Effect of acetylcholine on time-independent currents in sheep cardiac Purkinje fibers

Mechanisms for vagal modulation of ventricular repolarization and of coronary occlusion-induced lethal arrhythmias in cats

Our goal was to better understand the mechanisms underlying muscarinic receptor actions on the ventricle in vivo. Therefore, we studied the effects of vagal stimulation on ventricular repolarization and of vagal tone on lethal arrhythmias induced by 30 minutes of left anterior descending coronary artery ligation in anesthetized cats. Experimental groups included normal control cats subjected only to coronary ligation and cats pretreated with atropine, pertussis toxin (PTX), or propranolol. All cats received bilateral cervical vagal stimulation (Vstim) at 1, 3, and 5 Hz for 1 minute at 10-minute intervals. Before coronary ligation, Vstim slowed sinus rate, prolonged the PR interval, and lowered blood pressure. Most important from the point of view of electrophysiological function was a vagally induced acceleration of ventricular repolarization in paced and unpaced hearts, which could be explained by the effects of acetylcholine (ie, shortening the subepicardial muscle action potentials). The effect on repolarization was blocked by atropine or PTX but not by propranolol. The extent of sinus slowing and acceleration of repolarization was directly related to the level of functional PTX-sensitive G protein (P < .05). Coronary occlusion was performed during atrial pacing such that the heart rate in all groups was equal. The incidence of ventricular fibrillation (VF) was 10% in the control group and 50% and 54% in atropine and PTX groups, respectively (P < .05). During atrial pacing before coronary occlusion, a vagal index was calculated as percent QTc shortening during Vstim. When the vagal index was 13% to 26%, the incidence of VF during occlusion was zero. When the vagal index was 0% to 12%, VF was 52% (P < .01). Conclusions are as follows: (1) Vstim accelerates ventricular repolarization in cats via a pathway that incorporates a PTX-sensitive G protein and involves an altered gradient between epicardium and endocardium. (2) Removal of vagal tone during ischemia favors VF, as predicted by a vagal index.

Acetylcholine reverses effects of beta-agonists on pacemaker current in canine cardiac Purkinje fibers but has no direct action. A difference between primary and secondary pacemakers

We have investigated the actions of acetylcholine in the absence and presence of the beta-agonist isoproterenol in cardiac Purkinje fibers. beta-Agonists, like isoproterenol, increase the magnitude of the pacemaker current (If) in cardiac myocytes by shifting its activation voltage more positive on the voltage axis. We find that acetylcholine has no effect on If in the absence of isoproterenol. However, if If is first increased by beta-agonist stimulation, acetylcholine can then return If to control levels. This effect on If is exerted through muscarinic receptors since atropine prevents this action of acetylcholine. Functionally, this action of acetylcholine can guarantee the maintenance of ventricular pacemakers when there is high parasympathetic tone but can also prevent extra ventricular beats when sympathetic and parasympathetic tone are both high.

Indirect effects of acetylcholine on the electrogenic sodium pump in bull-frog atrial muscle fibres

1. Effects of acetylcholine (ACh) on the activity of electrogenic Na+ pump in bullfrog atrial muscle fibres were examined using the single sucrose-gap voltage clamp technique. 2. In the K+-free solution, 10 microM-ACh induced a large outward current (ACh-induced current) with an increase in the membrane conductance. 3. The amplitude of the ACh-induced current decreased to 15% of the control 10 min after application of 1 microM-ouabain, suggesting the contribution of electrogenic Na+ pump to the ACh-induced current. The remaining ACh-induced current was not affected even if the concentration of ouabain was increased ten times. 4. The K+-activated current induced by an activation of the electrogenic Na+ pump was suppressed or reversed its direction during the course of the ACh-induced current. 5. The ACh-induced current was completely inhibited by applications of either atropine or barium ions while the K+-activated current was not affected. 6. Both ouabain-sensitive and -insensitive ACh-induced currents were decreased when the membrane was hyperpolarized and eliminated around -95 mV. 7. The ouabain-sensitive component was decreased by increasing the external K+ concentration [K+]o; the proportions of this current to ACh-induced current in 0.5, 0.75, 1 and 2 mM [K+]o were 54, 42, 34 and 14%, respectively. 8. The current-voltage (i-v) relation obtained in 2 or 4 mM [K+]o, where the currents carried by Na+ and Ca2+ were blocked by application of 1 microM-TTX and 1 mM-Cd2+, exhibits marked inward-going rectification but does not show a clear N-shaped feature. Ba2+ (1 mM) induced an inward current at the holding potential (-80 mV) and eliminated the inward-going rectification of the membrane. 9. These results suggest that the increase in the K+ permeability by ACh increases the concentration of K+ immediately outside of the membrane, which in turn stimulates the electrogenic Na+ pump mechanism. The physiological significance of the action of ACh on the electrogenic Na+ pump in bull-frog atrium is discussed in relation to the background K+ current (IK,1).

