Cyclic nucleotide-gated channels: an expanding new family of ion channels

Beta subunits of the olfactory cyclic nucleotide-gated channel form a nitric oxide activated Ca2+ channel

Cyclic nucleotide-gated channels are important in visual and olfactory transduction, and possibly in other neuronal functions. These channels have a complex permeability to Ca2+ ions that may be important in their cellular functions. They are composed of two different subunits, alpha and beta, that have been cloned and expressed, but the beta subunit alone cannot be activated by cyclic nucleotides, confounding the analysis of its characteristics. However, we found that nitric oxide can activate the homomeric expressed beta subunit, and the resulting channel possesses many properties of the L-type Ca2+ channels, including high permeability to Ca2+ ions and sensitivity to Ca2+ channel blockers. Thus, the Ca2+ permeability characteristics of native channels are mostly conferred by properties of the beta subunit, and the beta subunit alone can act as a NO-sensitive Ca2+ channel. A nearly identical conductance activated by NO is present in the membrane of rat vomeronasal neurons, indicating that homomeric beta channels exist in vivo.

Role of inward K+ channel located at carrot plasma membrane in signal cross-talking of cAMP with Ca2+ cascade

Treatment of cultured carrot cells with dibutyryl cAMP or forskolin resulted in the appreciable decrease in extracellular K+ concentration. This decrease was found to be transient and the concentration of the ion in the culture medium restored to the original level within few minutes. The cAMP-induced decrease in K+ level in the medium was almost completely inhibited when carrot cells were incubated in the presence of K+ channel blockers, CsCl and tetraethylammonium chloride. Appreciable amounts of 45Ca2+ were discharged from 45Ca2+-loaded inside-out vesicles of carrot plasma membrane by the stimulation with cAMP, however, the release of the ion was significantly inhibited in the presence of the K+ channel blockers. The release of 45Ca2+ from the vesicles was also observed when K+ current was evoked with an ionophore, valinomycin, even in the absence of cAMP. These results suggest that the gating of some of the inward K+ channels located at plasma membrane of cultured carrot cells is controlled by cytoplasmic concentration of cAMP and the inward K+ current across the plasma membrane induced by the nucleotide elicits Ca2+ influx into the cells possibly by the activation of voltage-dependent Ca2+ channels.

Effect of englitazone on KATP and calcium-activated non-selective cation channels in CRI-G1 insulin-secreting cells

1. The effects of englitazone sodium, an antidiabetic agent, on ion channel activity in the CRI-G1 insulin secreting cell line was examined by use of the patch clamp technique. 2. Application of englitazone to the outside of CRI-G1 cells in the whole-cell recording configuration produced concentration-dependent inhibition of KATP currents with an IC50 value of 8 microM. The inhibition of the K+ current was not affected by the removal of Mg2+ ions from or the addition of trypsin to the solution bathing the intracellular surface of the cell membrane. 3. Englitazone also inhibited KATP channel activity in recordings from inside out excise membrane patches. The concentration-dependence of inhibition was identical to that observed in whole-cell recordings and was voltage-independent. Single channel recordings confirmed that neither the absence or presence of Mg2+ ions nor the addition of trypsin at the intracellular surface of the membrane influenced the inhibition of KATP channels by englitazone. 4. Englitazone also inhibited Ca(2+)-activated non-selective cation (NSCa) channels in inside-out patches in a concentration-dependent and voltage-independent manner with an IC50 value of 10 microM. In comparison, the non-sulphonylurea KATP channel blocker ciclazindol produced a slight voltage-dependent inhibition of the NSCa channel at a concentration of 20 microM. 5. In whole-cell recordings englitazone, at a relatively high concentration (50 microM) in comparison with that required to block KATP and NSCa channels, inhibited voltage-activated Ca2+ currents by 33% but did not inhibit voltage-activated K+ and Na+ currents. 6. It is concluded that englitazone is a novel blocker of NSCa and KATP channels. The inhibition of KATP channels occurs following procedures that dissociate sulphonylurea receptor coupling to the channel. The equipotent and voltage-independent inhibition of NSCa and KATP channels by englitazone may indicate a common mechanism of block.

