The alpha subunit of the GTP binding protein activates muscarinic potassium channels of the atrium

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Isoprenaline can activate the acetylcholine-induced K+ current in canine atrial myocytes via Gs-derived betagamma subunits

1. G protein betagamma subunits activate the acetylcholine-induced potassium current IK,ACh. There is no evidence of specificity at the level of the betagamma subunits. Therefore all G protein-coupled receptors in atrial myocytes should be able to activate IK,ACh. Paradoxically, it is often stated that isoprenaline does not activate IK,ACh. Rationales to explain this negative result include insufficient concentrations of Gs in the atrium or restricted access of Gs-derived betagamma subunits to the IK,ACh channel. We took advantage of a non-specific increase in Gs that results after infection with adenovirus. 2. Adenoviral infection unmasked a 1 microM isoprenaline-induced IK,ACh which was prevented by propranolol. Isoprenaline occasionally activated IK,ACh in uninfected and freshly dissociated atrial myocytes but the effect was larger and more consistent in infected myocytes. 3. Pertussis toxin pretreatment (100 ng ml-1 overnight) did not block the effect of isoprenaline. The effect of isoprenaline became persistent if cells were pretreated with cholera toxin (200 ng nl-1). 4. Signal transduction events distal to adenylyl cyclase were not involved in isoprenaline-induced IK,ACh. Forskolin (10 microM) did not activate IK,ACh. Inhibition of adenylyl cyclase with cytoplasmic application of 300 microM 2'-deoxyadenosine 3'-monophosphate did not prevent the activation of IK,ACh by isoprenaline. 5. Cytoplasmic application of a betagamma binding peptide derived from the C terminus of beta-adrenergic receptor kinase 1 (50 microM) prevented the effect of isoprenaline on IK,ACh. The peptide did not prevent the stimulation of the L-type calcium current by isoprenaline. 6. The results indicate that beta-adrenoceptors can activate IK,ACh in atrial myocytes through the release of betagamma subunits from Gs.

ATP-dependent activation of the atrial acetylcholine-induced K+ channel does not require nucleoside diphosphate kinase activity

Prior reports by others have shown that cytoplasmically applied ATP can activate the acetylcholine-induced K+ channel in inside-out atrial membrane patches when no guanine nucleotides are present in the solution bathing the cytosolic face of the membrane. A nucleoside diphosphate kinase mechanism was proposed to explain the activation by ATP. We show in the present study that cytoplasmic adenylylimidodiphosphate mimics the activation by ATP. Unlike ATP, the activation by adenylylimidodiphosphate does not subside on washout. Although commercially available adenylylimidodiphosphate is contaminated by guanylylimidodiphosphate, the activation by adenylylimidodiphosphate still occurs after HPLC purification to remove guanine nucleotide contamination. Adenylylimidodiphosphate does not support phosphotransferase activity by nucleoside diphosphate kinase. Therefore, nucleoside diphosphate kinase activity cannot explain the activation of atrial acetylcholine-induced K+ current by ATP and adenylylimidodiphosphate. We hypothesize that the activation by millimolar concentrations of ATP is due to binding of adenine nucleotide to the guanine nucleotide binding site of the G protein(s) responsible for stimulating the acetylcholine-induced K+ current.

Subclasses of vomeronasal receptor neurons: differential expression of G proteins (Gi alpha 2 and G(o alpha)) and segregated projections to the accessory olfactory bulb

Differential expression of G proteins (Gi alpha 2 and G(o alpha) and the separate central projections of Gi alpha 2- and G(o alpha)-immunoreactive (ir) vomeronasal receptor neurons were investigated in the mouse and rat using immunocytochemical methods. In the vomeronasal organ (VNO), receptor neurons with their cell bodies located in the middle layer (middle 1/3) of the vomeronasal sensory epithelium express Gi alpha 2. Axons of these Gi alpha 2-ir neurons can be followed from VNO to the anterior part, but not the posterior part, of the nerve-glomerular (N-GL) layer of the accessory olfactory bulb (AOB). Another population of receptor neurons, which are located in the deep layer (basal 1/3) of the vomeronasal sensory epithelium, express G(o alpha), and axons of the G(o alpha)-ir neurons can be traced to the posterior part, but not the anterior part, of the N-GL layers of the AOB. The axons of the two subclasses of receptor neurons are intermingled near the VNO and become segregated as they enter the AOB. Removal of the AOB results in retrograde degeneration of both Gi alpha 2-ir and G(o alpha)-ir receptor neurons in the VNO. These results suggest that at least two subclasses of receptor neurons exist in the VNO: the Gi alpha 2-ir neurons in the middle layer and the G(o alpha)-ir neurons in the deep layer of the VNO. The Gi alpha 2-ir neurons in the middle layer of the VNO project to the anterior part of the AOB, while the G(o alpha)-ir neurons in the deep layer of the VNO project to the posterior half of the AOB. These results are similar to our previous observations in the gray short-tailed opossum, suggesting that the existence of at least two subclasses of receptor neurons in the vomeronasal epithelium with differential projections to the AOB is a conserved feature among mammals.

