Single Atrial Cells Research Articles

Effects of Thiopental on Ca2+ Currents and Intracellular Ca2+ Transient in Single Atrial Cells from Guinea Pig

The effects of thiopental on Ca²⁺ currents and intracellular Ca²⁺ transient in single atrial cells from guinea pigs were studied by means of a whole-cell voltage-clamp method and Ca²⁺-sensitive fluorescent dye. Thiopental inhibited L-type voltage-dependent Ca²⁺ currents in a concentration-dependent manner (IC₅₀ = 2.8·10^–5 mol/l). Moreover, the mode of Ca²⁺ current inhibition by thiopental showed no use dependency. Electrical stimulation-induced intracellular Ca²⁺ transient was significantly suppressed by 10^–5 mol/l thiopental. However, the caffeine-sensitive Ca²⁺ releasing pathway from sarcoplasmic reticulum was not affected by thiopental. Our results indicate that thiopental inhibits L-type Ca²⁺ currents, but not release of Ca²⁺ from sarcoplasmic reticulum. These results suggest that the negative inotropic action of thiopental is mainly due to inhibition of L-type Ca²⁺ channels in guinea pig atrial myocytes.

Modulation of Muscarinic K+ Channel by Protein Kinase C in Ischemic Rat Atrial Myocytes

Background and Objectives：Recent studies have shown that many kinds of K + channels, including the muscarinic K + channel (KACh), are activated in the ischemic heart. It is known that these channels can be modulated by phosphorylation. However, little is known about the function of the KACh in ischemic hearts. In this study, we examined whether the KACh channel is mediated by protein kinase C (PKC) activation in rat atrial myocytes under ischemic conditions. Materials and Methods：Single atrial cells of adult rat heart were prepared by collagenase digestion. Channel activity of KACh was recorded by cell-attached configuration from single atrial cells under ischemic conditions, using a patch clamp technique. To simulate ischemia, adenosine or potassium cyanide (KCN) was applied to atrial myocytes, and Western blot was performed to specify PKC isoforms. Results：Adenosine and KCN markedly increased KACh channel activity. The responses to adenosine and KCN were increased 3-fold at mean open time from that observed with control. Channel activity of KACh was blocked by pretreatment with PKC antagonists such as sphingosine, Go 6976, and rottlerin. PKC α and PKC β I isoform levels were increased in the membrane fraction of ischemic heart, indicating that ischemic stress might trigger translocation of cytosolic PKC to the cell membrane. Conclusion：These results show that KACh channels are modulated by PKC activation under ischemic conditions induced by adenosine or KCN. Therefore, the channels can protect the heart from ischemic stress by increasing channel activity. (Korean Circulation J 2005;35:812-820)

Anti-cholinergic effect of verapamil on the muscarinic acetylcholine receptor-gated K+ channel in isolated guinea-pig atrial myocytes.

Effects of verapamil on the acetylcholine (ACh)-induced K+ current were examined in single atrial cells, using the tight-seal whole-cell clamp technique. The pipette solution contained guanosine-5'-triphosphate (GTP) or guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma S, a non-hydrolysable GTP analogue). In GTP-loaded cells, ACh induced a specific K+ current, which is known to be mediated by pertussis toxin-sensitive GTP-binding (G) proteins. Verapamil (0.1-100 microM) depressed the ACh-induced K+ current in a concentration-dependent fashion. In GTP-gamma S-loaded cells, the K+ current remained persistently after wash-out of ACh, probably due to irreversible activation of G proteins by GTP-gamma S. Verapamil (0.1-100 microM) also depressed the intracellular GTP-gamma S-induced K+ current. However, the magnitude of verapamil-depression of the K+ current in GTP-gamma S-loaded cells was significantly smaller than that in GTP-loaded cells at concentrations between 1 and 10 microM of the drug. From these results, it is suggested that verapamil may block not only the function of muscarinic ACh receptors but also of G proteins and/or the K+ channel itself and thereby depress the ACh-induced K+ current in isolated atrial myocytes.

Chronic administration of β-adrenoceptor blockers prolongs action potentials and refractoriness in single human atrial cells

Chronic administration of β-adrenoceptor blockers prolongs action potentials and refractoriness in single human atrial cells

EP receptor-mediated inhibition by prostaglandin E(1) of cardiac L-type Ca(2+) current of rabbits.

