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

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

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.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

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.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

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.

Simultaneous activation of p38 MAPK and p42/44 MAPK by ATP stimulates the K+ current ITREK in cardiomyocytes

Living cells exhibit multiple K(+) channel proteins; among these is the recently reported atypical two-pore domain K(+) channel protein TREK-1. Most K(+) currents are modulated by neurohormones and under various pathological conditions. Here, in rat ventricular cardiomyocytes using the whole-cell patch-clamp technique, we characterize for the first time a native TREK-1-like current (I(TREK)) that is activated by ATP, a purine agonist applied at a micromolar range. This current is sensitive to arachidonic acid, intracellular acidosis, and various K(+) current inhibitors. Reverse transcription-polymerase chain reaction reveals the presence of a TREK-1-like mRNA in rat cardiomyocytes that shows 93% identity with mouse TREK-1. ATP effects are greatly attenuated in the presence of arachidonic acid or HCO(-)(3)-induced intracellular acidosis. Using a series of inhibitors, we further demonstrate that the ATP-induced stimulation of I(TREK) implies the activation of cytosolic phospholipase A(2) and the release of arachidonic acid. These events require the simultaneous involvement of p38 MAPK and p42/44 MAPK, respectively, via a cAMP-dependent protein kinase and a tyrosine kinase pathway, whereas the two MAPKs conjugate to activate a mitogen- and stress-activated protein kinase (MSK-1). Our results thus demonstrate the occurrence of a TREK-1-like current in cardiac cells whose activation by purine agonists implies a dual-MAPK cytosolic pathway.

Cardioprotective effect of diadenosine tetraphosphate (AP4A) preservation in hypothermic storage and its relation with mitochondrial ATP-sensitive potassium channels

BACKGROUND

The preconditioning effect of diadenosine tetraphosphate (AP4A) was reported in ischemia/reperfused hearts, but its effect in heart preservation was unknown. According to the possible role of mitochondrial ATP-sensitive potassium channel (mK(ATP) channel) in the effect of ischemic preconditioning, the contribution of mK(ATP) channel to the effect of AP4A was tested.

METHODS

Isolated rat hearts were arrested and preserved by Eurocollin's (EC) solution at 4 degrees C for 8 hr. AP4A (80 microM) or AP4A with the 5-hydroxydecanoic acid (100 microM), a selective inhibitor of the mK(ATP) channel, was added into the EC solution. The preischemic and postischemic cardiac functions were evaluated on a buffer-perfused Langendorff apparatus before storage and after 20 min of reperfusion.

RESULTS

AP4A administration improved the recovery of poststorage cardiac functions (the rate-pressure production, left ventricular systolic pressure, heart rate, coronary flow rate, and derivative of left ventricular systolic pressure; P<0.05) and reduced the leakage of lactate dehydrate and creatine kinase during reperfusion, compared with EC alone. Those effects of AP4A were completely reversed by 5-hydroxydecanoic acid administration in combination subjects.

CONCLUSION

AP4A administration protects the heart through opening of the mK(ATP) channel during hypothermic preservation. Thus, addition of AP4A into cardioplegia may be a novel method of ischemic preconditioning in the transplantation context.

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.

P2Y receptor subtypes differentially couple to inwardly-rectifying potassium channels

Subtypes of P2Y receptors are well characterized with respect to their agonist profile but little is known about differences in their intracellular signalling properties. When expressed in Xenopus oocytes, both P2Y2 and P2Y6 receptors effectively couple to endogenous Ca2+-dependent Cl--channels. However, only P2Y2 receptors increased currents mediated by inward-rectifier K+ channels of the Kir3.0 subfamily. This increase in Kir-current was sensitive to pertussis toxin, while activation of Ca2+-dependent Cl--channels was not. In contrast, suramin, a P2 receptor antagonist, inhibited activation of both channels. These observations suggest that, in contrast to P2Y6, P2Y2 receptors couple to two different classes of G proteins.