Activation of the muscarinic K+ channel by P2-purinoceptors via pertussis toxin-sensitive G proteins in guinea-pig atrial cells

1. Whole-cell voltage clamp and cell-attached patch-clamp techniques were applied to single atrial myocytes enzymatically dissociated from adult guinea-pig hearts. 2. In whole-cell clamp conditions, external applications, of ATP activated the muscarinic K+ (KACh) current, identified by its inward rectification, its reversal potential near the calculated K+ equilibrium potential (EK) and its relaxation properties during step changes of whole-cell membrane potential. Theophylline, an antagonist for Pi-purinoceptors, did not affect the action of ATP on the KACh current, indicating that the response was evoked through P2-purinoceptors. 3. The concentration-response relationship for ATP was well described by a Hill equation with a half-maximal concentration of 1.84 microM and a Hill coefficient of 0.94. ATP (100 microM) produced a maximal increase of the KACh current to 10.92 microA microF-1, which corresponds to 44.9 and 80.9% of the maximal increases evoked by ACh (10 microM) and adenosine (100 microM), respectively. 4. The activation of KACh current gradually declined to a steady level despite the continuous presence of ATP (desensitization). Recovery from the desensitization was relatively rapid with a half-time of approximately 1.5 min. 5. The activation of KACh current by ATP was completely abolished by pre-incubating myocytes with pertussis toxin (PTX, 5 micrograms ml-1), indicating that P2-purinoceptors are coupled to PTX-sensitive G proteins to activate the KACh channel. 6. In the cell-attached patch recording, ATP (5 microM) applied to the pipette solution enhanced the activity of a channel with single-channel conductance of 52.7 +/- 0.9 pS (mean +/- S.E.M., n = 10), reversal potential near EK and mean open time of 1.1 +/- 0.1 ms. These conductance and kinetic properties are identical to those of the KACh channel in the heart. In contrast, ATP applied to the bath solution did not significantly affect the basal activity of KACh channel openings. These observations suggest that the mechanism coupling the P2-purinoceptor to the activation of the KACh channel involves membrane-delimited component(s) rather than soluble second messenger(s). 7. These results strongly suggest a direct coupling of the P2-purinoceptor to the KACh channel through PTX-sensitive G proteins, analogous to the coupling mechanism of the muscarinic ACh receptor and Pi-purinoceptor to this channel.

Identification of structural elements involved in G protein gating of the GIRK1 potassium channel

Chimeras of GIRK1 and IRK1, a G protein-insensitive inward rectifier, are activated by coexpression of G beta gamma if they contain either the N-terminal or part of the C-terminal hydrophilic domain of GIRK1. The N-terminal domain of GIRK1 also facilitates the fast rates of activation and deactivation following m2 muscarinic receptor stimulation. The hydrophobic core of GIRK1 (M1-H5-M2) is important for determining the brief single-channel open times typical of GIRK1 but not important for determining G beta gamma sensitivity. Coexpression with CIR revealed that the gating properties associated with different GIRK1 domains could not have arisen from altered ability to form heteromultimers. These results implicate specific regions of GIRK1 in G protein activation and suggest that GIRK1 may be closely linked to the m2 muscarinic receptor-G protein complex.

Evidence that direct binding of G beta gamma to the GIRK1 G protein-gated inwardly rectifying K+ channel is important for channel activation

Activation of G protein-gated K+ channels by G protein-coupled receptors contributes to parasympathetic regulation of heart rate in the atrium and inhibitory postsynaptic potentials in the peripheral and central nervous system. Having found that G beta gamma activates the cloned GIRK1 channel, we now report evidence for direct binding of G beta gamma to both the N-terminal hydrophilic domain and amino acids 273-462 of the C-terminal domain of GIRK1. These direct interactions are physiologically important because synthetic peptides derived from either domain reduce the G beta gamma binding as well as the G beta gamma activation of the channel. Moreover, the N-terminal domain may also bind trimeric G alpha beta gamma, raising the possibility that physical association of G protein-coupled receptors, G proteins, and K+ channels partially accounts for their compartmentalization and hence rapid and specific channel activation by receptors.

Cardiac muscarinic potassium channel activity is attenuated by inhibitors of G beta gamma

The cardiac muscarinic potassium channel (IK.ACh) is activated by a G protein upon receptor stimulation with acetylcholine. The G protein subunit responsible for activation (G alpha versus G beta gamma) has been disputed. We used G beta gamma inhibitors derived from the beta-adrenergic kinase 1 (beta ARK1) to assess the relative importance of G beta gamma in IK.ACh activation. In rabbit atrial myocytes, IK.ACh had a conductance of 49 +/- 6.2 pS. In inside-out patches, the mean open time was 1.60 +/- 0.57 ms, mean time constant (tau o) was 1.59 +/- 0.53 ms, and mean closed time was 3.02 +/- 1.35 ms (n = 38). beta ARK1 is a G beta gamma-sensitive enzyme that interacts with G beta gamma through a defined sequence near its carboxyl terminus. A 28-amino-acid peptide derived from the carboxyl terminus of beta ARK1 (peptide G) increased the closed time to 10.04 ms (P < .001) and decreased opening probability (NPo) by 71% (P < .001). Fusion proteins containing the entire carboxyl terminus of beta ARK1, glutathione S-transferase beta ARK1ct and hexahistidine beta ARK1ct, decreased NPo by 67% (P = .03) and 48% (P = .009), respectively. They also both significantly increased the closed time. None of the inhibitors affected mean open time or channel amplitude. A control peptide derived from a neighboring region of beta ARK1 had no significant effect on IK.ACh activity. These results provide further evidence for the role of G beta gamma in the activation of IK.ACh.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Differential time course for desensitization to muscarinic effects on K+ and Ca2+ channels