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.

The nitric oxide system in insects

It is well established that nitric oxide (NO) acts as a signalling molecule in the nervous system of both mammals and insects. In contrast to classical transmitters, the membrane-permeant NO can act on neighbouring targets limited by half-life and diffusion barriers. This type of diffuse signalling seems to be evolutionarily highly conserved and recent findings concerning the characterization and function of the NO system in insects are summarized in this review. Firstly, the properties and the localization of the NO forming enzyme, the NO synthase (NOS), are described. In the nervous system the brain contains by far the highest NOS activity. As an evolutionary peculiarity, a blood-feeding bug exhibits high NOS activity in the salivary glands. Secondly, the soluble guanylate cyclase (sGC), a major target of NO action, and cGMP-regulated enzymes like cGMP-dependent protein kinase and cyclic nucleotide gated channels are described. Anatomical organization of the NO/cGMP system in insects reveals evidence for a cellular separation of the release site and target site of NO, although in the antennal lobes of the locust an exception from this rule exists. Thirdly, the implication of the NO system in neuronal function in insects is described. In the honeybee, the NO/cGMP system in the antennal lobes is implicated in the processing of adaptive mechanisms during chemosensory processing, and recent findings support a specific role of the NO system in memory formation. Discussion of the results in insects with regard to properties and functions of the vertebrate NO system is attempted.

Ultraviolet B radiation acts through the nitric oxide and cGMP signal transduction pathway to stimulate melanogenesis in human melanocytes

Ultraviolet B (UVB) radiation is the main physiological stimulus for human skin pigmentation; however, the molecular mechanisms underlying this process are still unclear. Recently, nitric oxide (NO) and cGMP have been involved in mediation of skin erythema induced by UVB. Therefore, we investigated the role of NO and cGMP in UVB-induced melanogenesis. In this study, we demonstrated that UVB stimulation of melanogenesis was mimicked by exogenous NO donors. Additionally, we showed that NO stimulated cGMP synthesis and that cGMP was also a potent stimulator of melanogenesis. Furthermore, the inhibition of the melanogenic effect of NO by guanylate cyclase inhibitor demonstrated that NO mediated its effect through the activation of guanylyl cyclase. Interestingly, 1 min after UVB irradiation, we observed a significant increase in cGMP content in melanocytes. The effects of UVB on cGMP production and on melanogenesis were blocked by both guanylate cyclase and NO synthase inhibitors. Additionally, inhibition of cGMP-dependent kinase also prevented the stimulation of melanogenesis by UVB and NO. Therefore, we concluded that NO and cGMP production is required for UVB-induced melanogenesis and that cGMP mediated its melanogenic effects mainly through the activation of cGMP-dependent kinase.

Expression of photoreceptor cyclic nucleotide-gated cation channel alpha subunit (CNGCalpha) in the liver and skeletal muscle

Glucagon and beta-adrenergic agents increase cAMP levels and stimulate Ca2+ influx in liver cells. There is no consensus as to the mechanism by which these hormones stimulate the influx of Ca2+. Using mouse retinal rod CNGCalpha cDNA probes, we cloned rat liver and skeletal muscle, and human hepatic CNGCalpha subunit sequences showing 97-100% identity with the human rod channel. In order to assess channel activity, the effect of cyclic nucleotides on free intracellular Ca2+ levels of isolated hepatocytes was measured. Dibutyryl-cAMP was more effective in increasing free Ca2+ levels than dibutyryl-cGMP. These data indicate that the CNGCalpha subunit is expressed in both the liver and skeletal muscle possibly mediating hormonal effects on ion fluxes.

