Calcium-dependent inactivation of light-sensitive channels in Drosophila photoreceptors

Whole-cell voltage clamp recordings were made from photoreceptors of dissociated Drosophila ommatidia under conditions when the light- sensitive channels activate spontaneously, generating a "rundown current" (RDC). The Ca2+ and voltage dependence of the RDC was investigated by applying voltage steps (+80 to -100 mV) at a variety of extracellular Ca2+ concentrations (0-10 mM). In Ca(2+)-free Ringer large currents are maintained tonically throughout 50-ms-long voltage steps. In the presence of external Ca2+, hyperpolarizing steps elicit transient currents which inactivate increasingly rapidly as Ca2+ is raised. On depolarization inactivation is removed with a time constant of approximately 10 ms at +80 mV. The Ca(2+)-dependent inactivation is suppressed by 10 mM internal BAPTA, suggesting it requires Ca2+ influx. The inactivation is absent in the trp mutant, which lacks one class of Ca(2+)-selective, light-sensitive channel, but appears unaffected by the inaC mutant which lacks an eye-specific protein kinase C. Hyperpolarizing voltage steps applied during light responses in wild- type (WT) flies before rundown induce a rapid transient facilitation followed by slower inhibition. Both processes accelerate as Ca2+ is raised, but the time constant of inhibition (12 ms with 1.5 mM external Ca2+ at -60 mV) is approximately 10 times slower than that of the RDC inactivation. The Ca(2+)-mediated inhibition of the light response recovers in approximately 50-100 ms on depolarization, recovery being accelerated with higher external Ca2+. The Ca2+ and voltage dependence of the light-induced current is virtually eliminated in the trp mutant. In inaC, hyperpolarizing voltage steps induced transient currents which appeared similar to those in WT during early phases of the light response. However, 200 ms after the onset of light, the currents induced by voltage steps inactivated more rapidly with time constants similar to those of the RDC. It is suggested that the Ca(2+)-dependent inactivation of the light-sensitive channels first occurs at some concentration of Ca2+ not normally reached during the moderate illumination regimes used, but that the defect in inaC allows this level to be reached.     

The trp gene is essential for a light-activated Ca2+ channel in Drosophila photoreceptors.

Invertebrate phototransduction is an important model system for studying the ubiquitous inositol-lipid signaling system. In the transient receptor potential (trp) mutant, one of the most intensively studied transduction mutants of Drosophila, the light response quickly declines to baseline during prolonged intense light. Using whole-cell recordings from Drosophila photoreceptors, we show that the wild-type response is mediated by at least two functionally distinct classes of light-sensitive channels and that both the trp mutation and a Ca2+ channel blocker (La3+) selectively abolish one class of channel with high Ca2+ permeability. Evidence is also presented that Ca2+ is necessary for excitation and that Ca2+ depletion mimics the trp phenotype. We conclude that the recently sequenced trp protein represents a class of light-sensitive channel required for inositide-mediated Ca2+ entry and suggest that this process is necessary for maintained excitation during intense illumination in fly photoreceptors.     

Light-activated single channel currents in Limulus ventral nerve photoreceptors

Light-activated single channel currents were measured in Limulus ventral photoreceptors in the cell-attached configuration at 14°C. The results show three channel types with conductances of 6.2, 10.4 and 28.7 pS. The most active channels have the 10 pS conductance; the open time histograms of these channels could be best fitted by the sum of two exponentials with time constants (and weights) of 0.58 ms (0.78) and 4.32 ms (0.22), suggesting two populations of channels or two open states. The mean open time was 1.38 ms. The open time histogram of the channels with the 29 pS conductance could be best fitted by a single exponential with a time constant of 3.35 ms. First latencies of the 10 pS channels were between 40 and 280 ms but those of the 29 pS conductance channels were 300 ms. These findings suggest that the two channel types are gated by two different intracellular transmitters or mechanisms. Offprint requests to: K. Nagy     

Noise analysis of the glutamate-activated current in photoreceptors.