The time course of muscarinic effects on K and Ca currents was investigated at 22-24 degrees C in guinea-pig atrial myocytes, using the whole-cell voltage clamp. At a holding potential of -40 or -50 mV, short exposures to 100 microM acetylcholine (ACh) or carbachol (CCh) reproducibly induced outward K currents (IK,ACh). During long exposures to these agonists, IK,ACh faded with time. In cells not dialysed with guanosine triphosphate (GTP), IK,ACh could dissipate completely following 15-20 min of agonist exposure. After agonist washout, lost sensitivity was not recovered. In cells dialysed with GTP (0.2-1 mM), IK,ACh still faded but normal sensitivity to agonists was restored with washout. Fade of IK,ACh was not prevented by intracellular heparin or dextran, excluding the involvement of either beta-adrenergic or muscarinic receptor kinase. IK,ACh induced by bethanechol or adenosine also faded, and subsequent CCh application after washout revealed a diminished response. Intracellular guanosine-5'-o-(3-thiotriphosphate (GTP gamma S) induced IK,ACh which faded, and subsequent exposure to CCh was without effect. Equally, after full desensitization with CCh, GTP gamma S failed to induce IK,ACh. The Ca current (ICa) was activated by voltage steps to 0 mV and increased with 1-3 microM isoproterenol. This increase could be reversed by CCh, even when IK,ACh had completely faded. Prolonged muscarinic agonist exposure sometimes also caused fade of the effect on ICa, which always occurred after loss of IK,ACh. The results show that desensitization is heterologous and may involve the guanine nucleotide-binding (G) protein. The differential desensitization to the effects on IK,ACh and ICa suggests the involvement of two different signalling pathways in the muscarinic control of K and Ca channels.

A region of the muscarinic-gated atrial K+ channel critical for activation by G protein beta gamma subunits

Complementary DNAs encoding two types of inwardly rectifying K+ channels, GIRK1 and IRK1, have been cloned from rat atrium and mouse macrophage, respectively. GIRK1 expressed in Xenopus oocytes was activated by acetylcholine when m2 muscarinic acetylcholine receptor was coexpressed. The acetylcholine-induced activation of GIRK1 was enhanced by coexpression with the G protein beta 1 gamma 2 subunit but not the beta 1 gamma 1 or alpha subunits. Deletion of the C-terminus of GIRK1 impaired the channel activation associated with the beta 1 gamma 2 subunit. Moreover, replacement of the C-terminus of IRK1 with that of GIRK1 produced a chimera channel that was activated by the beta 1 gamma 2 subunit, whereas intact IRK1 was not activated by the beta 1 gamma 2 subunit. These findings define the C-terminus of GIRK1 as a regulatory region for the G protein beta gamma subunit.

Inhibitory effects of botulinum toxin on 5-HT1C receptor-induced Cl- current in Xenopus oocytes

Several low molecular weight G proteins have been identified, but their functional roles remain unclear. To clarify the involvement of low molecular weight G protein in receptor-stimulated turnover of polyphosphoinositide (PI) turnover, influences of botulinum toxins on serotonin (5-HT)-stimulated Cl- current mediated by PI turnover were investigated using Xenopus oocytes injected with rat brain mRNA. Treatment with botulinum toxin C, D or purified ADP-ribosyltransferase of botulinum toxin (botulinum toxin C3 enzyme) inhibited the 5-HT-induced Cl- current in oocytes, and ADP-ribosylated 23 kDa proteins. Both botulinum toxin C3 enzyme-induced inhibition of the current and ADP-ribosylation were suppressed by pretreatment with antibotulinum toxin C3 enzyme antibody. Botulinum toxin D treatment of oocytes was ineffective in the response of Cl- current induced by injection of 50 pmol inositol 1,4,5-trisphosphate and 50 pmol Ca2+. It is suggested that low molecular weight G proteins ADP-ribosylated by botulinum toxin C3 enzyme are involved in phospholipase C activation in Xenopus oocytes.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Activation of muscarinic K+ current in guinea-pig atrial myocytes by a serum factor

1. Atrial myocytes obtained by enzymatic perfusion of hearts from adult guinea-pigs and cultured for 0-14 days were studied using the whole-cell voltage-clamp technique. 2. Superfusion of the myocytes with diluted sera (1:100 to 1:10,000) from different species (human, horse, guinea-pig) evoked an inward rectifying K+ current. The voltage-dependent properties of this current were identical to those of the K+ current activated by acetylcholine (IK(ACh)). Current density in the presence of horse serum (1:100) approximately corresponded to the non-desensitizing fraction of IK(ACh) during superfusion with 1-2 x 10(-6) M ACh. 3. During a maximal serum-evoked current, application of ACh (10(-6) M) failed to evoke additional K+ current. After switching superfusion from serum-containing to serum-free solution, the K+ current decayed 1-2 orders of magnitude slower than ACh-activated IK(ACh). During the decay of the serum-evoked current, a proportional increase in responsiveness to ACh was recorded. During submaximal activation of K+ current by serum, a saturating concentration of ACh resulted in a total current that was identical to the current evoked by ACh alone minus the desensitizing component. Thus, activation of K+ current by serum caused desensitization of IK(ACh). From these results it is concluded that sera contain a factor that activates the same population of K+ channels as ACh. 4. Irreversible activation of IK(ACh) by ACh in myocytes dialysed with the GTP-analogue GTP-gamma-S abolished sensitivity to serum and vice versa. 5. The effect of serum was not modified by atropine (10(-6) M) which completely blocked the response to 2 x 10(-6) M ACh. Furthermore, theophylline (1 mM), which completely inhibited IK(ACh) activation by adenosine (100 microM), failed to inhibit the effect of serum. Thus, neither muscarinic nor purinergic (A1) receptors are involved. 6. The peptide somatostatin (10(-6) M) and the alpha 1-agonist phenylephrine (1 microM) which previously have been shown to cause activation of IK(ACh) channels, in the present study failed to evoke any measurable current, which excludes the involvement of the corresponding receptors. 7. Pre-incubation of the cells with pertussis toxin completely abolished IK(ACh) evoked by ACh, adenosine and serum, suggesting that the activating factor, like the classical agonists, causes opening of IK(ACh) channels via a G protein (Gi, GK). 8. The potency of serum to activate IK(ACh) was not reduced by dialysis, suggesting the molecular mass of the unknown factor to be > or = 5 kDa. No activating potency was found in the dialysing solutions.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Ion channels are signaling molecules and by themselves perform no work. In this regard they are unlike the usual membrane enzyme effectors for G proteins. The pathways of G protein receptor, G protein and ion channels are, therefore, purely informational in function. Because a single G protein may have several ion channels as effectors, the effects should be coordinated and this seems to be the case. Inhibition of Ca2+ current and stimulation of K+ currents would have a greater impact than either alone. Additional flexibility is provided by spontaneous noise in the complexes of G protein receptor, G protein, and ion channel. By having a non-zero setpoint, the range of control is extended and the responses become bi-directional.

