Modulation of the muscarinic K+ channel by P2-purinoceptors in guinea-pig atrial myocytes

P2Y Receptors in Synaptic Transmission and Plasticity: Therapeutic Potential in Cognitive Dysfunction

ATP released from neurons and astrocytes during neuronal activity or under pathophysiological circumstances is able to influence information flow in neuronal circuits by activation of ionotropic P2X and metabotropic P2Y receptors and subsequent modulation of cellular excitability, synaptic strength, and plasticity. In the present paper we review cellular and network effects of P2Y receptors in the brain. We show that P2Y receptors inhibit the release of neurotransmitters, modulate voltage- and ligand-gated ion channels, and differentially influence the induction of synaptic plasticity in the prefrontal cortex, hippocampus, and cerebellum. The findings discussed here may explain how P2Y₁ receptor activation during brain injury, hypoxia, inflammation, schizophrenia, or Alzheimer's disease leads to an impairment of cognitive processes. Hence, it is suggested that the blockade of P2Y₁ receptors may have therapeutic potential against cognitive disturbances in these states.

Regulatory mechanisms underlying the modulation of GIRK1/GIRK4 heteromeric channels by P2Y receptors

The muscarinic K(+) channel (I (K,ACh)) is a heterotetramer composed of GIRK1 (Kir3.1) and GIRK4 (Kir3.4) subunits of a G protein-coupled inwardly rectifying channel, and plays an important role in mediating electrical responses to the vagal stimulation in the heart. I (K,ACh) displays biphasic changes (activation followed by inhibition) through the stimulation of the purinergic P2Y receptors, but the regulatory mechanism involved in these modulation of I (K,ACh) by P2Y receptors remains to be fully elucidated. Various P2Y receptor subtypes and GIRK1/GIRK4 (I (GIRK)) were co-expressed in Chinese hamster ovary cells, and the effect of stimulation of P2Y receptor subtypes on I (GIRK) were examined using the whole-cell patch-clamp method. Extracellular application of 10 μM ATP induced a transient activation of I (GIRK) through the P2Y(1) receptor, which was completely abolished by pretreatment with pertussis toxin. ATP initially caused an additive transient increase in ACh-activated I (GIRK) (via M(2) receptor), which was followed by subsequent inhibition. This inhibition of I (GIRK) by ATP was attenuated by co-expression of regulator of G-protein signaling 2, or phosphatidylinositol-4-phosphate-5-kinase, or intracellular phosphatidylinositol 4,5-bisphosphate loading, but not by the exposure to protein kinase C inhibitors. P2Y(4) stimulation also persistently suppressed the ACh-activated I (GIRK). In addition, I (GIRK) evoked by the stimulation of the P2Y(4) receptor exhibited a transient activation, but that evoked by the stimulation of P2Y(2) or P2Y(12) receptor showed a rather persistent activation. These results reveal (1) that P2Y(1) and P2Y(4) are primarily coupled to the G(q)-phospholipase C-pathway, while being weakly linked to G(i/o), and (2) that P2Y(2) and P2Y(12) involve G(i/o) activation.

ATP as modulator of carbacholine effect on contractility of rat myocardium in postnatal ontogeny

We studied combined effect of 2-m-ATP, P(2) receptor agonist, and carbacholine, muscarinic M(2) cholinoreceptor agonist, on contractility of rat myocardium during the postnatal ontogeny. Activation of P(2) receptors can stimulate or attenuate the effects of carbacholine depending on animal age. 2-m-ATP potentiates the inhibitory effect of carbacholine on myocardial contractility in 14- and 100-day-old rats. In 21-day-old rats, activation of P(2) receptors prevented the negative effect of carbacholine on myocardial contractility. Activation of muscarinic M(2) receptors inhibited the inotropic effect of purine in all age groups.

Regulation of the muscarinic K+ channel by extracellular ATP through membrane phosphatidylinositol 4,5-bisphosphate in guinea-pig atrial myocytes

