Acetylcholine increases resting membrane potassium conductance in atrial but not in ventricular muscle during muscarinic inhibition of Ca++-dependent action potentials in chick heart

Separation methods used in the determination of choline and acetylcholine

Cholinergic neurotransmission has been the subject of intensive investigations in recent years due to increasing recognition of the importance of its roles in physiology, pathology and pharmacology. The fact that the disposition of a neurotransmitter may reflect its functional status has made the measurement of acetylcholine and/or its precursors and metabolites in biological fluids an integral part of cholinergic research. With evolving complexity in experimental approaches and designs, and correspondingly increasing demand on sensitivity, specificity and accuracy matching advancements in sophistication in analytical methods have been made. The present review attempts to survey the array of analytical techniques that have been adopted for the measurement of acetylcholine or its main precursor/metabolite choline ranging from simple bioassays, radioenzymatic assays, gas chromatography (GC) with flame ionization detection, GC with mass spectrometry (GC-MS) detection, high-performance liquid chromatography (HPLC) with electrochemical detection (ED), HPLC with MS (HPLC-MS) to the sophisticated combination of micro-immobilized enzymatic reactor, microbore HPLC and modified electrode technology for the detection of ultra-low levels with particular emphasis on the state of the art techniques.

Accentuated antagonism in canine subendocardium is not altered by chronic exercise

Acetylcholine often affects cardiac action potential repolarization only during augmented adrenergic tone, i.e., the phenomenon of accentuated antagonism. Since chronic exercise involves repeated changes in autonomic outflow, we determined whether it also influenced adrenergic/cholinergic interactions in isolated canine cardiac tissue. Using standard micro-electrode techniques in thin ventricular subendocardial slices isolated from exercised (EX: 8-10 wk daily exercise) and sedentary (SED): 8-10 wk cage rest) dogs, we examined transmembrane potential responses to isoproterenol (ISO: 10(-8), 10(-7), 10(-6) M) and to ISO in the presence of ACH (10(-5) M). Control transmembrane characteristics at BCL = 500 ms were similar for EX (N = 8 dogs) and SED (N = 9 dogs). ISO (10(-6) M) decreased action potential duration at 50% repolarization (APD50): EX = -29 +/- 15 ms; SED = 11 ms and at 90% repolarization (APD90): EX = -37 +/- 17 ms; and SED = -24 +/- 14 ms (P > 0.05, EX vs SED). ACH alone did not alter APD. With ACH (10(-5) M), delta APD50 with ISO (10(-6) M) was -5 +/- ms and 0 +/- 5 ms for EX and SED, respectively; delta APD90 was -8 +/- 4 ms and -8 +/- 7 ms for EX and SED, respectively (P > 0.05, EX vs SED). Thus, ACH antagonized ISO-mediated acceleration of repolarization equally in both groups. Chronic daily exercise does not influence adrenergic/cholinergic interactions at the cellular level.

Characterization of the acetylcholine-sensitive muscarinic K+ channel in isolated feline atrial and ventricular myocytes

M2-cholinergic receptor activation by acetylcholine (ACh) is known to cause a negative inotropic and chronotropic action in atrial tissues. This effect is still controversial in ventricular tissues. The ACh-sensitive muscarinic K+ channel (IK(ACh)) activity was characterized in isolated feline atrial and ventricular myocytes using the patch-clamp technique. Bath application of ACh (1 microM) caused shortening of action potential duration without prior stimulation with catecholamines in atrial and ventricular myocytes. Resting membrane potential was slightly hyperpolarized in both tissues. These effects of ACh were greater in atrium than in ventricle. ACh increased whole-cell membrane current in atrial and ventricular myocytes. The current-voltage (I-V) relationship of the ACh-induced current in ventricle exhibited inward-rectification whose slope conductance was smaller than that in atrium. In single channel recording from cell-attached patches, IK(ACh) activity was observed when ACh was induced in the pipette solution in both tissues. The channel exhibited a slope conductance of 47 +/- 1 pS (mean +/- SD, n = 14) in atrium and 47 +/- 2 pS (n = 10) in ventricle (not different statistically; NS). The open times were distributed according to a single exponential function with mean open lifetime of 2.0 +/- 0.3 msec (n = 14) in atrium and 1.9 +/- 0.3 msec (n = 10) in ventricle (NS); these conductance and kinetic properties were similar between the two tissues. However, the relationship between the concentration of ACh and single channel activity showed a higher sensitivity to ACh in atrium (IC50 = 0.03 microM) than in ventricle (IC50 = 0.15 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Computer simulation of the electrotonic modulation of pacemaker activity in the sinoatrial node by atrial muscle

