Currents carried by monovalent cations through cyclic GMP-activated channels in excised patches from salamander rods

Structural basis of properties, mechanisms, and channelopathy of cyclic nucleotide-gated channels

Recent years have seen an outpouring of atomic or near atomic resolution structures of cyclic nucleotide-gated (CNG) channels, captured in closed, transition, pre-open, partially open, and fully open states. These structures provide unprecedented molecular insights into the activation, assembly, architecture, regulation, and channelopathy of CNG channels, as well as mechanistic explanations for CNG channel biophysical and pharmacological properties. This article summarizes recent advances in CNG channel structural biology, describes key structural features and elements, and illuminates a detailed conformational landscape of activation by cyclic nucleotides. The review also correlates structures with findings and properties delineated in functional studies, including nonselective monovalent cation selectivity, Ca2+ permeation and block, block by L-cis-diltiazem, location of the activation gate, lack of voltage-dependent gating, and modulation by lipids and calmodulin. A perspective on future research is also offered.

CNG channel structure, function, and gating: a tale of conformational flexibility

Cyclic nucleotide-gated (CNG) channels are key to the signal transduction machinery of certain sensory modalities both in vertebrate and invertebrate organisms. They translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through sensory cells. Despite CNG and voltage-gated potassium channels sharing a remarkable amino acid sequence homology and basic architectural plan, their functional properties are dramatically different. While voltage-gated potassium channels are highly selective and require membrane depolarization to open, CNG channels have low ion selectivity and are not very sensitive to voltage. In the last few years, many high-resolution structures of intact CNG channels have been released. This wealth of new structural information has provided enormous progress toward the understanding of the molecular mechanisms and driving forces underpinning CNG channel activation. In this review, we report on the current understanding and controversies surrounding the gating mechanism in CNG channels, as well as the deep intertwining existing between gating, the ion permeation process, and its modulation by membrane voltage. While the existence of this powerful coupling was recognized many decades ago, its direct structural demonstration, and ties to the CNG channel inherent pore flexibility, is a recent achievement.

The permeation mechanism of organic cations through a CNG mimic channel

Several channels, ranging from TRP receptors to Gap junctions, allow the exchange of small organic solute across cell membrane. However, very little is known about the molecular mechanism of their permeation. Cyclic Nucleotide Gated (CNG) channels, despite their homology with K+ channels and in contrast with them, allow the passage of larger methylated and ethylated ammonium ions like dimethylammonium (DMA) and ethylammonium (EA). We combined electrophysiology and molecular dynamics simulations to examine how DMA interacts with the pore and permeates through it. Due to the presence of hydrophobic groups, DMA enters easily in the channel and, unlike the alkali cations, does not need to cross any barrier. We also show that while the crystal structure is consistent with the presence of a single DMA ion at full occupancy, the channel is able to conduct a sizable current of DMA ions only when two ions are present inside the channel. Moreover, the second DMA ion dramatically changes the free energy landscape, destabilizing the crystallographic binding site and lowering by almost 25 kJ/mol the binding affinity between DMA and the channel. Based on the results of the simulation the experimental electron density maps can be re-interpreted with the presence of a second ion at lower occupancy. In this mechanism the flexibility of the channel plays a key role, extending the classical multi-ion permeation paradigm in which conductance is enhanced by the plain interaction between the ions.

A ring of threonines in the inner vestibule of the pore of CNGA1 channels constitutes a binding site for permeating ions

Cyclic nucleotide-gated (CNG) channels and K+ channels have a significant sequence identity and are thought to share a similar 3D structure. K+ channels can accommodate simultaneously two or three permeating ions inside their pore and therefore are referred to as multi-ion channels. Also CNGA1 channels are multi-ion channels, as they exhibit an anomalous mole fraction effect (AMFE) in the presence of mixtures of 110 mM Li+ and Cs+ on the cytoplasmic side of the membrane. Several observations have identified the ring of Glu363 in the outer vestibule of the pore as one of the binding sites within the pore of CNGA1 channels. In the present work we identify a second binding site in the selectivity filter of CNGA1 channels controlling AMFE. Here, we show also that Cs+ ions at the intracellular side of the membrane block the entry of Na+ ions. This blockage is almost completely removed at high hyperpolarized voltages as expected if the Cs+ blocking site is located within the transmembrane electric field. Indeed, mutagenesis experiments show that the block is relieved when Thr359 and Thr360 at the intracellular entrance of the selectivity filter are replaced with an alanine. In T359A mutant channels AMFE in the presence of intracellular mixtures of Li+ and Cs+ is still present but is abolished in T360A mutant channels. These results suggest that the ring of Thr360 at the intracellular entrance of the selectivity filter forms another ion binding site in the CNGA1 channel. The two binding sites composed of the rings of Glu363 and Thr360 are not independent; in fact they mediate a powerful coupling between permeation and gating, a specific aspect of CNG channels.

Sodium selectivity of semicircular canal duct epithelial cells

Background

Sodium absorption by semicircular canal duct (SCCD) epithelial cells is thought to contribute to the homeostasis of the volume of vestibular endolymph. It was previously shown that the epithelial cells could absorb Na⁺ under control of a glucocorticoid hormone (dexamethasone) and the absorptive transepithelial current was blocked by amiloride. The most commonly-observed target of amiloride is the epithelial sodium channel (ENaC), comprised of the three subunits α-, β- and γ-ENaC. However, other cation channels have also been observed to be sensitive in a similar concentration range. The aim of this study was to determine whether SCCD epithelial cells absorb only Na⁺ or also K⁺ through an amiloride-sensitive pathway. Parasensory K⁺ absorption could contribute to regulation of the transduction current through hair cells, as found to occur via vestibular transitional cells [S. H. Kim and D. C. Marcus. Regulation of sodium transport in the inner ear. Hear.Res. doi:10.1016/j.heares.2011.05.003, 2011].

Results

We determined the molecular and functional expression of candidate cation channels with gene array (GEO GSE6197), whole-cell patch clamp and transepithelial recordings in primary cultures of rat SCCD. α-, β- and γ-ENaC were all previously reported as present. The selectivity of the amiloride-sensitive transepithelial and cell membrane currents was observed in Ussing chamber and whole-cell patch clamp recordings. The cell membrane currents were carried by Na⁺ but not K⁺, but the Na⁺ selectivity disappeared when the cells were cultured on impermeable supports. Transepithelial currents across SCCD were also carried exclusively by Na⁺.

Conclusions

These results are consistent with the amiloride-sensitive absorptive flux of SCCD mediated by a highly Na⁺-selective channel, likely αβγ-ENaC. These epithelial cells therefore absorb only Na⁺ via the amiloride-sensitive pathway and do not provide a parasensory K⁺ efflux from the canals via this pathway. The results further provide caution to the culture of epithelial cells on impermeable surfaces.

Voltage profile along the permeation pathway of an open channel

For ion channels, the transmembrane potential plays a critical role by acting as a driving force for permeant ions. At the microscopic level, the transmembrane potential is thought to decay nonlinearly across the ion permeation pathway because of the irregular three-dimensional shape of the channel's pore. By taking advantage of the current structural and functional understanding of cyclic nucleotide-gated channels, in this study we experimentally explore the transmembrane potential's distribution across the open pore. As a readout for the voltage drop, we engineered cysteine residues along the selectivity filter and scanned the sensitivity of their modification rates by Ag(+) to the transmembrane potential. The experimental data, which indicate that the majority of the electric field drops across the selectivity filter, are in good agreement with continuum electrostatic calculations using a homology model of an open CNG channel. By focusing the transmembrane potential across the selectivity filter, the electromotive driving force is coupled with the movement of permeant ions in the filter, maximizing the efficiency of this process.

