A derivative of amiloride blocks both the light-regulated and cyclic GMP-regulated conductances in rod photoreceptors

Cation dependencies and turnover rates of the human K⁺-dependent Na⁺-Ca²⁺ exchangers NCKX1, NCKX2, NCKX3 and NCKX4

The Solute Carrier Family 24 (SLC24) belongs to the CaCA super family of Ca(2+)/cation antiporters and codes for five different K(+)- dependent Na(+)- Ca(2+) exchangers (NCKX1-5). NCKX proteins play a critical role in Ca(2+) homeostasis in a wide variety of biological processes such as vision, olfaction, enamel formation, Melanocortin-4-receptor-dependent satiety and skin pigmentation. NCKX transcripts are widely found throughout the brain. In this study we examine the differences between NCKX1-4 in terms of cation dependencies. We measured changes to Ca(2+) influx via the reverse exchange mode while manipulating external Ca(2+) or K(+) or internal Na(+) concentrations (External Ca(2+) Dependence, External K(+) Dependence and Internal Na(+) Dependence respectively); we also looked at the effect of external Na(+)/Ca(2+) competition and 3' 4'-Dichlorobenzamil on the transport of ions in HEK 293 cell lines. A fluorescence based assay was used to determine differences in transport kinetics of the four membrane spanning exchangers using the Michaelis-Menten constant (Km). Our results show that there are no significant differences between the NCKX isoforms to explain the variation in the specific expression pattern of these exchangers.

Functional and structural properties of the NCKX2 Na(+)-Ca (2+)/K (+) exchanger: a comparison with the NCX1 Na (+)/Ca (2+) exchanger

Na(+)/Ca(2+)-K(+) exchangers (NCKX), alongside the more widely known Na(+)/Ca(2+) exchangers (NCX), are important players in the cellular Ca(2+) toolkit. But, unlike NCX, much less is known about the physiological roles of NCKX, while emergent evidence indicates that NCKX has highly specialized functions in cells and tissues where it is expressed. As their name implies, there are functional similarities in the properties of the two Ca(2+) exchanger families, but there are specific differences as well. Here, we compare and contrast their key functional properties of ionic dependence and affinities, as well as report on the effects of KB-R7943 - a compound that is widely used to differentiate the two exchangers. We also review structural similarities and differences between the two exchangers. The aim is to draw attention to key differences that will aid in differentiating the two exchangers in physiological contexts where both exist but perhaps play distinct roles.

Functional cGMP-gated channels in cerebellar granule cells

Cyclic nucleotide-gated channels (CNGCs) are important transducers of external signals in sensory processes. These channels are ubiquitously expressed in a variety of neurons, and are necessary to transduce signals for growth cone guidance and plasticity. Here, we demonstrate that the CNGC subunits (CNGA1 and CNGB1, presumably the 1b isoform) are expressed in rat cerebellar granule cells and that they combine to form functional channels. The expression of the mRNAs that encode these proteins is maximal after 7 days in cell culture, when the channels are expressed at synapses and co-localize with the synaptic marker synapsin I. These ligand-gated channels are functional and can be blocked by Mg(2+) or L-cis-diltiazem. Moreover, channel opening in response to increases in intracellular cGMP results in Ca(2+) entry into the cell. Chronic blockade (96 h) of these channels with L-cis-diltiazem significantly decreases the number of functional boutons, as determined by their capacity to load and unload the styryl dye FM1-43 when stimulated. Moreover, the unloading kinetics is modified from a biphasic to a monophasic profile in a subset of synaptic boutons. These channels are also expressed in early developmental stages, both in the soma and in emerging processes, and CNGA1 can be detected in growth cones. Pharmacological blockade of these channels with L-cis-diltiazem causes an overall change in growth cone morphology, impairing the formation of lamellipodia between filopodia and increasing the number of filopodia. J

The styryl dye FM1-43 suppresses odorant responses in a subset of olfactory neurons by blocking cyclic nucleotide-gated (CNG) channels

Many olfactory receptor neurons use a cAMP-dependent transduction mechanism to transduce odorants into depolarizations. This signaling cascade is characterized by a sequence of two currents: a cation current through cyclic nucleotide-gated channels followed by a chloride current through calcium-activated chloride channels. To date, it is not possible to interfere with these generator channels under physiological conditions with potent and specific blockers. In this study we identified the styryl dye FM1-43 as a potent blocker of native olfactory cyclic nucleotide-gated channels. Furthermore, we characterized this substance to stain olfactory receptor neurons that are endowed with cAMP-dependent transduction. This allows optical differentiation and pharmacological interference with olfactory receptor neurons at the level of the signal transduction.