Requirement of nitric oxide for induction of genes whose products are involved in nitric oxide metabolism in Rhodobacter sphaeroides 2.4.3

During denitrification, freely diffusible nitric oxide (NO) is generated for use as a terminal electron acceptor. NO is produced by nitrite reductase (Nir) and reduced to nitrous oxide by nitric oxide reductase (Nor). Using Nir and Nor-deficient mutants of Rhodobacter sphaeroides 2.4.3, we have shown that the endogenous production of NO or the addition of exogenous NO induces transcription of certain genes encoding Nir and Nor. A Nor-deficient strain was found to be capable of expressing wild type levels of nirK-lacZ and norB-lacZ fusions in medium unamended with nitrogen oxides. When this experiment is performed in the presence of hemoglobin, fusion expression is eliminated. NO and the NO-generator, sodium nitroprusside, can induce expression of both fusions in a strain lacking Nir and the consequent ability to produce NO. Sodium nitroprusside cannot induce expression of nirK-lacZ in a strain lacking the transcriptional activator NnrR (nitrite and nitric oxide reductase regulator). Addition of the cyclic nucleotides cAMP and 8-bromoguanosine-cGMP does not result in expression of either fusion. These results demonstrate that denitrifying bacteria produce NO as a signal molecule to activate expression of the genes encoding proteins required for NO metabolism.

Mutations in a cyclic nucleotide-gated channel lead to abnormal thermosensation and chemosensation in C. elegans

The C. elegans tax-4 mutants are abnormal in multiple sensory behaviors: they fail to respond to temperature or to water-soluble or volatile chemical attractants. We show that the predicted tax-4 gene product is highly homologous to vertebrate cyclic nucleotide-gated channels. Tax-4 protein expressed in cultured cells functions as a cyclic nucleotide-gated channel. The green fluorescent protein (GFP)-tagged functional Tax-4 protein is expressed in thermosensory, gustatory, and olfactory neurons mediating all the sensory behaviors affected by the tax-4 mutations. The Tax-4::GFP fusion is partly localized at the sensory endings of these neurons. The results suggest that a cyclic nucleotide-gated channel is required for thermosensation and chemosensation and that cGMP is an important intracellular messenger in C. elegans sensory transduction.

Anti-Ig-induced calcium influx in rat B lymphocytes mediated by cGMP through a dihydropyridine-sensitive channel

In contrast to excitable tissues where calcium channels are well characterized, the nature of the B lymphocyte calcium channel is unresolved. Here, we demonstrate by single cell analysis of freshly isolated rat B cells that the anti-immunoglobulin (Ig)-induced calcium influx takes place through a channel which shares pharmacologic and serologic properties with the L-type calcium channel found in excitable tissues. It is sensitive to the dihydropyridines nicardipine and Bay K 8644, to calciseptine, and to an anti-peptide antibody raised against the alpha1 subunit of the L-type calcium channel, but is voltage-insensitive. Anti-alpha1 and anti-alpha2 antibodies stain B but not T lymphocytes. Application of a cGMP agonist, measurement of cGMP levels in anti-Ig-stimulated B cells, and examining the effect of a guanylyl cyclase inhibitor on the anti-Ig response show that cGMP mediates the influx. This possibly involves a cGMP-dependent protein kinase. The anti-Ig-induced response is not abolished by prior treatment of B cells with a high dose of thapsigargin. These findings undermine the widely held belief of a categorical divide between excitable and non-excitable tissue calcium channels, demonstrate the limitations of the capacitative calcium influx theory, and point to a distinction between the calcium response mechanisms utilized by B and T lymphocytes.

Molecular cloning and expression of the Modulatory subunit of the cyclic nucleotide-gated cation channel

The cDNA of three variants of a cyclic nucleotide-gated (CNG) channel modulatory subunit (CNG4c-CNG4e) has been cloned. CNG4c, CNG4d, and CNG4e differ slightly from each other within an amino-terminal sequence that was originally reported as part of the bovine retinal glutamic acid-rich protein (GARP). The core region of CNG4 is homologous to the second subunit of the human rod photoreceptor channel (hRCNC2b), suggesting that both proteins are alternatively spliced products of the bovine and human homologue of the same gene. CNG4 transcripts are present in retina, testis, kidney, heart, and brain. Expression of CNG4 in HEK293 cells did not lead to detectable currents. Coexpression of CNG4 with the principal subunit of the bovine testis CNG channel (CNG3) resulted in currents which differed in several aspects from that induced by CNG3 alone. The heterooligomeric CNG3/CNG4 and the homooligomeric CNG3 channels were modified by Ca2+-calmodulin and some calmodulin antagonists. The results suggest that CNG4 forms functional heterooligomeric channels with CNG3 in vitro and probably also in intact tissues.