Prostaglandin E(1) (PGE(1)) has cardioprotective effects on the ischemic-reperfused heart. To clarify the mechanisms underlying the protective action of PGE(1) on myocardium, we examined the effect of PGE(1) on the L-type Ca(2+) current (I(Ca)) using single atrial cells from rabbits. PGE(1) did not show a significant effect on basal I(Ca) but inhibited the I(Ca) prestimulated by isoproterenol (Iso, 30 nM). This inhibition was concentration dependent (EC(50) = 0.027 microM). Both sulprostone, a specific PGE receptor subtype (EP(1) and EP(3)) agonist, and 11-deoxy-PGE(1), an EP(3) agonist, inhibited the Iso-stimulated I(Ca), similar to PGE(1). Pretreatment with pertussis toxin (PTX) abolished the PGE(1) inhibition of I(Ca). Both the application of forskolin plus IBMX and intracellular dialysis with 8-bromoadenosine 3',5'-cyclic monophosphate eliminated the effect of PGE(1). PGE(1) did not show any further inhibition of I(Ca) when the effect of Iso was almost fully antagonized by acetylcholine. Methylene blue (guanylate cyclase inhibitor), KT-5823 (cGMP-dependent protein kinase inhibitor), and erythro-9-(2-hydroxy-3-nonyl)adenine (type II phosphodiesterase inhibitor) did not significantly change the inhibitory effect of PGE(1). These findings suggest that 1) PGE(1) inhibits Iso-stimulated I(Ca) by binding to the EP(3) receptor and 2) the PTX-sensitive and cAMP-dependent pathway is involved in the PGE(1) inhibition of I(Ca), but the nitric oxide-cGMP-dependent pathway is not. The PGE(1)-induced antiadrenergic effect shown in this study may contribute to the PGE(1) protection of myocardium against ischemia.

P2 purinoceptors contribute to ATP-induced inhibition of L-type Ca2+ current in rabbit atrial myocytes.

Adenine compounds, including adenosine-5'-triphosphate (ATP) and adenosine (Ado), exert inhibitory effects on myocardium via P1 (subtype A1) purinoceptors. However, ATP per se is a potent activator of P2 purinoceptors. Our aim was to elucidate the respective roles of P1 and P2 purinoceptors in the actions of ATP on L-type calcium current (ICa) in rabbit atrial cells. A whole cell clamp technique was used to record ICa in single atrial cells from the rabbit heart. ATP (0.1 mumol/1-3 mmol/l) produced an inhibitory effect on ICa prestimulated by isoproterenol (ISO, 30 nmol/l), even in the presence of Ado (1 mmol/l). Both 1,3-dipropyl-8-cyclopentylxanthine (A1 blocker) and suramin (P2 blocker) partially blocked the ATP-induced inhibition of ICa, while their co-application nearly completely abolished the effect of ATP. ATP-gamma S (30 mumol/l) inhibited ISO-stimulated ICa significantly, and this inhibition was completely blocked by suramin. alpha, beta-Methylene-ADP, an inhibitor of hydrolysis of AMP to Ado, eliminated the suramin-resistant component of ICa inhibition by ATP. Pretreatment with pertussis toxin (PTX) abolished the ATP inhibition of ICa. Both intracellular dialysis with 8Br cAMP and the application of forskolin plus 3-isobutyl-1-methylxanthine also eliminated the effect of ATP. Both P1 and P2 purinoceptors are involved in the ATP inhibition of ISO-stimulated ICa in rabbit atrial cells. The P1 stimulation by ATP results from hydrolysis of ATP to Ado. Both the P2- and the P1-mediated effects of ATP and Ado, respectively. involve a PTX-sensitive and cAMP-dependent pathway.

Effects of relaxin on rat atrial myocytes. II. Increased calcium influx derived from action potential prolongation.