Gating in CNGA1 channels

The aminoacid sequences of CNG and K(+) channels share a significant sequence identity, and it has been suggested that these channels have a common ancestral 3D architecture. However, K(+) and CNG channels have profoundly different physiological properties: indeed, K(+) channels have a high ionic selectivity, their gating strongly depends on membrane voltage and when opened by a steady depolarizing voltage several K(+) channels inactivate, whereas CNG channels have a low ion selectivity, their gating is poorly voltage dependent, and they do not desensitize in the presence of a steady concentration of cyclic nucleotides that cause their opening. The purpose of the present review is to summarize and recapitulate functional and structural differences between K(+) and CNG channels with the aim to understand the gating mechanisms of CNG channels.

Movements of native C505 during channel gating in CNGA1 channels

We investigated conformational changes occurring in the C-linker and cyclic nucleotide-binding (CNB) domain of CNGA1 channels by analyzing the inhibition induced by thiol-specific reagents in mutant channels Q409C and A414C in the open and closed state. Cd(2+) (200 microM) inhibited irreversibly mutant channels Q409C and A414C in the closed but not in the open state. Cd(2+) inhibition was abolished in the mutant A414C(cys-free), in the double mutant A414C + C505T and in the tandem construct A414C + C505T/CNGA1, but it was present in the construct A414C + C505(cys-free). The cross-linker reagent M-2-M inhibited mutant channel Q409C in the open state. M-2-M inhibition in the open state was abolished in the double mutant Q409C + C505T and in the tandem construct Q409C + C505T/CNGA1. These results show that C(alpha) of C505 in the closed state is located at a distance between 4 and 10.5 A from the C(alpha) of A414 of the same subunit, but in the open state C505 moves towards Q409 of the same subunit at a distance that ranges from 10.5 to 12.3 A from C(alpha) of this residue. These results are not consistent with a 3-D structure of the CNGA1 channel homologous to the structure of HCN2 channels either in the open or in the closed state.

A comparison of electrophysiological properties of the CNGA1, CNGA1tandem and CNGA1cys-free channels

Three constructs are used for the analysis of biophysical properties of CNGA1 channels: the WT CNGA1 channel, a CNGA1 channel where all endogenous cysteines were removed (CNGA1cys-free) and a construct composed of two CNGA1 subunits connected by a small linker (CNGA1tandem). So far, it has been assumed, but not proven, that the molecular structure of these ionic channels is almost identical. The I/V relations, ionic selectivity to alkali monovalent cations, blockage by tetracaine and TMA+ were not significantly different. The cGMP dose response and blockage by TEA+ and Cd2+ were instead significantly different in CNGA1 and CNGA1cys-free channels, but not in CNGA1 and CNGA1tandem channels. Cd2+ blocked irreversibly the mutant channel A406C in the absence of cGMP. By contrast, Cd2+ did not block the mutant channel A406C in the CNGA1cys-free background (A406Ccys-free), but an irreversible and almost complete blockage was observed in the presence of the cross-linker M-4-M. Results obtained with different MTS cross-linkers and reagents suggest that the 3D structure of the CNGA1cys-free differs from that of the CNGA1 channel and that the distance between homologous residues at position 406 in CNGA1cys-free is longer than in the WT CNGA1 by several Angstroms.

Nonselective cation channels activated by the stimulation of muscarinic receptors in mammalian gastric smooth muscle

Muscarinic receptors play key roles in the control of gastrointestinal smooth muscle activity. However, specific physiological functions of each subtype remain to be determined. Single cell RT-PCR experiments showed that all five subtypes of muscarinic receptors were present in circular smooth muscle cells of the guinea-pig gastric antrum. Nonselective cation channels (NSCC) activated by ACh or CCh are coupled to pertussis toxin (PTX)-sensitive Go protein through m4 subtype as well as m2 and m3 subtypes in guinea-pig stomach. CCh-activated currents (I(CCh)), especially the steady-state I-V relationship of I(CCh) showed a chracteristic U-shaped curve; reversal potential of around 0 mV and inward rectification at around +15 mV and a negative slope conductance at negative potential range. Under physiological conditions, the measured single channel conductance of NSCC was approximately 25 pS. The single channel conductance was modulated by external monovalent and divalent cations including Na+, Cs+, Li+, and Ca2+ through changing both the open probability and unitary conductance. Through the NSCC, Ca2+ can move into the cell from extracellular solution as well as Na+. Calculated fractional Ca2+ current of I(CCh) (f(Ca)) was around 1% at the 2 mM [Ca2+]o and at the 4 mM [Ca2+]o, f(Ca) was 2.3%. Quinidine blocked I(CCh) potently in a reversible manner; IC50 was 0.25 microM. There were two kinds of I(CCh) modulations through Ca(2+)-dependent pathways in guinea-pig gastric smooth muscle cells; 1) Facilitation of I(CCh) via Ca2+/CaM-dependent MLCK pathway, 2) Desensitization of I(CCh) via Ca(2+)-dependent PKC pathway. In the mouse stomach, all seven types of TRPC mRNA were detected with RT-PCR. On the basis of electrophysiological, pharmacological, and molecular biological experiments, we reported the mTRPC5 as a candidate for the NSCC activated by muscarinic stimulation in mouse stomach.

Influence of permeant ions on gating in cyclic nucleotide-gated channels

Cyclic nucleotide-gated channels are key components in the transduction of visual and olfactory signals where their role is to respond to changes in the intracellular concentration of cyclic nucleotides. Although these channels poorly select between physiologically relevant monovalent cations, the gating by cyclic nucleotide is different in the presence of Na(+) or K(+) ions. This property was investigated using rod cyclic nucleotide-gated channels formed by expressing the subunit 1 (or alpha) in HEK293 cells. In the presence of K(+) as the permeant ion, the affinity for cGMP is higher than the affinity measured in the presence of Na(+). At the single channel level, subsaturating concentrations of cGMP show that the main effect of the permeant K(+) ions is to prolong the time channels remain open without major changes in the shut time distribution. In addition, the maximal open probability was higher when K(+) was the permeant ion (0.99 for K(+) vs. 0.95 for Na(+)) due to an increase in the apparent mean open time. Similarly, in the presence of saturating concentrations of cAMP, known to bind but unable to efficiently open the channel, permeant K(+) ions also prolong the time channels visit the open state. Together, these results suggest that permeant ions alter the stability of the open conformation by influencing of the O-->C transition.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Anomalous mole-fraction effects in recombinant and native cyclic nucleotide-gated channels in rat olfactory receptor neurons

Anomalous mole-fraction effects (AMFE) were studied, using the inside-out configuration of the patchclamp technique, in both recombinant wild-type alpha-homomeric rat olfactory adenosine 3',5'-cyclic monophosphate (cAMP)-gated channels (rOCNC1) expressed in human embryonic kidney cells (HEK 293) and native cyclic nucleotide-gated (CNG) channels in acutely isolated rat olfactory receptor neurons. Single-channel and macroscopic currents were activated by 200 microM and 500 microM cAMP, respectively. Macroscopic currents, measured with mixtures of Na(+)-NH(4)(+) or Cs(+)-Li(+) in the cytoplasmic bathing solution, displayed AMFE in the rOCNC1 channels at both positive and negative membrane potentials. The rOCNC1 single-channel conductance showed a distinct minimum (or maximum) in an 80% Na(+)-20% NH(4)(+) mixture (or a 60% Cs(+)-40% Li(+) mixture), but only at positive membrane potentials. Macroscopic measurements in native olfactory CNG channels with mixtures of Na(+)-NH(4)(+) indicated similar AMFE. These results suggest that both native CNG channels and recombinant alpha-homomeric channels allow several ions to be present simultaneously within the channel pore. They also further validate the dominant role of the alpha-subunit in permeation through these channels, provide the first evidence to suggest that rOCNC1 channels have multi-ion properties and further justify the use of the rOCNC1 channel as an effective model for structure-function studies of ion permeation and selectivity in olfactory CNG channels.