Covalent activation of retinal rod cGMP-gated channels reveals a functional heterogeneity in the ligand binding sites

Ion channels gated by the binding of multiple ligands play a critical role in synaptic transmission and sensory transduction. It has been difficult to resolve the contribution of individual binding events to channel gating because ligands are continuously binding and unbinding at each site. In examining the allosteric mechanism of retinal rod cGMP-gated channels, we have circumvented this problem by making use of a cGMP derivative, 8-p-azidophenacylthio-cGMP (APT-cGMP), that can be covalently tethered to the binding sites in the presence of long-wavelength UV light. In excised membrane patches, a population of channels was isolated that contained covalently-attached ligands at all but one site. Activation of these channels by cGMP revealed a previously unknown heterogeneity in the ligand-binding sites. The dose-response relations were much shallower than predicted by single-site activation models, but were well described by models in which there are two populations of sites, in roughly equal proportion, that bind cGMP with apparent affinities that differ by a factor of 25. The two apparent affinities, incorporated into a four-site model of the channel, provided an accurate description of the patch's original dose-response relation. A comparison of results on native and expressed channels suggests that the heterogeneity in the native channel arises at least in part from the presence of two different cGMP-binding subunits.

Direct activation of the olfactory cyclic nucleotide-gated channel through modification of sulfhydryl groups by NO compounds

The activation of a cyclic nucleotide-gated channel is the final step in sensory transduction in olfaction. Normally, this channel is opened by the intracellular cyclic nucleotide second messenger cAMP or cGMP. However, in single channel recordings we found that donors of nitric oxide, a putative intercellular messenger, could directly activate the native olfactory neuron channel. Its action was independent of the presence of the normal ligand and did not involve the cyclic nucleotide binding site, suggesting an alternate site on the molecule that is critical in channel gating. The biochemical pathway appears to utilize nitric oxide in one of its alternate redox states, the nitrosonium ion, transnitrosylating a free sulfhydryl group belonging to a cysteine residue tentatively identified as being in the region linking the S6 transmembrane domain to the ligand binding domain.

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.

The renal cGMP-gated cation channel: its molecular structure and physiological role

Cyclic nucleotide-gated cation channels, which are permeable to monovalent and divalent cations, are expressed in a number of tissues. cDNAs encoding cGMP-gated cation channel subunits have been cloned in retinal rods, cones, olfactory neuroepithelium, pineal gland, aorta, testis, heart, and most recently kidney. Patch clamp studies have identified and characterized cGMP-gated cation channels in the cortical collecting duct (CCD) and inner medullary collecting duct (IMCD). cGMP-gated cation channels in kidney share many biophysical and molecular properties with the retinal rod cGMP-gated channel. However, unlike the retinal rod channel, the cGMP-gated cation channel in kidney is inhibited by cGMP and stimulated by increased calcium levels. In the IMCD the cGMP-gated cation channel mediates electrogenic sodium absorption which is inhibited by ANP via cGMP. Recently, cGMP-gated cation channel poly(A+) RNA has been identified in other nephron segments by RT-PCR and in situ PCR hybridization. Furthermore, cGMP-gated cation channel protein has also been identified in all nephron segments by Western blot analysis. These observations suggest that cGMP-gated cation channels, or closely related gene products, may play an important physiological role in all nephron segments. Hormones that increase intracellular cGMP may regulate sodium, and perhaps calcium, uptake in nephron segments proximal to the IMCD. Increases in cell sodium and calcium may regulate other transport and signaling pathways.