Relaxin produces positive inotropic and chronotropic effects in rat hearts. The effect of relaxin on the action potential duration (APD) of single quiescent rat atrial cells was investigated with a whole cell patch clamp. Relaxin induced a significant, dose-dependent prolongation of the APD. This effect was maximal at 200 ng/ml (nominal concentration of 33.6 nM), which caused, on average, a 57% increase in the time taken to reach 90% repolarization. The effect of relaxin was blocked by the protein kinase A inhibitor 5-24 amide, indicating that its effect is mediated by an adenosine 3',5'-cyclic monophosphate-dependent mechanism. The increased APD induced by relaxin caused an enhanced entrance of calcium, with the charge carried through voltage-activated calcium channels increased by approximately 25%. This increase was not due to a direct modulation of calcium currents (20); rather, it was a consequence of the longer period of cellular depolarization. Our findings that relaxin increased the APD and therefore increased the calcium influx in atrial myocytes could explain the positive inotropic effects induced by relaxin in atrial preparations.

SD‐3212, a new class I and IV antiarrhythmic drug: a potent inhibitor of the muscarinic acetylcholine‐receptor‐operated potassium current in guinea‐pig atrial cells

1. By use of patch-clamp techniques, the effects of SD-3212, a novel antiarrhythmic drug, on the calcium current (Ica), the sodium current (INa) and the muscarinic acetylcholine-receptor-operated potassium current (IK.ACh) were examined and compared with those of bepridil in guinea-pig single atrial cells. 2. SD-3212 inhibited ICa and INa in a concentration-dependent manner. The IC50 values of SD-3212 for inhibition of ICa and INa were 1.29 microM and 3.92 microM, respectively. The steady state inactivation curves of ICa and INa were shifted in the hyperpolarizing direction in the presence of 1 microM SD-3212. Similar inhibition of ICa and INa was also observed with bepridil. The IC50 values of bepridil for depression of ICa and INa were 1.55 microM and 4.43 microM, respectively. 3. The muscarinic acetylcholine-receptor-operated potassium current (IK.ACh) was activated by the extracellular application of 1 microM carbachol in the GTP-loaded cells or by the intracellular loading of GTP gamma S, a nonhydrolysable GTP analogue. SD-3212 potently inhibited the carbachol- and GTP gamma S-induced IK.ACh and the IC50 values were 0.38 microM and 0.20 microM, respectively. These IC50 values were very close and about 10 times lower than those for inhibiting ICa and INa. Bepridil also suppressed the carbachol- and GTP gamma S-induced IK.ACh with the IC50 values of 0.69 microM and 0.84 microM, respectively. 4. In guinea-pig atrial cells stimulated at 0.2 Hz, carbachol at a concentration of 1 microM markedly shortened action potential duration. Both SD-3212 (0.1-1 microM) and bepridil (1-10 microM) reversed the action potential shortening in a concentration-dependent manner. The antagonizing effect of SD-3212 on the carbachol-induced action potential shortening was more potent than that of bepridil. 5. These results suggest that SD-3212 inhibits IK.ACh by depressing the function of the potassium channel itself and/or associated GTP-binding proteins. SD-3212 is a unique antiarrhythmic drug, which potently inhibits IK.Ach in addition to its class I and IV effects. SD-3212 and bepridil may be useful for the termination and prevention of vagally-induced atrial flutter and fibrillation.

Calcium currents and contraction in frog atrial trabeculae

The properties of two types of calcium current (ICa(T) and ICa(L)) and their relationship with contraction were studied in isolated frog atrial trabeculae using the double mannitol gap technique. In Ringer solution containing TTX the calcium currents observed are similar to the ICa(T) and ICa(L) found in single frog atrial cells. The results obtained by using inorganic calcium channel blockers have shown that the phasic contraction appears only when ICa(T) and ICa(L) are simultaneously present. It is proposed that ICa(T) loads the sarcoplasmic reticulum (SR) while ICa(L) triggers Ca2+ release from the SR.