Inhibition of ATP-induced increase in muscarinic K+ current by trypsin, alkaline pH, and anions

In atrial cells, the open probability of G protein-activated ACh-sensitive K+ (KACh) channels can be increased approximately fivefold by intracellular ATP (ATPi). Using inside-out patches, we examined how proteases, changes in intracellular pH, and different anions affect G protein-mediated activation and ATP-induced stimulation of the KACh channel. Treatment with trypsin (0.5 mg/ml) removed the GTP dependence of the KACh channel and abolished the ATP-induced stimulation. Intracellular GTP activated KACh channels at all intracellular pH values tested (6.0-8.0), with the concentration at which half-maximal activation (K1/2) occurred ranging from 0.3 (pH 8.0) to 6.7 (pH 6.0) microM. However, the ATPi-induced increase in KACh channel activity was inhibited at pH 8. 0 (K1/2 = pH 7.4). All anions tested except sulfate, phosphate, fluoride, and iodide supported GTP-induced activation. Of the anions that supported GTP-induced activation, only citrate blocked the ATP-induced stimulation of the KACh channel. These results indicate that the GTP- and ATP-mediated effects on the KACh channel use separate signaling pathways. The ATP-mediated effect involves a trypsin- and pH-sensitive mechanism.

Extracellular ATP directly gates a cation-selective channel in rabbit airway ciliated epithelial cells

1. A membrane conductance activated by extracellular ATP was identified and characterized in freshly dissociated rabbit airway ciliated cells using the whole-cell and outside-out patch configurations of the patch-clamp technique. 2. In solutions designed to maximize currents through voltage-gated calcium channels, there were no indications of voltage-gated Ba2+ currents. 3. Extracellular ATP (but not UTP or ADP) activated a membrane conductance which remained activated for several minutes in the presence of ATP. The conductance was permeable to monovalent and divalent cations with approximate relative permeabilities (P) for PBa : PCs : PTEA of 4 : 1 : 0.1. Permeability to Cl- was negligible. 4. Including GDP-beta-S in the intracellular solution did not inhibit the effects of ATP, nor did GTP-gamma-S irreversibly activate the conductance. 5. In outside-out membrane patches, with GDP-beta-S in the pipette solution, ATP activated ion channels which had a chord conductance of approximately 6 pS in symmetrical 150 mM CsCl solutions at -120 mV. 6. Suramin (100 microM) inhibited the whole-cell currents activated by ATP (200 microM) by 93 +/- 3 %. Similar effects of suramin were observed on ATP-activated channels in outside-out membrane patches. 7. Extracellular ATP had a priming action on the response to subsequent exposure to ATP. At -40 mV, the time to half-maximal current activation (t1/2) was 46 +/- 9 s during the first exposure to 200 microM ATP and decreased to 5 +/- 3 s during a second exposure to the same concentration of ATP. The priming action of ATP was not inhibited by including GDP-beta-S in the intracellular solution. 8. The initial rate of activation increased with the concentration of ATP, and was voltage sensitive. During the first exposure to 200 microM ATP, t1/2 at +40 mV was 4-fold longer than t1/2 at -40 mV. 9. Half-maximal activation of the conductance shifted from 210 +/- 30 to 14 +/- 4 microM added ATP when CaCl2 in the extracellular solution was reduced from 1.58 to 0. 01 mM. The Hill coefficient for ATP was 1.2 in both solutions.10. These observations suggest that a form of ATP uncomplexed with divalent cations directly gates an ion channel (P2X receptor) in rabbit airway ciliated cells, which serves as a pathway for Ca2+ influx. This purinoceptor may contribute to sustained ciliary activation during prolonged exposures to ATP.