1 The present study was designed to examine the functional role of membrane phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) in the regulation of the muscarinic K(+) channel (I(K,ACh)) by extracellular ATP and adenosine in guinea-pig atrial myocytes, using the whole-cell patch-clamp method. 2 Bath application of ATP in micromolar concentrations typically evoked a transient activation of I(K,ACh); a rapid activation phase was consistently followed by a progressive decline even to the baseline level despite the continued presence of ATP. This progressive decline of I(K,ACh) was significantly attenuated either by blockade of phospholipase C (PLC) with compound 48/80 (100 microM) or by addition of PtdIns(4,5)P(2) (50 microM) to the cell inside, suggesting that depletion of membrane PtdIns(4,5)P(2) via PLC activation is mainly, if not totally, responsible for the progressive decline of I(K,ACh) during the presence of ATP. 3 When atrial myocytes were exposed to wortmannin (50 microM) following ATP (50 microM) application to impair the resynthesis of PtdIns(4,5)P(2), the activation of I(K,ACh) evoked by subsequently applied ATP (50 microM) was greatly reduced. Activation of I(K,ACh) by adenosine (100 microM) was partially reduced by pretreatment of atrial myocytes with ATP (100 microM) and was largely abolished by a further addition of wortmannin (50 microM) in the presence of ATP (100 microM). These results support the view that the activation of I(K,ACh) by ATP and adenosine depends on membrane PtdIns(4,5)P(2) that is subject to reduction by extracellular ATP. 4 The present study thus provides functional evidence to suggest that extracellular ATP activates PLC and thereby depletes membrane PtdIns(4,5)P(2) that is critically involved in the activation process of I(K,ACh) by its agonists ATP and adenosine in guinea-pig atrial myocytes.

Regulation of cardiac IKs potassium current by membrane phosphatidylinositol 4,5-bisphosphate

Regulation of the slowly activating component of delayed rectifier K+ current (IKs) by membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PtdIns-(4,5)P2) was examined in guinea pig atrial myocytes using the whole-cell patch clamp method. IKs was elicited by depolarizing voltage steps given from a holding potential of -50 mV, and the effect of various test reagents on IKs was assessed by measuring the amplitude of tail current elicited upon return to the holding potential following a 2-s depolarization to +30 mV. Intracellular application of 50 microM wortmannin through a recording pipette evoked a progressive increase in IKs over a 10-15-min period to 208.5 +/- 14.6% (n = 9) of initial magnitude obtained shortly after rupture of the patch membrane. Intracellular application of anti-PtdIns(4,5)P2 monoclonal antibody also increased the amplitude of IKs to 198.4 +/- 19.9% (n = 5). In contrast, intracellular loading with exogenous PtdIns(4,5)P2 at 10 and 100 mum produced a marked decrease in the amplitude of IKs to 54.3 +/- 3.8% (n = 5) and 44.8 +/- 8.2% (n = 5), respectively. Intracellular application of neomycin (50 microM) or aluminum (50 microM) evoked an increase in the amplitude of IKs to 161.0 +/- 13.5% (n = 4) and 150.0 +/- 8.2% (n = 4), respectively. These results strongly suggest that IKs channel is inhibited by endogenous membrane PtdIns(4,5)P2 through the electrostatic interaction with the negatively charged head group on PtdIns(4,5)P2. Potentiation of IKs by P2Y receptor stimulation with 50 microM ATP was almost totally abolished when PtdIns(4,5)P2 was included in the pipette solution, suggesting that depletion of membrane PtdIns(4,5)P2 is involved in the potentiation of IKs by P2Y receptor stimulation. Thus, membrane PtdIns(4,5)P2 may act as an important physiological regulator of IKs in guinea pig atrial myocytes.

P2X4, P2Y1 and P2Y2 receptors on rat alveolar macrophages

ATP receptors present on rat alveolar macrophages (NR8383 cells) were identified by recordings of membrane current, measurements of intracellular calcium, RT-PCR and immunocytochemistry. In whole-cell recordings with a sodium-based internal solution, ATP evoked an inward current at -60 mV. This reversed at 0 mV. The EC50 for ATP was 18 microM in normal external solution (calcium 2 mm, magnesium 1 mm). The currents evoked by 2',3-O-(4-benzoyl)benzoyl-ATP were about five-fold smaller than those observed with ATP. ADP, UTP and alphabeta-methylene-ATP (alphabetameATP) (up to 100 microM) had no effect. ATP-evoked currents were potentiated up to ten-fold by ivermectin and were unaffected by suramin (30-100 microM), pyridoxal-phosphate-6-azophenyl-(2,4-sulphonic acid) (30-100 microM), and brilliant blue G (1 microM). In whole-cell recordings with a potassium-based internal solution and low EGTA (0.01 mm), ATP evoked an inward current at -60 mV that was followed by larger outward current. ADP and UTP (1-100 microM) evoked only outward currents; these reversed polarity at the potassium equilibrium potential and were blocked by apamin (10 nm). Outward currents were also blocked by the phospholipase C inhibitor U73122 (1 microM), and they were not seen with higher intracellular EGTA (10 mm). Suramin (30 microM) blocked the outward currents evoked by ATP and UTP, but not that evoked by ADP. PPADS (10 microM) blocked the ADP-evoked outward current without altering the ATP or UTP currents. RT-PCR showed transcripts for P2X subunits 1, 4 and 7 (not 2, 3, 5, 6) and P2Y receptors 1, 2, 4 and 12 (not 6). Immunocytochemistry showed strong P2X4 receptor expression partly associated with the membrane, weak P2X7 staining that was not associated with the cell membrane, and no P2X1 receptor immunoreactivity. We conclude that rat alveolar macrophages express (probably homomeric) P2X4 receptors, but find no evidence for other functional P2X subtypes. The P2Y receptors are most likely P2Y1 and P2Y2 and these couple through phospholipase C to an increase in intracellular calcium and the opening of SK type potassium channels.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Enhanced negative chronotropy by inhibitory receptors in transgenic heart overexpressing beta(2)-adrenoceptors

