interestingly, a rapid temperature drop after heating markedly enhanced the PKD2L1 currents at both single-channel and whole-cell levels. The rebound activation of the PKD2L1 channel was due to an increase in the open probability but not in the single-channel conductance. These results suggest that heating opens but subsequently inactivates PKD2L1 channels, which is essential for the rebound activation of the channel after heating.