Activation of a cAMP-regulated Ca(2+)-signaling pathway in pancreatic beta-cells by the insulinotropic hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone that is currently under investigation for use in the treatment of diabetes mellitus. To investigate the Ca2+ signaling pathways by which GLP-1 may stimulate the secretion of insulin from pancreatic beta-cells, we examined its effects on the concentration of free intracellular Ca2+ ([Ca2+]i) while simultaneously determining what action it exerts on ion channel function. Measurements of [Ca2+]i were obtained from single rat beta-cells and from beta TC6 and HIT-T15 insulinoma cells loaded with the Ca2+ indicator fura-2, and changes in membrane potential and current were monitored using the perforated patch clamp technique. We report a previously undocumented action of GLP-1 and analogs of cAMP (8-bromo-cAMP, Sp- or Rp-adenosine 3',5'-cyclic monophosphothionate triethylamine) to raise [Ca2+]i that is attributable to the activation of a prolonged inward current designated here as IcAMP. Activation of IcAMP is associated with an increased membrane conductance, membrane depolarization, and triggers large increases of [Ca2+]i. IcAMP is primarily a Na+ current that is blocked by extracellularly applied La3+ or by intracellular administration of Ca2+ chelators (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl, EGTA) and which exhibits a reversal potential of about -26 mV. We propose that IcAMP results from the opening of nonselective cation channels that are activated by intracellular Ca2+ and cAMP and which might play an important role in the regulation of insulin secretion from pancreatic beta-cells.

Activation of a cAMP-regulated Ca(2+)-signaling pathway in pancreatic beta-cells by the insulinotropic hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone that is currently under investigation for use in the treatment of diabetes mellitus. To investigate the Ca2+ signaling pathways by which GLP-1 may stimulate the secretion of insulin from pancreatic beta-cells, we examined its effects on the concentration of free intracellular Ca2+ ([Ca2+]i) while simultaneously determining what action it exerts on ion channel function. Measurements of [Ca2+]i were obtained from single rat beta-cells and from beta TC6 and HIT-T15 insulinoma cells loaded with the Ca2+ indicator fura-2, and changes in membrane potential and current were monitored using the perforated patch clamp technique. We report a previously undocumented action of GLP-1 and analogs of cAMP (8-bromo-cAMP, Sp- or Rp-adenosine 3',5'-cyclic monophosphothionate triethylamine) to raise [Ca2+]i that is attributable to the activation of a prolonged inward current designated here as IcAMP. Activation of IcAMP is associated with an increased membrane conductance, membrane depolarization, and triggers large increases of [Ca2+]i. IcAMP is primarily a Na+ current that is blocked by extracellularly applied La3+ or by intracellular administration of Ca2+ chelators (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl, EGTA) and which exhibits a reversal potential of about -26 mV. We propose that IcAMP results from the opening of nonselective cation channels that are activated by intracellular Ca2+ and cAMP and which might play an important role in the regulation of insulin secretion from pancreatic beta-cells.

Cyclic GMP contact points within the 63-kDa subunit and a 240-kDa associated protein of retinal rod cGMP-activated channels

Ion channels from retinal rods and a variety of other cells are directly gated by cyclic nucleotides. The rod channel is known to contain a 63-kDa subunit, and there is molecular genetic evidence for the existence, in human retina, of a second subunit with a deduced molecular mass of about 100 kDa. When purified from bovine rods, the channel consists of the 63-kDa subunit and a 240-kDa associated protein that has been shown recently to contain a version of the cloned second subunit as part of a larger complex. We had previously shown that a photoaffinity analog of cGMP, 8-(p-azidophenacylthio)-[32P]cGMP, specifically labels both the 63- and 240-kDa proteins. Here the analog was used to identify cGMP-binding regions and amino acid contact points within these proteins. The specific labeling of the 63-kDa subunit was localized to a 66 amino acid fragment (Tyr-515-Met-580) that is contained entirely within a 110 amino acid region proposed to be the cGMP-binding site on the basis of homology with other cyclic nucleotide-binding proteins. Within this fragment, amino acid residues Val-524, Val-525, and Ala-526 were found to contain label. These residues are part of a larger hydrophobic cluster that appears to line the binding pocket. The results also indicate that the 240-kDa protein contains a similar cGMP-binding site. Sequencing of a specifically labeled 8-kDa fragment through 16 amino acid residues indicated that the fragment was derived from the portion of the 240-kDa complex that contains the second subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous, ligand-independent activity of the cGMP-gated ion channels in cone photoreceptors of fish