Reprogramming progeny cells of embryonic RPE to produce photoreceptors: development of advanced photoreceptor traits under the induction of neuroD

PURPOSE

In examining the prospect of producing functional photoreceptors by reprogramming the differentiation of RPE progeny cells, this study was conducted to investigate whether reprogrammed cells can develop highly specialized ultrastructural and physiological traits that characterize retinal photoreceptors.

METHODS

Cultured chick RPE cells were reprogrammed to differentiate along the photoreceptor pathway by ectopic expression of neuroD. Cellular ultrastructure was examined with electron microscopy. Cellular physiology was studied by monitoring cellular free calcium (Ca(2+)) levels in dark-adapted cells in response to light and in light-bleached cells in response to 9-cis-retinal.

RESULTS

Reprogrammed cells were found to localize red opsin protein appropriately to the apex. These cells developed inner segments rich in mitochondria, and while in culture, some formed rudimentary outer segments, analogous to those of developing photoreceptors in the retina. In response to light, reprogrammed cells reduced their Ca(2+) levels, as observed with developing retinal photoreceptors in culture. Further, on exposure to 9-cis-retinal, the light-bleached, reprogrammed cells increased their Ca(2+) levels, reminiscent of visual cycle recovery.

CONCLUSIONS

These results indicate the potential of reprogrammed cells to develop advanced ultrastructural and physiological traits of photoreceptors and point to reprogramming progeny cells of embryonic RPE as a possible alternative in producing developing photoreceptors.

The light-activated signaling pathway in SCN-projecting rat retinal ganglion cells

In mammals, the master circadian clock resides in the suprachiasmatic nuclei (SCN) of the hypothalamus. The period and phase of the circadian pacemaker are calibrated by direct photic input from retinal ganglion cells (RGCs). SCN-projecting RGCs respond to light in the absence of rod- and cone-driven synaptic input, a property for which they are termed intrinsically photosensitive. In SCN-projecting RGCs, light activates a nonselective cationic current that displays inward and outward rectification. The goal of the present study was to investigate the identity of the light-activated ion channel and the intracellular signaling pathway leading to its activation. We considered two candidate channels, cyclic nucleotide-gated (CNG) channels and transient receptor potential (TRP) channels, which mediate vertebrate and invertebrate phototransduction, respectively. We report that the intrinsic light response relies upon a G-protein-dependent process. Although our data indicate that cyclic nucleotides modulate the signaling pathway, CNG channels do not appear to conduct the light-activated current because (i) cyclic nucleotides in the pipette solution do not activate a conductance or completely block the light response, (ii) CNG channel blockers fail to inhibit the light response, (iii) the effects of internal and external divalent cations are inconsistent with their effects on CNG channels, and (iv) immunohistochemistry reveals no CNG channels in SCN-projecting RGCs. Finally, we show that the pharmacology of the light-activated channel resembles that of some TRPC channel family members; the response is blocked by lanthanides and ruthenium red and SK&F 96365, and is enhanced by flufenamic acid and 1-oleoyl-2-acetyl-sn-glycerol. Furthermore, immunohistochemical experiments reveal that TRPC6 is expressed in many RGCs, including those that express melanopsin.

The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

The SLC24 Na+/Ca2+-K+ exchanger family: vision and beyond

Na(+)/Ca(2+)-K(+) exchange (NCKX) was first discovered in the outer segments of vertebrate rod photoreceptors (ROS), where it is the only mechanism for extruding the Ca(2+) that enters ROS via the light-sensitive and cGMP-gated channels. ROS NCKX1 is the only NCKX gene family member studied extensively in situ. ROS NCKX1 cDNAs have been cloned subsequently from a number of species including man and shown to be the first member of a new gene family ( SLCA24). Three further members of the human NCKX gene family have been cloned subsequently ( NCKX2- 4) by homology with NCKX1, while a partial sequence of a fifth human NCKX gene has appeared in the data base. NCKX-related genes have also been identified in lower animals including fruit flies, worms and sea urchins. NCKX2 is expressed in the brain, in retinal cone photoreceptors and in retinal ganglion cells, while NCKX3 and NCKX4 show a broader expression pattern. In situ NCKX1 and heterologously expressed NCKX2 operate at a 4Na(+):1Ca(2+)+1 K(+) stoichiometry; both NCKX1 and NCKX2 are bidirectional transporters normally extruding Ca(2+) from the cell (forward exchange), but also able to carry Ca(2+) into the cell (reverse exchange) when the transmembrane Na(+) gradient is reversed. Sequence changes have been observed for both NCKX1 and NCKX2 in patients with retinal diseases, but a definitive association with retinal disease has not been shown.