The properties of carbachol-activated nonselective cation channels at the single channel level in guinea pig gastric myocytes

We investigated the properties of carbachol (CCh)-activated nonselective cation channels (NSC(CCh)) at the single channel level in the gastric myocytes of guinea pigs using a magnified whole-cell mode or an outside-out mode. The channel activity (NPo) recorded in a magnified whole-cell mode increased with depolarization (from -120 to -20 mV) and had the half activation potential of -81 mV under the symmetrical 140 mM Cs+ condition. The single channel conductance depended upon the extracellular monovalent cations with the order of Cs+ (35 pS) > Na+ (25 pS) > Li+ (21 pS). The channel activities markedly diminished or disappeared when external Cs+ was replaced with Na+ or N-methyl-D-glucamate (NMDG+). With Cs+ and Na+ as external cations, the channel showed a monotonic increase in NPo with the increased mole fraction of Cs+ over Na+, and it had an intermediate conductance value in solution containing 67% Cs+ with 33% Na+. These data suggested that the extracellular monovalent cations regulate the whole-cell current of NSC(CCh) by modulating both the open state probability and the unitary conductance, and there is one binding site for the extracellular cations within the pore.

The interaction of Na(+) and K(+) in the pore of cyclic nucleotide-gated channels

The permeability ratio between K(+) and Na(+) ions in cyclic nucleotide-gated channels is close to 1, and the single channel conductance has almost the same value in the presence of K(+) or Na(+). Therefore, K(+) and Na(+) ions are thought to permeate with identical properties. In the alpha-subunit from bovine rods there is a loop of three prolines at positions 365 to 367. When proline 365 is mutated to a threonine, a cysteine, or an alanine, mutant channels exhibit a complex interaction between K(+) and Na(+) ions. Indeed K(+), Rb(+) and Cs(+) ions do not carry any significant macroscopic current through mutant channels P365T, P365C and P365A and block the current carried by Na(+) ions. Moreover in mutant P365T the presence of K(+) in the intracellular (or extracellular) medium caused the appearance of a large transient inward (or outward) current carried by Na(+) when the voltage command was quickly stepped to large negative (or positive) membrane voltages. This transient current is caused by a transient potentiation, i.e., an increase of the open probability. The permeation of organic cations through these mutant channels is almost identical to that through the wild type (w.t.) channel. Also in the w.t. channel a similar but smaller transient current is observed, associated to a slowing down of the channel gating evident when intracellular Na(+) is replaced with K(+). As a consequence, a rather simple mechanism can explain the complex behavior here described: when a K(+) ion is occupying the pore there is a profound blockage of the channel and a potentiation of gating immediately after the K(+) ion is driven out. Potentiation occurs because K(+) ions slow down the rate constant K(off) controlling channel closure. These results indicate that K(+) and Na(+) ions do not permeate through CNG channels in the same way and that K(+) ions influence the channel gating.

Cyclic nucleotide-gated channels mediate membrane depolarization following activation of store-operated calcium entry in endothelial cells

Calcium agonists induce membrane depolarization in endothelial cells through an unknown mechanism. Present studies tested the hypothesis that pulmonary artery endothelial cells express a cyclic nucleotide-gated (CNG) cation channel activated by store-operated calcium entry to produce membrane depolarization. In the whole-cell configuration, voltage-clamped cells revealed a large non-inactivating, outwardly rectifying cationic current in the absence of extra- or intracellular Ca(2+) that was reduced upon replenishment of Ca(2+). The inward current was non-selective for K(+), Na(+), Cs(+), and Rb(+) and was not inhibited by high tetraethylammonium concentrations. cAMP and cGMP stimulated the current and changed the cation permeability to favor Na(+). Moreover, 8-bromo-cAMP stimulated the current in voltage-clamped cells in the perforated patch mode. The cationic current was inhibited by the CNG channel blocker LY83,583, and reverse transcriptase-polymerase chain reaction cloning identified expression of a CNG channel resembling that seen in olfactory neurons. Activation of store-operated calcium entry using thapsigargin increased a current through the CNG channel. Stimulation of the current paralleled pulmonary artery endothelial cell membrane depolarization, and both the current and membrane depolarization were abolished using LY83,583. Taken together, these data demonstrate activation of store-operated calcium entry stimulates a CNG channel producing membrane depolarization. Such membrane depolarization may contribute to slow feedback inhibition of store-operated calcium entry.

Divalent cation selectivity is a function of gating in native and recombinant cyclic nucleotide-gated ion channels from retinal photoreceptors

The selectivity of Ca2+ over Na+ is approximately 3.3-fold larger in cGMP-gated channels of cone photoreceptors than in those of rods when measured under saturating cGMP concentrations, where the probability of channel opening is 85-90%. Under physiological conditions, however, the probability of opening of the cGMP-gated channels ranges from its largest value in darkness of 1-5% to essentially zero under continuous, bright illumination. We investigated the ion selectivity of cGMP-gated channels as a function of cyclic nucleotide concentration in membrane patches detached from the outer segments of rod and cone photoreceptors and have found that ion selectivity is linked to gating. We determined ion selectivity relative to Na+ (PX/PNa) from the value of reversal potentials measured under ion concentration gradients. The selectivity for Ca2+ over Na+ increases continuously as the probability of channel opening rises. The dependence of PCa/PNa on cGMP concentration, in both rods and cones, is well described by the same Hill function that describes the cGMP dependence of current amplitude. At the cytoplasmic cGMP concentrations expected in dark-adapted intact photoreceptors, PCa/PNa in cone channels is approximately 7.4-fold greater than that in rods. The linkage between selectivity and gating is specific for divalent cations. The selectivity of Ca2+ and Sr2+ changes with cGMP concentration, but the selectivity of inorganic monovalent cations, Cs+ and NH4+, and organic cations, methylammonium+ and dimethylammonium+, is invariant with cGMP. Cyclic nucleotide-gated channels in rod photoreceptors are heteromeric assemblies of alpha and beta subunits. The maximal PCa/PNa of channels formed from alpha subunits of bovine rod channels is less than that of heteromeric channels formed from alpha and beta subunits. In addition, Ca2+ is a more effective blocker of channels formed by alpha subunits than of channels formed by alpha and beta subunits. The cGMP-dependent shift in divalent cation selectivity is a property of alphabeta channels and not of channels formed from alpha subunits alone.

Ca2+ permeation in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels conduct Na+, K+ and Ca2+ currents under the control of cGMP and cAMP. Activation of CNG channels leads to depolarization of the membrane voltage and to a concomitant increase of the cytosolic Ca2+ concentration. Several polypeptides were identified that constitute principal and modulatory subunits of CNG channels in both neurons and non-excitable cells, co-assembling to form a variety of heteromeric proteins with distinct biophysical properties. Since the contribution of each channel type to Ca2+ signaling depends on its specific Ca2+ conductance, it is necessary to analyze Ca2+ permeation for each individual channel type. We have analyzed Ca2+ permeation in all principal subunits of vertebrates and for a principal subunit from Drosophila melanogaster. We measured the fractional Ca2+ current over the physiological range of Ca2+ concentrations and found that Ca2+ permeation is determined by subunit composition and modulated by membrane voltage and extracellular pH. Ca2+ permeation is controlled by the Ca2+-binding affinity of the intrapore cation-binding site, which varies profoundly between members of the CNG channel family, and gives rise to a surprising diversity in the ability to generate Ca2+ signals.