Effect of tetrodotoxin, lidocaine, and quinidine on the transient inward current of sheep Purkinje fibres

The effect of tetrodotoxin (TTX), lidocaine, and quinidine on the transient inward current (TI) was studied in voltage-clamped sheep cardiac Purkinje fibres. The TI was induced by elevation of extracellular Ca or addition of strophanthidin. Reduction of external Na had a biphasic effect on the steady state TI magnitude; a moderate (less than 50%) reduction of external Na had an enhancing effect on the TI; a further decrease of extracellular Na was accompanied by a decline of TI amplitude. The TI could not be induced in Na-free medium (external Ca less than or equal to 9.0 mM). TTX, lidocaine, and quinidine reduced the magnitude of the TI in a dose-dependent way. The blocking effect of these agents could be compensated for by a moderate (less than 50%) reduction of external Na or an elevation of extracellular Ca. It is suggested that the blocking effect of TTX, lidocaine, and quinidine on the TI is due to a reduction of intracellular Na, which causes a decay of intracellular Ca via the Na-Ca exchange mechanism.

Use of the cell-attached patch clamp technique to examine regulation of single cardiac K channels by cyclic GMP

Cyclic nucleotides play a central role in the modulation of ion channels in a variety of tissues, including the heart. In order to determine the possible role of cyclic GMP (cGMP) in the regulation of the background K channel activity of cardiac cells, the effect of 8-Br-cGMP on the inwardly-rectifying K channels of cultured ventricular myocytes from embryonic chick hearts was examined. 8-Br-cGMP (10(-4) to 10(-3) M) inhibited these single channel currents within 3 to 10 min. Spontaneous recovery of the currents occurred with prolonged (greater than or equal to 15 min) exposure to 8-Br-cGMP, but this recovery was accompanied by altered channel behavior. Thus, a new long-lasting open state of the channel appeared, in addition to the open state observed prior to 8-Br-cGMP addition. Superfusion of the cells with the muscarinic agonist carbamylcholine (10(-5) M) also resulted in inhibition of the currents, which suggests that the cGMP-mediated inhibition of these channels may occur under physiological conditions. Thus, it appears that cGMP may be an important modulator of the background K conductance (and excitability) of cardiac cells.

Currents through ionic channels in multicellular cardiac tissue and single heart cells

Ionic channels are elementary excitable elements in the cell membranes of heart and other tissues. They produce and transduce electrical signals. After decades of trouble with quantitative interpretation of voltage-clamp data from multicellular heart tissue, due to its morphological complexness and methodological limitations, cardiac electrophysiologists have developed new techniques for better control of membrane potential and of the ionic and metabolic environment on both sides of the plasma membrane, by the use of single heart cells. Direct recordings of the behavior of single ionic channels have become possible by using the patch-clamp technique, which was developed simultaneously. Biochemists have made excellent progress in purifying and characterizing ionic channel proteins, and there has been initial success in reconstituting some partially purified channels into lipid bilayers, where their function can be studied.

