Membrane-delimited cell signaling complexes: direct ion channel regulation by G proteins

Rapid steroid hormone actions via membrane receptors

Steroid hormones regulate a wide variety of physiological and developmental functions. Traditional steroid hormone signaling acts through nuclear and cytosolic receptors, altering gene transcription and subsequently regulating cellular activity. This is particularly important in hormonally-responsive cancers, where therapies that target classical steroid hormone receptors have become clinical staples in the treatment and management of disease. Much progress has been made in the last decade in detecting novel receptors and elucidating their mechanisms, particularly their rapid signaling effects and subsequent impact on tumorigenesis. Many of these receptors are membrane-bound and lack DNA-binding sites, functionally separating them from their classical cytosolic receptor counterparts. Membrane-bound receptors have been implicated in a number of pathways that disrupt the cell cycle and impact tumorigenesis. Among these are pathways that involve phospholipase D, phospholipase C, and phosphoinositide-3 kinase. The crosstalk between these pathways has been shown to affect apoptosis and proliferation in cardiac cells, osteoblasts, and chondrocytes as well as cancer cells. This review focuses on rapid signaling by 17β-estradiol and 1α,25-dihydroxy vitamin D3 to examine the integrated actions of classical and rapid steroid signaling pathways both in contrast to each other and in concert with other rapid signaling pathways. This new approach lends insight into rapid signaling by steroid hormones and its potential for use in targeted drug therapies that maximize the benefits of traditional steroid hormone-directed therapies while mitigating their less desirable effects.

Role of ERα36 in membrane-associated signaling by estrogen

Traditionally, steroid hormones such as the vitamin D3 metabolites, testosterone and dihydrotesterone, and 17β-estradiol act through cytosolic and nuclear receptors that directly interact with DNA to alter gene transcription and regulate cellular development. However, recent studies focused on rapid and membrane effects of steroid hormones have given invaluable insight into their non-classical mechanisms of action. In some cases, the traditional receptors were implicated as acting also in the plasma membrane as membrane-associated receptors. However, recent data have demonstrated the presence of an alternative splicing variant to traditional estrogen receptor α known as ERα36, which is present in the plasma membranes of several different cell types including several cancer cell types and even in some normal cells including cartilage and bone cells. The physiological effects that result from the membrane activation of ERα36 may vary from one cell type to another, but the mechanism of action appears to use similar pathways such as the activation of various protein kinases and phospholipases leading to the activation of signaling cascades that result in rapid, non-genomic responses. These rapid responses can affect cell proliferation and apoptotic signaling, indirectly activate downstream genomic signaling through phosphorylation cascades of transcription factors, and crosstalk with classical pathways via interaction with classical receptors. This review describes the data from the last several years and discusses the non-classical, rapid, and membrane-associated cellular responses to steroid hormones, particularly 17β-estradiol, through the classical receptors ERα and ERβ and various non-classical receptors, especially estrogen receptor-α36 (ERα36).

Modulation of Kv3.4 channel N-type inactivation by protein kinase C shapes the action potential in dorsal root ganglion neurons

Fast inactivation of heterologously expressed Kv3.4 channels is dramatically slowed upon phosphorylation of the channel's N-terminal (N-type) inactivation gate by protein kinase C (PKC). However, the presence and physiological importance of this exquisite modulation in excitable tissues were unknown. Here, we employed minimally invasive cell-attached patch-clamping, single-cell qPCR and specific siRNAs to unambiguously demonstrate that fast-inactivating Kv3.4 channels underlie a robust high voltage-activated A-type K(+) current (I(AHV)) in nociceptive dorsal root ganglion neurons from 7-day-old rats. We also show that PKC activation with phorbol 12,13-dibutyrate (PDBu) causes a 4-fold slowing of Kv3.4 channel inactivation and, consequently, accelerates the repolarization of the action potential (AP) by 22%, which shortens the AP duration by 14%. G-protein coupled receptor (GPCR) agonists eliminate I(AHV) fast inactivation in a membrane-delimited manner, suggesting a Kv3.4 channel signalling complex. Preincubation of the neurons with the PKC inhibitor bisindolylmaleimide II inhibits the effect of GPCR agonists and PDBu. Furthermore, activation of PKC via GPCR agonists recapitulates the effects of PDBu on the AP. Finally, transfection of the neurons with Kv3.4 siRNA prolongs the AP by 25% and abolishes the GPCR agonist-induced acceleration of the AP repolarization. These results show that Kv3.4 channels help shape the repolarization of the nociceptor AP, and that modulation of Kv3.4 channel N-type inactivation by PKC regulates AP repolarization and duration. We propose that the dramatic modulation of I(AHV) fast inactivation by PKC represents a novel mechanism of neural plasticity with potentially significant implications in the transition from acute to chronic pain.

The role of G proteins in the activity and ethanol modulation of glycine-induced currents in rat neurons freshly isolated from the ventral tegmental area

In freshly isolated neurons of the ventral tegmental area of young rats, we first examined the role of G proteins in the functional modulation of the glycine receptor (GlyR). GTP-gamma-S [guanosine-5'-0-(2-thiotriphosphate)] (2 mM) or GDP-beta-S [guanosine 5'-0-(2-thiodiphosphate)] (2 mM) was added to the pipette solution of whole-cell recordings to regulate G protein activities. GTP-gamma-S enhanced the amplitude of glycine-induced current (I(Gly)), suggesting modulation of GlyRs via a G protein-coupled pathway. GDP-beta-S suppressed I(Gly), suggesting that basal G protein activity positively modulates the GlyRs. We next examined effects of G proteins in ethanol potentiation of GlyR function. Activation of G proteins with 2 mM GTP-gamma-S attenuated, but did not eliminate, ethanol-induced potentiation of I(Gly). These results suggest that GTP-gamma-S and ethanol share the same pathway of activating GlyRs. When G proteins are maximally activated by GTP-gamma-S, the action of ethanol was partially occluded. When 2 mM GDP-beta-S was added in pipette solution, ethanol-induced potentiation of I(Gly) was significantly attenuated, suggesting that GDP-beta-S partially blocked the action of ethanol. However, the inability of GTP-gamma-S (or GDP-beta-S) to eliminate completely the potentiating effect of ethanol indicates that some other factors, in addition to G proteins, may also contribute to the action of ethanol on GlyRs.