Molecular determinants of a Ca2+-binding site in the pore of cyclic nucleotide-gated channels: S5/S6 segments control affinity of intrapore glutamates

Cyclic nucleotide-gated (CNG) channels play an important role in Ca2+ signaling in many cells. CNG channels from various tissues differ profoundly in their Ca2+ permeation properties. Using the voltage-dependent Ca2+ blockage of monovalent current in wild-type channels, chimeric constructs and point mutants, we have identified structural elements that determine the distinctively different interaction of Ca2+ with CNG channels from rod and cone photoreceptors and olfactory neurons. Segments S5 and S6 and the extracellular linkers flanking the pore region are the only structural elements that account for the differences between channels. Ca2+ blockage is strongly modulated by external pH. The different pH dependence of blockage suggests that the pKa of intrapore glutamates and their protonation pattern differ among channels. The results support the hypothesis that the S5-pore-S6 module, by providing a characteristic electrostatic environment, determines the protonation state of pore glutamates and thereby controls Ca2+ affinity and permeation in each channel type.

Physical origin of selectivity in ionic channels of biological membranes

This paper shows that the selectivity properties of monovalent cation channels found in biological membranes can originate simply from geometrical properties of the inner core of the channel without any critical contribution from electrostatic interactions between the permeating ions and charged or polar groups. By using well-known techniques of statistical mechanics, such as the Langevin equations and Kramer theory of reaction rates, a theoretical equation is provided relating the permeability ratio PB/PA between ions A and B to simple physical properties, such as channel geometry, thermodynamics of ion hydration, and electrostatic interactions between the ion and charged (or polar) groups. Diffusive corrections and recrossing rates are also considered and evaluated. It is shown that the selectivity found in usual K+, gramicidin, Na+, cyclic nucleotide gated, and end plate channels can be explained also in the absence of any charged or polar group. If these groups are present, they significantly change the permeability ratio only if the ion at the selectivity filter is in van der Waals contact with them, otherwise these groups simply affect the channel conductance, lowering the free energy barrier of the same amount for the two ions, thus explaining why single channel conductance, as it is experimentally observed, can be very different in channels sharing the same selectivity sequence. The proposed theory also provides an estimate of channel minimum radius for K+, gramicidin, Na+, and cyclic nucleotide gated channels.

Calcium modulation of ligand affinity in the cyclic GMP-gated ion channels of cone photoreceptors

To investigate modulation of the activation of cGMP-gated ion channels in cone photoreceptors, we measured currents in membrane patches detached from the outer segments of single cones isolated from striped bass retina. The sensitivity of these channels to activation by cGMP depends on the history of exposure to divalent cations of the membrane's cytoplasmic surface. In patches maintained in 20 microM Ca++ and 100 microM Mg++ after excision, the current amplitude dependence on cGMP is well described by a Hill equation with average values of K1/2, the concentration necessary to activate half the maximal current, of 86 microM and a cooperativity index, n, of 2.57. Exposing the patch to a solution free of divalent cations irreversibly increases the cGMP sensitivity; the average value of K1/2 shifts to 58.8 microM and n shifts to 1.8. Changes in cGMP sensitivity do not affect other functional parameters of the ion channels, such as the interaction and permeation of mono- and divalent cations. Modulation of cGMP activation depends on the action of an endogenous factor that progressively dissociates from the channel as Ca++ concentration is lowered below 1 microM. The activity of the endogenous modulator is not well mimicked by exogenously added calmodulin, although this protein competes with the endogenous modulator for a common binding site. Thus, the modulation of cGMP affinity in cones depends on the activity of an unidentified molecule that may not be calmodulin.

Modulation of rod, but not cone, cGMP-gated photoreceptor channels by calcium-calmodulin

Inside-out patches containing cGMP-gated channels were excised from catfish rod or cone outer segments and held under voltage clamp. The net cGMP-dependent currents elicited by saturating and subsaturating concentrations of cGMP at +/-30 mV were measured and the dependence of current upon cGMP concentration was determined. The apparent affinity of the channel for its ligand was estimated by fitting these data with the Hill equation. The concentration of cGMP required to give half the maximum current (K1/2) in rod and cone channels at +30 mV was approximately 28 microM and approximately 37 microM, respectively, and was weakly voltage dependent. Thus, cone channels have an intrinsically higher K1/2 than rod channels. For both types of channel, the Hill coefficient was approximately 2.3. In the presence of calcium-calmodulin, the apparent affinity of the rod channel for cGMP decreased by about twofold, but the apparent affinity of the cone channels was unaffected. These results indicate that the open probability of the cone channel for its ligand cannot be modulated by calmodulin. This represents the first significant departure between rod and cone photoreceptors in mechanisms used by phototransduction and suggests that the beta subunit of the cone channel must be different from that of the rod channel.

Ion selectivity predictions from a two-site permeation model for the cyclic nucleotide-gated channel of retinal rod cells

We developed a two-site, Eyring rate theory model of ionic permeation for cyclic nucleotide-gated channels (CNGCs). The parameters of the model were optimized by simultaneously fitting current-voltage (IV) data sets from excised photoreceptor patches in electrolyte solutions containing one or more of the following ions: Na+, Ca2+, Mg2+, and K+. The model accounted well for 1) the shape of the IV relations; 2) the binding affinity for Na+; 3) reversal potential values with single-sided additions of Ca2+ or Mg2+ and biionic KCl; and 4) the K1 and voltage dependence for divalent block from the cytoplasmic side of the channel. The differences between the predicted K1's for extracellular block by Ca2+ and Mg2+ and the values obtained from heterologous expression of only the alpha-subunit of the channel suggest that the beta-subunit or a cell-specific factor affects the interaction of divalent cations at the external but not the internal face of the channel. The model predicts concentration-dependent permeability ratios with single-sided addition of Ca2+ and Mg2+ and anomalous mole fraction effects under a limited set of conditions for both monovalent and divalent cations. Ca2+ and Mg2+ are predicted to carry 21% and 10%, respectively, of the total current in the retinal rod cell at -60 mV.

Block of the cGMP-gated cation channel of catfish rod and cone photoreceptors by organic cations

Tetraalkylammonium compounds and other organic cations were used to probe the structure of the internal and external mouths of the pore of cGMP-gated cation channels from rod and cone photoreceptors. Both rod and cone channels were blocked by tetramethyl- through tetrapentylammonium from the intracellular side in a voltage-dependent fashion at millimolar to micromolar concentrations. The dissociation constant at 0 mV (KD(O)) decreased monotonically with increasing carbon chain length from approximately 80 mM (TMA) to approximately 80 microM (TPeA), where the dissociation constant in rod channels is approximately 50% that of cone channels. N-Methyl-D-glucamine and the buffer Tris also blocked the cone channel in a voltage-dependent fashion at millimolar concentrations, but with lower affinity than similarly sized tetraalkylammonium blockers. Block by tetrahexylammonium (THxA) was voltage-independent, suggesting that the diameter of the intracellular mouth of these channels is less than the size of THxA but larger than TPeA. The location of the binding site for intracellular blockers was approximately 40% across the voltage-drop from the intracellular side. The addition of one carbon to each of the alkyl side chains increased the binding energy by approximately 4 kJ mol-1, consistent with hydrophobic interactions between the blocker and the pore. Cone, but not rod, channels were blocked by millimolar concentrations of extracellular TMA. The location of the extracellular binding site was approximately 13% of the voltage drop from the extracellular side. In cone channels, the two blocker binding sites flank the location of the cation binding site proposed previously.