1. We studied the electrical conductance of membrane patches detached from the outer segment of single cone photoreceptors isolated from striped bass retina. 2. Only a single class of ion channels exists in the plasma membrane of the cone outer segments; they are gated by cytoplasmic cGMP and select cations over anions, but distinguish poorly among cations. In the absence of added cGMP and of divalent cations, however, membrane patches detached from the outer segments exhibit a small conductance that ideally selects cations over anions, but distinguishes poorly between Na+ and Li+. 3. The cGMP-independent conductance does not arise from the effect of residual cGMP that may remain associated with the detached membrane, because treatment of the patch with cGMP-specific phosphodiesterase does not affect this conductance. 4. The cGMP-independent conductance is pharmacologically indistinguishable from that activated by cGMP. Ca2+ and L-cis-diltiazem block both conductances at comparable concentrations and with similar quantitative characteristics. 5. We analysed the noise of Ca(2+)- or L-cis-diltiazem-dependent macroscopic currents both in the presence and in the absence of cGMP. In the presence of cGMP, the power density spectrum of the noise is well fitted by the sum of two Lorentzian components. The same function with similar corner frequencies fits the noise of the cGMP-independent currents. However, the total power in the current fluctuations is smaller in the absence of cGMP than in its presence; also, the ratio of the zero frequency asymptotes of the low over the high frequency components, S1(0)/Sh(0), is larger in the absence of cGMP than in its presence.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of calcium-activated nonselective cation channel activity by cyclic nucleotides in the rat insulinoma cell line, CRI-G1

The regulation of a calcium-activated nonselective cation (Ca-NS+) channel by analogues of cyclic AMP has been investigated in the rat insulinoma cell line, CRI-G1. The activity of the channel is modulated by cyclic AMP in a complex way. In the majority of patches (83%) tested concentrations of cyclic AMP of 10 microM and above cause an inhibition of channel activity which is immediately reversible on washing. In contrast, lower concentrations of cyclic AMP, between 0.1 and 1.0 microM, produce a transient activation of channel activity in most patches (63%) tested. One group of analogues, including N6-monobutyryl cyclic AMP and N6, 2'-O-dibutyryl cyclic AMP reduced the activity of the Ca-NS+ channel at all concentrations tested and 2'-O-Monobutyryl cyclic AMP produced inhibition in all patches tested except one, at all concentrations. A second group produced dual concentration-dependent effects on Ca-NS+, low concentrations stimulating and high concentrations inhibiting channel activity. 6-Chloropurine cyclic AMP and 8-bromo cyclic AMP produced effects similar to those of cyclic AMP itself. In contrast, 8-[4-chlorophenylthio] cyclic AMP also showed a dual action, but with a high level of activation at all concentrations tested up to 1 mM. Ca-NS+ channel activity was also predominantly activated by low concentrations of Sp-cAMPS. The activating effects of both Sp-cAMPS and cyclic AMP are antagonized by Rp-cAMPS, which by itself only produced a weak inhibition of Ca-NS+ channel activity even at concentrations of 10 microM and above. The results are discussed in terms of a model in which cyclic AMP, and other cyclic nucleotides, modulate the activity of the Ca-NS+ channel by binding to two separate sites.

Cyclic nucleotide gated channels

Recent studies have revealed that cyclic nucleotide gated channels have a variety of forms and functions. These channels are now thought to be heteromultimers of at least two kinds of subunits and to undergo functional modulation. Ion permeation involves at least two ion-binding sites, and recent work on the alpha subunit suggests that many structural regions are involved in the control of channel gating. The continued use of both molecular and physiological approaches promises to further our understanding of how these channels work and how they are involved in cellular function.