Role for the third intracellular loop in cell surface stabilization of the alpha2A-adrenergic receptor

Previous studies have shown that alpha2A-adrenergic receptor (alpha2A-AR) retention at the basolateral surface of polarized MDCKII cells involves its third intracellular (3i loop). The present studies examining mutant alpha2A-ARs possessing short deletions of the 3i loop indicate that no single region can completely account for the accelerated surface turnover of the Delta3ialpha2A-AR, suggesting that the entire 3i loop is involved in basolateral retention. Both wild-type and Delta3i loop alpha2A-ARs are extracted from polarized Madin-Darby canine kidney (MDCK) cells with 0.2% Triton X-100 and with a similar concentration/response profile, suggesting that Triton X-100-resistant interactions of the alpha2A-AR with cytoskeletal proteins are not involved in receptor retention on the basolateral surface. The indistinguishable basolateral t(1)/(2) for either the wild-type or nonsense 3i loop alpha2A-AR suggests that the stabilizing properties of the alpha2A-AR 3i loop are not uniquely dependent on a specific sequence of amino acids. The accelerated turnover of Delta3i alpha2A-AR cannot be attributed to alteration in agonist-elicited alpha2A-AR redistribution, because alpha2A-ARs are not down-regulated in response to agonist. Taken together, the present studies show that stabilization of the alpha2A-AR on the basolateral surface of MDCKII cells involves multiple mechanisms, with the third intracellular loop playing a central role in regulating these processes.

Establishment of beta-adrenergic modulation of L-type Ca2+ current in the early stages of cardiomyocyte development

beta-Adrenergic modulation of the L-type Ca2+ current (ICaL) was characterized for different developmental stages in murine embryonic stem cell-derived cardiomyocytes using the whole-cell patch-clamp technique at 37 degreesC. Cardiomyocytes first appeared in embryonic stem cell-derived embryoid bodies grown for 7 days (7d). ICaL was insensitive to isoproterenol, forskolin, and 8-bromo-cAMP in very early developmental stage (VEDS) cardiomyocytes (from 7+1d to 7+2d) but highly stimulated by these substances in late developmental stage (LDS) cardiomyocytes (from 7+9d to 7+12d), indicating that all signaling cascade components became functionally coupled during development. In early developmental stage (EDS) cells (from 7+3d to 7+5d), the stimulatory response to forskolin and 8-bromo-cAMP was relatively weak. The forskolin effect was strongly augmented by ATP-gamma-S. At this stage, basal ICaL was stimulated by the nonselective phosphodiesterase (PDE) inhibitor isobutylmethylxanthine, by PDE inhibitors selective for the PDE II, III, and IV isoforms, as well as by the phosphatase inhibitor okadaic acid. Stimulation of ICaL by the catalytic subunit of the cAMP-dependent protein kinase A (PKA) was found to be similar (about 3 times) throughout development and in adult mouse ventricular cardiomyocytes, indicating that no structural changes of the Ca2+ channel related to phosphorylation occurred during development. ICaL was stimulated by isoproterenol in the presence of a PKA inhibitor and GTP-gamma-S in LDS but not VEDS cardiomyocytes, suggesting the development of a membrane-delimited stimulatory pathway mediated through the stimulatory GTP binding protein, Gs. We conclude that uncoupling and/or low expression of Gs protein accounted for the ICaL insensitivity to beta-adrenergic stimulation in VEDS cardiomyocytes. Furthermore, in EDS cells at the 7+4d stage, the reduced beta-adrenergic response is due, at least in part, to high intrinsic PDE and phosphatase activities.

Angiotensin II type 1 receptor-modulated signaling pathways in neurons

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.

Muscarine modulates Ca2+ channel currents in rat sensorimotor pyramidal cells via two distinct pathways

We used the whole cell patch-clamp technique and single-cell reverse transcription-polymerase chain reaction (RT-PCR) to study the muscarinic receptor-mediated modulation of calcium channel currents in both acutely isolated and cultured pyramidal neurons from rat sensorimotor cortex. Single-cell RT-PCR profiling for muscarinic receptor mRNAs revealed the expression of m1, m2, m3, and m4 subtypes in these cells. Muscarine reversibly reduced Ca2+ currents in a dose-dependent manner. The modulation was blocked by the muscarinic antagonist atropine. When the internal recording solution included 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid (EGTA) or 10 mM bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), the modulation was rapid (tauonset approximately 1.2 s). Under conditions where intracellular calcium levels were less controlled (0.0-0.1 mM BAPTA), a slowly developing component of the modulation also was observed (tauonset approximately 17 s). Both fast and slow components also were observed in recordings with 10 mM EGTA or 20 mM BAPTA when Ca2+ was added to elevate internal [Ca2+] ( approximately 150 nM). The fast component was due to a reduction in both N- and P-type calcium currents, whereas the slow component involved L-type current. N-ethylmaleimide blocked the fast component but not the slow component of the modulation. Preincubation of cultured neurons with pertussis toxin (PTX) also greatly reduced the fast portion of the modulation. These results suggest a role for both PTX-sensitive G proteins as well as PTX-insensitive G proteins in the muscarinic modulation. The fast component of the modulation was reversed by strong depolarization, whereas the slow component was not. Reblock of the calcium channels by G proteins (at -90 mV) occurred with a median tau of 68 ms. We conclude that activation of muscarinic receptors results in modulation of N- and P-type channels by a rapid, voltage-dependent pathway and of L-type current by a slow, voltage-independent pathway.