Effect of changing temperature on the ionic permeation through the cyclic GMP-gated channel from vertebrate photoreceptors

Native cGMP-gated channels were studied in rod outer segments of the larval tiger salamander Ambystoma tigrinum. The alpha subunit of the cGMP-gated channel from bovine rods, here referred to as the wild type (w.t.), and mutant channels were heterologously expressed in Xenopus laevis oocytes. These channels were studied in excised membrane patches in the inside-out configuration and were activated by the addition of 100 or 500 microM cGMP. The effect of temperature on the ionic permeation was studied. The macroscopic current flowing through the native channel at +100 mV had an activation energy of 35.8, 30, 31.8, 34.5, 41.3, and 22.4 kJ mol-1 in the presence of Li+, Na+, K+, Rb+, Cs+, and NH4+, respectively. The macroscopic current flowing through the w.t. channel at +100 mV had an activation energy of 45.2, 38.2, 37.5, 47.3, 49.4, and 38.9 kJ mol-1 in the presence of Li+, Na+, K+, Rb+, Cs+, and NH4+, respectively. The activation energy of the macroscopic current flowing through the native and w.t. channels did not vary significantly when the ionic concentration of the permeant ion was changed between 2.5 and 110 mM. The activation energy of the single-channel current of the w.t. channel at +100 mV was 40.4 and 33 kJ mol-1 for Na+ and NH4+, respectively. The reversal potential of biionic solutions changed significantly with temperature. These results can be used to obtain an estimate of the enthalpic and entropic contributions to the barrier of the Gibbs free energy experienced by an ion during its permeation through the open channel. These estimates indicate that the ionic permeation and selectivity of the cGMP-gated channel are controlled both by enthalpic and entropic factors and that the selectivity of the native channel for Li+ over Na+ is primarily caused by entropic effects.

Molecular diversity of cyclic nucleotide-gated cation channels

Cyclic nucleotide-gated cation channels (CNG channels) form a multi-gene family consisting of at least five distinct members (CNG1-5). Expression studies have indicated that only CNG1-3 are able to form functional homooligomeric channels. Although structurally related, the cDNAs of CNG4-5 fail to induce cyclic nucleotide-dependent currents when expressed alone. However, when co-expressed with CNG1-3 they confer some of the physiologically observed properties of native CNG channels which are absent from the homooligomeric CNG1-3 channels. CNG channels are expressed in several tissues and cell types pointing to a general function of these channels in a wide variety of cellular systems. There is now increasing evidence that a major function of CNG channels may consist in providing a second messenger-regulated pathway for Ca2+ influx.

Ca2+ flux in retinal rod and cone outer segments: differences in Ca2+ selectivity of the cGMP-gated ion channels and Ca2+ clearance rates

In intact rod and cone photoreceptors of various vertebrate species, depolarization in the dark to > or = +20 mV specifically activates the cGMP-dependent conductance in the outer segment. This activation reflects a voltage-dependent decrease in cytoplasmic Ca2+ and the consequent activation of a Ca(2+)-dependent guanylyl cyclase. The conductance activation in cones is much faster in time course and larger in extent than that in rods. Simulations of the experimental results suggest that these differences arise from differences in Ca2+ homeostasis in the rod and cone outer segments. Direct measurements demonstrate that, indeed, the Ca2+ permeability of the cGMP-gated channels is higher in cones than in rods. Also, as was previously known, the rate of Ca2+ efflux from cone outer segments is higher than that in rods. Therefore, a given light-dependent change in membrane current should cause a much larger and much quicker decrease in Ca2+ concentration in cones than in rods. The activity of every Ca(2+)-dependent biochemical event in the outer segment should, hence, change more rapidly and to a larger extent in cones than in rods. We propose that these kinetic and stoichiometric differences in the function of Ca(2+)-dependent processes is important in explaining the difference in the transduction signal of the two receptor types.

A 240 kDa protein represents the complete beta subunit of the cyclic nucleotide-gated channel from rod photoreceptor

The cyclic nucleotide-gated channel from rod photoreceptors is composed of two distinct subunits (alpha and beta). The properties of the alpha subunit, which can form functional channels by itself, are modified by coexpression with a homologous polypeptide, designated the beta subunit. However, the alpha subunit from rod photoreceptor membranes copurifies with a 240 kDa protein that is significantly larger than this putative beta subunit. We now demonstrate by peptide sequencing and by cloning and functional expression of cDNA that the 240 kDa protein represents the complete beta subunit with an unusual bipartite structure. The N-terminal part is essentially identical to a glutamic acid-rich protein (GARP), whereas the C-terminal part is highly homologous to the previously cloned human "beta subunit." Expression of the complete beta subunit in HEK 293 cells results in a polypeptide with the same apparent molecular weight as the 240 kDa protein of the native rod channel. Coexpression of the alpha subunit with the full-length beta subunit yields hetero-oligomeric channels with properties characteristic of the native channel.

The multi-ion nature of the cGMP-gated channel from vertebrate rods

1. Native cGMP-gated channels were studied in rod outer segments of the larval tiger salamander, Ambystoma tigrinum. The alpha-subunit of the cGMP-gated channel, here referred to as the wild type (WT), and mutant channels were heterologously expressed in Xenopus laevis oocytes. These channels were studied in excised membrane patches in the inside-out configuration and were activated by the addition of 100 or 500 microM cGMP. The current carried by monovalent cations was measured under voltage-clamp conditions. 2. In the presence of 110 mM Na+ in the extracellular medium and different amounts of Na+ in the intracellular medium, the I-V relations of the native channel could be described by a single-site model with a profile of Gibbs free energy with two barriers and a well. A similar result was obtained in the presence of 110 mM Li+ in the extracellular medium and different amounts of Li+ in the intracellular medium. The well depth was 1.4RT (where R is the gas constant and T is the absolute temperature) for both Li+ and Na+. 3. The I-V relations of the native channel in the presence of 110 mM Na+ on one side of the membrane and 110 mM Li+ on the other side could not be described by the same single-site model with identical values of barriers and well obtained in the presence of Li+ or Na+ alone: the well for Li+ had to be at least 4RT. 4. In the presence of mixtures of 110 mM Li+ and Cs+ on the cytoplasmic side of the membrane, an anomalous mole fraction effect was observed both in the native and the WT channel. No anomalous behaviour was seen in the presence of Li(+)-Na+ and Li(+)-NH4+ mixtures. 5. The anomalous mole fraction effect with mixtures of Li+ and Cs+ was not observed in the channel where glutamate 363 was mutated to a glutamine (E363Q) or an asparagine (E363N). When glutamate 363 was mutated to an aspartate (E363D), the anomalous mole fraction effect with mixtures of Li+ and Cs+ was still observed, although significantly reduced. 6. When lysine 346, arginine 369, aspartate 370 and glutamate 372 were neutralized by mutation to glutamine, the ion permeation through the mutant channels and the WT channel had largely similar properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Family of cyclic nucleotide gated ion channels

A variety of different cyclic nucleotide gated ion channels have recently been identified using molecular cloning and electrophysiological techniques. Current research is focussed on the specific molecular determinants that endow these channels with their distinctive character of gating, selectivity and modulation. In some cases, it has been possible to identify the specific physiological roles of different cyclic nucleotide gated channels. Their interactions with Ca2+ and calmodulin are particularly important, and determine the specific functions these channels subserve in distinct cells.