Transgenic (TG) mice overexpressing beta(2)-adrenoceptors (AR) in the heart have enhanced beta-adrenergic activity. Since the degree of beta-adrenergic activation influences the negative chronotropic control of heart rate (HR), we studied the inhibitory effect of cholinergic and purinergic stimulation on HR in TG and wild-type (WT) control mice. Bradycardia in response to vagal nerve stimulation and administration of acetylcholine or adenosine was studied in anesthetised animals and perfused hearts. Basal HR was significantly higher in TG than WT mice (P<0.01). Electrical stimulation of vagal nerves (1-32 Hz) induced a Hz-dependent reduction in HR and the response was more pronounced in TG than WT groups (P<0.01). In perfused hearts, HR reduction by acetylcholine (ACh) was more pronounced with EC(50) 110-fold lower in TG than WT hearts. Adenosine-induced bradycardia, which was abolished by a P(1) antagonist, was more pronounced in TG hearts. After pre-treatment with pertussis toxin (PT, 100 microg/kg), bradycardia by vagal nerve stimulation or ACh remained unchanged in WT, but markedly inhibited in TG hearts (both P<0.01). Conversely, inhibiting guanylyl cyclase with LY83583 (30 microM) or nitric oxide synthase with L-NMMA (100 microM) attenuated HR reduction by vagal nerve stimulation in WT but not in TG hearts. Immunobloting assay showed similar G(ialpha2) abundance in TG and WT hearts. Thus, cardiac overexpression of beta(2)AR with high beta-adrenergic activity leads to hypersensitivity of inhibitory receptors controlling HR due to increase in activity of PT-sensitive G-proteins.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Purine and pyrimidine nucleotides inhibit a noninactivating K+ current and depolarize adrenal cortical cells through a G protein-coupled receptor

Bovine adrenal zona fasciculata (AZF) cells express a noninactivating K+ current (IAC) that sets the resting membrane potential and may mediate depolarization-dependent cortisol secretion. External ATP stimulates cortisol secretion through activation of a nucleotide receptor. In whole-cell patch clamp recordings from bovine AZF cells, we found that ATP selectively inhibited IAC K+ current by a maximum of 75.7 +/- 3% (n = 13) with a 50% inhibitory concentration of 1.3 microM. A rapidly inactivating A-type K+ current was not inhibited by ATP. Other nucleotides, including ADP and the pyrimidines UTP and UDP, also inhibited IAC, whereas 2-methylthio-ATP (2-MeSATP) and CTP were completely ineffective. The rank order of potency for six nucleotides was UTP = ADP > ATP > UDP >> 2-MeSATP = CTP. At maximally effective concentrations, UTP, ADP, and UDP inhibited IAC current by 81.4 +/- 5.2% (n = 7), 70.7 +/- 7.2% (n = 4), and 65.2 +/- 7.9% (n = 5), respectively. Inhibition of IAC by external ATP was reduced from 71. 3 +/- 3.2% to 22.8 +/- 4.5% (n = 18) by substituting guanosine 5'-O-2-(thio) diphosphate for GTP in the patch pipette. Inhibition of IAC by external ATP (10 microM) was markedly suppressed (to 17.3 +/- 5.5%, n = 9) by the nonspecific protein kinase antagonist staurosporine (1 microM) and eliminated by substituting the nonhydrolyzable ATP analog 5-adenylyl-imidodiphosphate or UTP for ATP in the pipette. ATP-mediated inhibition of IAC was not altered by the kinase C antagonist calphostin C, the calmodulin inhibitory peptide, or by buffering the intracellular (pipette) Ca++ with 20 mM 1,2-bis-(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid. In current clamp recordings, ATP and UTP (but not CTP) depolarized AZF cells at concentrations that inhibited IAC K+ current. These results demonstrate that bovine AZF cells express a nucleotide receptor with a P2Y3 agonist profile that is coupled to the inhibition of IAC K+ channels through a GTP-binding protein. The inhibition of IAC K+ current and associated membrane depolarization are the first cellular responses demonstrated to be mediated through this receptor. Nucleotide inhibition of IAC proceeds through a pathway that is independent of phospholipase C, but that requires ATP hydrolysis. The identification of a new signaling pathway in AZF cells, whereby activation of a nucleotide receptor is coupled to membrane depolarization through inhibition of a specific K+ channel, suggests a mechanism for ATP-stimulated corticosteroid secretion that depends on depolarization-dependent Ca++ entry. This may be a means of synchronizing the stress-induced secretion of corticosteroids and catecholamines from the adrenal gland.