Inhibitory action of diltiazem on voltage-gated calcium channels in cone photoreceptors

The benzothiazepine, diltiazem, is commonly used as an inhibitor of vascular L-type Ca channels, and is a clinically important anti-anginal and antihypertensive medication. In the retina, diltiazem also inhibits cyclic-nucleotide gated (CNG) channels, including the cGMP-gated channels in photoreceptors, and has been suggested to be a neuroprotectant in an animal model of retinitis pigmentosa, a degenerative disease of photoreceptors. In contrast to CNG channels, the actions of diltiazem on photoreceptor Ca channels have not been studied. We show that D-cis-diltiazem can block Ca channels in cone photoreceptors and that the potency and efficacy of cone photoreceptor Ca channel inhibition by this drug is unconventional. Over the concentration range of 5-500 microM diltiazem, the dose response curve was biphasic with a high affinity saturation level of approximately 30% block in the 20-50 microM range (IC(50)=4.9 microM) and a low affinity saturation block (near 100%) with concentrations up to 500 microM (IC(50)=100.4 microM). The degree of block was found to be equivalent when Bay K 8644 was used to increase Ca channel current, indicating that the levels of block do not result from multiple Ca channel subtypes having differing sensitivities to diltiazem. Calcium imaging showed that the relatively low efficacy of the high-affinity Ca channel block was not due to the species of charge-carrying divalent cation nor that it was associated with dialysis of cellular contents. These data contribute to an emerging perspective that the photoreceptor Ca channel has properties unique from other L-type channels, an important consideration should these channels become a target for testing putative neuroprotective therapies.

NO upregulation of a cyclic nucleotide-gated channel contributes to calcium elevation in endothelial cells

We investigated whether nitric oxide (NO) upregulates a cyclic nucleotide-gated (CNG) channel and whether this contributes to sustained elevation of intracellular calcium levels ([Ca(2+)](i)) in porcine pulmonary artery endothelial cells (PAEC). Exposure of PAEC to an NO donor, NOC-18 (1 mM), for 18 h increased the protein and mRNA levels of CNGA2 40 and 50%, respectively (P < 0.05). [Ca(2+)](i) in NO-treated cells was increased 50%, and this increase was maintained for up to 12 h after removal of NOC-18 from medium. Extracellular calcium is required for the increase in [Ca(2+)](i) in NO-treated cells. Thapsigargin induced a rapid cytosolic calcium rise, whereas both a CNG and a nonselective cation channel blocker caused a faster decline in [Ca(2+)](i), suggesting that capacitive calcium entry contributes to the elevated calcium levels. Antisense inhibition of CNGA2 expression attenuated the NO-induced increases in CNGA2 expression and [Ca(2+)](i) and in capacitive calcium entry. Our results demonstrate that exogenous NO upregulates CNGA2 expression and that this is associated with elevated [Ca(2+)](i) and capacitive calcium entry in porcine PAEC.

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.

Cyclic nucleotide-gated channels contribute to the cholinergic plateau potential in hippocampal CA1 pyramidal neurons

Plateau potentials are prolonged membrane depolarizations that are observed in hippocampal pyramidal neurons when spiking and Ca(2+) entry occur in combination with muscarinic receptor activation. In this study, we used whole-cell voltage clamping to study the current underlying the plateau potential and to determine the cellular signaling pathways contributing to this current. When combined with muscarinic stimulation, depolarizing command potentials that evoked Ca(2+) influx elicited a prolonged tail current (I(tail)) that had an extrapolated reversal potential of -20 mV. I(tail) was not observed when intracellular Ca(2+) levels were chelated with 10 mm intracellular BAPTA, and I(tail) was reversibly depressed in low external sodium. When I(tail) was evoked at intervals >3 min, current amplitudes were stable for up to 1 hr. However, at shorter intervals, I(tail) was refractory, with a time constant of recovery of 43.5 sec. The inhibitors of soluble guanylate cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinequinone depressed I(tail) and zaprinast, which blocks cGMP-specific phosphodiesterase, enhanced I(tail), suggesting that a component of I(tail) was activated by cGMP. The inhibitors of cyclic nucleotide-gated (CNG) channels l-cis-diltiazem and 2',4'-dichlorobenzamil reversibly depressed I(tail). However, protein kinase G inhibition had no effect. Therefore, these results indicate that a component of I(tail) is attributable to activation of CNG channels. We conclude that Ca(2+) influx when combined with muscarinic receptor activation activates soluble guanylate cyclase and increases cGMP levels. The increased cGMP activates CNG channels and leads to prolonged depolarization. The cation conductance of the CNG channel contributes to the prolonged depolarization of the plateau potential.