Stimulatory and inhibitory regulation of calcium-activated potassium channels by guanine nucleotide-binding proteins

The regulation of membrane ion channels by guanine nucleotide-binding proteins (G proteins) has been described in numerous tissues. This regulation has been shown to involve the membrane-delimited stimulatory action of G proteins on ion channels. We now show that single calcium-activated potassium channels (KCa channels) in airway smooth muscle cells are both stimulated and inhibited by G proteins in membrane patches. We demonstrate that the beta-adrenergic agonist isoproterenol stimulates channel activity via the alpha subunit of the stimulatory G protein of adenylyl cyclase, Gs, and that channel opening is inhibited by the action of the muscarinic agonist methacholine, acting via a pertussis toxin-sensitive G protein. Isoproterenol stimulated and methacholine inhibited channel activity in the same outside-out patches when GTP was present at the cytosolic surface of the patch. In inside-out patches, addition of GTP and guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]) augmented channel activity when isoproterenol was included in the patch pipette, and inhibited channel activity when methacholine was included in the pipette. Consistent with these results, in the presence of GTP[gamma S], the alpha subunit of Gs (alpha s.GTP[gamma S] complex) opened KCa channels in a dose-dependent manner, whereas in the presence of guanosine 5'-[beta-thio]diphosphate, alpha s had no effect. By contrast, application of activated alpha i or alpha o proteins did not inhibit channel activity in inside-out patches, indicating that channel inhibition is more complex than a simple alpha subunit/channel interaction, similar to the complex inhibitory regulation of adenylyl cyclase. These results suggest that hormonal regulation of KCa channels shares substantial features with the regulation of adenylyl cyclase and demonstrate that a single ion channel may serve as the regulatory target for the membrane-delimited action of stimulatory and inhibitory G proteins. Moreover, they demonstrate a potentially important functional pathway by which beta-adrenergic and other Gs-linked receptors stimulate relaxation of smooth muscle, independent of cAMP-dependent protein phosphorylation.

On the mechanism of G protein beta gamma subunit activation of the muscarinic K+ channel in guinea pig atrial cell membrane. Comparison with the ATP-sensitive K+ channel

The mechanism of G protein beta gamma subunit (G beta gamma)-induced activation of the muscarinic K+ channel (KACh) in the guinea pig atrial cell membrane was examined using the inside-out patch clamp technique. G beta gamma and GTP-gamma S-bound alpha subunits (G alpha *'s) of pertussis toxin (PT)-sensitive G proteins were purified from bovine brain. Either in the presence or absence of Mg2+, G beta gamma activated the KACh channel in a concentration-dependent fashion. 10 nM G beta gamma almost fully activated the channel in 132 of 134 patches (98.5%). The G beta gamma-induced maximal channel activity was equivalent to or sometimes larger than the GTP-gamma S-induced one. Half-maximal activation occurred at approximately 6 nM G beta gamma. Detergent (CHAPS) and boiled G beta gamma preparation could not activate the KACh channel. G beta gamma suspended by Lubrol PX instead of CHAPS also activated the channel. Even when G beta gamma was pretreated in Mg(2+)-free EDTA internal solution containing GDP analogues (24-48 h) to inactivate possibly contaminating G i alpha *'s, the G beta gamma activated the channel. Furthermore, G beta gamma preincubated with excessive GDP-bound G o alpha did not activate the channel. These results indicate that G beta gamma itself, but neither the detergent CHAPS nor contaminating G i alpha *, activates the KACh channel. Three different kinds of G i alpha * at 10 pM-10 nM could weakly activate the KACh channel. However, they were effective only in 40 of 124 patches (32.2%) and their maximal channel activation was approximately 20% of that induced by GTP-gamma S or G beta gamma. Thus, G i alpha * activation of the KACh channel may not be significant. On the other hand, G i alpha *'s effectively activated the ATP-sensitive K+ channel (KATP) in the ventricular cell membrane when the KATP channel was maintained phosphorylated by the internal solution containing 100 microM Mg.ATP. G beta gamma inhibited adenosine or mACh receptor-mediated, intracellular GTP-induced activation of the KATP channel. G i alpha *'s also activated the phosphorylated KATP channel in the atrial cell membrane, but did not affect the background KACh channel. G beta gamma subsequently applied to the same patch caused prominent KACh channel activation. The above results may indicate two distinct regulatory systems of cardiac K+ channels by PT-sensitive G proteins: G i alpha activation of the KATP channel and G beta gamma activation of the KACh channel.