Potassium ion channels and human disease: phenotypes to drug targets?

A significant difficulty faced by the pharmaceutical industry is the initial identification and selection of macromolecular targets upon which de novo drug discovery programs can be initiated. A drug target should have several characteristics: known biological function; robust assay systems for in vitro characterization and high-throughput screening; and be specifically modified by and accessible to small molecular weight compounds in vivo. Ion channels have many of these attributes and can be viewed as suitable targets for small molecule drugs. Potassium (K+) ion channels form a large and diverse gene family responsible for critical functions in numerous cell types, tissues and organs. Recent discoveries, facilitated by genomics technologies combined with advanced biophysical characterization methods, have identified novel K+ channels that are involved in important physiologic processes, or mutated in human inherited disease. These findings, coupled with a rapidly growing body of information regarding modulatory channel subunits and high resolution channel structures, are providing the critical information necessary for validation of K+ channels as drug targets.

Activation of a nonspecific cation current in rat cultured retinal pigment epithelial cells: involvement of a G(alpha i) subunit protein and the mitogen-activated protein kinase signalling pathway

1. Whole-cell patch-clamp recording techniques were used to investigate the G protein subtype and related signalling molecules involved in activation of a nonspecific cation (NSC) current in rat cultured retinal pigment epithelial (RPE) cells. 2. Under control conditions, in 130 mM NaCl with K+ aspartate in the pipette, cytosolic dialysis with guanosine-5'-O-(3-triphosphate) (GTPgammaS, 0.1 mM) activated a large non-inactivating NSC current in 80% of the cells recorded from. 3. Loading RPE cells with antibodies (10 microg-ml(-1)) against the alpha subunit of all PTX-sensitive G proteins (G(alpha i/o/t/z)) reduced NSC current activation to 11%, while loading RPE cells with antibodies directed specifically against the alpha subunits of the Gi subclass (G(alpha i-3)) completely abolished current activation. In RPE cells loaded with anti-G(alpha s) activation of the NSC current was unaffected. 4. Investigation of the potential downstream mediators in the G(alpha i) NSC channel pathway revealed that activation of the cation conductance was unaffected by treatment of RPE cells with the selective protein kinase C inhibitor GF 109203X (3 microM) or the selective CaM kinase II inhibitor KN-93 (50 microM). However, NSC current activation was delayed and the current amplitude reduced in the presence of the nonselective kinase inhibitor H-7 (100 microM) or the selective inhibitor of MAPKK (MEK) activation, PD 98059 (50 microM). 5. In the absence of GTPgammaS, the NSC current was not activated by superfusion of the cells with the cyclic GMP kinase activator dibutyryl-cyclic GMP or with the adenylate cyclase activator forskolin. 6. These results support the involvement of a G protein of the G(alpha i) subclass in the activation of a NSC current in rat RPE cells, and suggest a potential modulatory role for MAP kinase-dependent phosphorylation in current regulation.

Modulation of reconstituted ATP-sensitive K(+)-channels by GTP-binding proteins in a mammalian cell line

1. The action of GTP-binding proteins on ATP-sensitive potassium (KATP) channels was investigated. KATP channels were expressed in a mammalian cell line (COS-1 cells) by cotransfecting vectors carrying the sulphonylurea receptor (SUR1) and BIR (Kir6.2), a member of the inward rectifier K+ channel family. G proteins were also tested on KATP channels composed of an isoform of SUR1, SUR2A, in combination with Kir6.2. 2. The alpha and beta gamma subunits of the GTP binding protein G1 were tested separately in inside-out patches under continuous recording. G alpha-11 increases the activity of SUR1-Kir6.2 and SUR2A-Kir6.2 channels by 200 and by 30%, respectively. 3. G alpha-12 does not increase the activity of SUR1-Kir6.2 channels, but increase the activity of SUR2A-Kir6.2 channels by 30%. 4. Control experiments showed that GTP gamma S, a specific activator of G proteins, and heat-inactivated G alpha-11 do not increase the single channel activity. 5. No effects of the other subunits (beta gamma) from either G11 or G12 on the single channel activity were observed. 6. The protein kinase C inhibitors H7 and an inhibitory peptide (FARKGALRQKNV) had no effect on the modulatory action of G alpha-11 on SUR1-Kir6.2 channels. 7. We conclude that both types of reconstituted KATP channels are modulated by G proteins.

The role of G proteins in the activity and mercury modulation of GABA-induced currents in rat neurons

The role of G proteins in the functional modulation and potentiation by mercury chloride of the GABA(A) receptor-channel complex in rat dorsal root ganglion neurons was studied by using the whole-cell patch clamp technique. Stimulation of Gs proteins by application of GTP-gamma-S in the patch pipette or by incubation of neurons with cholera toxin reduced GABA-induced currents, suggesting modulation of GABA-induced currents via a Gs-protein-coupled pathway. GDP-beta-S in the pipette solution or pretreatment of dorsal root ganglion neurons with pertussis toxin suppressed GABA-induced currents, suggesting that basal Gi/Go-protein activity positively modulates the GABA(A) receptor-channel complex. Mercury chloride potentiation of GABA-activated currents was blocked by application of GTP-gamma-S in the patch pipette or by incubation of neurons with cholera toxin. Mercury chloride potentiation of GABA-activated currents was blocked by application of GDP-beta-S in the patch pipette or by incubation of neurons with pertussis toxin. G proteins, probably Gi/Go proteins, underlie the mercury chloride potentiation of GABA-induced currents.