Cyclic nucleotide-gated channels contribute to the cholinergic plateau potential in hippocampal CA1 pyramidal neurons

Plateau potentials are prolonged membrane depolarizations that are observed in hippocampal pyramidal neurons when spiking and Ca(2+) entry occur in combination with muscarinic receptor activation. In this study, we used whole-cell voltage clamping to study the current underlying the plateau potential and to determine the cellular signaling pathways contributing to this current. When combined with muscarinic stimulation, depolarizing command potentials that evoked Ca(2+) influx elicited a prolonged tail current (I(tail)) that had an extrapolated reversal potential of -20 mV. I(tail) was not observed when intracellular Ca(2+) levels were chelated with 10 mm intracellular BAPTA, and I(tail) was reversibly depressed in low external sodium. When I(tail) was evoked at intervals >3 min, current amplitudes were stable for up to 1 hr. However, at shorter intervals, I(tail) was refractory, with a time constant of recovery of 43.5 sec. The inhibitors of soluble guanylate cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and 6-anilino-5,8-quinolinequinone depressed I(tail) and zaprinast, which blocks cGMP-specific phosphodiesterase, enhanced I(tail), suggesting that a component of I(tail) was activated by cGMP. The inhibitors of cyclic nucleotide-gated (CNG) channels l-cis-diltiazem and 2',4'-dichlorobenzamil reversibly depressed I(tail). However, protein kinase G inhibition had no effect. Therefore, these results indicate that a component of I(tail) is attributable to activation of CNG channels. We conclude that Ca(2+) influx when combined with muscarinic receptor activation activates soluble guanylate cyclase and increases cGMP levels. The increased cGMP activates CNG channels and leads to prolonged depolarization. The cation conductance of the CNG channel contributes to the prolonged depolarization of the plateau potential.

Participation of a K(+) channel modulated directly by cGMP in the speract-induced signaling cascade of strongylocentrotus purpuratus sea urchin sperm

Speract, a decapeptide from Strongylocentrotus purpuratus sea urchin eggs, transiently stimulates a membrane guanylyl cyclase and activates a K(+)-selective channel that hyperpolarizes sperm. However, previous studies of sperm and of sperm membrane vesicles reached conflicting conclusions about the mechanisms that open these channels. We find that speract hyperpolarizes and increases the cGMP content of flagellar vesicles. We confirm previous findings that intravesicular GTPgammaS and GTP enhance this hyperpolarization, but not GDPbetaS. The G protein activators AlF(-)(4) and mastoparan also are ineffective. Thus, it is unlikely that a G protein participates in the speract response. In contrast, hyperpolarization responses to speract are increased by 3-isobutyl-1-methylxanthine, which preferentially inhibits cGMP-selective phosphodiesterases of sperm, and the 8Br-cGMP derivative hyperpolarizes vesicles in the absence of speract. The responses to speract and to 8Br-cGMP have similar ionic selectivities (K(+) > Rb(+) > > Li(+) > Na(+)) and sensitivities to the channel blockers 4-aminopiridine and 3, 4-dichlorobenzamil, indicating that they likely result from opening of the same K(+) channel. Inhibitors that preferentially inhibit cAMP-selective phosphodiesterases do not alter responses to speract, and permeant cAMP analogs do not hyperpolarize vesicles. In addition, inhibitors of protein kinases and phosphatases fail to alter vesicle hyperpolarization by speract. The increase in vesicular cGMP content produced by speract therefore may directly mediate opening of the channel that hyperpolarizes sperm membrane vesicles. Similar mechanisms presumably operate in intact sperm.

Lack of a role for cyclic nucleotide gated cation channels in lung liquid absorption in fetal sheep