Activation of Guinea Pig Atrial Muscarinic K+ Channels by Nucleoside Triphosphates in the Absence of Acetylcholine

Atrial Muscarinic K+ Channel Activation. Introduction: We studied the possible association of a nucieoside diphosphate kinase (NDPK) to the atrial muscarinic receptor‐G proten‐K+ channel triad and its functional importance in the activation of muscarinic K channels under basal conditions, i.e., in the absence of acetylcholine (ACh). Methods and Results: We performed cell‐attached and inside‐out patch clamp experiments in single atrial cells isolated from guinea pig hearts. Muscarinic K+ channels in inside‐out patches can be fully activated in the absence of ACh and guanosine triphosphate (GTP), provided that adenosine triphosphate (ATP)‐Mg2 + is present at the intracellular side of the membrane. The K 50 for ATP and Mg2 + are 224 μM and 105 μ respectively. The activation by ATP‐ Mg2+ is the result of a phosphorylating reaction. Other nucleoside triphosphates (NTPs) such as ITP, GTP, UTP, and CTP can substitute for ATP, but their efficiency is lower than that of ATP. The NTP‐dependent activation can be explained by a membrane‐bound NDPK that directly phosphorylates GDP that is bound to the G protein, GK. This NDPK‐catalyzed transsphosphory lation to activate GK is distinct from the classical agonist‐induced GDP‐to‐GTP exchange. The ATP‐dependent activation can be blocked as a result of competition between different intracellular nucleotides for the binding sites on NDPK. Conclusion: Under physiological conditions and in the absence of ACh, tbe block by G nucleotides and ADP will prevent full activation of the channels by intracellular ATP (present in millimolar concentrations). However, sporadic transphosphorylation of G K‐bound GDP may contribute to tbe basal activity of the channels and hence to the basal K* conductance of atrial cells. (J Cardiovasc Electrophysiol, Vol. 3, pp. 457–473, October 1992)

Pacemaker current in single cells and in aggregates of cells dissociated from the embryonic chick heart.

1. We have measured the time-dependent pacemaker current, I(f), in single cells, or small clusters of two or three cells dissociated from embryonic chick hearts with the whole-cell patch clamp technique, and in multicellular reaggregates of dissociated cells with the two-microelectrode voltage clamp technique. 2. We observed time-dependent current (I(f)) in the -90 to -60 mV range from aggregates of ventricular cells, as in our earlier results from this preparation, which we previously attributed to the potassium ion current mechanism, IK2. We also observed I(f) in single atrial cells and aggregates of atrial cells. 3. The range of activation of I(f) was -120 to -90 mV in atrial preparations (either single cells or aggregates). The activation range of I(f) in ventricular cells was also -120 to -90 mV which is approximately 30 mV negative to the I(f) activation range in ventricular cell aggregates. The reasons for this shift of I(f) in ventricular preparations are unknown. 4. The I(f) component clearly underlies the spontaneous pacemaker depolarization which is observed in ventricular heart cell aggregates during the first week of embryonic development. However, I(f) is not a significant factor underlying spontaneous activity in atrial preparations. The pacemaker current in these cells is a net inward background component, which is significantly reduced in amplitude with development, as is the I(f) component in the ventricle.

Muscarinic agonist-induced actions on potassium and calcium channels in atrial myocytes: differential desensitization.

Desensitization to the effects of acetylcholine (ACh) or carbachol (CCh) on ACh-regulated potassium current (IK(ACh)) and the voltage-dependent L-type, calcium current (ICa(L)) was studied in single guinea-pig atrial cells under voltage clamp at room temperature (22-24 degrees C). At a holding potential of -40 mV, CCh (100 microM) activated IK(ACh) repeatedly and reproducibly when applied for 15-s periods with 60-s intervals between applications. Prolonged (10 min) exposure to CCh caused a time-dependent decline of IK(ACh) and a marked reduction (desensitization) of the response to subsequent application of CCh. In the absence of guanosine triphosphate (GTP) in the pipette, only partial resensitization occurred within 20 min after CCh washout. The desensitization to CCh could also be obtained when adenosine (20 microM) was used to activate this current. Therefore, this desensitization must be heterologous. In the presence of isoproterenol (ISO, 3 microM), ICa(L) increased. Acetylcholine continued to inhibit this current at a time when its activation of IK(ACh) had become desensitized. Muscarinic inhibition of ICa(L) eventually desensitized but only after IK(ACh) desensitization had occurred. Because of its heterologous nature, as well as of the ability of ACh to desensitize in the presence of intrapipette dextran, which is reported to inhibit beta-adrenergic receptor kinase (beta ARK), it seems unlikely that beta ARK-dependent phosphorylation of the muscarinic receptor (mAChR) accounted for desensitization. The differential desensitization time course is consistent with the hypothesis that different subunits of Gi, the inhibitory guanine nucleotide binding protein, not only transduce the agonist effects of ACh on IK(ACh) and ICa(L) but are also subjected to different inactivation/regulation processes.