Background K+ current in isolated canine cardiac Purkinje myocytes

The current-voltage (I-V) relation of the background current, IK1, was studied in isolated canine cardiac Purkinje myocytes using the whole-cell, patch-clamp technique. Since Ba2+ and Cs+ block IK1, these cations were used to separate the I-V relation of IK1 from that of the whole cell. The I-V relation of IK1 was measured as the difference between the I-V relations of the cell in normal Tyrode (control solution) and in the presence of either Ba2+ (1 mM) or Cs+ (10 mM). Our results indicate that IK1 is an inwardly rectifying K+ current whose conductance depends on extracellular potassium concentration. In different [K+]0's the I-V relations of IK1 exhibit crossover. In addition the I-V relation of IK1 contains a region of negative slope (even when that of the whole cell does not). We also examined the relationship between the resting potential of the myocyte, Vm, and [K+]0 and found that it exhibits the characteristic anomalous behavior first reported in Purkinje strands (Weidmann, S., 1956, Elektrophysiologie der Herzmuskelfaser, Med. Verlag H. Huber), where lowering [K+]0 below 4 mM results in a depolarization.

Pertussis toxin-insensitive phosphoinositide hydrolysis, membrane depolarization, and positive inotropic effect of carbachol in chick atria

Muscarinic agonists can stimulate rather than inhibit cardiac muscle in some preparations. In left atria from hatched chicks, treatment with pertussis toxin reversed the membrane action of carbachol from hyperpolarization to depolarization and reversed the inotropic effect of carbachol from negative to positive. Acetylcholine also depolarized the membrane and increased the force of contraction in atria from pertussis-toxin-treated chicks although oxotremorine did not. These cholinergic responses were blocked by atropine but not by adrenoceptor antagonists, suggesting that they are mediated via muscarinic receptors and are not due to actions of endogenously released catecholamines. Muscarinic receptor stimulation leads to two distinct biochemical responses in chick atria: inhibition of adenylate cyclase and activation of phosphoinositide (PI) hydrolysis. The former is lost in atria from pertussis-toxin-treated chicks, whereas the PI response persists. The pharmacologic characteristics of the PI response resemble those of the depolarization and positive inotropic response. Both are insensitive to blockade by pertussis toxin, require high concentrations of carbachol, and are elicited by acetylcholine but not by oxotremorine. The present study suggests that muscarinic agonist-induced PI turnover may be responsible for the membrane depolarization and positive inotropic effects of carbachol and acetylcholine; that an increase in Na+ conductance underlies these responses; and that it is stimulated either by an increase of intracellular calcium mobilized by inositol triphosphate and/or by activation by protein kinase C.

Characterization of the acetylcholine-induced potassium current in rabbit cardiac Purkinje fibres

Acetylcholine (ACh) induces a K+ current in rabbit cardiac Purkinje fibres. The question was studied whether ACh produces this effect by modifying the properties of K+ channels pre-existing in the absence of the neurotransmitter or whether it induces the formation of a different type of K+ channels. The relaxation properties of the ACh-induced current and its blockade by Cs+ and Ba2+ have been investigated using voltage clamp. During hyperpolarizing or depolarizing voltage pulses of moderate amplitude, the ACh-induced current is time independent. For large voltage pulses, time-dependent changes of the ACh-induced current are observed. These latter changes can be explained by intracellular K+ accumulation/depletion phenomena or by the effects of ACh on time-dependent currents (e.g. the late outward current, ix). Cs+ and Ba2+ block the ACh-induced current. The block produced by 20 mM-Cs+ is instantaneous and increases with hyperpolarization, i.e. it is voltage dependent. The block produced by Ba2+ at high concentrations (greater than 1 mM) is also instantaneous but complete at all potentials studied, and thus voltage independent. At these concentrations, either ion also blocks the background inward rectifier (iK1) current in a similar way. Low [Ba2+] (less than 0.1 mM) cause a block of the ACh-induced current which is instantaneous and little voltage dependent. The block of iK1 in contrast is time and voltage dependent for the same concentrations. These results indicate that the ACh-induced K+ current is different from the background iK1 current.

Changes by acetylcholine of membrane currents in rabbit cardiac Purkinje fibres

The ionic mechanisms underlying the effects of acetylcholine (ACh) on electrophysiological properties of rabbit cardiac Purkinje fibres have been analysed using the two-micro-electrode voltage-clamp technique on short preparations. In normal Tyrode solution, ACh shifted the membrane currents in the outward direction at potentials positive to the K+ equilibrium potential, EK, and in the inward direction at potentials negative to EK. When ACh effects were studied in various [K+]o the reversal potential for the ACh-induced current was changed in a way expected for a pure K+ electrode. The results indicate an increase of an inward-rectifying K+ conductance by ACh. ACh also decreased the magnitude of the late outward current (ix). The activation curve for this current was not modified, but the fully activated current was decreased. The effect of ACh on ix was more pronounced in the presence of catecholamines. The slow inward current (isi) was not changed by ACh in control conditions. However, the increase produced by beta-adrenergic stimulation in this current was suppressed by ACh in a concentration-dependent way.