The glutamate-activated current in photoreceptors has been attributed both to a sodium/glutamate transporter and to a glutamate-activated chloride channel. We have further studied the glutamate-activated current in single, isolated photoreceptors from the tiger salamander using noise analysis on whole-cell patch-clamp recordings. In cones, the current is generated by chloride channels with a single-channel conductance of 0.7 pS and an open lifetime of 2.4 ms. The number of channels per cell is in the range of 10,000-20,000. Activation of the channels requires the presence of both glutamate and sodium. The single-channel conductance and the open lifetime of the channel are independent of the external concentration of glutamate and sodium. External glutamate and sodium affect only the opening rate of the channels. D,L-Threo-3-hydroxyaspartate (THA), a glutamate-transport blocker, is shown to be a partial agonist for the channel. The single-channel conductance is the same regardless of whether glutamate or THA is the ligand, but the open lifetime of the channel is only 0.8 ms with THA as ligand. The glutamate-activated current in rods has a similar single-channel conductance (0.74 pS) and open lifetime (3 ms). We propose a kinetic model, consistent with these results, to explain how a transporter can simultaneously act both as a sodium/glutamate-gated chloride channel and a glutamate/sodium cotransporter.     

Genetic dissection of light-induced Ca2+ influx into Drosophila photoreceptors

Invertebrate photoreceptors use the inositol-lipid signaling cascade for phototransduction. A useful approach to dissect this pathway and its regulation has been provided by the isolation of Drosophila visual mutants. We measured extracellular changes of Ca2+ [delta Ca2+]o in Drosophila retina using Ca(2+)-selective microelectrodes in both the transient receptor potential (trp) mutant, in which the calcium permeability of the light-sensitive channels is greatly diminished and in the inactivation-but-no-afterpotential C (inaC) mutant which lacks photoreceptor-specific protein kinase C (PKC). Illumination induced a decrease in extracellular [Ca2+] with kinetics and magnitude that changed with light intensity. Compared to wild-type, the light-induced decrease in [Ca2+]o (the Ca2+ signal) was diminished in trp but significantly enhanced in inaC. The enhanced Ca2+ signal was diminished in the double mutant inaC;trp indicating that the effect of the trp mutation overrides the enhancement observed in the absence of eye-PKC. We suggest that the decrease in [Ca2+]o reflects light-induced Ca2+ influx into the photoreceptors and that the trp mutation blocks a large fraction of this Ca2+ influx, while the absence of eye specific PKC leads to enhancement of light-induced Ca2+ influx. This suggestion was supported by Ca2+ measurements in isolated ommatidia loaded with the fluorescent Ca2+ indicator, Ca Green-5N, which indicated an approximately threefold larger light-induced increase in cellular Ca2+ in inaC relative to WT. Our observations are consistent with the hypothesis that TRP is a light activated Ca2+ channel and that the increased Ca2+ influx observed in the absence of PKC is mediated mainly via the TRP channel.     

Constitutive activity of the light-sensitive channels TRP and TRPL in the Drosophila diacylglycerol kinase mutant, rdgA

Mutations in the Drosophila retinal degeneration A (rdgA) gene, which encodes diacylglycerol kinase (DGK), result in early onset retinal degeneration and blindness. Whole-cell recordings revealed that light-sensitive Ca2+ channels encoded by the trp gene were constitutively active in rdgA photoreceptors. Early degeneration was rescued in rdgA;trp double mutants, lacking TRP channels; however, the less Ca2+-permeable light-sensitive channels (TRPL) were constitutively active instead. No constitutive activity was seen in rdgA;trpI;trp mutants lacking both classes of channel, although, like rdgA;trp, these still showed a residual slow degeneration. Responses to light were restored in rdgA;trp but deactivated abnormally slowly, indicating that DGK is required for response termination. The findings suggest that early degeneration in rdgA is caused by uncontrolled Ca2+ influx and support the proposal that diacylglycerol or its metabolites are messengers of excitation in Drosophila photoreceptors.     

Translocation of the Drosophila transient receptor potential-like (TRPL) channel requires both the N- and C-terminal regions together with sustained Ca2+ entry

In Drosophila photoreceptors the transient receptor potential-like (TRPL), but not the TRP channels undergo light-dependent translocation between the rhabdomere and cell body. Here we studied which of the TRPL channel segments are essential for translocation and why the TRP channels are required for inducing TRPL translocation. We generated transgenic flies expressing chimeric TRP and TRPL proteins that formed functional light-activated channels. Translocation was induced only in chimera containing both the N- and C-terminal segments of TRPL. Using an inactive trp mutation and overexpressing the Na(+)/Ca(2+) exchanger revealed that the essential function of the TRP channels in TRPL translocation is to enhance Ca(2+)-influx. These results indicate that motifs present at both the N and C termini as well as sustained Ca(2+) entry are required for proper channel translocation.     