Endothelin activates voltage-dependent Ca2+ current by a G protein-dependent mechanism in rabbit cardiac myocytes

1. Endothelin is a vasoactive peptide released from vascular endothelial cells which has potent cardiac inotropic effects. We examined the effect of endothelin on the verapamil-sensitive Ca2+ current (ICa) in enzymatically dispersed rabbit ventricular myocytes. 2. Using the whole-cell voltage clamp technique with a standard dialysing pipette solution, the application of extracellular endothelin (20 nM) did not increase the peak ICa, but in fact caused a small reversible decline (903 +/- 109 pA without endothelin, 727 +/- 95 pA with endothelin (means +/- S.E.M., n = 14, P less than 0.05)). 3. If GTP (100 microM) was added to the pipette solution, the extracellular application of endothelin (0.2 or 20 nM) caused a large, reproducible increase in peak ICa (871 +/- 85 pA without endothelin, 1230 +/- 110 pA with 20 nM-endothelin (n = 10, P less than 0.05). The endothelin enhancement of ICa occurred after a delay of approximately 3-4 min at room temperature. 4. The GTP requirement for the endothelin effect on ICa suggests that its effect may be mediated through a G protein-dependent pathway. To investigate this further, experiments were performed with pipette solutions containing guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), a GDP analogue which inhibits G protein cycling. With the addition of GDP beta S (0.5-5.0 mM) to the pipette solution (along with 100 microM-GTP), the effect of endothelin on peak ICa was blocked (1062 +/- 86 pA without endothelin, 1170 +/- 134 pA with endothelin (n = 11, P greater than 0.05)). 5. Incubation of myocytes with pertussis toxin (500 ng/ml) prevented the partial ACh-induced reversal of the isoprenolol enhancement of ICa. However, this identical treatment failed to block the endothelin enhancement of the voltage-dependent Ca2+ current (n = 4). 6. Taken together, these results confirm that while the effect of endothelin in rabbit cardiac ventricular myocytes is mediated through a G protein-dependent pathway, the G protein involved is pertussis toxin-insensitive.

Role of the GTP-binding protein Gs in the beta-adrenergic modulation of cardiac Ca channels

In the heart, the guanosine 5'-triphosphate (GTP)-binding protein Gs is activated by hormone binding to beta-adrenergic receptors and stimulates the intracellular cyclic adenosine 3',5'-monophosphate (cAMP) pathway that leads to phosphorylation of L-type Ca channels by the cAMP-dependent protein kinase A. Additionally, Gs can modulate cardiac Ca channels directly in cell-free systems. In order to examine the question of whether these pathways could be separated functionally and whether they act independently or synergistically on L-type Ca channels in intact cells, the whole-cell Ca current (ICa) and the respective current density were measured in guinea-pig ventricular myocytes at 0 mV. The following results were obtained. First, typically, the ICa density increased from 12 to 40 microA/cm2 following application of 1 microM isoproterenol (ISP) to myocytes bathed in solutions containing 1.8 mM CaCl2. However, 1 microM ISP enhanced ICa only from 9 to 17 microA/cm2 after inhibition of the protein kinase A by dialysis of 0.5 mM Rp-cAMPs (the Rp-isomer of adenosine 3',5'-monophosphorothioate) in the presence of 0.5 mM GTP. Withdrawal of GTP from the dialysate attenuated the effects of ISP on ICa. Thus, Rp-cAMPS unmasks a GTP-dependent component of the beta-adrenergic stimulation of ICa, which probably reflects the direct stimulation of Ca channels by Gs under block of cAMP-dependent phosphorylation. Second, in cells under dialysis with 100 or 200 microM cAMP, bath application of 20-40 microM 3-isobutyl-1-methylxanthine (IBMX) enhanced the ICa density to about 41 microA/cm2 indicating saturation of the cAMP pathway. Under this condition, 1 microM ISP was without significant effect on ICa. This result may suggest that direct Gs stimulation is rather ineffective on Ca channels after maximal cAMP-dependent phosphorylation. Alternatively, maximal stimulation of the cAMP pathway may also interfere with the activation of the Gs pathway in intact myocytes. Third, simultaneous application of 1 microM ISP and 40 microM IBMX enhanced ICa up to densities of around 75 microA/cm2 during cell dialysis with 100 microM cAMP, an effect much stronger than that exerted by IBMX alone under similar conditions. Since it seems likely that Gs is activated more quickly, than the cAMP pathway during application of the ISP/IBMX mixture, the latter result suggests that a direct effect of Gs may act to prime L-type Ca channels for cAMP-dependent phosphorylation during beta-adrenergic stimulation of cardiac myocytes.

Control of K+ channels by G proteins

Heterotrimeric G3 proteins are though to couple receptors to ionic channels via cytoplasmic mediators such as cGMP in the case of retinal rods, cAMP in the case of olfactory cells, and the cAMP cascade in the case of cardiac myocytes. G protein-mediated second messenger effects on K+ channels are dealt with elsewhere in this series. Recently, membrane-delimited pathways have been uncovered and an hypothesis proposed in which the alpha subunits of G proteins directly couple receptors to ionic channels, particularly K+ channels. While direct coupling has not been proven, the membrane-delimited nature has been established for specific G proteins and their specific K+ channel effectors.

G protein-mediated ion channel activation

Guanine nucleotide binding proteins couple a wide variety of receptors to ion channels via both "direct" or membrane-delimited and "indirect" second messenger-mediated pathways. This tutorial summarizes current approaches to defining the mechanisms of guanine nucleotide binding protein-mediated ion channel activation. Two well-characterized ion channels in the heart, namely, the beta-adrenergic receptor-activated calcium channel and the muscarinic receptor-activated potassium channel, are used to illustrate the criteria that can distinguish between direct and indirect guanine nucleotide binding protein-transduced pathways.