Cyclic nucleotide-gated channels in visual and olfactory transduction

Rod and cone photoreceptors are the light detectors in the visual system whereas olfactory receptor cells are the odorant detectors in the olfactory system. Despite the two very different types of stimuli, light in photoreceptors, and odorant molecules in olfactory receptor cells, the mechanisms of visual and olfactory transduction appear to have many homologies. Both stimuli trigger a chain of enzymatic events that terminates in a change in the concentration of a cyclic nucleotide: a decrease in the concentration of cGMP in photoreceptors, and an increase in the concentration of cAMP in olfactory receptor cells. These cyclic nucleotides directly gate cation channels and therefore a change in their concentration induced by the external stimulus is converted into an electrical signal. The analysis of the ionic selectivity properties of cyclic nucleotidegated channels in retinal rods, cones and in olfactory receptor cells shows that there are many similarities between these channels. They do not appreciably select between alkali monovalent cations and can be permeated and blocked by divalent cations. Their ionic permeation properties are consistent with the presence of a cation-binding site of high-field strength in the pore.

The permeation of organic cations through cAMP-gated channels in mammalian olfactory receptor neurons

The permeation of monovalent organic cations through adenosine 3',5'-cyclic monophosphate-(cAMP) activated channels was studied by recording macroscopic currents in excised inside-out membrane patches from the dendritic knobs of isolated mammalian olfactory receptor neurons (ORNs). Current-voltage relations were measured when bathing solution Na+ was replaced by monovalent organic cations. Permeability ratios relative to Na+ ions were calculated from changes in reversal potentials. Some of the small organic cations tested included ammonium (NH4+), hydroxylammonium and formamidinium, with relative permeability ratios of 1.41, 2.3 and 1.01 respectively. The larger methylated and ethylated ammonium ions studied included: DMA (dimethylammonium), TMA (tetramethylammonium) and TEA (tetraethylammonium) and they all had permeability ratios larger than 0.09. Even large cations such as choline, arginine and tris(hydroxymethyl)aminomethane (Tris) were appreciably permeant through the cAMP-activated channel with permeability ratios ranging from 0.19 to 0.7. The size of the permeating cations, as assessed by molecular weight, was a good predictor of the permeability. The permeability sequence of the cAMP-activated channel in our study was PNH4 > PNa > PDMA > PTMA > PCholine > PTEA. Higher permeability ratios of hydroxylammonium, arginine and tris(hydroxymethyl)aminomethane cannot be explained by ionic size alone. Our results indicate that: (i) cAMP-activated channels poorly select between monovalent cations; (ii) the pore dimension must be at least 6.5 x 6.5 A, in order to allow TEA and Tris to permeate and (iii) molecular sieving must be an important mechanism for the permeation of large organic ions through the channels with specific ion binding playing a smaller role than in other structurally similar channels. In addition, the results clearly indicate that cyclic nucleotide-gated (CNG) channels in different cells are not the same, the olfactory CNG channel being different from that of the photoreceptors, particularly with respect to the permeation of large organic cations, which the ORN channels allow to permeate readily.

Conductance and kinetics of single cGMP-activated channels in salamander rod outer segments

1. The conductance and kinetics of single 3',5'-cyclic guanosine monophosphate (cGMP)-activated channels of retinal rod outer segments were studied in inside-out membrane patches. The size of the single channel currents was increased by using low concentrations of divalent cations. 2. At saturating cGMP concentration, the current flickered at high frequency. Occasionally, the current was interrupted by closures lasting tens or hundreds of milliseconds. At +50 mV the maximum current during an opening was slightly more than 1 pA, but the open channel level was poorly resolved due to the speed of the gating transitions. 3. Amplitude histograms confirmed the presence of a sublevel of current, roughly a quarter the size of the peak current, at low cGMP concentrations. The fraction of time in the sublevel decreased with increasing cGMP concentration, suggesting that the sublevel may be due to opening by the partially liganded channel. 4. Consistent with previous macroscopic current recordings, single channel activation by cGMP had an apparent dissociation constant of 8.6 microM, and a Hill coefficient of 2.8. 5. At saturating cGMP concentrations, the channel was modelled as a two-state system with the following parameters. The open channel conductance was 25 pS. The opening rate constant, beta, was 1.5 x 10(4) s-1 at 0 mV, and had a voltage sensitivity equivalent to the movement of 0.23 electronic charges outward through the membrane electric field. The closing rate constant, alpha, was 2.1 x 10(4) s-1 and was voltage insensitive. Assuming that the open-state chord conductance was voltage independent, the inferred voltage dependence of beta largely accounted for the outward rectification in the steady-state macroscopic current-voltage relation of multichannel patches, at saturating cGMP concentration.

Differences in transduction between rod and cone photoreceptors: an exploration of the role of calcium homeostasis

Rod and cone photoreceptors respond to light with distinct sensitivity and kinetics. Recent biochemical and electrophysiological studies demonstrate that the enzymes of the phototransduction cascade are similar, but not identical, in these two photoreceptor types. In contrast, light or voltage stimulation generates changes in the cytoplasmic concentration of Ca2+ in the outer segment that are far larger and faster in cones than in rods. This distinction reflects rod-cone differences in each of the elements that control Ca2+ homeostasis: cell volume, the rate of Ca2+ clearance from the outer segment, the cytoplasmic Ca2+ buffering, and the Ca2+ influx through cGMP-gated ion channels.

A single negative charge within the pore region of a cGMP-gated channel controls rectification, Ca2+ blockage, and ionic selectivity

Ca2+ ions control the cGMP-gated channel of rod photoreceptor cells from the external and internal face. We studied ion selectivity and blockage by Ca2+ of wild-type and mutant channels in a heterologous expression system. External Ca2+ blocks the inward current at micromolar concentrations in a highly voltage-dependent manner. The blockage at negative membrane voltages shows a steep concentration dependence with a Hill coefficient of approximately 2. The blockage from the internal face requires approximately 1000-fold higher Ca2+ concentrations. Neutralization of a glutamate residue (E363) in the putative pore region between transmembrane segments H4 and H5 induces outward rectification and changes relative ion conductances but leaves relative ion permeabilities nearly unaffected. The current blockage at -80 mV requires approximately 2000-fold higher external Ca2+ concentrations and the voltage dependence is almost abolished. These results demonstrate that E363 represents a binding site for monovalent and divalent cations and resides in the pore lumen.