Activation of muscarinic K+ channels by extracellular ATP and UTP in rat atrial myocytes

The effects of extracellular adenine and pyrimidine nucleotides on the acetylcholine-activated K+ channels (KACh) in rat cardiac myocytes were compared and examined by using the patch-clamp technique. In perforated-patch whole-cell recording experiments, extracellular adenosine triphosphate (ATP) reversibly caused an increase in K+ current. 8-Cyclopentyl-1,3-dipropylxanthine (CPX; 1 microM), a potent A1-adenosine-receptor antagonist, only partially antagonized the ATP-induced increase in K+ current, whereas glibenclamide (30 microM) had no effect. In cell-attached mode, adenosine and ATP activated single channels that had nearly identical conductance (29 pS) and open time (1.53 ms). These results suggest that adenosine and ATP can activate the same population of K+ channels. Uridine triphosphate (UTP; 100 microM) also caused an increase in steady-state K+ current. In cell-attached mode, the addition of UTP to the recording pipette solution (not in the bath solution) activated the channel current. The single-channel conductance and open time for UTP-induced channel current were 27 pS and 1.57 ms, respectively. These values were similar to those for the K+ channels activated by adenosine or ATP. The rank order of potency for the activation of KACh channels was adenosine = ATP > UTP. The addition of CPX (1 microM) to the pipette solution attenuated the ATP-induced channel activity by approximately 70% and fully prevented activation by AMPCPP, a less hydrolyzable ATP analog but did not cause any effect on UTP-induced channel activity. In pertussis toxin-treated cardiac myocytes, no any activity of UTP-induced KACh-channel current was observed. Our results demonstrate that extracellular ATP and UTP can directly activate KACh-channel current. This activation also was linked to pertussis toxin-sensitive G protein. The effect of extracellular ATP is mainly caused by the action on binding to A1-adenosine receptor, whereas the effect of extracellular UTP may be mediated possibly by P2u-purinergic (or 5'-nucleotide) receptor.

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.

Nucleotide receptors

Adenosine 5'-triphosphate (ATP) and/or related nucleotides act at both ionotropic (P2X) and metabotropic (P2Y) receptors. P2X receptor subunits (P2X1-P2X7) form ligand-gated cation channels, as homomultimers or heteromultimers. Recent work indicates that P2X3 subunits participate in channels expressed by nociceptive sensory neurons, and that the second of the two transmembrane domains of each subunit contributes to the ion permeation pathway. P2X7 subunits form large cytolytic pores in addition to cation channels; they have been found in macrophages and brain microglia. P2Y receptors form a distinct subset of G-protein-coupled receptors; most couple through G proteins to phospholipase C, but inhibition of adenylate cyclase and N-type Ca2+ channels, and activation of K+ channels also occurs. Expressed P2Y receptors have generally been distinguished pharmacologically by the rank order of effectiveness of agonists; some prefer pyrimidines to purines. Recent studies suggest that it is important to use purified nucleotides in such classifications. Several P2Y receptors have a very widespread tissue distribution.

Dual effects of extracellular ATP on the muscarinic acetylcholine receptor-operated K+ current in guinea-pig atrial cells

Adenosine 5'-triphosphate (ATP) is stored in sympathetic and parasympathetic nerve terminals and co-released with norepinephrine and acetylcholine during nerve stimulation. In the heart in situ parasympathetic nerve is tonically stimulated and the activated muscarinic acetylcholine-receptor-operated K+ current (I(K,ACh)) plays an important role in the repolarization of the atrial action potential, the sinoatrial node automaticity and the atrioventricular conduction. In the present study, effects of extracellular ATP on the I(K,ACh) activated by carbachol or adenosine were examined in isolated guinea-pig atrial cells by use of the patch-clamp technique. ATP (10 microM) per se produced a transient activation of I(K,ACh) in atrial cells held at -40 mV. When I(K,ACh) was preactivated by 1 microM carbachol or 10 microM adenosine, ATP (1-100 microM) produced a transient increase followed by a sustained decrease of the current. These ATP-induced biphasic changes of I(K,ACh) were abolished by suramin (100 microM) or reactive blue-2 (30 microM), but not by theophylline (500 microM), indicating the involvement of P2 purinoceptors. ATP also enhanced and then partially reversed the action potential shortening induced by carbachol or adenosine in current-clamped atrial cells. Extracellular ATP did not increase but decreased the openings of the single K(ACh) channel that were recorded by use of a pipette solution containing 1 microM carbachol in the cell-attached mode. Thus, P2 purinoceptor stimulation produces dual effects of ATP on the pre-activated I(K,ACh) and may modulate the chronotropic and inotropic responses during autonomic nerve stimulation.