Single amino acid change in the fifth transmembrane segment of the TRP Ca2+ channel causes massive degeneration of photoreceptors

The trp gene encodes subunits of a highly Ca(2+)-permeable class of light-activated channels of Drosophila photoreceptors. The recently characterized mutation in this gene, Trp(P365), is semidominant and causes massive degeneration of photoreceptors by making the TRP channel constitutively active. We show that a single amino acid change, Phe-550 to Ile, near the beginning of the fifth transmembrane domain of TRP channel subunits is necessary to induce, and sufficient to closely mimic, the original mutant phenotypes of Trp(P365). Hypotheses are presented as to why the amino acid residues at position 550 and its immediate vicinity might be important in influencing the regulation of the TRP channel and why the substitution of Phe for Ile at this position, in particular, could result in constitutive activity of the channel.     

TRP and calcium stores in Drosophila phototransduction.

Phototransduction in Drosophila is mediated by the ubiquitous phosphoinositide cascade, leading to opening of the TRP and TRPL channels, which are prototypical members of a novel class of membrane proteins. Drosophila mutants lacking the TRP protein display a response to light that declines to the dark level during illumination. It has recently been suggested that this response inactivation results from a negative feedback by calcium-calmodulin, leading to closure of the TRPL channels. It is also suggested that in contrast to other phosphoinositide-mediated systems, Ca2+ release from internal stores is neither involved in channel activation nor in phototransduction in general. We now show that inactivation of the light response in trp photoreceptors is enhanced upon reduction of the intracellular Ca2+ concentration. Furthermore, in Ca(2+)-free medium, when there is no Ca2+ influx into the photoreceptors, we demonstrate a significant elevation of intracellular Ca2+ upon illumination. This elevation correlates with ability of the cells to respond to light. Accordingly, malfunctioning of Ca2+ stores, either by Ca2+ deprivation or by application of the Ca2+ pump inhibitor, thapsigargin, confers a trp phenotype on wild type flies. The results indicate that the response inactivation in trp cells results from Ca2+ deficiency rather than from Ca(2+)-dependent negative feedback. The results also indicate that there is light-induced release of Ca2+ from intracellular stores. Furthermore, the response to light is correlated to Ca2+ release, and normal function of the stores is required for prolonged excitation. We suggest that phototransduction in Drosophila depends on Ca(2+)-release mediated signalling and that TRP is essential for the normal function of this process.     

Mechanisms of up-regulation of single calcium channels by serotonin in Helix pomatia neurons

Action of serotonin (5-HT) on single Ca(2+) channel activity was studied in identified neurons of snail Helix pomatia. Only one type of Ca(2+) channels of 5 pS unitary conductance was determined under patch-clamp cell-attached mode. Kinetic analysis have shown a monotonically declining distribution of channel open times (OT) with mean time constant of 0.2 ms. The distribution of channel closed times (CT) could be fitted by double-exponential curve with time constants 1 and 12 ms. We established that 5-HT acts on Ca(2+) channel activity indirectly via cytoplasm. 5-HT prolonged the OT (up to 0.3 ms) and shortened the CT proportionally for both constants to 0.4 and 6 ms correspondingly. A conclusion is made that enhancement of Ca(2+) macro-current by 5-HT is determined by kinetic changes, increase of the number of active channels, and increase of the probability of OT. At the same time the transmitter did not affect the unitary channel conductance.     

Ion channels activated by L-glutamate and GABA in cultured cerebellar neurons of the rat

Glutamate and GABA-receptor channels were investigated in explants of rat cerebellum grown in cell culture. The patch-clamp technique was used to examine neurons under whole cell clamp and the properties of channels were derived by analysis of glutamate and GABA-evoked current noise. In addition, single channel currents activated by glutamate were recorded from isolated outside-out patches of membrane. We found evidence for at least two types of glutamate receptor-channels in cerebellar cells. Some neurons exhibited a channel of 50 pS conductance with a Lorentzian noise spectrum of 5.9 ms time constant. Single channels were readily resolved both in whole cell clamp and excised patches. Other neurons possessed low conductance channels which produced two component spectra. Estimates of the single channel conductance gave a value of about 140 fS. GABA channel noise obtained from these cells was also fitted by two component spectra which gave single channel conductance of 16 pS.     

