Effects of lidocaine and on slow response and depressed fast response action potentials of canine cardiac Purkinje fibers

Disease may decrease resting potential of cardiac fibers, thereby depressing the upstroke velocity of the action potential, causing slow conduction and reentry. A decrease in resting potential may also cause automaticity. We studied the effects of lidocaine (5 and 20 mg/l) on canine Purkinje fibers with reduced membrane potentials with either depressed Na+-dependent upstrokes (depressed fast responses) or with slow inward (Ca++) current-dependent upstrokes (slow responses). Depressed fast responses were produced by elevating [K+]0 in the perfusate, reducing membrane potential to around -60 mV, without abolishing excitability. Slow responses were produced by either perfusing fibers with a Na+-free, Ca++-rich solution, or by perfusing them with a high [K+]0 Tyrode's solution containing norepinephrine. Lidocaine had a marked depressant effect on depressed fast response action potentials. The drug markedly decreased Vmax and conduction velocity. It sometimes decreased action potential amplitude and caused conduction block. Resting potential was not changed. On the other hand, lidocaine had little effect on slow response action potentials. Resting potential, Vmax and action potential amplitude were not altered nor was conduction changed. The rate of spontaneous impulse initiation was slightly reduced by 5 mg/l of lidocaine but not by 20 mg/l. We conclude that lidocaine does not exert its antiarrhythmic effect by directly depressing the slow inward current but may be antiarrhythmic because it depresses an already depressed fast inward current and can cause conduction block.     

Electrophysiologic, inotropic and antiarrhythmic effects of propafenone, 5-hydroxypropafenone and N-depropylpropafenone

We compared the electrophysiologic, inotropic and antiarrhythmic properties of propafenone and two metabolites, 5-hydroxy (5-OH) propafenone and N-depropyl (N-DP) propafenone. In 18 canine Purkinje fibers with normal maximum diastolic potentials, all drugs (1 x 10(-8) to 1 x 10(-5) M) reduced action potential amplitude and duration. However, propafenone and 5-OH propafenone reduced Vmax in a use-dependent fashion at a lower concentration than N-DP propafenone. In 16 Purkinje fibers, slow response action potentials were induced by 22 mM K+ and isoproterenol, 1 x 10(-6) M. Vmax was comparably reduced by all compounds at 1 x 10(-5) M, but action potential amplitude was not affected by 5-OH propafenone. In 16 other Purkinje fibers in which automaticity at low levels of membrane potential was induced by BaCl2 (0.25 mM), only 5-OH propafenone was effective in slowing the automatic rate at therapeutic concentrations (3 micrograms/ml). In 15 guinea pig papillary muscles, all three drugs had negative inotropic effects at concentrations greater than or equal to 1 x 10(-6) M. In conscious dogs with sustained ventricular tachycardia 24 hr after infarction, we injected propafenone or a metabolite through an atrial cannula. At similar plasma levels, neither propafenone (n = 6) nor N-DP propafenone (n = 6) suppressed the arrhythmia, whereas 5-OH propafenone eliminated ventricular tachycardia in four of six dogs, and was more effective against monomorphic than polymorphic ventricular tachycardia. Hence, the two major metabolites of propafenone have important electrophysiologic effects, and 5-OH propafenone is more potent than the parent compound as a antiarrhythmic drug in the 24-hr Harris dog.     

Effects of dimethylpropranolol (UM-272) on the electrophysiological properties of guinea-pig ventricular muscles

The effects of dimethylpropranolol (UM-272) on transmembrane action potentials were examined in isolated right ventricular papillary muscles of the guinea pig. UM-272 (10(-5) to 3 X 10(-4) M) caused a dose-dependent decrease in the Vmax of the action potential. At 3 X 10(-4) M, a slight decrease in the amplitude of action potential was also observed. The resting potential and the action potential duration were not affected by the drug. In the presence of UM-272, trains of stimuli at rates higher than 0.1 Hz led to an exponential decline in Vmax (onset rate, 0.13-0.28 per action potential) to a new plateau level. This use-dependent block was augmented at the higher stimulation frequency. The time constant for the recovery of Vmax from the use-dependent block (offset) was 7.1 to 7.3 sec. In depolarized papillary muscles with 8 or 10 Mm [K+]0, the inhibitory action of UM-272 on Vmax of the first action potential after a long quiescent period (tonic block) was augmented markedly, but the rates of onset and offset of the use-dependent block were similar to those in normally polarized preparations under 5 mM [K+]0. The curves relating membrane potential and Vmax in preparations stimulated infrequently were shifted by 7.2 mV with UM-272 at 10(-4) M in the direction of more negative potentials. These findings suggest that UM-272 has kinetically similar use-dependent inhibitory action of the fast sodium channels of cardiac muscles as other Class Ia antiarrhythmic drugs like quinidine or procainamide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Time course of the electrophysiological effects of quinidine on canine cardiac Purkinje fibers: concentration dependence and comparison with lidocaine and disopyramide