5HT4 receptors couple positively to tetrodotoxin-insensitive sodium channels in a subpopulation of capsaicin-sensitive rat sensory neurons

The distribution of tetrodotoxin (TTX)-sensitive and -insensitive Na+ currents and their modulation by serotonin (5HT) and prostaglandin E2 (PGE2) was studied in four different types of dorsal root ganglion (DRG) cell bodies (types 1, 2, 3, and 4), which were previously identified on the basis of differences in membrane properties (). Types 1 and 2 DRG cells expressed TTX-insensitive Na+ currents, whereas types 3 and 4 DRG cells exclusively expressed TTX-sensitive Na+ currents. Application of 5HT (1-10 microM) increased TTX-insensitive Na+ currents in type 2 DRG cells but did not affect Na+ currents in type 1, 3, or 4 DRG cells. The 5HT receptor involved resembled the 5HT4 subtype. It was activated by 5-methoxy-N,N-dimethyltryptamine (10 microM) but not by 5-carboxyamidotryptamine (1 microM), (+)-8-hydroxydipropylaminotetralin (10 microM), or 2-methyl-5HT (10 microM), and was blocked by ICS 205-930 with an EC50 of approximately 2 microM but not by ketanserin (1 microM). PGE2 (4 or 10 microM) also increased Na+ currents in varying portions of cells in all four groups. The effect of 5HT and PGE2 on Na+ currents was delayed for 20-30 sec after exposure to 5HT, suggesting the involvement of a cytosolic diffusible component in the signaling pathway. The agonist-mediated increase in Na+ current, however, was not mimicked by 8-chlorophenylthio-cAMP (200 microM), suggesting the possibility that cAMP was not involved. The data suggest that the 5HT- and PGE2-mediated increase in Na+ current may be involved in hyperesthesia in different but overlapping subpopulations of nociceptors.

Two parallel signaling pathways couple 5HT1A receptors to N- and L-type calcium channels in C-like rat dorsal root ganglion cells

The coupling of serotonin receptors to Ca2+ channels was studied in a subpopulation of acutely isolated rat dorsal root ganglion (DRG) cell bodies (type 1 DRG cells), which have membrane properties similar to C-type nociceptive sensory neurons. In these cells, serotonin (5HT) inhibited high-threshold Ca2+ channel current and decreased action potential duration. The inhibitory effects of 5HT and the 5HT1A agonist 8-OH-DPAT were shown to be antagonized by the 5HT1A antagonists spiperone and pindolol, respectively, indicating involvement of a 5HT1A receptor. Several observations suggest that 5HT1A receptors couple to N- and L-type Ca2+ channels by two different signaling pathways in type 1 DRG cells. The inhibition of Ca2+ channel currents produced by 10 microM 5HT occurred in two phases, an initial slowing of current activation rate (kinetic slowing), which was complete within 10 s, and a simultaneous reduction in steady state current amplitude (steady state inhibition), which peaked in approximately 1 min. The kinetic slowing, but not steady state inhibition, was reversed by a positive prepulse to +70 mV (prepulse). Blockade of N-type Ca2+ channels selectively reduced the kinetic slowing and its reversal by prepulses. Chelation of intracellular Ca2+ or blockade of L-type Ca2+ channels selectively reduced the steady state inhibition. Recordings using the cell-attached patch configuration suggest that steady state inhibition required a component that was diffusible in the cytosol, while kinetic slowing occurred via a membrane delimited pathway. The application of 5HT to the cell body outside the patch pipette reduced macroscopic Ca2+ channel currents in 33% of small-diameter DRG cells tested, indicating the participation of a cytosolic diffusible component. Application of 5HT (a membrane impermeant compound) outside the patch pipette produced steady state inhibition only, whereas similar application of membrane permeant 5HT1A agonists, 8-OH-DPAT or 5-methoxy-N,N-dimethyl-tryptamine, produced kinetic slowing and steady state inhibition. Together these data suggest that 5HT1A receptors couple negatively to Ca2+ channels via two pathways: a membrane-delimited pathway that couples to N-channels and actuates voltage-sensitive kinetic slowing and a pathway dependent on a cytosolic diffusible component and free intracellular Ca2+, which couples to L channels and actuates steady state inhibition.

Cholera and pertussis toxins increase acidification of endocytic vesicles without altering ion conductances

Acidification of endocytic vesicles, driven by the vacuolar H+ pump, is affected by parallel ion transporters. Because adenosine 3',5'-cyclic monophosphate (cAMP) and heterotrimeric G proteins may alter ion transporters, I tested whether cholera and pertussis toxins affected acidification of rat liver endosomes. Fluorescein-labeled dextran-loaded "10-min" endosomes from cholera toxin-treated rats exhibited ATP-dependent rates of acidification in the presence and absence of Cl- or K+ that were approximately 60-120% (P < 0.05) faster than rates from control endosomes. This increase was greater for "older" "20-min" endosomes and less for 'early" "2-min" endosomes. Ion transport functions of 10-min and 20-min toxin-exposed endosomes were similar to those of 2-min control endosomes. Cholera toxin also increased ATP-dependent steady-state intravesicular H+ concentration by 38-218% (P < 0.05). Pertussis toxin increased endosome acidification rates by 20-54% (P < 0.05). Both toxins increased liver cAMP content, and endosomes prepared from perfused livers exposed to 0.75 mM dibutyryl cAMP exhibited similar increases in acidification rates. These studies indicate that both cholera and pertussis toxins markedly alter the function of rat liver endosomes. The mechanism is unlikely to reflect major changes in vesicle ion transporters but rather may indicate either an increase in the number of H+ pumps per endosome and/or changes in fusion, remodeling, and maturation of early endocytic vesicles in response to cAMP.