P2 receptor modulation of voltage-gated potassium currents in Brown adipocytes

Using patch voltage-clamp techniques, we find there are two components to the voltage-gated potassium current (IKv) in rat brown adipocytes. The components differ in their gating and responses to purinergic stimulation, but not their pharmacology. IKv-A recovers from inactivation at physiological membrane potentials, while IKv-B inactivation recovers at more negative potentials. Both currents are >90% blocked by similar concentrations of quinine and tetraethylammonium, but not by beta-dendrotoxin, charybdotoxin, or apamin. The two current components are differentially modulated by extracellular ATP. ATP shifts the voltage dependence of IKv-A inactivation negative by 38 +/- 5 mV (n = 35, +/-SEM) and shifts activation by -14 +/- 2 mV in whole-cell experiments. ATP did not affect the steady state inactivation voltage dependence of IKv-B, but did apparently convert IKv-A into IKv-B. The pharmacology of the inactivation shift is consistent with mediation by a P2 purinergic receptor. Purinergic stimulation of perforated-patch clamped cells causes hyperpolarizing shifts in the window current of IKv-A by shifting inactivation -18 +/- 4 mV and activation -7 +/- 2 mV (n = 16). Since perforated-patch recordings will most closely resemble in vivo cell responses, this ATP-induced shift in the window current may facilitate IKv activation when the cell depolarizes. IKv activity is necessary for the proliferation and differentiation of brown adipocytes in culture (Pappone, P.A., and S.I. Ortiz-Miranda. 1993. Am. J. Physiol. 264:C1014-C1019) so purinergic modulation of IKv may be important in altering adipocyte growth and development.

Selective enhancement of the slow component of delayed rectifier K+ current in guinea-pig atrial cells by external ATP

1. The effects of external ATP on the rapidly and slowly activating components (IKr and IKs, respectively) of the delayed rectifier K+ current (IK) in guinea-pig atrial myocytes were determined using the whole-cell configuration of the patch-clamp technique. 2. An envelope of tails test was conducted by applying depolarizing pulses to +40 mV from a holding potential of -40 mV for various durations between 50 ms and 2 s under control conditions and during exposure to 50 microM ATP. The ATP-induced IK, obtained by digital subtraction, exhibited a constant ratio (0.37) of the tail current to time-dependent current, regardless of the pulse duration. This current ratio was compatible with the predicted ratio of the driving force at +40 and -40 mV for a non-rectifying K+ conductance, suggesting that the ATP-induced IK is due primarily to IKs. 3. The amplitude of IKr isolated from the IK enhanced by ATP, determined as an E-4031 (5 microM)-sensitive current, was similar to the control magnitude of IKr, thus showing that external ATP did not cause an increase in IKr. 4. The voltage-dependent activation of the ATP-induced IK during 500 ms depolarizing test pulses could be described by a Boltzmann equation with a half-activation voltage (V1/2) of 11.5 mV and slope factor (k) of 12.0 mV, which were close to those of IKs (V1/2 of 12.1 mV and k of 12.3 mV), determined as an E-4031-resistant IK, under the same isochronal (500 ms) activation conditions. 5. These results provide evidence to suggest that extracellular ATP selectively potentiates the slow component of IK (IKs), with no measurable effects on IKr, in guinea-pig atrial myocytes.