Enhancement of the ATP-sensitive K+ current by extracellular ATP in rat ventricular myocytes. Involvement of adenylyl cyclase-induced subsarcolemmal ATP depletion

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells, where they regulate cardiac function during metabolic impairment. The present study analyzes the effects of extracellular ATP (ATPc), a P2-purinergic agonist that can be released under various conditions in the myocardial cell bed, on KATP current (IK-ATP) in rat ventricular myocytes. Under the whole-cell patch-clamp configuration at a physiological level of intracellular ATP, applying ATPc in the micromolar range did not activate IK-ATP. However, dialyzing the cell with a low-ATP (100 mumol/L) pipette solution elicited a slowly, quasilinearly increasing IK-ATP that was markedly enhanced by applying ATPe in the presence of a Purinergic antagonist. The effect was reversible on washing out the agonist. The IK-ATP enhancement was inhibited by cholera toxin treatment of the myocytes, suggesting that a Gs protein was involved to mediate the effect. Experiments on excised patches allowed us to exclude a membrane-delimited G protein-dependent pathway. Rather, the results suggested that ATPe activates the adenylyl cyclase, since its inhibition by 2'-deoxyadenosine 3'-monophosphate and SQ-22536, which respectively interact with the purine and catalytic site of the cyclase, strongly reduced the ATPe-induced IK-ATP enhancement, whereas neither compound affected IK-ATP in inside-out patches. Inhibition of cAMP-dependent protein kinase by protein kinase inhibitor peptide 5-24 did not alter the purinergic effect. The findings suggests that ATPe triggers the activation of adenylyl cyclase, which causes a subsarcolemmal ATP depletion sufficient to enhance IK-ATP as it develops during low-ATP dialysis of rat ventricular myocytes.

Purinergic facilitation of ATP-sensitive potassium current in rat ventricular myocytes

1. The effects of different purinergic agonists on the cardiac adenosine 5'-triphosphate (ATP)-sensitive potassium current (IK(ATP)), appearing during dialysis of rat isolated ventricular myocytes with a low-ATP (100 microM) internal solution under whole-cell patch-clamp conditions, were examined in the presence of a P1 purinoceptor antagonist. 2. The extracellular application of ATP in the micromolar range induced, besides known inward currents through cationic and chloride channels, the facilitation of IK(ATP) once IK(ATP) had already been partially activated during the low-ATP dialysis. 3. Analogues of ATP, alpha, beta-methyleneadenosine 5'-triphosphate (alpha, beta meATP), 2-methylthioadenosine triphosphate (2MeSATP), adenosine 5'-O-3-thiotriphosphate (ATP gamma S) similarly facilitated IK(ATP). UTP and ADP were very weak agonists while AMP and adenosine had no detectable effect. 4. The half-maximal stimulating concentration (C50) of alpha, beta meATP, an analogue that did not elicite the interfering inward cationic current was 1.5 microM. Similar apparent C50 (1-2 microM) were observed for ATP and analogues tested with somewhat less maximal effect of ATP gamma S. 5. Suramin, a nonselective P2-purinoceptor antagonist, altered IK(ATP) at the relatively high concentration required to inhibit purinoceptors. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a supposedly predominantly P2x-purinoceptor antagonist, at micromolar concentration inhibited the transient inward current but did not block the facilitation of IK(ATP). 6. Our results demonstrate that ATP and its analogues facilitate IK(ATP) in rat ventricular myocytes by stimulation of non-P1-, non-P2x-purinoceptors.