Alpha 1-adrenoceptor activation of a non-selective cation current in rabbit portal vein by 1,2-diacyl-sn-glycerol

1. The transduction mechanisms involved in the activation and modulation of the noradrenaline-activated cation current (Icat) were investigated with whole-cell patch clamp techniques in rabbit portal vein smooth muscle cells. 2. Intracellular application of guanosine 5-O-(3-thiotriphosphate) (GTP gamma S, 500 microM) evoked a 'noisy' inward current at -50 mV with a similar current-voltage relationship and reversal potential to the current evoked by bath application of noradrenaline (100 microM). Guanosine 5-O-(2-thiodiphosphate) (GDP beta S, 1 mM) markedly inhibited noradrenaline-activated Icat. 3. The phospholipase C (PLC) inhibitor U73122 inhibited the amplitude of the noradrenaline-activated Icat in a concentration- and time-dependent manner and the IC50 was about 180 nM. U73122 had similar effects on the cation current evoked by GTP gamma S. 4. Intracellular application of myo-inositol 1,4,5-trisphosphate (IP3, 100 microM) from the patch pipette did not activate any membrane current in cells where intracellular calcium concentration ([Ca2+]i) was buffered to 14 nM, but subsequent addition of noradrenaline evoked Icat. 5. Bath application of the 1,2-diacyl-sn-glycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG, 10 microM) activated Icat, whereas the phorbol ester phorbol 12,13-dibutyrate (PDBu, 0.1-5 microM) failed to activate Icat, in every cell examined. Icat activated by OAG after bath application of PDBu was not significantly different from OAG-activated Icat in the absence of PDBu. The DAG lipase inhibitor RHC80267 (10 microM) activated Icat in some cells, whereas the DAG kinase inhibitor R59949 (10 microM) never activated Icat. 6. Bath application of the protein kinase C inhibitor chelerythrine (1-10 microM) had no effect on either OAG-or noradrenaline-activated Icat. 7. It is concluded that noradrenaline activates Icat via a G-protein coupled to PLC and that the resulting DAG product plays a central role in the activation of cation channels via a protein kinase C-independent mechanism.

D2 dopamine receptors reduce N-type Ca2+ currents in rat neostriatal cholinergic interneurons through a membrane-delimited, protein-kinase-C-insensitive pathway

Dopamine has long been known to regulate the activity of striatal cholinergic interneurons and the release of acetylcholine. Yet, the cellular mechanisms by which this regulation occurs have not been elucidated. One way in which dopamine might act is by modulating voltage-dependent Ca2+ channels. To test this hypothesis, the impact of dopaminergic agonists on Ca2+ channels in neostriatal cholinergic interneurons was studied by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reactions. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly, all interneurons tested (90%, n = 17) coexpressed D2 (short and long isoforms) and D1b (D5) dopamine receptor mRNAs. D1a receptor mRNA was found in only a small subset (20%) of the sample and D3 and D4 receptor mRNAs were undetectable. D2 receptor agonists rapidly and reversibly reduced N-type Ca2+ currents. D1b/D1a receptor activation had little or no effect on Ca2+ currents. The D2 receptor antagonist sulpiride blocked the effect of D2 agonists. Dialysis with guanosine-5'-O-(2-thiodiphosphate) or brief exposure to the G protein (Gi/o) alkylating agent N-ethylmaleimide also blocked the D2 modulation. The reduction in N-type currents was neither accompanied by kinetic slowing nor significantly reversed by depolarizing prepulses. The D2 receptor effects were mediated by a membrane-delimited pathway, because the modulation was not seen in cell-attached patches when agonist was applied to the bath and was not disrupted by perturbations in cytosolic signaling pathways known to be linked to D2 receptors. Activation of M2 muscarinic receptors occluded the D2 modulation, suggesting a shared signaling element. However, activation of protein kinase C attenuated the M2 modulation without significantly affecting the D2 modulation. Taken together, our results suggest that activation of D2 dopamine receptors in cholinergic interneurons reduces N-type Ca2+ currents via a membrane-delimited, Gi/o class G protein pathway that is not regulated by protein kinase C. This signaling pathway may underlie the ability of D2 receptors to reduce striatal acetylcholine release.

Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases

The whole cell recording technique was used to examine an outwardly rectifying chloride current activated by hypotonic shock in bovine pigmented ciliary epithelial (PCE) cells. Removal of internal and external Ca2+ did not affect the activation of these currents, but they were abolished by the phospholipase C inhibitor neomycin. The current was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-disothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a voltage-dependent manner, but tamoxifen, dideoxyforskolin, and quinidine did not affect it. This blocking profile differs from that of the volume-sensitive chloride channel in neighboring nonpigmented ciliary epithelial cells (Wu, J., J. J. Zhang, H. Koppel, and T. J. C. Jacob, J. Physiol, Lond. 491: 743-755, 1996), and this difference implies that the volume responses of the two cell types are mediated by different chloride channels (Jacob, T. J. C., and J. J. Zhang. J. Physiol. Lond. In press). Intracellular administration of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to PCE cells induced a transient, time-independent, outwardly rectifying chloride current that closely resembled the current activated by hypotonic shock. DIDS produced a voltage-dependent block of the GTP gamma S-activated current similar to the block of the hypotonically activated current. Intracellular neomycin completely prevented activation of this current as did incubation of the cells in calphostin C. and inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTP gamma S but extended the duration of the response. Inhibition of phospholipase A2 (PLA2) with p-bromophenacyl bromide prevented the activation of the hypotonically induced current and also inhibited the current once activated by hypotonic solution. The findings imply that the hypotonic response in PCE cells is mediated by both phospholipase C (PLC) and PLA2. Both phospholipases generate arachidonic acid, and, in addition, the PLC pathway regulates the PLA2 pathway via a PKC-dependent phosphorylation of PLA2.