Effects of calcitonin gene-related peptide on membrane currents in mammalian cardiac myocytes.

We examined the effects of calcitonin gene-related peptide (CGRP) on the membrane currents of single atrial and ventricular cells of guinea pig heart. The tight-seal whole-cell voltage-clamp technique was used. In atrial cells, like isoproterenol, CGRP increased the L-type Ca channel current (ICa.L) in a concentration-dependent manner. Human CGRP-(8-37), a putative CGRP receptor antagonist, completely abolished the CGRP-induced increase of ICa.L. Although the effects of CGRP were similar to those of isoproterenol, propranolol, a beta-adrenergic receptor antagonist, did not affect the CGRP-induced increase of ICa.L. After ICa.L had been maximally activated by isoproterenol (2 microM) or intracellular cyclic adenosine 5'-monophosphate (100 microM), CGRP failed to increase ICa.L. Acetylcholine antagonized the effects of CGRP on ICa.L. Unlike the effects on atrial cells, CGRP had no significant effects on the membrane currents of ventricular myocytes. These results indicate that CGRP increases ICa.L via adenylate cyclase activation by binding to specific membrane receptors in cardiac atrial myocytes. Furthermore, CGRP receptors are expressed in atrial cells but probably not in ventricular cells.

Reduced beta-agonist sensitivity in single atrial cells from failing human hearts

Human myocytes were isolated from right atrial appendage, and contractile responses to inotropic agents were studied. Responses to inotropic agents of cells isolated from patients with mild heart disease [New York Heart Association (NYHA) classes I and II] were compared with those of myocytes from rabbit atria. Maximally effective concentrations of calcium, forskolin, and isoproterenol increased contraction amplitude to a similar extent (11.9, 11.5, and 11.3% change in cell length, respectively), but histamine produced a smaller effect (7.1%). The maximum responses of rabbit atrial cells to calcium (18.5%) and isoproterenol (15.0%) were significantly greater than human. In human cells, the velocity of contraction or relaxation was accelerated more by isoproterenol (P less than 0.05) or forskolin (P less than 0.01) than by high calcium. Only relaxation velocity was increased by isoproterenol in rabbit cells (P less than 0.05). Rabbit myocytes contracted and relaxed 10-30% faster than human (P less than 0.05). Cells from the atria of patients with New York Heart Association (NYHA) heart failure class III or IV were less responsive to isoproterenol than those from class I or II (P less than 0.01). Omitting data from patients who had been taking calcium-channel blockers or beta-adrenoceptor agonist or antagonist drugs did not affect the conclusions. Analysis of variance revealed a significantly greater between-patient than within-patient variation (P less than 0.001), indicating that cells from the same patient have a tendency to respond in a similar way. Responses to high calcium did not differ among NYHA classes. The effect of forskolin was not reduced in NYHA class III, although there was a decreased response in cells from two patients in NYHA class IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Inward current generated by Na‒Ca exchange during the action potential in single atrial cells of the rabbit

To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange.

Somatostatin decreases the calcium inward current in guinea‐pig atria

1. The effects of somatostatin on mechanical and electrophysiological responses were studied in guinea-pig atrial muscle preparations and single cells. 2. Somatostatin (greater than or equal to 10(-8) M) decreased the twitch contraction in a concentration-dependent manner in electrically driven left atria and spontaneously beating right atria. However, the beating rate was not affected. 3. The negative inotropic effect of somatostatin was transient. Desensitization to this agent developed slowly during continuous exposure to the peptide. 4. In single atrial cells, somatostatin significantly shortened the action potential duration, but the resting potential and action potential amplitude were not affected. 5. Under whole cell voltage-clamp conditions, somatostatin decreased the calcium inward current without affecting the sodium and potassium currents. 6. These results suggest that somatostatin selectively acts on the calcium channel of guinea-pig atrial cells to reduce the calcium inward current, which in turn gives rise to the negative inotropic effect.