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

1. Activation of muscarinic K+ (KACh) channels by P2-purinergic agonists, such as ATP, decreases monotonically in the continued presence of agonist. We investigated the mechanisms underlying this process of decline in guinea-pig atrial myocytes using the patch-clamp technique. 2. External ATP reversibly depressed the acetylcholine (ACh, 5.5-11 microM)-induced KACh current in a concentration-dependent manner with a half-maximal inhibitory concentration (IC50) of 5.4 microM. 3. External ATP irreversibly reduced guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S)-induced KACh current both in control and pertussis toxin (PTX)-pretreated cells, suggesting (i) that the ATP-induced inhibition of KACh current occurred at some step(s) downstream from the activation of the PTX-sensitive G protein, GK, and (ii) that a PTX-insensitive G protein was involved in the signal transduction pathway. 4. The potency order of ATP analogues in reducing KACh current was ATP > or = 2-methylthio-ATP > or = alpha, beta-methylene-ATP, indicating involvement of a P2Y-type purinoceptor. 5. In the cell-attached patch recording, ATP (100 microM) applied to the bath solution reduced the activity of the KACh channels activated by ACh in the pipette, in two out of eight experiments, suggesting the possible involvement of cytosolic second messengers in the inhibition of KACh channels. 6. The ATP-induced reduction of KACh current was not affected by a protein kinase C inhibitor, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride (H-7), suggesting that this response was not mediated by the activation of protein kinase C. 7. These results demonstrate that, in addition to the membrane-delimited activation through GK, external ATP causes an inhibition of the KACh channel probably by activating a PTX-insensitive G protein and cytosolic second messenger(s), which may underlie the monotonic decrease of the ATP-activated KACh current.

Enhancement of delayed rectifier K+ current by P2-purinoceptor stimulation in guinea-pig atrial cells

1. We studied the effects of P2-purinoceptor stimulation on the delayed rectifier K+ current (IK) in guinea-pig atrial myocytes using a whole-cell voltage-clamp technique. 2. External application of ATP increased IK, evoked by a 500 ms depolarizing pulse from a holding potential of -40 mV, under conditions in which the L-type Ca2+ channel was blocked; the effect was dose dependent with a half-maximal concentration (K1/2) of 0.95 microM. ATP (50 microM) produced a maximal increase of IK of about a factor of 2. 3. External ADP also enhanced IK in a dose-dependent manner with a K1/2 of 3.65 microM, whereas adenosine (100 microM) failed to evoke this response. Theophylline (500 microM), a blocker of the Pi-purinoceptor, did not antagonize the stimulating action of ATP on IK. These results indicate that IK was enhanced via P2-purinoceptors. 4. External ATP or ADP did not produce a significant change in the current kinetics of IK. 5. Pre-incubation of the atrial myocytes with pertussis toxin (PTX, 5 micrograms ml-1) did not affect the stimulating action of ATP on IK, indicating that PTX-sensitive G proteins did not mediate the ATP action. 6. The enhancement of IK by ATP developed slowly; the effects usually reached a maximum approximately 30-60 s after the application of ATP. This suggests the involvement of a diffusible cytosolic second messenger(s) in the response. ATP could further increase IK after maximal enhancement by isoprenaline (0.5-1.0 microM), suggesting that the intermediate steps were independent of cyclic AMP-dependent protein kinase (protein kinase A). 7. Potentiation of IK by ATP was not attenuated by either (i) pretreatment of the cells with 5 microM 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine dihydrochloride (H-7) or (ii) intracellular perfusion of 20 mM 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that protein kinase C and intracellular Ca2+ did not mediate the response. 8. It is concluded that the activation of P2-purinoceptors increases IK through intracellular mechanisms independent of protein kinase A, protein kinase C or intracellular free Ca2+ in guinea-pig atrial myocytes.