Redox modulation of L-type calcium channels in ferret ventricular myocytes. Dual mechanism regulation by nitric oxide and S-nitrosothiols

The effects of NO-related activity and cellular thiol redox state on basal L-type calcium current, ICa,L, in ferret right ventricular myocytes were studied using the patch clamp technique. SIN-1, which generates both NO. and O2-, either inhibited or stimulated ICa,L. In the presence of superoxide dismutase only inhibition was seen. 8-Br-cGMP also inhibited ICa,L, suggesting that the NO inhibition is cGMP-dependent. On the other hand, S-nitrosothiols (RSNOs), which donate NO+, stimulated ICa,L. RSNO effects were not dependent upon cell permeability, modulation of SR Ca2+ release, activation of kinases, inhibition of phosphatases, or alterations in cGMP levels. Similar activation of ICa,L by thiol oxidants, and reversal by thiol reductants, identifies an allosteric thiol-containing "redox switch" on the L-type calcium channel subunit complex by which NO/O2- and NO+ transfer can exert effects opposite to those produced by NO. In sum, our results suggest that: (a) both indirect (cGMP-dependent) and direct (S-nitrosylation/oxidation) regulation of ventricular ICa,L, and (b) sarcolemma thiol redox state may be an important determinant of ICa,L activity.

IRK(1-3) and GIRK(1-4) inwardly rectifying K+ channel mRNAs are differentially expressed in the adult rat brain

Molecular cloning together with functional characterization has shown that the newly identified family of inwardly rectifying K+ channels consists of several closely related members encoded by separate genes. In this report we demonstrate the differential mRNA expression and detailed cellular localization in the adult rat brain of seven members of the IRK and GIRK subfamilies. Using both radiolabeled cRNA riboprobes and specific oligonucleotide probes directed to nonconserved regions of both known and newly isolated rat brain cDNAs, in situ hybridization revealed wide distribution with partly overlapping expression of the mRNAs of IRK1-3 and GIRK1-4. Except for the low levels of GIRK4 transcripts observed, the overall distribution patterns of the other GIRK subunits were rather similar, with high levels of expression in the olfactory bulb, hippocampus, cortex, thalamus, and cerebellum. Marked differences in expression levels existed only in some thalamic, brainstem, and midbrain nuclei, e.g., the substantial nigra, superior colliculus, or inferior olive. In contrast, IRK subunits were expressed more differentially: all mRNAs were abundant in dentate gyrus, olfactory bulb, caudate putamen, and piriform cortex. IRK1 and IRK3 were restricted to these regions, but they were absent from most parts of the thalamus, cerebellum, and brainstem, where IRK2 was expressed predominantly. Because channel subunits may assemble as heteromultimers, additional functional characterization based on overlapping expression patterns may help to decipher the native K+ channels in neurons and glial cells.

Angiotensin II type 1 receptor modulation of neuronal K+ and Ca2+ currents: intracellular mechanisms

Angiotensin II (ANG II) elicits an ANG II type 1 (AT1) receptor-mediated decrease in voltage-dependent K+ current (Ik) and an increase in voltage-dependent Ca2+ current (ICa) in neurons cocultured from newborn rat hypothalamus and brain stem. Modulation of these currents by ANG II involves intracellular messengers that result from an AT1 receptor-mediated stimulation of phosphoinositide hydrolysis. For example, the effects of ANG II on IK and ICa were abolished by phospholipase C antagonists. The reduction in IK produced by ANG II was attenuated by either protein kinase C (PKC) antagonists or by chelation of intracellular Ca2+. By contrast, PKC antagonism abolished the stimulatory effect of ANG II on ICa. Superfusion of the PKC activator phorbol 12-myristate 13-acetate produced effects on IK and ICa similar to those observed after ANG II. Furthermore, intracellular application of inositol 1,4,5-trisphosphate (IP3) elicited a significant reduction in IK. This suggests that the AT1 receptor-mediated changes in neuronal K+ and Ca2+ currents involve PKC (both IK and ICa) and IP3 and/or intracellular Ca2+ (IK).

Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart

In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.

Effects of Psychotria colorata alkaloids in brain opioid system

An ethnopharmacological survey showed that home remedies prepared with flowers and fruits of Psychotria colorata are used by Amazonian peasants as pain killers. Psychopharmacological in vivo evaluation of alkaloids obtained from leaves and flowers of this species showed a marked dose-dependent naloxone-reversible analgesic activity, therefore suggesting an opioid-like pharmacological profile. This paper reports an inhibitory effect of P. colorata flower alkaloids on [3H]naloxone binding in rat striata as well as a decrease in adenylate cyclase basal activity. The alkaloids did not affect [3H] GMP-PNP binding. These findings provide a neurochemical basis for the opioid-like activity previously detected in vivo and point to Psychotria alkaloids as a potential source of new bioactive opiate derivatives.

Characterization and regulation of a chloride channel from bovine tracheal epithelium