Gating, selectivity and blockage of single channels activated by cyclic GMP in retinal rods of the tiger salamander

1. Patches in the inside-out configuration were excised from the membrane of outer and inner segments of the larval tiger salamander, Ambystoma tigrinum. The current flowing through single channels opened by cyclic GMP was studied with the voltage clamp technique. 2. Amplitude histograms of current recordings from patches containing only one flickering channel, excised from the inner segment and in the presence of 100 microM cyclic GMP, could be fitted by a theoretical scheme in which the single channel conductance was at least 55 pS at +40 mV and at least 45 pS at -40 mV. The mean open time was no longer than the time constant of our recording system, about 35 microseconds. Similar results were obtained by analysis of the amplitude histograms of patches from the outer segment containing many channels, and in the presence of 1-5 microM cyclic GMP. 3. In membrane patches excised from the outer segment, reducing the temperature from 24 to 8 degrees C did not reduce the flickering, but changed the amplitude histograms of current fluctuations activated by 1 microM cyclic GMP in a way consistent with a decrease of 50% in the single channel conductance and a decrease of 50% in the open probability. 4. In the presence of 1 microM cyclic GMP at +60 mV, when Na+ was replaced by NH4+ or K+, brief outward current transients flowing through single channels were observed. When Na+ was replaced with Li+, Rb+ or Cs+, current transients were very small. 5. The shape of the power spectrum of current fluctuations induced by 1 microM cyclic GMP at +60 mV did not change when the permeating ion was Na+, K+ or NH4+. Analysis of the amplitude histogram did not show any effect of the tested monovalent cations on the open probability or on channel gating. At +60 mV, the estimated single channel currents were at least 4, 2.8 and 2 pA for NH4+, Na+ and K+ respectively. 6. The addition of 0.5 or 1 mM Ca2+ to the medium bathing the cytoplasmic side of the membrane greatly reduced the frequency of openings, but single channel activity could still be observed. The blocking effect of 1 mM Ca2+ on the channel activity induced by 2 microM cyclic GMP could be counterbalanced by increasing the cyclic GMP concentration. The addition of 0.5 or 1 mM Ca2+ did not change the shape of power spectra obtained at membrane voltages between -100 and +100 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Single-channel properties of cloned cGMP-activated channels from retinal rods

Single-channel properties of a cloned channel activated by cyclic GMP have been analysed. The mRNA encoding for the channel was injected into oocytes of Xenopus laevis and the current flowing through a single ionic channel activated by cGMP was studied in excised patches under voltage-clamp conditions. The ionic channel activated by cGMP had a single-channel conductance of 32 +/- 2 pS at +120 mV and 25 +/- 4 pS at -120 mV, and its conductance was not significantly affected by increasing the cGMP concentration from 20 microM to 200 microM. The single-channel currents in the presence of NH+4, Na+, K+, Li+ and Rb+ in the medium bathing the cytoplasmic side of the membrane at +140 mV were 5.3, 4.7, 3.8, 1.3 and 0.8 pA, respectively. The single-channel current in the presence of Cs+ was less than 0.5 pA. Ca2+ and Mg2+ (both 0.5 mM) in the presence of 100 microM cGMP did not appreciably affect the channel activity at membrane potentials more negative than -80 mV, whereas at +100 mV they reduced the single-channel conductance by about threefold. The ionic selectivity and the blockage by divalent cations of the native channel found in amphibian rods and in the cloned channel from bovine rods are quite similar. However, the cloned channel has well-resolved openings, especially at positive membrane voltages, whereas the native channel is characterized by a continuous flickering between the open and closed state.

Cyclic nucleotide-activated channels in carp olfactory receptor cells

When applied from the cytoplasmic side, cyclic 3',5'-adenosine and guanosine monophosphates reversibly increased the ion permeability of inside-out patches of carp olfactory neuron plasma membrane. The cAMP (cGMP)-induced permeability via cAMP (cGMP) concentration was fitted by Hill's equation with the exponents of 1.07 +/- 0.15 (1.12 +/- 0.05) and EC50 = 1.3 +/- 0.6 microM (0.9 +/- 0.3 microM). Substitution of NaCl in the bathing solution by chlorides of other alkali metals resulted in a slight shift of reversal potential of the cyclic nucleotide-dependent (CN) current, which indicates a weak selectivity of the channels. Permeability coefficients calculated by Goldman-Hodgkin-Katz's equation corresponded to the following relation: PNa/PK/PLi/PRb/PCs = 1:0.98:0.94:0.70:0.61. Ca2+ and Mg2+ in physiological concentrations blocked the channels activated by cyclic nucleotides (CN-channels). In the absence of divalent cations the conductance of single CN-channels was equal to 51 +/- 9 pS in 100 mM NaCl solution. Channel density did not exceed 1 micron-2. The maximal open state probability of the channel (Po) tended towards 1.0 at a high concentration of cAMP or cGMP. Dichlorobenzamil decreased Po without changing the single CN-channel' conductance. CN-channels exhibited burst activity. Mean open and closed times as well as the burst duration depended on agonist concentration. A kinetic model with four states (an inactivated, a closed and two open ones) is suggested to explain the regularities of CN-channel gating and dose-response relations.

Divalent effects on cGMP-activated currents in excised patches from amphibian photoreceptors

The light-sensitive current in photoreceptors is conducted by a single class of ion channels gated by the binding of multiple molecules of cytoplasmic cGMP. Both Na and Ca ions enter the outer segment through this channel and Ca behaves as a blocking ion, greatly reducing the influx of Na. Because intracellular Ca functions as the cytosolic messenger for light adaptation, and this channel is the major entry point for Ca into the outer segment, we seek a better understanding of the selectivity properties of the channel and how they affect intracellular Ca levels. In these studies, we added divalent cations to the cytoplasmic face of an excised patch at constant, symmetrical [Na]. Our results suggest a novel high-affinity divalent binding site at the internal face of the channel. At constant low levels of cGMP, the addition of 10-100 nM cytoplasmic Ca or Mg attenuated the current 5- to 10-fold. There is also a low-affinity site, midway through the transmembrane field; saturation of this site reduces the divalent-free current approximately 100-fold. The presence of a high-affinity cytoplasmic site raises the question of whether Ca regulates the photoreceptor current through a direct interaction with the channel perhaps altering the channel selectivity or kinetics.

The permeability of the cGMP-activated channel to organic cations in retinal rods of the tiger salamander

1. The permeability of the channel activated by guanosine 3',5'-cyclic monophosphate (cGMP) to many organic monovalent cations was determined by recording macroscopic currents in excised inside-out patches of plasma membrane from isolated retinal rod outer segments of the tiger salamander. 2. Current-voltage relations were measured when the NaCl of the bathing medium was replaced by salts of organic cations. Permeability ratios relative to Na+ ions were calculated with the Goldman-Hodgkin-Katz potential equation from the measured changes of reversal potentials. 3. Hydroxylammonium+, hydrazinium+ and methylammonium+, which are molecules of very similar shape and size, permeate the channel with very different permeability ratios: 5.92, 1.99 and 0.60 respectively. 4. Methylated and ethylated ammonium+ compounds were investigated. It was found that, not only methylammonium+, but also dimethylammonium+ and ethylammonium+ were permeant with permeability ratios of 0.6, 0.14 and 0.16 respectively. Trimethylammonium+, tetramethylammonium+, diethylammonium+, triethylammonium+, and tetraethylammonium+ were not permeant. 5. Guanidinium+ and its derivatives formamidinium+, aminoguanidinium+, acetamidinium+ and methylguanidinium+ were all permeant with permeability ratios 1.12, 1.00, 0.63, 0.36 and 0.33 respectively. 6. The cGMP-activated channel was found to be permeable to at least thirteen organic cations. Molecular models of the permeant cations indicate that the cross-section of the narrowest part of the pore must be at least as large as a rectangle of 0.38 x 0.5 nm dimensions.