Effects of acetylcholine on electrophysiological properties of rabbit cardiac Purkinje fibers

The action of acetylcholine (10(-9)-10(-4) M) was investigated in isolated rabbit cardiac Purkinje fibers, using standard microelectrode recording of transmembrane potentials and two-microelectrode voltage clamp technique. In nonstimulated fibers, acetylcholine hyperpolarized the diastolic membrane potential and slowed or suppressed spontaneous activity. The hyperpolarization was more pronounced in low potassium solutions and in depolarized fibers; it was less marked in the presence of cesium (2 X 10(-2) M), and was suppressed by barium (3-5 X 10(-3) M). In stimulated fibers, acetylcholine shortened the action potential duration and shifted the plateau level to more negative values; this effect was influenced little by the stimulation frequency and not by chloride removal from the perfusing solution. In voltage-clamped preparations, acetylcholine shifted the holding current in the outward direction at potentials less negative than EK, while it shifted the current in the inward direction at potentials more negative than EK. The changes induced by acetylcholine were concentration-dependent (apparent KM: 1.5 X 10(-7) M); they were mimicked by carbachol (10(-8)-10(-5) M) and blocked by atropine (10(-8)-10(-7) M). The time course of the effects was biphasic: a maximum was reached in the first minute after addition of acetylcholine; thereafter, the effect decayed to a steady value. On removal of acetylcholine, a transient inversion of the changes produced by acetylcholine was observed, the magnitude of which depended on the acetylcholine concentration used and on the duration of exposure to acetylcholine. This time course was not abolished by pretreatment with physostigmine (10(-6) M), manganese ions (2 X 10(-3) M), or with adrenoceptor blockers [propranolol (2 X 10(-7) M) and/or phentolamine (10(-7)-10(-6) M)]. The results show that rabbit Purkinje fibers are as sensitive to acetylcholine as atrial preparations. The changes produced by acetylcholine are suggestive of an increase in an inward rectifying potassium ion conductance and are mediated by muscarinic receptor stimulation. The secondary decay in the effects of acetylcholine and their inversion on washout can be explained by a desensitization mechanism if it is assumed that the acetylcholine-sensitive channel is already functional in the absence of acetylcholine and is modulated in its conductance and/or open state probability by acetylcholine.

Parasympathetic effects on electrophysiologic properties of cardiac ventricular tissue

The physiologic importance of parasympathetic influence on the sinoatrial and atrioventricular nodes is well established, but the importance of parasympathetic modulation of ventricular function remains controversial. Recognized effects of muscarinic cholinergic stimulation on ventricular automaticity and ventricular repolarization, the ability of muscarinic cholinergic agonists to antagonize catecholamine effects in the ventricle and proposed mechanisms for these effects are described. Anatomic studies have demonstrated a great abundance of cholinergic nerve endings in association with the ventricular conducting system. Stimulation of the vagus nerve or addition of muscarinic cholinergic agonists suppresses ventricular automaticity in most species and antagonizes isoproterenol-induced action potential shortening and isoproterenol-restored slow response action potentials. In vivo, interactions between the parasympathetic and sympathetic nervous systems occur at multiple levels. Muscarinic cholinergic agonists inhibit release of norepinephrine from sympathetic nerve terminals, inhibit catecholamine-stimulated adenylate cyclase activity and alter cyclic guanosine monophosphate (GMP) and possibly cyclic adenosine monophosphate (AMP) levels. Evidence is also presented that, in vivo, parasympathetic effects on ventricular electrical function might influence the pathophysiologic milieu responsible for initiation or termination of certain ventricular arrhythmias. Vagal influences appear to be protective against certain digitalis-induced arrhythmias and protective in certain experimental acute myocardial infarctions. In human beings, there appears to be tonic vagal tone in the ventricle and vagal stimulation terminates certain types of ventricular tachycardia. The evidence presented supports a physiologic role of parasympathetic stimulation in altering ventricular electrical function.