1. The patch-clamp technique was used to characterize chloride channels from the apical membranes of bovine tracheal epithelial cells. Application of GTP gamma S or NaF to excised patches revealed the existence of a novel type of Cl- channel regulated by G-proteins in a membrane-delimited manner. 2. The channel had a linear current-voltage relationship, with a conductance of 100-120 pS. Its open probability was independent of voltage. 3. The channel was highly anion selective (permeability ratio, PNa/PCl = 0.06 +/- 0.04) and had the halide permeability sequence: I- > Br- > or = Cl- > F-, corresponding to the Eisenman I sequence. This suggested that neither ionic size nor diffusion rate determined ion permeation through the channel. 4. The mole fraction behaviour was studied using fluoride and chloride ions. Mixtures of ions produced currents that would be expected from the linear combination of the two ions acting independently, indicating relatively simple permeation through the pore and compatible with a single ion binding site. 5. The channel was inhibited by the stilbene disulphonates SITS (4-acetamido-4'-isothiocyanatostilbene-2, 2'-disulphonic acid) and DNDS (4,4'-dinitrostilbene-2,2'-sulphonic acid). SITS introduced voltage dependence to channel gating and indicated the possible involvement of lysine residues in the channel permeation pathway. 6. NaF was unable to activate Cl- channels in the presence of the aluminum chelator, deferoxamine mesylate. This indicates that Al3+ ions play an important role in chloride channel activation by fluoride. NaF activation was not dependent on the presence of calcium ions. 7. The channel was insensitive to alkaline phosphatase and to the specific inhibitors of protein phosphatase types I and 2A, okadaic acid and calyculin A. 8. The channels could be activated by GTP gamma S or by NaF in the presence of the phospholipase A2 inhibitor quinacrine, indicating that this enzyme is not involved in channel regulation.

Direct control of a large conductance K(+)-selective channel by G-proteins in adrenal chromaffin granule membranes

We report here the presence of a Ca(2+)-independent K(+)-channel of large conductance in adrenal chromaffin cell secretory vesicle membranes which is controlled by inhibitory as well as stimulatory heterotrimeric GTP-binding proteins. Using antibodies against specific alpha subunits for immunoblot analysis, we were able to identify the presence of the inhibitory G(i)2 and G(i)3 subtypes, as well as the stimulatory G(o) and Gs subtypes, but not G(i)1 in adrenal chromaffin granules. Furthermore, functional analysis of the K(+)-channel incorporated into planar lipid bilayers showed that GDP beta S and GTP gamma S have opposite effects on channel activity inducing interconversions between a low and a high open-probability state. Consistent with these findings, the same antibodies antagonized the effects of the nonhydrolyzable analogues on the open probability of the K(+)-channel.

Membrane phospholipids and adrenergic receptor function

We have reviewed the effects on adrenergic receptors by membrane phospholipid alterations secondary to oxidative stress and phospholipases' activity. Experimental evidences indicate that the function of both alpha- and beta-adrenoceptors is regulated by their phospholipid microdomain; however, the underlying mechanism is still undefined. No information seems to be available on the influence of phospholipids on alpha 2-adrenoceptors and on all adrenoceptors' subtypes. Thus, further studies are necessary to clarify the role of membrane phospholipids in regulating the function of each member of the adrenergic receptor superfamily.

Effects of Linalool on glutamatergic system in the rat cerebral cortex

Linalool is a monoterpene compound reported to be a major component of essential oils in various aromatic species. Several Linalool-producing species are used in traditional medical systems, including Aeolanthus suaveolens G. Dom (Labiatae) used as anticonvulsant in the Brazilian Amazon. Psychopharmacological in vivo evaluation of Linalool showed that this compound have dose-dependent marked sedative effects at the Central Nervous System, including hypnotic, anticonvulsant and hypothermic properties. The present study reports an inhibitory effect of Linalool on Glutamate binding in rat cortex. It is suggested that this neurochemical effect might be underlining Linalool psychopharmacological effects. These findings provide a rational basis for many of the traditional medical use of Linalool producing plant species.

Modulation of brain Na+ channels by a G-protein-coupled pathway

Na+ channels in acutely dissociated rat hippocampal neurons and in Chinese hamster ovary (CHO) cells transfected with a cDNA encoding the alpha subunit of rat brain type IIA Na+ channel (CNaIIA-1 cells) are modulated by guanine nucleotide binding protein (G protein)-coupled pathways under conditions of whole-cell voltage clamp. Activation of G proteins by 0.2-0.5 mM guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), a nonhydrolyzable GTP analog, increased Na+ currents recorded in both cell types. The increase in current amplitude was caused by an 8- to 10-mV negative shift in the voltage dependence of both activation and inactivation. The effects of G-protein activators were blocked by treatment with pertussis toxin or guanosine 5'-[beta-thio]diphosphate (GDP[beta S]), a nonhydrolyzable GDP analog, but not by cholera toxin. GDP[beta S] (2 mM) alone had effects opposite those of GTP[gamma S], shifting Na(+)-channel gating 8-10 mV toward more-positive membrane potentials and suggesting that basal activation of G proteins in the absence of stimulation is sufficient to modulate Na+ channels. In CNaIIA-1 cells, thrombin, which activates pertussis toxin-sensitive G proteins in CHO cells, caused a further negative shift in the voltage dependence of Na(+)-channel activation and inactivation beyond that observed with GTP alone. The results in CNaIIA-1 cells indicate that the alpha subunit of the Na+ channel alone is sufficient to mediate G protein effects on gating. The modulation of Na+ channels via a G-protein-coupled pathway acting on Na(+)-channel alpha subunits may regulate electrical excitability through integration of different G-protein-coupled synaptic inputs.

Angiotensin II type 2 receptor stimulation of neuronal K+ currents involves an inhibitory GTP binding protein

Angiotensin II (ANG II) elicits an ANG II type 2 (AT2) receptor-mediated increase in outward K+ current (IK; delayed rectifier K+ current) in neurons cocultured from rat hypothalamus and brain stem. Here we have shown that the AT2-receptor-mediated stimulation of neuronal IK by ANG II (100 nM) was abolished by pretreatment of cultures with pertussis toxin (PTX; 200 ng/ml) and by intracellular application of an antibody against the inhibitory guanine nucleotide (GTP) binding protein (anti-Gi alpha, 1:200). Antibodies against other GTP binding proteins (anti-Go alpha, 1:50 and 1:200; anti-Gq/11 alpha, 1:200) did not alter the AT2-receptor-mediated stimulation of neuronal IK by ANG II (100 nM). Furthermore, this effect of ANG II (100 nM) was inhibited by the serine/threonine phosphatase inhibitor okadaic acid (1-10 nM) and by anti-type 2A protein phosphatase (PP2A) antibodies but not by the tyrosine phosphatase inhibitor sodium orthovanadate (1 mM). Thus we have identified key components (Gi and PP2A) of the signal transduction pathway that is responsible for the AT2-receptor-mediated stimulation of neuronal K+ currents.