We used standard microelectrode techniques to observe the time course of the appearance and disappearance of the cellular electrophysiologic effects of antiarrhythmic drugs during drug infusion and after washout. The slopes of phases 0 (Vmax), 2(V2) and 3(V3) of the action potential of canine Purkinje fibers were followed during 30 min of infusion of quinidine (0.2 - 1 x 10(-5) M), disopyramide (1 x 10(-5) M) or lidocaine (1 x 10(-5) M) and then during 60 min of washout with drug-free Tyrode's solution. All three drugs significantly reduced V3 and increased V2; quinidine and disopyramide also significantly reduced Vmax. The onset of the effects of quinidine and disopyramide on Vmax, V2 and V3 occurred at similar rates. Both the onset and disappearance of the effects of lidocaine were more rapid than those of quinidine and disopyramide. This may have been related to the greater lipid solubility of lidocaine with a heptane: water partition coefficient of 0.85 for lidocaine compared with 0.16 for disopyramide and 0.06 for quinidine. The effects of quinidine (1 x 10(-5) M) on V3 reversed much more slowly upon washout (T1/257 +/- 12 min, mean +/- S.E.) than the effects of quinidine on Vmax (T1/218 +/- 3 min, P less than .01) and V2 (T1/215 +/- 3 min, P less than .01). Concentration-response data showed that the time course of washout of the effects of quinidine was independent of drug concentration. These data suggest that rapidity of antiarrhythmic drug action is related to lipophilicity and that the effect of quinidine on V3 is due to action at a different cellular site from its effects on Vmax and V2.     

Effect of lidocaine hydrochloride on membrane conductance in mammalian cardiac purkinje fibers

Lidocaine depresses automaticity in cardiac Purkinje fibers by decreasing the slope of slow diastolic depolarization, but the mechanisms of this effect are poorly understood. To test the proposal that the antiautomatic effect of lidocaine might be mediated by an increase in membrane potassium conductance, transmembrane voltage (V(m)) was measured in Purkinje fibers perfused with sodium-deficient Tyrode containing choline as the major cation. V(m) was varied by altering the external potassium concentration, [K](o), from 0.5 to 150 mM before and after lidocaine, 2.14 x 10(-5) M, a concentration considered equivalent to clinical plasma antiarrhythmic levels. In Purkinje fibers, resting V(m) varies linearly with [K](o) plotted on a logarithmic scale from 4 to 150 mM, approximately as predicted by the Nernst equation. At [K](o) of 0.5-2.7 mM, resting V(m) diverges from the predicted potassium equilibrium potential (V(K)) resulting in an increased driving force for the outward K(+) current (V(m) - V(K)). In choline Tyrode at [K](o) of 2.7 mM or less, lidocaine caused a significant increase in V(m), the change being a positive linear function of (V(m) - V(K)) with a P < 0.01. This effect was more striking in Purkinje fibers with a V(m) reduced by stretch. These findings imply that lidocaine increased membrane chord conductance for the potassium ion (gK).Current-voltage relationships using intracellular current pulses were performed in choline Tyrode at [K](o) of 0.5, 2.0, and 4.0 mM and, at each [K](o), lidocaine was found to increase membrane slope conductance (GK). The increase in GK was even more apparent when the current-voltage relationships in long Purkinje fibers was corrected for cable complications or when experiments were done in short Purkinje fibers. To minimize complications due to membrane rectifier properties, GK was measured using intracellular application of small hyperpolarizing current pulses as V(m) was decreased from -90 to -60 mv by increasing the [K](o) from 3 to 15 mM before and after lidocaine. Lidocaine increased the GK over this range of V(m).These results suggest that lidocaine increases membrane potassium conductance within the range of V(m) where the pacemaker potential is seen, an action which can account for its ability to suppress automaticity, and, in part, for its ability to prevent reentrant arrhythmias.     

Bupivacaine and lidocaine blockade of calcium-mediated slow action potentials in guinea pig ventricular muscle

The effects of bupivacaine and lidocaine on the slow-rising action potentials, induced by superfusion with high-K+ (26 mM) Tyrode's solution with 10(-6) M isoproterenol, were studied in guinea pig ventricular muscle. The concentrations of bupivacaine or lidocaine necessary to result in a 50% depression of the maximum upstroke velocity were about 10(-5) and 10(-4) M, respectively. The concentrations necessary to completely block the slow action potentials were about 10(-4) M for bupivacaine and 5 X 10(-4) M for lidocaine. The relative potencies of lidocaine to bupivacaine in abolishment of the slow action potentials are in the ratio of 1:5. These results suggest that local anesthetics do affect the slow Ca++ channel of ventricular muscle.     