Serotonin increases an anomalously rectifying K+ current in the Aplysia neuron R15

Previous work has shown that serotonin causes an increase in K+ conductance in the identified Aplysia neuron R15. This response is mediated by cAMP-dependent protein phosphorylation. The results presented here show that the K+ channel modulated by serotonin is an anomalous or inward rectifier (designated IR) that is present in R15 together with the three other distinct K+ channels previously described for this cell. Several lines of evidence indicate that this inward rectifier is partially activated in the resting cell and is further activated by serotonin. Voltage clamp analysis of resting and serotonin-evoked membrane currents at various external K+ concentrations shows that both currents have reversal potentials close to the potassium equilibrium potential, exhibit similar dependences in magnitude on external K+ concentration, and display marked anomalous rectification. The effects of particular monovalent and divalent cations are also similar on the resting and serotonin-evoked currents. Rb+, Cs+, and Ba2+ block both currents while Tl+ can substitute for K+ as a charge carrier and channel activator in both. These properties are characteristics of anomalous rectifiers in other systems. Furthermore, measurement of the voltage dependence of inactivation for the fast transient K+ current shows that this current cannot account for the anomalously rectifying K+ conductance in R15. The inward rectifier is therefore a separate current mediated by its own channels, the activity of which can be modulated by serotonin.

The effect of intracellular cyclic nucleotides and calcium on the action potential and acetylcholine response of isolated cardiac cells

Ventricular and atrioventricular nodal cells from guinea pig and rabbit hearts were isolated by perfusing the heart with collagenase (Langendorff perfusion). In these cells the cyclic nucleotides cAMP and cGMP or Ca and EGTA were injected through a microelectrode by pressure (0.5-3 kg/cm2). The effect of injection on both the action potential and the hyperpolarization induced by acetylcholine was studied. The following results were obtained. 1. cAMP prolonged the ventricular action potential and shifted the plateau to more positive potentials. The configuration of the A-V nodal action potential was not detectably changed by cAMP injection, but the spontaneous rate was increased. 2. cGMP first shortened the ventricular action potential. In most experiments this effect was followed by long lasting prolongation of the action potential. 3. Both extracellular and intracellular application of dibutyryl cGMP shortened the ventricular action potential but did not produce a subsequent prolongation. However, prolongation was observed on injection of GMP, the direct metabolite. 4. Injection of cGMP in nodal cells did not hyperpolarize the membrane nor slow the spontaneous rate; rather, an increase in rate was observed. 5. The acetylcholine-induced hyperpolarization was not altered in amplitude or time course by the injection of cAMP, cGMP, Ca or EGTA. 6. The results support the hypothesis that cGMP might be involved in the control of voltage-controlled ionic channels but suggest that it does not play a role as a mediator of the classical muscarinic action i.e. the activation of a specific potassium channel by acetylcholine.

Effects of Cs on acetylcholine induced current. Is ik1 increased by acetylcholine in frog atrium?

1.20 mM Cs+ ions reduce background current and decrease drastically the K+ depletion process, probably as a consequence of the reduction of iK1. The background current-voltage relationship becomes linear. 2. 20 mM Cs+ ions completely abolish the current induced by acetylcholine. 3. The possibility that the K+ current induced by acetylcholine is due to an increase of iK1 is discussed.

Effects of acetylcholine on time-dependent currents in sheep cardiac Purkinje fibers

Voltage clamp experiments were carried out on sheep Purkinje fibers to determine the effect of Ach on the time-dependent currents. On the pacemaker current (iK2) Ach 10(-6) mol . l(-1) had the following effects: shift of the activation curve by a few mV in the depolarizing direction, without change in the rectifier ratio. The potential dependence of the time constants for activation and deactivation was influenced in a similar way as the activation curve. Ach had no effect on the positive dynamic current (iqr) of the late plateau outward current (ix). The slow inward current (isi) as well as the transient inward current (T.I.) were reduced in amplitude and slowed in time course by Ach. The changes in pacemaker current are important in explaining the increased rate of diastolic depolarization in the presence of Ach. The decrease of slow inward current by Ach cannot be made responsible for the plateau shift or the prolongation of the action potential.