Open channel block by Ca2+ underlies the voltage dependence of drosophila TRPL channel

The light-activated channels of Drosophila photoreceptors transient receptor potential (TRP) and TRP-like (TRPL) show voltage-dependent conductance during illumination. Recent studies implied that mammalian members of the TRP family, which belong to the TRPV and TRPM subfamilies, are intrinsically voltage-gated channels. However, it is unclear whether the Drosophila TRPs, which belong to the TRPC subfamily, share the same voltage-dependent gating mechanism. Exploring the voltage dependence of Drosophila TRPL expressed in S2 cells, we found that the voltage dependence of this channel is not an intrinsic property since it became linear upon removal of divalent cations. We further found that Ca(2+) blocked TRPL in a voltage-dependent manner by an open channel block mechanism, which determines the frequency of channel openings and constitutes the sole parameter that underlies its voltage dependence. Whole cell recordings from a Drosophila mutant expressing only TRPL indicated that Ca(2+) block also accounts for the voltage dependence of the native TRPL channels. The open channel block by Ca(2+) that we characterized is a useful mechanism to improve the signal to noise ratio of the response to intense light when virtually all the large conductance TRPL channels are blocked and only the low conductance TRP channels with lower Ca(2+) affinity are active.     

TRPC1 proteins confer PKC and phosphoinositol activation on native heteromeric TRPC1/C5 channels in vascular smooth muscle: comparative study of wild-type and TRPC1-/- mice

Ca(2+)-permeable cation channels consisting of canonical transient receptor potential 1 (TRPC1) proteins mediate Ca(2+) influx pathways in vascular smooth muscle cells (VSMCs), which regulate physiological and pathological functions. We investigated properties conferred by TRPC1 proteins to native single TRPC channels in acutely isolated mesenteric artery VSMCs from wild-type (WT) and TRPC1-deficient (TRPC1(-/-)) mice using patch-clamp techniques. In WT VSMCs, the intracellular Ca(2+) store-depleting agents cyclopiazonic acid (CPA) and 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) both evoked channel currents, which had unitary conductances of ～2 pS. In TRPC1(-/-) VSMCs, CPA-induced channel currents had 3 subconductance states of 14, 32, and 53 pS. Passive depletion of intracellular Ca(2+) stores activated whole-cell cation currents in WT but not TRPC1(-/-) VSMCs. Differential blocking actions of anti-TRPC antibodies and coimmunoprecipitation studies revealed that CPA induced heteromeric TRPC1/C5 channels in WT VSMCs and TRPC5 channels in TRPC1(-/-) VSMCs. CPA-evoked TRPC1/C5 channel activity was prevented by the protein kinase C (PKC) inhibitor chelerythrine. In addition, the PKC activator phorbol 12,13-dibutyrate (PDBu), a PKC catalytic subunit, and phosphatidylinositol-4,5-bisphosphate (PIP(2)) and phosphatidylinositol-3,4,5-trisphosphate (PIP(3)) activated TRPC1/C5 channel activity, which was prevented by chelerythrine. In contrast, CPA-evoked TRPC5 channel activity was potentiated by chelerythrine, and inhibited by PDBu, PIP(2), and PIP(3). TRPC5 channels in TRPC1(-/-) VSMCs were activated by increasing intracellular Ca(2+) concentrations ([Ca(2+)](i)), whereas increasing [Ca(2+)](i) had no effect in WT VSMCs. We conclude that agents that deplete intracellular Ca(2+) stores activate native heteromeric TRPC1/C5 channels in VSMCs, and that TRPC1 subunits are important in determining unitary conductance and conferring channel activation by PKC, PIP(2), and PIP(3).     