Electrotonic interaction between the sinoatrial (SA) node and surrounding atrial muscle was investigated in a computer simulation using a modified Oxsoft HEART model (Oxsoft, Oxford, UK). When an SA node cell model was coupled to a passive atrial membrane model (RC circuit) with various coupling conductances (Gc), there was a Gc-dependent prolongation of spontaneous cycle length (SCL). At a sufficiently high value of Gc, the spontaneous activity was finally stopped. A nonlinear relationship between Gc and SCL was obtained, similar to that observed in experiments on rabbit SA node cells. When the muscarinic potassium current (iK,ACh) was activated in the SA node cell model, the coupling-induced inhibition of pacemaker activity was potentiated. Although coupling current and iK,ACh were additive, their effects on SCL were more than additive because of the nonlinear dependence of SCL on net current. A decrease in the input resistance of the atrial membrane model to stimulate the activation of iK,ACh in atrial muscle was also shown to potentiate the coupling-induced inhibition of SA node spontaneous activity.

Two distinct types of inwardly rectifying K+ channels in bull-frog atrial myocytes

1. Single atrial myocytes were enzymatically isolated from the bull-frog as previously described (Hume & Giles, 1981), and patch-clamp techniques were used in an attempt to identify and separate two inwardly rectifying K+ channels in this tissue. 2. Single-channel measurements consistently demonstrated the existence of two different resting K+ channels, which both exhibited strong inward rectification. The unitary conductances of these K+ channels were 34 +/- 4 and 22 +/- 3 pS (mean +/- S.D., at 22-24 degrees C) when measured with 110 mM-K+ in the pipette solution, and their mean open times were 0.87 +/- 0.33 and 129.9 +/- 49.4 ms, respectively. 3. In the absence of acetylcholine (ACh) in the pipette, openings of the larger channels with the shorter open times occurred at a very low frequency. When ACh was present in the patch pipette, the activity of this channel increased significantly, although the single-channel conductance and gating behaviour were very similar either with or without ACh in the pipette. 4. The zero-current voltage (extrapolated from the inward currents through these types of channels) depended on the extracellular K+ concentration. [K+]o, in the fashion expected for a predominantly K(+)-selective channel: it shifted by 58 mV for a tenfold change in [K+]o. Very similar results were obtained from whole-cell voltage-clamp measurements (53 mV for a tenfold change in [K+]o). 5. The conductance of both types of K+ channels depended on [K+]o. The single-channel conductances were 25 +/- 3 and 13 +/- 2 pS with 50 mM [K+]o, and 19 +/- 4 and 9 +/- 2 pS with 20 mM [K+]o, respectively. 6. These results demonstrate that two types of resting inwardly rectifying K+ channels can be identified in single atrial myocytes. One of these is an inwardly rectifying K+ channel (IK1) previously identified in whole-cell voltage-clamp experiments (Hume & Giles, 1983). The second channel is the muscarinic receptor-regulated K+ channel (IK(ACh) which was first described in mammalian nodal and atrial cells. 7. N-Ethylmaleimide (NEM), a reagent which alkylates sulphydryl groups, affects these two types of K+ channels differentially. In the cell-attached patch configuration, bath application of NEM (50 microM) completely abolished the activity of IK(ACh), without affecting the IK1 channel activity. 8. To obtain further evidence that these two currents, IK1 and IK(ACh), were different, the inside-out patch-clamp technique was used.(ABSTRACT TRUNCATED AT 400 WORDS)

A long lasting Ca2+-activated outward current in guinea-pig atrial myocytes

Among other characteristics, the steady-state current-voltage relationship of patch-clamped single atrial myocytes from guinea-pig hearts is defined by an outward current hump in the potential region -15 to +40 mV. This hump was reversibly suppressed by Co2+ (3 mM) or nitrendipine (5 microM) and enhanced by Bay K 8644 (5 microM). The maintained outward current component suppressed by Co2+ extended between -15.2 +/- 1.9 mV and +39.5 +/- 1.7 mV (mean +/- SEM of 14 cells) and has an amplitude of 95.7 +/- 9.4 pA at +10 mV. In isochronal I-V curves, the hump was already visible at 400 ms with essentially the same amplitude as at 1500 ms. The Co2+-sensitive outward current underlying the hump was poorly time-dependent during 1.5 s voltage pulses but slowly relaxed upon repolarization. Tail currents reversed near the K+ equilibrium potential under our experimental conditions. The current hump of the steady-state I-V curve was also abolished by caffeine (10 mM) or ryanodine (3 microM), both drugs that interfere with sarcoplasmic reticulum function. Apamin (1 microM) or quinine (100 microM) but not TEA (5-50 mM) markedly reduced its amplitude. However, at similar concentrations as required to inhibit the hump, both apamin and quinine appeared to be poorly specific for Ca2+-activated K+ currents in heart cells since they also inhibited the L-Type Ca2+ current. It is concluded that a long lasting Ca2+-activated outward current, probably mainly carried by K+ ions but not sensitive to TEA, exists in atrial myocytes which is responsible for the current hump of the background I-V curve.