Intracellular magnesium affects I(K) in single frog atrial cells.

Voltage-clamp experiments on single frog (Rana pipiens) atrial cells using whole cell recording techniques revealed that the addition of MgCl2 to the 150 mM KCl patch pipette solution influenced the voltage- and time-dependent potassium current (IK). After rupture of the membrane patch under the tip of the pipette, IK increased with time when the pipette solution was magnesium free, but decreased slightly when the solution contained 1.5 mM MgCl2. More dramatic decreases in IK occurred when the solution contained 3.0 or 10 mM MgCl2. In addition to suppressing the magnitude of IK, the activation rate of this current was enhanced by 10 mM MgCl2 but was not affected by 1.5 or 3 mM MgCl2. Other chloride salts containing mono-, di-, or trivalent cations were used to demonstrate that the effects of MgCl2 on IK were not related to alterations in ionic strength, osmolality, or chloride concentration produced by adding MgCl2 to the pipette solution. Our results suggest that changes in the intracellular magnesium concentration influence IK as the pipette solution exchanges with the intracellular fluid.

AN-132, a new class I anti-arrhythmic agent, depresses the acetylcholine-induced K + current in atrial myocytes

AN-132, a newly developed anti-arrhythmic agent, effectively depressed the acetylcholine (ACh)-induced K + current (I K.ACh), as measured with the tight-seal, whole-cell clamp technique in single atrial cells of guinea-pig. When GTP-γS was loaded into the cell through a pipette, I K.ACh was activated persistently, probably due to irreversible activation of G proteins by GTP-γS. AN-132 was much less potent to depress I K.ACh in GTP-γS-loaded cells than in GTP-loaded cells. In conclusion, AN-132 may block mainly the cardiac muscarinic ACh receptors to suppress I K.ACh.

Anti-cholinergic effects of quinidine, disopyramide, and procainamide in isolated atrial myocytes: mediation by different molecular mechanisms.

Effects of quinidine, disopyramide, and procainamide on the acetylcholine (ACh)-induced K+ channel current were examined in single atrial cells, using the tight-seal, whole-cell clamp technique. The pipette solution contained guanosine-5'-triphosphate (GTP) or guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma S, a nonhydrolysable GTP analogue). In GTP-loaded cells, not only ACh but also adenosine induced a specific K+ channel current via GTP-binding proteins (G) by activating muscarinic ACh or adenosine receptors. Quinidine and disopyramide depressed the ACh-induced K+ current quite effectively. Procainamide had a weak inhibitory effect. Quinidine also depressed adenosine-induced K+ current, while the effect of disopyramide on adenosine-induced current was much smaller than that on ACh-induced current. In GTP-gamma S-loaded cells, the K+ channel was uncoupled from the receptors and was activated irreversibly, probably due to direct activation of G proteins by GTP-gamma S. Quinidine depressed the GTP-gamma S-induced K+ current just as in the cases of ACh- and adenosine-induced currents of GTP-loaded cells. Disopyramide had only a weak inhibitory effect and procainamide showed no effect. From these results, it is strongly suggested that the major mechanisms underlying the anti-cholinergic effects of quinidine, disopyramide, and procainamide are different; quinidine may inhibit the muscarinic K+ channel itself and/or G proteins, while disopyramide and high doses of procainamide may mainly block functions of muscarinic ACh receptors in atrial myocytes.

Activation of atrial muscarinic K+ channels by low concentrations ofβγ subunits of rat brain G protein

Effects of G protein beta gamma subunits from rat brain on cardiac K+ channel was examined in single atrial cells of guinea-pig, using patch clamp techniques. We found that 10 pM concentration of rat brain beta gamma subunits preparation could activate the atrial muscarine receptor-gated K+ channel (IK.ACh). Neither the detergent, CHAPS, used to suspend beta gamma nor the boiled beta gamma preparation activated IK.ACh. Furthermore, preincubation of beta gamma subunits preparation in Mg2+-free solution, which easily inactivated alpha-GTP-gamma S, did not affect beta gamma-activation of IK.ACh. We concluded, therefore, that beta gamma subunits themselves can activate IK.ACh.