Kinetic properties of single-ion channels activated by light in Limulus ventral nerve photoreceptors

Previous results on Limulus ventral photoreceptors have suggested that besides inositol trisphosphate, another unknown transmitter may also work in the transduction cascade. This assumption has been supported by the finding of two light-activated channel types. The present report furnishes further evidence of the dual transmitter mechanism in phototransduction by analyzing the kinetic properties and voltage dependency of these cation channels with conductances of 12 pS and 30 pS. Single-channel currents were recorded in Limulus ventral nerve photoreceptors in cell-attached configuration at 14°C. At V _m + 80 mV the open-time histograms of both channels were fit best by the sum of two exponentials; time constants (and weights) were: 0.81 ms (0.62) and 6.20 ms (0.38) for the 12 pS channels and 2.38 ms (0.43) and 19.4 ms (0.57) for the 30 pS channels. At this potential the mean open times were 2.7 ms for the 12 pS and 13.3 ms for the 30 pS channels, about two-times larger than at hyperpolarizing potentials. The deactivation kinetics were also different for the two channels. The time constants of the decay of the channel activity, after switching off the light, were 2.5 s for the 12 pS and 12.9 s for the 30 pS channels. The 12 pS channel exhibits bursting and subconductance states at positive potentials. The subconductances are about 20%, 46% and 72% of the fully open state. Results show that the two types of light-activated channels have different kinetic parameters, voltage dependence and gating mechanisms. The two channels are suggested to be gated by different transmitters or processes. It is proposed that for the 30 pS channel the transmitter could be calcium ion or a calcium-dependent transmitter.     

Lanthanum mimicks the trp photoreceptor mutant of Drosophila in the blowfly Calliphora

The effect of lanthanum on the light response of blowfly (Calliphora erythrocephala) photoreceptors was studied. The electrophysiological behaviour of the photoreceptors in the presence of La can be summarized as follows: 1. Upon long stimulation the photoreceptors responded with a 'transient receptor potential', i.e. the cells depolarized at the onset of the stimulus and then repolarized to (or below) the resting potential. This effect was dependent on stimulus intensity and occurred only at high intensities. During illumination membrane noise was reduced. 2. The light-induced changes in membrane potential were paralleled by changes in membrane resistance. 3. The time course of the receptor response was slowed down. 4. Light adaptation led to an increase in response latency. 5. The recovery of the receptor response after light adaptation was slowed down. 6. The sensitivity of the receptor cells measured by the response to short light stimuli was reduced. In summary, the electrophysiological behaviour of Calliphora photoreceptors in the presence of La was very similar to that of the photoreceptors of the trp (transient receptor potential) mutant of Drosophila melanogaster. This result suggests that La and trp mutation affect the same cellular processes in the photoreceptors.     

Sensing with TRP channels

Drosophila melanogaster flies carrying the trp (transient receptor potential) mutation are rapidly blinded by bright light, because of the absence of a Ca2+-permeable ion channel in their photoreceptors. The identification of the trp gene and the search for homologs in yeast, flies, worms, zebrafish and mammals has led to the discovery of a large superfamily of related cation channels, named TRP channels. Activation of TRP channels is highly sensitive to a variety of chemical and physical stimuli, allowing them to function as dedicated biological sensors that are essential in processes such as vision, taste, tactile sensation and hearing.     

Calcium influx via TRP channels is required to maintain PIP2 levels in Drosophila photoreceptors

The trp (transient receptor potential) gene encodes a Ca2+ channel responsible for the major component of the phospholipase C (PLC) mediated light response in Drosophila. In trp mutants, maintained light leads to response decay and temporary total loss of sensitivity (inactivation). Using genetically targeted PIP2-sensitive inward rectifier channels (Kir2.1) as biosensors, we provide evidence that trp decay reflects depletion of PIP2. Two independent mutations in the PIP2 recycling pathway (rdgB and cds) prevented recovery from inactivation. Abolishing Ca2+ influx in wild-type photoreceptors mimicked inactivation, while raising Ca2+ by blocking Na+/Ca2+ exchange prevented inactivation in trp. The results suggest that Ca2+ influx prevents PIP2 depletion by inhibiting PLC activity and facilitating PIP2 recycling. Without this feedback one photon appears sufficient to deplete the phosphoinositide pool of approximately 4 microvilli.     

Properties of Acetylcholine Receptor Ion Channels in the Acutely Dissociated Neurons of the Marginal Division in the Rat Striatum