Pronounced cholinergic but only moderate purinergic effects in isolated atrial and ventricular heart muscle from cats

1. The effects of cholinergic and purinergic stimulation on action potential, force of contraction and 86Rb efflux were investigated in cat atrial and/or ventricular heart muscle. 2. Acetylcholine and carbachol exerted a concentration-dependent negative inotropic effect in cat atrial heart muscle. Carbachol 10 mumol l-1 completely abolished the force of contraction and increased the rate constant of 86Rb efflux 2-3 fold, whereas the action potential duration was shortened to about 1/10 of its length under control conditions. 3. The effects of acetylcholine and carbachol in cat atrial heart muscle were mimicked, qualitatively, by adenosine and its analogues 5'-(N-ethyl)-carboxamido-adenosine (NECA) and (-)-N6-(R-phenyl-isopropyl)-adenosine (R-PIA). Maximal purinergic effects, however, amounted to about 15-50% in comparison to those of cholinergic stimulation. 4. In cat ventricular heart muscle, cholinergic or purinergic stimulation had no significant effects on the force of contraction in the absence of a cyclic AMP-dependent positive inotropic effect. Carbachol antagonized the positive inotropic effect elicited by either 3-isobutyl-1-methylxanthine, isoprenaline or cyclic 8-(4-chlorphenylthio)adenosine-3':5'-monophosphate; NECA and R-PIA were less effective. The inhibition by carbachol of the effects of isoprenaline was not related to a change in the rate constant of 86Rb efflux. 5. It is concluded that the effects of cholinoceptor and purinoceptor agonists in the cat heart involve a change in the potassium conductance in the atrium, whereas the effects in the ventricle may be related to changes of intracellular cyclic AMP levels. It seems reasonable to assume that, in comparison to cholinergic stimulation, a low density of purinoceptors in the cat heart is responsible for the relatively weak effects of adenosine agonists in this species.

Positive inotropic and chronotropic effects of trypsin and some other proteolytic enzymes in the guinea-pig heart

1 In atrial preparations of the young guinea-pig (body weight 150-250 g), five proteolytic enzymes (trypsin, chymotrypsin, bacterial-Al-proteinase (nagarse), bromelain and kallikrein) produced concentration-dependent positive inotropic and chronotropic effects, while they exerted only minimal effects on the papillary muscle preparations. 2 To characterize the effects, further experiments were conducted in atrial preparations using trypsin. There was a strong tendency for tachyphylaxis: a second exposure to the same concentration of trypsin resulted in considerably smaller positive inotropic and chronotropic effects. The positive inotropic and chronotropic effects of this substance were not affected by propranolol (5 X 10(-7)M). However, an accumulation of cyclic AMP was observed and the positive inotropic and chronotropic effects were potentiated by aminophylline (10(-4)M) in association with an augmentation of the accumulation of cyclic AMP. In preparations partially depolarized with high K+ (22mM) medium (contractions ceased under this condition) trypsin 100 micrograms ml-1 reinstated the contraction. Treatment of the preparation with aprotinin (200 u ml-1) resulted in a strong inhibition of the positive inotropic and chronotropic effects. 3 Islet activating protein (IAP), a specific inhibitor of the 'inhibition specific' guanine nucleotide binding regulatory protein of the adenylate cyclase system, did not produce significant inhibition of the positive inotropic and chronotropic effects of trypsin, whereas it produced a complete inhibition of the negative inotropic and chronotropic effects of carbachol. 4. These results suggest that the positive inotropic and chronotropic effects ofproteolytic enzymes are intimately connected with the proteolytic activities through which adenylate cyclase is activated to produce an accumulation of cyclic AMP within the myocardium. The destruction of the 'inhibition specific' guanine nucleotide regulatory protein of the adenylate cyclase was not substantiated as a mechanism of activation of the adenylate cyclase.