1. Late gestation fetal sheep were chronically catheterised in utero to allow measurement of the rate of production of lung liquid (Jv) from 132-143 days gestation (term, 147 days), and to test the hypothesis that cyclic nucleotide gated cation channels mediate a component of fetal lung liquid absorption. 2. In eight experiments, 0.5 microg min-1 adrenaline caused a significant (P < 0.005) reduction in Jv from +18. 12 +/- 3.52 to -10.27 +/- 5.26 ml h-1. Dichlorobenzamil (a blocker of cyclic nucleotide gated cation channels) at 1.5 x 10-5 M did not significantly inhibit the adrenaline-induced lung liquid absorption (Jv dichlorobenzamil, -5.77 +/- 2.78 ml h-1; P > 0.1) when the data were grouped, but did exert a significant gestational effect (r = 0. 90, P < 0.01). Subsequent addition of 10-4 M amiloride (a blocker of epithelial sodium channels) abolished the adrenaline-induced absorption of lung liquid (mean Jv amiloride, +6.45 +/- 1.59 ml h-1; P < 0.01 relative to Jv adrenaline and P < 0.005 relative to Jv dichlorobenzamil). 3. In seven experiments, 0.5 microg min-1 adrenaline caused a significant (P < 0.0005) reduction in Jv from +18.95 +/- 2. 98 to -10.08 +/- 3.75 ml h-1. Amiloride (10-4 M) inhibited the adrenaline response (Jv amiloride, +5.46 +/- 1.09 ml h-1; P < 0.005). However, subsequent addition of 1.5 x 10-5 M dichlorobenzamil had no additive effect to that of amiloride (Jv dichlorobenzamil, +4.58 +/- 0.93 ml h-1; P > 0.1). 4. In six experiments, the cGMP analogue 8-Br-cGMP at 10-4 M caused a significant (P < 0.05) reduction in Jv from +15.20 +/- 2.81 to +11.63 +/- 1.71 ml h-1. Amiloride (10-4 M) did not block the effect of 8-Br-cGMP (Jv amiloride, +14.00 +/- 2.49 ml h-1; not significantly different from 8-Br-cGMP). Subsequent addition of 1.5 x 10-5 M dichlorobenzamil also did not block the effect of 8-Br-cGMP (Jv dichlorobenzamil, +11.37 +/- 1.22 ml h-1; not significantly different from either Jv amiloride or Jv 8-Br-cGMP). 5. We conclude that, in fetal sheep, neither adrenaline nor cGMP stimulate lung liquid absorption by actions on cyclic nucleotide gated cation channels, and that the effect of cGMP on fetal lung liquid secretion is minor and does not involve epithelial sodium channels. The effect of dichlorobenzamil, when given before amiloride, was probably due to an action on amiloride sensitive epithelial sodium channels.

Cyclic nucleotide-gated cation channels mediate sodium and calcium influx in rat colon

We found mRNA for the three isoforms of the cyclic nucleotide-gated nonselective cation channel expressed in the mucosal layer of the rat intestine from the duodenum to the colon and in intestinal epithelial cell lines in culture. Because these channels are permeable to sodium and calcium and are stimulated by cGMP or cAMP, we measured 8-bromo-cGMP-stimulated sodium-mediated short-circuit current (I(sc)) in proximal and distal colon and unidirectional (45)Ca(2+) fluxes in proximal colon to determine whether these channels could mediate transepithelial sodium and calcium absorption across the colon. Sodium-mediated I(sc), stimulated by 8-bromo-cGMP, were inhibited by dichlorobenzamil and l-cis-diltiazem, blockers of cyclic nucleotide-gated cation channels, suggesting that these ion channels can mediate transepithelial sodium absorption. Sodium-mediated I(sc) and net transepithelial (45)Ca(2+) absorption were stimulated by heat-stable toxin from Escherichia coli that increases cGMP. Addition of l-cis-diltiazem inhibited the enhanced transepithelial absorption of both ions. These results suggest that cyclic nucleotide-gated cation channels simultaneously increase net sodium and calcium absorption in the colon of the rat.

A novel role for cyclic nucleotide-gated cation channels in lung liquid homeostasis in sheep

1. Sheep lungs were artificially perfused in situ with warmed whole oxygenated sheep blood. The airspaces of the lungs were filled with liquid containing an impermeant tracer, to allow measurement of the rate of net transepithelial liquid movement under various conditions. 2. Dichlorobenzamil (1.5 x 10-5 M), a blocker of cyclic nucleotide-gated cation channels, inhibited the resting absorption of lung liquid in sheep aged 6 months (n = 5) (from -36.47 +/- 4.62 to -4.36 +/- 5.27 ml h-1, means +/- s.e.m.; P < 0.005, paired t test). Amiloride (10-4 M), a blocker of epithelial sodium channels, had no additive effect to that of dichlorobenzamil. 3. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.21 +/- 8.57 to -11.05 +/- 4.91 ml h-1; P < 0.05, one-tailed paired t test), and dichlorobenzamil (1.5 x 10-5 M) exerted an additive effect to that of amiloride resulting in secretion at +6.29 +/- 3.05 ml h-1 (P < 0. 01, paired t test). 4. In the lungs of sheep aged 6 weeks (n = 3), amiloride (10-4 M) also inhibited the resting absorption of liquid (from -26.36 +/- 14.05 to -5.17 +/- 8.27 ml h-1; P < 0.05, one-tailed paired t test); however, dichlorobenzamil (1.5 x 10-5 M) did not exert an additive effect to that of amiloride. 5. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.70 +/- 8.58 to -6.79 +/- 4.28 ml h-1; P < 0.05, paired t test), and pimozide (1.5 x 10-4 M), another blocker of cyclic nucleotide-gated cation channels, also exerted an additive effect to that of amiloride, resulting in secretion of lung liquid at +15.36 +/- 9.14 ml h-1 (P < 0.05, paired t test). 6. We conclude that cyclic nucleotide-gated cation channels mediate a component of lung liquid absorption in sheep aged 6 months (but not in sheep aged 6 weeks), and that a mechanism for lung liquid secretion (present in fetuses) is retained at 6 months of age.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor

All cellular signaling pathways currently known to elevate cGMP involve the activation of a guanylyl cyclase to synthesize cGMP. Here we describe an exception to this rule. In the vertebrate parietal eye, the photoreceptors depolarize to light under dark-adapted conditions, unlike rods and cones but like most invertebrate photoreceptors. We report that the signaling pathway for this response involves a rise in intracellular cGMP resulting from an inhibition of the phosphodiesterase that hydrolyzes cGMP. Furthermore, this phosphodiesterase is driven by an active G protein in darkness. These results indicate an antagonistic control of the phosphodiesterase by two G proteins, analogous to the Gs/Gi control of adenylyl cyclase. Our findings demonstrate an unusual phototransduction mechanism and at the same time indicate that signaling involving cyclic nucleotides is more elaborate than previously known.

Antagonists of the cGMP-gated conductance of vertebrate rods block the photocurrent in scallop ciliary photoreceptors

1. Hyperpolarizing scallop photoreceptors, like vertebrate rods, use cGMP as an internal messenger and their light-sensing structure is also of ciliary origin. To ascertain possible functional similarities between the light-sensitive conductances in the two classes of visual cells, we examined in scallop photoreceptors the effects of several antagonists of the photocurrent of rods. 2. Extracellular application of L-cis-diltiazem rapidly and reversibly suppressed the photocurrent. The effect was stereospecific and dose dependent, with a K1/2 of approximately 400 microM. Intracellular dialysis at lower doses (100-200 microM) also induced a substantial inhibition. 3. L-cis-Diltiazem reduced the light-activated conductance without shifting the intensity-response curve. Furthermore, the drug also blocked the current directly evoked by application of cGMP. These observations indicate that the inhibitory effects result from blockage of the conductance, rather than from impairment of the activating cascade. 4. The fractional blockage increased e-fold per approximately 55 mV depolarization, regardless of the side of drug application, as if the charged form of L-cis-diltiazem can only access the blocking site from the intracellular compartment. 5. The amiloride derivative 3',4'-dichlorobenzamil potently suppressed the photocurrent (K1/2 approximately 5 microM), without affecting its kinetics or operating range. Amiloride itself was also effective at higher concentrations. 6. The pharmacological resemblance of these light-dependent channels to those of rods and cones indicates that significant aspects of the transduction cascade are conserved across disparate sensory cells of ciliary origin.

Cyclic nucleotide-gated cation channels mediate sodium absorption by IMCD (mIMCD-K2) cells

The inner medullary collecting duct cell line, mIMCD-K2, absorbs Na+ by an amiloride-sensitive, electrogenic mechanism. The goal of the present study was to characterize the amiloride-sensitive, Na+ -conducting channels responsible for electrogenic Na+ absorption. To this end, we measured Na+ currents in single cells with the patch-clamp technique and Na+ currents across monolayers mounted in Ussing-type chambers. In whole cell patch-clamp experiments, amiloride-sensitive, inward Na+ currents were mediated by nonselective cation channels. In single-channel patch-clamp experiments, amiloride- and guanosine 3',5'-cyclic monophosphate (cGMP)-sensitive, 20-pS nonselective cation channels (i.e., CNG channels) were identified in the apical membrane. CNG channels were inhibited by amiloride, diltiazem, ethylisopropylamiloride (EIPA), and 8-bromo-cGMP and were permeable to Ca2+ and Mg2+. Epithelial Na+ channels were never observed in whole cell or single-channel recordings. Na+ absorption across confluent monolayers was inhibited with a rank order potency of benzamil > amiloride > phenamil >> EIPA > diltiazem. Our data are most consistent with the view that CNG channels mediate electrogenic Na+ absorption across mIMCD-K2 cells.