Cell-attached mode of patch clamp technique was employed to investigate the properties of acetylcholine (ACh)-induced ion channels in acutely dissociated neurons from the marginal division (MrD) of rat striatum. Two types of conductance states (25 pS and 60 pS) were recorded. The 25 pS channel (more than 80%) was the main type in the neurons of MrD and was described here. The amplitudes of inward currents increased with hyperpolorization and the reversing potential was about 0 mV. Both single short opening and long burst openings were observed in MrD neurons. Two time constants of these two kinds of ion channels are 0.29 ms, 1.84 ms and 1.96 ms, 18.24 ms, respectively. Average close time can be fitted with two exponential functions, the two time constants are 1.7 ms and 54 ms. Probability of channel opening is about 0.012 and no voltage-dependence was found. The properties of reversing potential, voltage-independence and the form of agonist to the ion channels indicated that the recorded channel currents flow through AChR channels. The mAChR is involved in slow synaptic transmission and Ach can not induce the opening of mAChR ion channel. The binding site of ACh to AChR and the nAChR ion channel are the same protein, ACh can only activate nAChR ion channel directly. Therefore, the recorded ion channels in the present study are nAChR ion channels. The results suggest that nAChR ion channels exist in the neurons of MrD and the MrD probably is involved in learning and memory mechanism of the brain.     

Characterization of the calcium-activated chloride channel in isolated guinea-pig hepatocytes

Macroscopic and unitary currents through Ca(2+)-activated Cl- channels were examined in enzymatically isolated guinea-pig hepatocytes using whole-cell, excised outside-out and inside-out configurations of the patch-clamp technique. When K+ conductances were blocked and the intracellular Ca2+ concentration ([Ca2+]i) was set at 1 microM (pCa = 6), membrane currents were observed under whole-cell voltage-clamp conditions. The reversal potential of the current shifted by approximately 60 mV per 10-fold change in the external Cl- concentration. In addition, the current did not appear when Cl- was omitted from the internal and external solutions, indicating that the current was Cl- selective. The current was activated by increasing [Ca2+]i and was inactivated in Ca(2+)-free, 5 mM EGTA internal solution (pCa > 9). The current was inhibited by bath application of 9- anthracenecarboxylic acid (9-AC) and 4,4'-diisothiocyanatostilbene-2,2'- disulfonic acid (DIDS) in a voltage-dependent manner. In single channel recordings from outside-out patches, unitary current activity was observed, whose averaged slope conductance was 7.4 +/- 0.5 pS (n = 18). The single channel activity responded to extracellular Cl- changes as expected for a Cl- channel current. The open time distribution was best described by a single exponential function with mean open lifetime of 97.6 +/- 10.4 ms (n = 11), while at least two exponentials were required to fit the closed time distributions with a time constant for the fast component of 21.5 +/- 2.8 ms (n = 11) and that for the slow component of 411.9 +/- 52.0 ms (n = 11). In excised inside-out patch recordings, channel open probability was sensitive to [Ca2+]i. The relationship between [Ca2+]i and channel activity was fitted by the Hill equation with a Hill coefficient of 3.4 and the half-maximal activation was 0.48 microM. These results suggest that guinea-pig hepatocytes possess Ca(2+)-activated Cl- channels.     

Nonselective currents and channels in plasma membranes of protoplasts from coats of developing seeds of bean

In developing bean (Phaseolus vulgaris) seeds, phloem-imported nutrients move in the symplast from sieve elements to the ground parenchyma cells where they are transported across the plasma membrane into the seed apoplast. To study the mechanisms underlying this transport, channel currents in ground parenchyma protoplasts were characterized using patch clamp. A fast-activating outward current was found in all protoplasts, whereas a slowly activating outward current was observed in approximately 25% of protoplasts. The two currents had low selectivity for univalent cations, but the slow current was more selective for K(+) over Cl(-) (P(K):P(Cl) = 3.6-4.2) than the fast current (P(K):P(Cl) = 1.8-2.5) and also displayed Ca(2+) selectivity. The slow current was blocked by Ba(2+), whereas both currents were blocked by Gd(3+) and La(3+). Efflux of K(+) from seed coat halves was inhibited 25% by Gd(3+) and La(3+) but was stimulated by Ba(2+) and Cs(+), suggesting that only the fast current may be a component in the pathway for K(+) release. An "instantaneous" inward current observed in all protoplasts exhibited similar pharmacology and permeability for univalent cations to the fast outward current. In outside-out patches, two classes of depolarization-activated cation-selective channels were observed: one slowly activating of low conductance (determined from nonstationary noise to be 2.4 pS) and another with conductances 10-fold higher. Both channels occurred at high density. The higher conductance channel in 10 mM KCl had P(K):P(Cl) = 2.8. Such nonselective channels in the seed coat ground parenchyma cell could function to allow some of the efflux of phloem-imported univalent ions into the seed apoplast.