Adenosine: a natural modulator of L-type calcium channels in atrial myocardium?

The mechanism of action of adenosine at the level of atrial myocardium has been a matter of debate. Electrophysiological studies showed that adenosine increases K+ efflux which may reduce Ca2+ influx, indirectly, by shortening the myocardial action potential. Recently some authors proposed that adenosine also depresses Ca2+ influx by a direct action on the L calcium channel, but, this effect being lower than that on voltage-dependent K+ channels, it was considered of minor importance. The effect of adenosine and its stable analogues was studied in the presence of the dihydropyridine Bay K 8644, a highly specific L-type calcium channel agonist, on isolated guinea-pig atria. The inotropic effect of the calcium channel activator was found to be antagonized by adenosine A1-receptor agonists. Binding studies showed that the effect on Bay K 8644 was not due to the interaction between adenosine analogues and dihydropyridines at the level of a common receptor site on L-type Ca2+ channels. Inhibitors of K+ channels did not antagonize the effect of adenosine analogues against Bay K 8644. Experimental conditions aimed to unmask an effect on slow Ca2+ currents (i.e. K+ depolarized paced atria), further supported that adenosine analogues may act in atria as negative modulators on L-type Ca2+ channels. Finally, the use of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly specific A1-receptor antagonist, demonstrated that the antagonism of Bay K 8644 by adenosine analogues is strictly dependent on A1 receptors. The above data support the possibility of a dual signal transduction pathway to ion channels (K+ and Ca2+) linked to A1 receptors in atrial myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of spontaneous opening of muscarinic K+ channels in rabbit atrium

1. Intracellular mechanism(s) for controlling the opening of muscarinic K+ channels in the absence of an applied muscarinic agonist were studied in rabbit atrium by applying the patch clamp technique to isolated single myocytes. 2. In the cell-attached patch configuration, currents due to the activity of both the muscarinic K+ channel and the inward rectifying K+ channel were recorded. However, while the inward rectifying K+ channel currents were observed in only ten patches of 211 examined, spontaneous opening (i.e. in the absence of a muscarinic agonist) of the muscarinic K+ channel currents was observed in all patches examined in these atrial cells. 3. The single-channel currents due to spontaneous opening of muscarinic K+ channels were identified on the basis of their very similar conductance and gating properties to the unitary events which have been recorded when 0.5 microM-acetylcholine is included in the pipette and 10 microM-GTP is present in the internal side of the patch membrane. 4. Although the spontaneous opening of the muscarinic K+ channels disappeared soon after excision of the patch membrane, this type of channel activity reappeared following application of ATP and MgCl2 to the internal side of the torn-off patch, as expected from previous publications. 5. The K+ channel activity induced by the ATP and Mg2+ (measured as the product of the number of channels, N, times the probability of opening, Po) was strongly dependent upon concentration of free Mg2+; it was half-maximal at 2.2 x 10(-4) M [Mg2+]i. However, after the muscarinic K+ channels had been activated by 100 microM-guanosine 5'-O-3-thiotriphosphate (GTP gamma S) together with ATP and Mg2+, an increase in the Mg2+ concentration from 5.5 x 10(-5) to 2 x 10(-3) M failed to enhance this channel activity. 6. Pertussis toxin, which is known to uncouple muscarinic receptors from associated G-proteins (G(i) or G(o)), failed to inhibit the ATP- and Mg(2+)-induced activation of this K+ channel in the absence agonists. 7. In experiments made to test whether the Mg(2+)-ATP requirement results from an obligatory phosphorylation reaction, ATP was replaced with adenylyl-imidodiphosphate (AMP-PNP), an analogue of ATP which is resistant to hydrolysis. This K+ channel activity was not present when ATP was replaced with AMP-PNP.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of adenosine receptor agonists on volume-activated ion transport in pig red cells

Swelling of pig red cells leads to an increase in a chloride-dependent K flux which can be potentiated by cAMP, whereas cell shrinking causes a selective increase in Na movement which is mediated by a Na/H exchanger. We examined the influence of adenosine and adenosine receptor agonists on the volume-sensitive, ouabain-resistant, chloride-dependent K flux, referred to as Rb flux and volume-activated Na/H exchange pathway. It was found that adenosine and adenosine receptor agonists inhibited the Rb flux. N6-cyclohexyl adenosine (CHA) has been found to be the most potent inhibitor with EC50 of approximately 4.5 microM followed by 2-chloroadenosine (Cl-ado) with EC50 of approximately 27 microM and 5'-(N-ethyl)-carboxamido-adenosine (NECA) with EC50 of approximately 185 microM. CHA also inhibits the cAMP-stimulated Rb flux. However, CHA does not alter the basal intracellular cAMP level nor the intracellular cAMP content raised by exogenously added cAMP. In contrast to the adenosine agonist action on the Rb flux, Na/H exchange, which is activated upon cell shrinkage, exhibits a slight stimulation in response to CHA. These findings suggest that the presence of A1 adenosine receptors on the surface of red cells influences the regulation of volume-activated ion transport.