Comparison of effects of acetylcholine on calcium and potassium currents in frog atrium and ventricle

1. Ca2+ and K+ currents were measured in single atrial and ventricular myocytes from frog heart with the whole-cell patch-clamp technique. 2. K+ currents were blocked with intra- and extracellular Cs+ and the fast Na+ current was blocked with tetrodotoxin (TTX). The Ca2+ current (ICa) was evoked by a depolarizing pulse from -80 to 0 mV. ICa was larger in ventricular (3.4 +/- 2.5 microA/cm2) than atrial (1.6 +/- 2.5 microA/cm2) myocytes. 3. Acetylcholine (ACh) had no effect on basal ICa when K+ currents were blocked with Cs+ or Ba2+. Isoprenaline increased ICa and ACh reduced the isoprenaline-stimulated current to basal levels. 4. In contrast, when K+ currents were not blocked, ACh reduced the net inward current and increased the outward current at the end of the depolarizing pulse. The outward current was studied in the presence of Cd2+ to block ICa. The steady-state current-voltage relationship inwardly rectified and reversed near the K+ reversal potential (EK). The magnitude of the steady-state ACh-activated K+ current at 0 mV was 1.0 +/- 0.7 microA/cm2 in ventricular cells and 3.67 +/- 1.7 microA/cm2 in atrial cells. 5. With depolarization, the outward current increased instantaneously and then decreased to a new steady level. The first phase of the decay occurred with a time constant similar to that of the activation of ICa. The Cd2+-sensitive current (corresponding to ICa) was obtained by subtracting currents in the presence and absence of Cd2+. The Cd2+-sensitive current was not affected by ACh. 6. The apparent effect of ACh on basal ICa can be explained quantitatively by activation of a time-dependent K+ current by ACh that contaminates ICa.

A quantitative analysis of the acetylcholine-activated potassium current in single cells from frog atrium

The K+ current activated by acetylcholine (ACh) in single cells from the frog atrium was analyzed using the whole-cell patch clamp technique. The ACh current was analyzed quantitatively by subtracting the currents elicited in response to voltage steps in the presence and absence of a steady bath-application of 1 microM ACh. The net ACh currents were voltage- and time-dependent. With depolarizing jumps, the ACh-activated current declined from an instantaneous peak to a new steady level. With hyperpolarizations, the instantaneous current change was followed by a time-dependent increase in current. The current relaxations were well fitted by the sum of two exponentials with time constants of approximately 20 ms and approximately 300 ms at -120 mV. Both time constants decreased with depolarization. The current-voltage relationship inwardly-rectified. This inward rectification was due both to a decrease in the single channel conductance and a decrease in the number of open channels with depolarization. The ACh-activated K+ current differs from the background K+ current in several respects. The kinetics of the background K+ current are much more rapid and the background K+ channel passes much less current in the outward direction than the ACh channel.

Mechanism of action of acetylcholine on calcium current in single cells from frog ventricle

Ca currents (ICa) were measured by whole-cell patch clamp in single cells isolated from frog ventricle in which K currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs. Inward currents elicited by depolarizing voltage steps from a holding potential of -80 mV were blocked completely by 0.5 mM-Cd. The quality of the voltage clamp was assessed using two patch electrodes on a single cell. One electrode was used in the voltage-clamp mode to measure membrane currents and the other in current-clamp to measure membrane potential. Ca currents as large as 2 nA were well clamped in cells as long as 210 micron. Acetylcholine (ACh) had no effect on ICa in the absence of beta-adrenergic stimulation but reduced to control levels ICa elevated by isoprenaline. Nanomolar concentrations of ACh were able to reduce significantly ICa elevated by 2 microM-isoprenaline. ACh had no effect on the shape of the I-V curve, on the reactivation (recovery from inactivation), or on the inactivation of ICa. Although isoprenaline increased ICa by an average of 6.5-fold, it had no effect on the shape of the I-V curve or on the inactivation at test potentials negative to +40 mV. However, isoprenaline slowed the half-reactivation time from a control value of 120 +/- 10 ms (mean +/- S.D.) to 153 +/- 12 ms at -80 mV. The effect of cyclic AMP on ICa was investigated using two patch electrodes, one filled with cyclic AMP. Maximal effects of cyclic AMP were observed with 5 microM-cyclic AMP in the pipette. Maximal ICa was recorded several minutes after breaking the patch with the second electrode. After removing the cyclic-AMP-containing electrode, ICa declined to control levels after approximately 10 min. 5 microM-cyclic AMP in the patch electrode increased ICa by an average of 6.9-fold, but had no effect on the shape of the I-V curve or on inactivation. Cyclic AMP had a slowing effect on reactivation (half-reactivation time = 155 +/- 24 ms) similar to that of isoprenaline. ACh (1-10 microM) did not reduce ICa elevated with cyclic AMP (0.1-20 microM-cyclic AMP in the pipette). With low concentrations of cyclic AMP in the pipette (0.1 microM), isoprenaline augmented ICa, but with 5 microM-cyclic AMP in the pipette, isoprenaline was incapable of increasing ICa further. These results suggest that the decrease of ICa produced by ACh can be explained solely by decreases in cyclic AMP levels.