cGMP stimulates sodium and chloride currents in rat tracheal airway epithelia

To test the hypothesis that guanosine 3',5'-cyclic monophosphate (cGMP) regulates ion transport in airway epithelial cells, we measured short-circuit current (I(sc)) and (22)Na+ fluxes in primary cultured rat tracheal epithelial cells. In Cl- -containing Ringer solution, I(sc) was increased by approximately 17 microA/cm2 after application of 1 mM 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP), whereas, in Cl- -free solutions, the Na+ -mediated component was approximately 5 microA/cm2, suggesting a cGMP stimulation of Cl-secretory current and a smaller Na+ absorptive current. Inward and net mucosal-to-serosal (22)Na+ flux was doubled in the presence of 2 mM 8-BrcGMP. To determine whether nucleotide-gated channels play a role in this transepithelial Na+ absorption, blockers of nucleotide-gated cation channels were used to inhibit I(sc). The cGMP-stimulated Na+-mediated I(sc) was blocked by as little as 500 nM dichlorobenzamil or 50 microM L-cis-diltiazem, which are known blockers for cyclic nucleotide-gated cation channels. These agents also blocked the basal (non-cGMP-stimulated) current when measured in the presence of 10 microM amiloride, which blocks current through 5-pS amiloride-sensitive Na+ channels. To document whether the distribution of nucleotide-gated nonselective cation channels was consistent with a role in airway epithelial transport, in situ hybridization was performed. In situ hybridization of mRNA encoding for nucleotide-gated cation channels was found in epithelial cell layers of rat trachea, bronchi, bronchioles, and alveolar cells but not in smooth muscle layers or tracheal cartilage. Reverse transcriptase-polymerase chain reaction, restriction enzyme analysis, and sequencing of the cDNA transcribed from mRNA of whole lung and tracheal epithelial cells indicate that a channel highly homologous to the retinal nucleotide-gated nonselective cation channel (CNG1) is present. Thus these data, along with evidence supporting the existence of signal transduction pathways elevating intracellular levels of cGMP, indicate that cGMP regulates transepithelial ion transport in lung epithelial tissues.

pH changes in frog rods upon manipulation of putative pH-regulating transport mechanisms

Rod intracellular pH (pHi) in the intact frog retina was measured fluorometrically with the dye 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein under treatments chosen to affect putative pH-regulating transport mechanisms in the plasma membrane. The purpose was to relate possible pHi changes to previously reported effects on photoresponses. In nominally bicarbonate-free Ringer, application of amiloride (1 mM) or substitution of 95 mM external Na+ by K+ or choline triggered monotonic but reversible acidifications, consistent with inhibition of Na+/H+ exchange. Bicarbonate-dependent mechanisms were characterized as follows: (1) Replacing half of a 12 mM phosphate buffer by bicarbonate caused a sustained rise of pHi. (2) Subsequent application of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2',2'-disulphonic acid (DIDS, 0.2 mM) set off a slow acidification. (3) Substitution of external Cl- by gluconate (95 mM) caused a rapid pHi rise both in normal Na+ and low-Na+ perfusion. (4) This effect was inhibited by DIDS. The results support a consistent explanation of parallel electrophysiological experiments on the assumption that intracellular acidifications reduce and alkalinizations (in a certain range) augment photoresponses. It is concluded that both Na+/H+ exchange and bicarbonate transport control rod pHi, modulating the light-sensitive current. Part of the bicarbonate transport is by Na(+)-independent HCO3-/Cl- exchange, but a further Na(+)-coupled bicarbonate import mechanism is implicated.

Changes in the light-sensitive current of salamander rods upon manipulation of putative pH-regulating mechanisms in the inner and outer segment

The light-sensitive current of dark-adapted rods isolated from the Ambystoma retina was recorded while either the inner or the outer segment (IS or OS) protruding from the suction pipette was exposed to treatments intended to reveal the physiological roles of pH-regulating transport mechanisms. Applied to the IS, both amiloride (presumed to block Na+/H+ exchange, 2 mM) and 4-4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) (presumed to block bicarbonate transport, 0.1 mM) generally abolished light sensitivity completely but reversibly, consistent with acidification of the IS. Yet, the circulating ("dark") current often persisted, implying that the OS was not acidified. Applied to the OS, amiloride depressed but DIDS increased the dark current and photoresponses. Given the fact that the current increases with rising OS-pHi, this suggests alkalinization, which could be due to DIDS inhibiting bicarbonate extrusion by HCO3-/Cl- exchangers in the OS. Consistent with this idea, replacing external Cl- by other anions increased the current as would be expected if HCO3-/Cl- exchange is reversed. We propose that the IS and OS manage their acid balances independently and with different sets of transport mechanisms. Acidosis in either compartment suppresses the photosensitivity of the rod, but by differing mechanisms.

Origin of the chloride current in olfactory transduction

In the cilia of amphibian olfactory receptor neurons, odorants elicit a receptor current that has two components: a cationic current through cAMP-gated channels and a Cl- current. Here, a cascade of ciliary currents that accounts for the total receptor current is demonstrated. In isolated olfactory cilia, cAMP sequentially activates two currents. The first is a primary cationic current through channels directly gated by cAMP. Part of this current is carried by Ca2+, which in turn activates a Cl- current. This secondary current is eliminated by the presence of Cl- channel inhibitors, replacement of Cl- with methanesulfonate-, removal of external Ca2+, or blockers of the cAMP-gated cationic channels. When cytoplasmic Ca2+ buffering is low, small cationic currents can activate Cl- currents that are 20 times larger.