Stimulus-secretion coupling and Ca2+ dynamics in pancreatic acinar cells

1. Unique spatiotemporal dynamics in cytosolic Ca2+ concentration, [Ca2+]c, were characterized in various cell types. In pancreatic acinar cells, physiological concentrations of cholecystokinin octapeptide, CCK-8, (< 10 pM) induce repetitive [Ca2+]c spikes commonly termed Ca2+ oscillation, whereas relatively higher concentrations (30 pM-1 nM) evoke biphasic [Ca2+]c dynamics; a rapid transient peak followed by a sustained increase. Much higher concentrations (> 1 nM) induce a large transient followed by a steep decay. 2. These [Ca2+]c dynamics correspond to secretory responses. Repetitive [Ca2+]c change is attributable to the upstroke of the bell-shaped dose-response relationship and the biphasic change is responsible for the downstroke of the relation (so called high-dose inhibited secretion). The large transient [Ca2+]c increase is associated with morphological changes such as bleb formation. 3. Possible interrelation between dose of secretagogues, secretory responses, [Ca2+]c dynamics, IP3 production, receptor occupation and morphological change will be discussed from both pharmacological and physiological points of view.

G-protein control of voltage dependence as well as gating of muscarinic metabotropic channels in guinea-pig ileum

1. Voltage-dependent properties of muscarinic receptor cationic current activated by carbachol in single smooth muscle cells have been studied using patch-clamp recording techniques. Cells were obtained by enzymic digestion from the longitudinal muscle layer of guinea-pig small intestine. 2. The inward cationic current showed a pronounced U-shaped current-voltage relationship (inward current negative). The relationship of cationic conductance to voltage could be described by a Boltzman distribution which was shifted 36 mV in the negative direction on the voltage axis by increasing fractional receptor occupancy (by increasing agonist concentration from 3 to 300 microM), and in the positive direction by desensitization during prolonged application of agonist. Cationic channels opened by low and high concentrations of carbachol at the same potential do not have identical properties. 3. Release of GTP within the cell, by flash photolysis of an inert caged precursor, had the same effect on the current-voltage relationship as increasing receptor occupancy by the agonist. Release of GDP beta S by flash photolysis had the opposite effect. 4. These various results could be explained if cationic channel opening upon receptor activation required binding of at least one alpha-GTP subunit, but the position of the activation curve on the voltage axis depended critically on the concentration of activated G-protein alpha-subunits in the cell.

Distribution and localization of a G protein-coupled inwardly rectifying K+ channel in the rat

The cellular distribution of the mRNA of the inwardly rectifying K+ channel KGA (GIRK1) was investigated in rat tissue by in situ hybridization. KGA was originally cloned from the heart and represents the first G protein-activated K+ channel identified. It is expressed in peripheral tissue solely in the atrium, but not in the ventricle, skeletal muscle, lung and kidney. In the central nervous system KGA is most prominently expressed in the Ammon's horn and dentate gyrus of the hippocampus, neocortical layers II-VI, cerebellar granular layer, olfactory bulb, anterior pituitary, thalamic nuclei and several distinct nuclei of the lower brainstem. The abundant expression of KGA in many CNS neurons supports its important role as a major target channel for G protein mediated receptor function.

A membrane-delimited pathway of G-protein regulation of the guard-cell inward K+ channel

GTP-binding protein (G-protein) regulation of inward rectifying K+ channels in the plasma membrane of Vicia (Vicia faba L.) guard cells has previously been demonstrated at the whole-cell level. However, whether a cytosolic signal transduction chain is required for G-protein regulation of K+ channels in Vicia guard cells, or in any plant cell type, remains unknown. In the present study, we assayed effects of several G-protein regulators on inward K+ channels in isolated inside-out membrane patches from Vicia guard cell protoplasts. Guanosine 5'-[gamma-thio]triphosphate, a nonhydrolyzable GTP analog that locks G proteins into their activated state, decreased the open state probability (Po) of single inward K+ channels. This decrease in Po was accompanied by an increase in one of the closed time constants of the K+ channel. Guanosine 5'-[beta-thio]diphosphate, a GDP analog that locks G proteins into their inactivated state, slightly increased the Po of the inward K+ channel and shortened the closed time constants. Pertussis toxin and cholera toxin, which ADP-ribosylate G proteins at different sites, decreased the Po of the inward K+ channel. Our data indicate that G proteins can act via a membrane-delimited pathway to regulate inward K+ channels in the guard-cell plasma membrane.

Kinetics of GABAB receptor-mediated inhibition of calcium currents and excitatory synaptic transmission in hippocampal neurons in vitro

The time courses of the gamma-aminobutyric acid type B (GABAB) receptor-mediated inhibition of excitatory synaptic transmission and of action potential-evoked calcium currents were studied in hippocampal neurons in vitro with step-like changes of a saturating baclofen concentration. Inhibition mediated by postsynaptic GABAB receptors was excluded pharmacologically. Both presynaptic inhibition and reduction of calcium currents developed and declined exponentially with similar time constants of about 0.2 and 3 s, respectively. The close correlation of the time courses indicates that fast, G protein-mediated depression of voltage-gated calcium channels and thus direct reduction of the presynaptic calcium influx may contribute to the GABAB receptor-induced inhibition of excitatory synaptic transmission in hippocampal neurons in vitro.