Interactions between divalent cations and the gating machinery of cyclic GMP-activated channels in salamander retinal rods

The effects of divalent cations on the gating of the cGMP-activated channel, and the effects of gating on the movement of divalent cations in and out of the channel's pore were studied by recording macroscopic currents in excised membrane patches from salamander retinal rods. The fractional block of cGMP-activated Na+ currents by internal and external Mg2+ as well as internal Ca2+ was nearly independent of cGMP concentration. This indicates that Mg2+ and Ca2+ bind with similar affinity to open and closed states of the channel. In contrast, the efficiency of block by internal Cd2+ or Zn2+ increased in proportion to the fraction of open channels, indicating that these ions preferentially occupy open channels. The kinetics of block by internal Ni2+, which competes with Mg2+ but blocks more slowly, were found to be unaffected by the fraction of channels open. External Ni2+, however, blocked and unblocked much more rapidly when channels were mostly open. This suggests that within the pore a gate is located between the binding site(s) for ions and the extracellular mouth of the channel. Micromolar concentrations of the transition metal divalent cations Ni2+, Cd2+, Zn2+, and Mn2+ applied to the cytoplasmic surface of a patch potentiated the response to subsaturating concentrations of cGMP without affecting the maximum current induced by saturating cGMP. The concentration of cGMP that opened half the channels was often lowered by a factor of three or more. Potentiation persisted after the experimental chamber was washed with divalent-free solution and fresh cGMP was applied, indicating that it does not result from an interaction between divalent cations and cGMP in solution; 1 mM EDTA or isotonic MgCl2 reversed potentiation. Voltage-jump experiments suggest that potentiation results from an increase in the rate of cGMP binding. Lowering the ionic strength of the bathing solution enhanced potentiation, suggesting that it involves electrostatic interactions. The strong electrostatic effect on cGMP binding and absence of effect on ion permeation through open channels implies that the cGMP binding sites on the channel are well separated from the permeation pathway.

Cation interactions within the cyclic GMP-activated channel of retinal rods from the tiger salamander

1. The ionic dependence of current through the 3',5'-cyclic guanosine monophosphate (cyclic GMP)-activated channels of salamander rods was studied in excised inside-out membrane patches from isolated outer segments. Voltage-clamp experiments on transducing rods were performed so that the channels in intact cells could be compared with those in excised patches. 2. The reversal potential of the cyclic GMP-induced patch current was close to the Na+ equilibrium potential when the concentration of NaCl on the cytoplasmic surface of a patch was varied at constant external NaCl concentration. Fitting the Goldman-Hodgkin-Katz equation indicated that the apparent ratio of permeabilities for Na+ and Cl- was at least 50. This confirms a previous report that the channel's Na+ permeability is much larger than its Cl- permeability. 3. Na+ currents through the channel did not obey the independence principle. The outward patch current at large positive potential began to saturate with increasing concentrations of internal Na+, as if permeation required Na+ to bind to a site with an apparent dissociation constant around 180 mM. 4. In symmetrical NaCl solutions containing very low concentrations of divalent cations the current-voltage relation measured from excised patches 50 microseconds after switching the voltage showed mild outward rectification. By 1 ms the rectification was more pronounced. The rectification at 50 microseconds is attributed to voltage dependence of Na+ permeation. The additional rectification at later times is attributed to voltage dependence of the channel's probability of being open, depolarization favouring the open state. 5. In symmetrical Mg2+ solutions the cyclic GMP-induced patch currents were smaller and the outward rectification was more pronounced. 6. Addition of Mg2+ or Ca2+ to an internal Na+ solution blocked the cyclic GMP-induced Na+ current through the channels, as if by occupying a single binding site with an affinity in the 0.1-2 mM range. Block by Mg2+ was voltage dependent, suggesting that the binding site was within the channel's transmembrane electric field. Raising the Mg2+ concentration on the external surface of the patch increased the apparent dissociation constant of block by internal Mg2+, as expected if external and internal Mg2+ compete for the same binding site. 7. Block by internal Ca2+ had an opposite and weaker voltage dependence than block by internal Mg2+. 8. In symmetrical solutions containing both Na+ and Mg2+ the outward rectification was more pronounced than in solutions containing Na+ alone. In solutions thought to be close to physiological the outward patch current increased e-fold for a depolarization of 24-30 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

A cyclic GMP-activated channel in dissociated cells of the chick pineal gland

Phototransduction in the vertebrate retina is dependent in part on a cyclic GMP-activated ionic channel in the plasma membrane of rods and cones. But other vertebrate cells are also photosensitive. Cells of the chick pineal gland have a photosensitive circadian rhythm in melatonin secretion that persists in dissociated cell culture. Exposure to light causes inhibition of melatonin secretion, and entrainment of the intrinsic circadian oscillator. Chick pinealocytes express several 'retinal' proteins, including arrestin, transducin and a protein similar to the visual pigment rhodopsin. Pinealocytes of lower vertebrates display hyperpolarizing responses to brief pulses of light. Thus it is possible that some of the mechanisms of phototransduction are similar in retinal and pineal photoreceptors. We report here the first recordings of cyclic GMP-activated channels in an extraretinal photoreceptor. Application of GMP, but not cyclic AMP, to excised inside-out patches caused activation of a 15-25 pS cationic channel. These channels may be essential for phototransduction in the chick pineal gland.

The cyclic nucleotide-gated channels of vertebrate photoreceptors and olfactory epithelium

Cation channels that are directly gated by guanosine 3', 5'-cyclic monophosphate (cGMP) control the flow of ions across the surface membrane of vertebrate rod and cone photoreceptor cells. A similar channel, gated by adenosine 3',5'-cyclic monophosphate (cAMP), exists in vertebrate olfactory sensory neurons. The channel polypeptide of rod photoreceptors has been identified and the amino acid sequence of the channel polypeptide in rod and olfactory cells has been determined by cloning cDNA. Although the cyclic nucleotide-gated channels functionally belong to the class of ligand-gated channels, they share some structural features with voltage-gated channels.

Blockage and permeation of divalent cations through the cyclic GMP-activated channel from tiger salamander retinal rods

1. Blockage and permeation of divalent cations through channels activated by guanosine 3',5'-cyclic monophosphate (cyclic GMP) were studied in membrane patches excised from retinal rods of the tiger salamander Ambystoma tigrinum by rapidly changing the ionic medium bathing the intracellular side of the excised membrane. 2. The Na+ current, observed when 110 mM-NaCl was present on both sides of the membrane patch, was reduced by the addition of 1 mM of the chloride salts of Ca2+, Mg2+, Sr2+, Ba2+ or Mn2+ to the bathing medium. The sequence of blocking potency at +60 mV was Mg2+ greater than Mn2+ approximately Ba2+ greater than Ca2+ greater than Sr2+, while at -60 mV it was Ba2+ greater than Ca2+ greater than Sr2+ greater than Mn2+ approximately Mg2+. For all divalent cations the blocking effect depended, in a complex way, on the membrane potential. 3. The blocking effect of Ca2+ and Mg2+ increased when the concentration of cyclic GMP was reduced from 100 to 5 microM. At -60 mV 1 mM-Ca2+ blocked about 34% of the Na+ current in the presence of 100 microM-cyclic GMP, while in the presence of 5 microM-cyclic GMP, 1 mM-Ca2+ blocked about 56% of the Na+ current. 4. When, in the presence of 100 microM-cyclic GMP, 110 mM-NaCl at the intracellular side was replaced by equiosmolar amounts of chloride salts of divalent cations (73.3 mM) a small outward current carried by divalent cations could be observed at large positive membrane potentials. At +60 mV the ratio between the current carried by Na+, Sr2+, Ca2+, Ba2+, Mg2+ and Mn2+ was 83.3:1.4:1:0.58:0.33:0.25. 5. In agreement with previous observations the dependence of the Na+ current on the concentration of cyclic GMP shows a clear co-operativity among cyclic GMP molecules.4+ cyclic GMP-gated channel in excised patches is similar to but not identical to the selectivity sequence of divalent cations through the channel in intact rods.