GTP-dependent regulation of myometrial KCa channels incorporated into lipid bilayers

The regulation of calcium-activated K (KCa) channels by a G protein-mediated mechanism was studied. KCa channels were reconstituted in planar lipid bilayers by fusion of membrane vesicles from rat or pig myometrium. The regulatory process was studied by exploring the actions of GTP and GTP gamma S on single channel activity. KCa channels had a conductance of 260 +/- 6 pS (n = 25, +/- SE, 250/50 mM KCl gradient) and were voltage dependent. The open probability (Po) vs. voltage relationships were well fit by a Boltzmann distribution. The slope factor (11 mV) was insensitive to internal Ca2+. The half activation potential (V1/2) was shifted -70 mV by raising internal Ca2+ from pCa 6.2 to pCa 4. Addition of GTP or GTP gamma S activated channel activity only in the presence of Mg2+, a characteristic typical of G protein-mediated mechanisms. The Po increased from 0.18 +/- 0.08 to 0.49 +/- 0.07 (n = 7, 0 mV, pCa 6 to 6.8). The channel was also activated (Po increased from 0.03 to 0.37) in the presence of AMP-PNP, a nonphosphorylating ATP analogue, suggesting a direct G protein gating of KCa channels. Upon nucleotide activation, mean open time increased by a factor of 2.7 +/- 0.7 and mean closed time decreased by 0.2 +/- 0.07 of their initial values (n = 6). Norepinephrine (NE) or isoproterenol potentiated the GTP-mediated activation of KCa channels (Po increased from 0.17 +/- 0.06 to 0.35 +/- 0.07, n = 10). These results suggest that myometrium possesses beta-adrenergic receptors coupled to a GTP-dependent protein that can directly gate KCa channels. Furthermore, KCa channels, beta-adrenergic receptors, and G proteins can be reconstituted in lipid bilayers as a stable, functionally coupled, molecular complex.

Basolateral K conductance: role in regulation of NaCl absorption and secretion

In this review we explore the possible role of basolateral K conductance (gK) in the regulation of salt absorption and secretion. This inquiry is prompted by a growing body of evidence which, taken together, suggests that basolateral gK is very labile and that alterations in basolateral gK may be a key feature in both stimulatory and inhibitory regulatory mechanisms. We first consider the role of basolateral gK in relation to models for salt absorption and secretion, particularly in relation to the maintenance of cellular charge balance and the obligatory coupling between the apical and basolateral membranes that is produced by transcellular current flow. Next, we review some of the experimental evidence that suggests that changes in basolateral gK are associated with transport regulation. The cellular mechanisms that are known to impact on K channel regulation are considered in a general way, and finally, we consider the use of integrated models for understanding possible coordinate regulation of apical and basolateral cell membranes.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Three different potassium channels in human atrium. Contribution to the basal potassium conductance

We applied the cell-attached and inside-out patch-clamp technique under symmetrical isotonic potassium conditions on single human (and guinea pig) atrial cells. The human cells were isolated by a modified method to that described earlier. Our aim was twofold: 1) to study the single-channel characteristics of potassium channels in human atrial single cells, present under basal conditions (iK1 and iK(ATP] or when stimulated with 10(-5) M acetylcholine; and 2) to calculate the contribution of these three channel types to the total basal potassium conductance in human atrial cells, and to compare the results with data on guinea pig atrial cells under the same conditions. We found that in human cells 58% of the patches (n = 42/74) contained acetylcholine-sensitive potassium channels: their conductance was 42 +/- 1.2 pS and mean open time (tau o) was 1.7 +/- 0.5 msec. They showed sporadic openings in the absence of agonist, and activation by acetylcholine was G-protein dependent. In 16% of the patches (n = 7/44), adenosine (10(-4) M) activated the same channels, but the activity was lower than when stimulated by acetylcholine. In 18% of the patches (n = 9/51), an iK1 channel was present (conductance, 27 pS; tau o, 8.7 msec), whereas in the cell-attached mode, ATP-dependent channels were never seen. However, they were present in half of the inside-out patches on washout of ATPi (conductance, 73 pS; tau o, 1.4 msec). The basal potassium conductance (i.e., in the absence of any exogenous hormone or neurotransmitter) was mainly due to iK1 channels in both human and guinea pig cells, a finding that is in contrast with previous reports. However, the potassium current that is induced by acetylcholine is much higher in guinea pig than in human isolated cells; a fraction of it would suffice to fully determine the resting potassium conductance in guinea pig atrial cells, whereas it can play only a modulatory role in human cells. This difference could be important in species-specific autonomic modulation and antiarrhythmic drug action.

The G protein-channel connection

Recent interest in the regulation of ion currents by hormones and neurotransmitters has focused on the role of G proteins as modulators. Which G proteins are involved? How is this regulation achieved? Initial results suggest that the pathways and mechanisms of action are complex and that delineation of this area of regulation has just begun.

Guanine nucleotide-binding protein, alpha i-3, directly activates a cation channel in rat renal inner medullary collecting duct cells

We examined whether GTP binding proteins (G proteins) regulate sodium conducting channels in the apical membrane of renal inner medullary collecting duct (IMCD) cells and thereby modulate sodium absorption. Patch clamp studies were conducted on inside-out patches of the apical membrane of IMCD cells grown in primary culture. Guanosine 5'-triphosphate (GTP) and the nonhydrolyzable GTP analogue, GTP gamma S, which activate G proteins, increased the open probability of the cation channel. In contrast, the nonhydrolyzable GDP analogue, GDP beta S, which decreases G protein activity, inhibited the channel. Pertussis toxin also reduced the open probability of the channel. Addition of the alpha *i-3 subunit of Gi to the solution bathing the cytoplasmic surface of the membrane increased the open probability in a dose-dependent manner (2-200 pM). The threshold concentration for activation by alpha *i-3 was 2 pM. Activation of the cation channel by alpha *i-3 was not mediated via a protein kinase. The IMCD is the first polarized epithelium in which an ion channel has been shown to be directly regulated by a G protein. Thus, G proteins are important elements in regulating sodium absorption by the IMCD.