Frequency-dependent actions of phenytoin in adult and young canine Purkinje fibers

Phenytoin has been reported to be particularly effective in the treatment of postoperative ventricular arrhythmias in children. The authors used standard microelectrode techniques to examine the developmental changes in the action of phenytoin on the transmembrane action potential of neonatal and adult canine Purkinje fibers. Their goals were to test whether developmental differences in phenytoin action on the action potential might explain the clinical observations and to evaluate the contribution of use-dependent reduction of Vmax and effects on slow responses to the antiarrhythmic action of phenytoin. In Tyrode's solution with [K+]0 = 4 mM, phenytoin at 5 and 10 micrograms/ml (concentrations comparable to therapeutic plasma levels) had no major effects on action potential characteristics or use dependence at either age. At [K+]0 = 6 mM, on decreasing the drive cycle length from 1300 to 300 msec, phenytoin reduced Vmax significantly and in a concentration-dependent manner. The magnitude of this action was similar at both ages. Conduction times were also significantly prolonged. The time constants for onset of (tau o) and recovery from (tau r) use-dependent block were similar in neonates and adults. The effects of phenytoin on slow responses were significant, although modest, at both ages, but there was no significant effect on conduction. This study indicates that in K+-depolarized Purkinje fibers, use-dependent reduction of the fast Na+ current is a major determinant of the antiarrhythmic action of phenytoin. In contrast to lidocaine and quinidine, no age-related changes in phenytoin action were found, underscoring the different developmental effects of individual antiarrhythmic drugs.     

Comparative in vivo and in vitro study of the cardiac effects of midalcipran and imipramine

Midalcipran is a new antidepressant drug inhibiting both noradrenaline and serotonin uptake without any postsynaptic and anticholinergic activities. Its cardiac effects were compared with those of imipramine, a tricyclic antidepressant drug. In anaesthetised guinea-pigs intravenous perfusion of imipramine and midalcipran (1 ml/min from a solution at 0.66 mg/ml) brought about ventricular arrhythmias, respectively at 16.5 and 26.4 mg/kg and cardiac arrest at 58 and 97 mg/kg. The safety index (ratio of i.v. lethal dose and ED50 evaluated by the yohimbine test) is 22 times wider for midalcipran than imipramine. In in vitro studies on guinea-pig ventricular myocardium, imipramine exerted a greater class 1 antiarrhythmic effect than midalcipran. The reduction of Vmax was significant at 3 X 10(-6) M for imipramine and 1 X 10(-5) M for midalcipran in normal (4 mM K+) and hyperpolarizing (2.7 mM K+) conditions. At the concentration of 1 X 10(-5) M midalcipran significantly lengthened, whereas imipramine non significantly shortened the action potential durations (APD50, APD90). The results provide confirmation of a lesser depression in sodium conductance with midalcipran as compared to imipramine. Therefore it is proposed that less adverse cardiac effects may be observed at therapeutic doses.     

Novel cardioprotective effects of TYB-3823 on ischemic damage in the working hearts of rats: comparison with lidocaine

The cardioprotective effects of a new antiarrhythmic drug, TYB-3823 [1-(2,6-dimethylphenyl)-dimethylaminoguanidine hydrochloride] were examined in the working hearts of rats and compared with those of lidocaine. Before ischemia, TYB-3823 at 5 x 10(-5) M produced a slight negative inotropic effect, resulting in a decrease in aortic flow and cardiac output. However, at lower concentrations (10(-6) and 10(-5) M), the drug had no significant effect on the functional cardiac parameters before ischemia. Lidocaine at such concentrations also had no effect. Global ischemia for 15 min decreased cardiac function rapidly which only recovered partially, with a delay, after reperfusion in the control hearts. Treatment with TYB-3823 accelerated the time course of recovery during reperfusion markedly, significantly improving functional cardiac parameters. However, lidocaine had little effect on recovery of function. Reperfusion-induced arrhythmia was equipotently inhibited by TYB-3823 and lidocaine. Leakage of cytosolic enzymes (lactate dehydrogenase, creatine phosphokinase and alpha-hydroxybutylic dehydrogenase) during reperfusion was inhibited more effectively by TYB-3823 than lidocaine. Light microscopic and electron microscopic examinations revealed that treatment with TYB-3823 protected against the histological damage induced by ischemia, such as hyaline degeneration of myocardium, absence of cross-striation and swelling of mitochondria. These results suggest that, unlike lidocaine, TYB-3823 causes a novel cardioprotective effect through unknown mechanisms in addition to its antiarrhythmic action.     