On the relation between ERG waves and retinal function: inverted rod photoresponses from the frog retina

In rod mass receptor photoresponses recorded across the isolated frog retina, a paradoxical cornea-positive wave may precede the response of normal polarity. We present a model which shows that the light-induced decrease in rod current can give rise to inverted or biphasic ERG signals if the distal part (tip) of the rod outer segment responds more slowly and/or less sensitively than the proximal part (base). The condition is that current entering at the tip is represented with greater weight in the ERG. The model reproduces recorded ERG waveforms well. It further predicts that if there is a light-insensitive conductance in the tip membrane, ERG photoresponses may be non-recordable although current photoresponses are only slightly reduced. The model reveals a type of complexity in the relation between mass potentials and underlying physiological processes which has not previously received attention.

IP3- and cAMP-induced responses in isolated olfactory receptor neurons from the channel catfish

Olfactory receptor neurons enzymatically dissociated from channel catfish olfactory epithelium were depolarized transiently following dialysis of IP3 or cAMP (added to the patch pipette) into the cytoplasm. Voltage and current responses to IP3 were blocked by ruthenium red, a blocker of an IP3-gated Ca(2+)-release channel in sarcoplasmic reticulum. In contrast, the responses to cAMP were not blocked by extracellularly applied ruthenium red, nor by L-cis-diltiazem or amiloride and two of its derivatives. The current elicited by cytoplasmic IP3 in neurons under voltage clamp displayed a voltage dependence different from that of the cAMP response which showed marked outward rectification. A sustained depolarization was caused by increased cytoplasmic IP3 or cAMP when the buffering capacity for Ca2+ of the pipette solution was increased, when extracellular Ca2+ was removed or after addition of 20-200 nM charybdotoxin to the bathing solution, indicating that the repolarization was caused by an increase in [Cai] that opened Ca(2+)-activated K+ channels. The results suggest that different conductances modulated by either IP3 or cAMP are involved in mediating olfactory transduction in catfish olfactory receptor neurons and that Ca(2+)-activated K+ channels contribute to the termination of the IP3 and cAMP responses.

Four cases of direct ion channel gating by cyclic nucleotides

Four different nucleotide-gated ion channels are discussed in terms of their biophysical properties and their importance in cell physiology. Channels activated directly by cGMP are present in vertebrate and invertebrate photoreceptors. In both cases cGMP increases the fraction of time the channel remains in the open state. At least three cGMP molecules are involved in channel opening in vertebrate photoreceptors and the concentration of the cyclic nucleotide to obtain the half maximal effect is about 15 microM. The light-dependent channel of both vertebrates and invertebrates is poorly cation selective. The vertebrate channel allows divalent cations to pass through 10-15-fold more easily than monovalent ions. In agreement with their preference for divalent cations, this channel is blocked by l-cis Dialtazem, a molecule that blocks certain types of calcium channels. In olfactory neurons a channel activated by both cAMP and cGMP is found and, as in the light-dependent channel, several molecules of the nucleotide are needed to open the channel with a half maximal effect obtained in the range of 1-40 microM. The channel is poorly cationic selective. A K+ channel directly and specifically activated by cAMP is found in Drosophila larval muscle. At least three cAMP molecules are involved in the opening reaction. Half-maximal effect is obtained at about 50 microM. This channel is blocked by micromolar amount of tetraethylammonium applied internally. Interestingly, this channel has a probability of opening 10-20-fold larger in the mutant dunce, a mutant that possesses abnormally elevated intracellular cAMP level, than in the wild type.

Evidence for Na+/H+ exchange in vertebrate rod photoreceptors

In this paper, we have documented our investigation of pH regulation in the rod photoreceptor of the toad, Bufo marinus. Unlike other neural tissues, the retina depends upon aerobic glycolysis to meet its energy requirements. A consequence of its reliance on glycolysis is a large metabolic production of protons (H+) which must be extruded by pH regulating mechanisms. Based on the work of previous authors, we propose that rods share these same characteristics with whole retina. Our results in rods are consistent with the hypothesis that under nominally bicarbonate-free conditions, an amiloride-sensitive Na+/H+ exchanger contributes to pH regulation in this cell.

Cyclic nucleotide-activated channels in the frog olfactory receptor plasma membrane

Patch clamp technique was used to record cyclic nucleotide-dependent current of the frog olfactory receptor cell plasma membrane. Data obtained indicate that the channels passing this current are permeable to Ca2+ or Mg2+ and moderately selective for monovalent cations according to the sequence Li+, Na+, K+ greater than Rb+ greater than Cs+ and are effectively blocked by 1-cis-diltiazem and 3',4'-dichlorobenzamil. The conductance of single cyclic nucleotide-gated channels in solutions with low Ca2+ and Mg2+ content is about 19 pS. The results demonstrate that cyclic nucleotide-activated channels of olfactory receptor cells are virtually identical to photoreceptor ones.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.