Ionic basis of the different action potential configurations of single guinea-pig atrial and ventricular myocytes

Single myocardial cells were enzymatically dispersed from guinea-pig atria and ventricles. At 25 degrees C, atrial cell action potentials differed significantly from ventricular cell action potentials in duration (atrial = 141 ms, ventricular = 497 ms) and over-shoot (atrial = +36 mV, ventricular = +42 mV). Action potentials of atrial and ventricular cells responded differently to changes in external K+ concentration ([K+]o). Elevation of [K+]o from 6 to 11 mM depolarized atrial cells but produced no significant change in action potential duration; similar changes in [K+]o depolarized ventricular cells and produced a significant shortening of the action potential duration. Voltage-clamp experiments were performed to investigate the ionic basis underlying the different action potential configurations of single atrial and ventricular myocytes. A single-micropipette voltage-clamp technique was used, employing either extremely small-tip diameter pipettes, without internal cell dialysis (Hume & Giles, 1983), or larger tip diameter pipettes, with internal dialysis (Hamill, Marty, Neher, Sakmann & Sigworth, 1981). Two significant differences in background K+ conductance in single atrial and ventricular myocytes were observed: (i) the isochronal (5 s) current-voltage relationship of single ventricular myocytes exhibited a region of prominent negative slope conductance and elevation of [K+]o produced cross-over; a negative slope conductance region was absent in atrial cells and elevation of [K+]o produced very little cross-over of isochronal current-voltage relationships, and (ii) hyperpolarizing voltage pulses applied from holding potentials of -50 mV elicited inward current in ventricular cells which decayed with time; similar voltage-clamp pulses in atrial cells elicited inward currents which fail to decay. Single K+ channel current measurements confirmed the existence of different resting K+ channel properties in single atrial and ventricular myocytes. Resting K+ channels in both cell types had similar single channel conductances (30-32 pS with [K+]o = 145 mM) but ventricular K+ channels had significantly slower gating kinetics compared to atrial K+ channels (ventricular K+ channel mean open time = 223 ms; atrial K+ channel mean open time = 1 ms at Vr (resting membrane potential) -20 mV). The plateau and duration of the guinea-pig ventricular action potential was insensitive to high concentrations of tetrodotoxin (3 X 10(-5) M) but extremely sensitive to external Ca2+ concentration ([Ca2+]o). The second inward Ca2+ current (iCa) density was estimated in small atrial and ventricular myocytes of similar diameter and length.(ABSTRACT TRUNCATED AT 400 WORDS)

Assay of muscarinic acetylcholine receptor function in cultured cardiac cells by stimulation of 86Rb+ efflux

An assay for the increase in potassium permeability mediated by muscarinic acetylcholine receptors (mAChR) in cultured cardiac cells is described, using the K+ ion substitute 86Rb+ as the tracer ion. Cardiac cells accumulate 86Rb+ from the extracellular medium in a Na+/K+ ATPase-dependent manner. Subsequent efflux of 86Rb+ in the absence and presence of muscarinic agonists follows kinetics similar to those previously reported for 42K+. The mAChR agonist carbamylcholine (carbachol) stimulated 86Rb+ efflux with an EC50 of 50 nM. The half-time for efflux is reduced by greater than 40% at maximally effective concentrations of agonist. Stimulation of 86Rb+ efflux by carbachol is blocked by the mAChR antagonist atropine with an IC50 of 15 nM. The stimulation of 86Rb+ efflux by carbachol is not affected by the presence of the Na+/K+ ATPase inhibitor ouabain. This assay provides a method for quantitating the mAChR-mediated increase in K+ permeability in cardiac cells without the use of 42K+.