Potassium channel blockers differentially affect carbachol and (-)-N6-phenylisopropyladenosine on guinea-pig atria

1. The effect of three different potassium channel blockers (tetraethylammonium, TEA; 4-aminopyridine, 4-AP; and apamin) and of variations in the concentration of K+ and Ca2+ in the medium, have been studied on the responses of guinea-pig isolated atria to (-)-N6-phenylisopropyladenosine (R-PIA), a stable adenosine A1-receptor agonist, and to carbachol, a muscarinic agonist. R-PIA and carbachol showed the same negative inotropic effects over a similar range of concentrations (3-300 microM), both in spontaneously beating and in electrically driven atria. 2. TEA (0.1 to 20 mM) and 4-AP (0.3 to 3 mM), both antagonized the negative inotropic and chronotropic effects of carbachol in a concentration-dependent manner. In contrast, these compounds failed to inhibit the effects induced by R-PIA. Apamin, a specific blocker of a low conductance Ca2+-activated K+ channel, was ineffective in accordance with the absence of these channels in atrial tissue. 3. TEA (0.1 to 20mM) inhibited the negative inotropic effect of carbachol, but not that of R-PIA, in atria paced and depolarized by a high K+ medium (22 mM). In this preparation Na+ current is abolished and the contraction induced by noradrenaline and electrical stimulation is solely dependent on Ca2+ influx currents. 4. Stepwise addition of Ca2+ to a calcium-depleted perfusing medium of electrically driven atria, induced a positive inotropic effect which was inhibited by R-PIA. In contrast, carbachol had no effect. 5. In agreement with our previous study, the data suggest that R-PIA acts on isolated atria by inhibiting Ca2+ influx through L-channels.

Constitutive mutants in the yeast pheromone response: ordered function of the gene products

The alpha factor pheromone inhibits the division of yeast a cells. A general method was developed for isolating mutants that exhibit constitutive activation of the pheromone response pathway. A dominant allele of the STE4 locus was recovered in addition to recessive mutations in the SCG1 gene. SCG1 and STE4 are known to encode G alpha and G beta homologs, respectively. Analysis of double mutants suggests that the STE4 gene product functions after the SCG1 product but before the STE5 product.

G-protein beta gamma-subunits activate the cardiac muscarinic K+-channel via phospholipase A2

Muscarinic receptors of cardiac pacemaker and atrial cells are linked to a potassium channel (IK.ACh) by a pertussis toxin-sensitive GTP-binding protein. The dissociation of G-proteins leads to the generation of two potential transducing elements, alpha-GTP and beta gamma. IK.ACh is activated by G-protein alpha- and beta gamma-subunits applied to the intracellular surface of inside-out patches of membrane. beta gamma has been shown to activate the membrane-bound enzyme phospholipase A2 in retinal rods. Arachidonic acid, which is produced from the action of phospholipase A2 on phospholipids, is metabolized to compounds which may act as second messengers regulating ion channels in Aplysia. Muscarinic receptor activation leads to the generation of arachidonic acid in some cell lines. We therefore tested the hypothesis that beta gamma activates IK.ACh by stimulation of phospholipase A2. When patches were first incubated with antibody that blocks phospholipase A2 activity, or with the lipoxygenase inhibitor, nordihydroguaiaretic acid, beta gamma failed to activate IK.ACh. Arachidonic acid and several of its metabolites derived from the 5-lipoxygenase pathway, activated the channel. Blockade of the cyclooxygenase pathway did not inhibit arachidonic acid-induced channel activation. We conclude that the beta gamma-subunit of G-proteins activates IK.ACh by stimulating the production of lipoxygenase-derived second messengers.

The G protein-gated atrial K+ channel is stimulated by three distinct Gi alpha-subunits

The guanine nucleotide-binding protein, Gi, which inhibits adenylyl cyclase, has recently been shown to have three subtypes of the alpha-subunit, termed Gi alpha-1, Gi alpha-2 and Gi alpha-3. They share 87-94% amino-acid sequence homology and so are difficult to separate from one another. Among other functions, purified preparations activate K+ channels but there is confusion over which of the subtypes activates the muscarinic K+ channels of the atrial muscle of the heart: Gi alpha-3, also termed Gk, has been shown to activate this channel but it is not clear whether Gi alpha-1 does or does not. To clarify this problem, we expressed the subtypes separately in Escherichia coli to eliminate contamination by other subtypes and tested the recombinant alpha- chains on atrial muscarinic K+ channels. Although we anticipated that only Gi alpha-3 would have Gk activity, to our surprise all three recombinant subtypes were active, from which we deduce that the Gi subtypes are multifunctional.

Specific guanine nucleotide binding by membranes from Cucurbita pepo seedlings

A microsomal membrane preparation from hypocotyls of dark-grown Cucurbita pepo L. (zucchini) seedlings contains specific high-affinity binding sites for the non-hydrolyzable GTP analog guanosine 5'-[gamma-thio]triphosphate (GTP-gamma-S). Both the binding affinity and the pattern of binding specificity for GTP and guanine nucleoside triphosphate analogs are shared with the more thoroughly characterized animal G-proteins that are known to be involved in signal transduction. The sensitivity of GTP-gamma-S binding to Mg+2 ions and temperature was similar to that reported for rabbit liver G-protein, although the plant complex dissociated more readily. GTP-gamma-S could be recovered unchanged from the binding complex. Proteins (Mr 33 and 50 kDa) present in zucchini membrane preparations were revealed by immunoblotting with antiserum specific for the alpha subunit of platelet GS. These may be homologous to animal G-proteins.