Cholinergic signaling in the rat pineal gland

1. Innervation of the mammalian pineal gland is mainly sympathetic. Pineal synthesis of melatonin and its levels in the circulation are thought to be under strict adrenergic control of serotonin N-acetyltransferase (NAT). In addition, several putative pineal neurotransmitters modulate melatonin synthesis and secretion. 2. In this review, we summarize what is currently known on the pineal cholinergic system. Cholinergic signaling in the rat pineal gland is suggested based on the localization of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), as well as muscarinic and nicotinic ACh binding sites in the gland. 3. A functional role of ACh may be regulation of pineal synaptic ribbon numbers and modulation of melatonin secretion, events possibly mediated by phosphoinositide (PI) hydrolysis and activation of protein kinase C via muscarinic ACh receptors (mAChRs). 4. We also present previously unpublished data obtained using primary cultures of rat pinealocytes in an attempt to get more direct information on the effects of cholinergic stimulus on pinealocyte melatonin secretion. These studies revealed that the cholinergic effects on melatonin release are restricted mainly to intact pineal glands since they were not readily detected in primary pinealocyte cultures.

Adenylyl cyclases and the interaction between calcium and cAMP signalling

Adenylyl cyclase is the prototypical second messenger generator. Nearly all of the eight cloned adenylyl cyclases are regulated by one or other arm of the phospholipase C pathway. Functional and ultrastructural investigations have shown that adenylyl cyclases are intimately associated with sites of calcium ion entry into the cell. Oscillations in cellular cyclic AMP levels are predicted to arise because of feedback inhibition of adenylyl cyclase by Ca2+. Such findings inextricably intertwine cellular signalling by cAMP and internal Ca2+ and extend the known regulatory modes available to cAMP.

Protein thiol modification and apoptotic cell death as cGMP-independent nitric oxide (NO) signaling pathways

Nitric oxide signaling is achieved through both cGMP-dependent and cGMP-independent mechanisms. The latter are exemplified by protein thiol modification followed by subsequent NAD(+)-dependent automodification of the glycolytic enzyme GAPDH, or by mechanisms inducing accumulation of the tumor suppressor gene p53 and causing apoptotic cell death. Both cGMP-independent actions are initiated using NO-releasing compounds and an active LPS/cytokine-inducible NO synthase. NO-synthase inhibitors block the release of NO and hinder downstream signaling mechanisms; they are therefore valuable pharmacological tools linking a defined cellular response to various NO actions. Signal transducing mechanisms elicited by NO can be studied using GAPDH as a representative example of NO-induced protein modification and are grouped as follows: --S-Nitrosylation reactions initiated by NO+ --NAD(+)-dependent, post-translational covalent automodification of GAPDH --Oxidative modification (thiol oxidation) and inhibition of GAPDH by NO-related agents, probably ONOO- GAPDH and several other protein targets may serve as molecular sensors of elevated NO concentrations and may transmit this message through posttranslational modification and oxidation-induced conformational changes as cGMP-independent NO signaling pathways. Toxicity of NO seems to be linked to both apoptosis and necrosis, depending on the chemistry of NO it undergoes in a given biological milieu. Toxicity manifests as a relative excess of NOx, metal-NO interactions, and ONOO- formation in relation to cellular defense systems. Although accumulation of the tumor-suppressor gene product p53 in response to NO opens a regulatory mechanism known to be involved in apoptotic cell death, cGMP-independent signaling pathways remain to be elucidated. As NO-dependent modification of GAPDH would imply down-regulation of glycolysis and concomitant energy production followed by cell death, our data so far do not support this assumption. In recent years, NO has proved to be a beneficial messenger with a potentially toxic activity. It will be challenging to investigate NO biochemistry in closer detail and to elucidate how NO targets biological systems, especially in relation to its pathophysiological role.