Electrophysiologic effects of ketanserin on canine Purkinje fibers, ventricular myocardium and the intact heart

We studied the actions of ketanserin (KT) on transmembrane action potentials (AP) of canine Purkinje fibers (PF) and ventricular muscle (VM) and on rhythm in vivo. PF AP duration (APD) was increased by KT (10(-8) to 10(-6) M) and shortened at 10(-5) M. KT effect on APD was greater during stimulation at longer cycle lengths and KT induced early afterdepolarizations in two of six PF at [K+]0 = 2.7 mM. Maximum diastolic potential, AP amplitude and Vmax were not changed by KT. In VM, KT (10(-8) to 10(-6) M) prolonged APD; but 10(-5) M KT did not shorten APD, reducing the difference in APD between VM and PF. KT had no effect on slow response Vmax or amplitude but prolonged APD. To analyze whether changes in Na plateau current or transient outward current contributed to KT effects on APD, we used tetrodotoxin (TTX) and 4-aminopyridine. TTX shortened APD and in its presence, KT (10(-5) M) induced no further shortening. In contrast, the effect of KT persisted in the presence of 4-aminopyridine. In six anesthetized, open chest dogs, KT prolonged the QT interval, but did not modify QRS duration and epicardial conduction time or induce arrhythmias. KT facilitated the onset of torsades de pointes during epicardial aconitine application.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduction of Vmax by QX-314 and benzocaine in neonatal and adult canine cardiac Purkinje fibers

The authors have previously shown that the use-dependent action of lidocaine on the Vmax of canine Purkinje fibers and on intraventricular conduction in the in situ heart undergoes significant developmental changes. In this study, they use standard microelectrode techniques to test whether these age-related differences are due to the charged, more hydrophilic form or to the uncharged, more lipophilic form of a local anesthetic. QX-314, a permanently charged lidocaine derivative, depressed Vmax to a significantly greater extent in adult than in neonatal Purkinje fibers. This difference was due to its use-dependent blocking action and not to its tonic blocking action. The kinetic time constant (tau on) for the development of use dependence was shorter in adults (90 +/- 9 vs. 134 +/- 15 beats; P less than .05), whereas the time constant for recovery from use dependence (tau off) was shorter in neonates (53 +/- 4 vs. 106 +/- 10 sec; P less than .05). QX-314 (3 X 10(-5) M) shifted the curve of Vmax vs. activation voltage in a hyperpolarizing direction by 16.2 +/- 2.4 mV in adults and 5.1 +/- 1.1 mV in neonates (P less than .05). In contrast, the uncharged tertiary amine benzocaine (1 X 10(-5)-5 X 10(-4) M) showed no developmental differences in its effects on Vmax. Adult and neonatal fibers showed comparable tonic block and no use-dependent block. These results extend those of the authors' previous studies and suggest that developmental differences in the action of local anesthetics depend primarily on the use-dependent action of the charged molecular form.     

Nicardipine actions on smooth muscle cells and neuromuscular transmission in the guinea-pig basilar artery

The effects of nicardipine on smooth muscle cells of the guinea-pig basilar artery were investigated by means of microelectrode and isometric tension recording methods. Nicardipine (1 X 10(-8) to 3 X 10(-6) M) did not modify the membrane potential and resistance of smooth muscle cells. The spike evoked by application of outward current pulse in the presence of tetraethylammonium (greater than 1 X 10(-3) M) was inhibited by 1 X 10(-9) M and was almost blocked by 3 X 10(-7) M nicardipine. Perivascular nerve stimulation evoked the excitatory junction potential which was slightly suppressed by 3 X 10(-6) M nicardipine. The contractions evoked by excess concentration of [K]0, NaCl-free solution or ATP was abolished and by 5-hydroxytryptamine was markedly inhibited in Ca-free ethylene glycol bis(beta-aminoethyl ether)N,N'-tetracetic acid-containing solution, but that induced by caffeine was only slightly inhibited. Nicardipine (greater than 3 X 10(-10) M) markedly inhibited the K-induced contraction noncompetitively as estimated from the Lineweaver-Burk's plot. The ATP-induced contractions were slightly inhibited by nicardipine (greater than 1 X 10(-8) M) to a lesser extent than the K-induced contraction. On the other hand, nicardipine (1 X 10(-6) M) had no effect on the NaCl-free-or 5-hydroxytryptamine-induced contraction. When nicardipine (1 X 10(-6) M) was applied during 2.5 mM Ca loading to muscle cells after depletion of the stored Ca, the subsequently generated caffeine-induced contraction was slightly inhibited due to inhibition of the passive Ca influx. These results indicate that nicardipine possesses a selective inhibitory action for the Ca channel, but the inhibition is limited to particular Ca influxes such as the voltage-dependent one, but not the receptor operated and NaCl-free-induced Ca influxes. This agent acts predominantly on the postjunctional muscle rather than the prejunctional nerve terminal.     

Electrophysiologic actions of clofilium and lidocaine in ischemically injured canine epicardium.

The electrophysiologic actions of the Class III antiarrhythmic drug, clofilium, and the Class IB antiarrhythmic drug, lidocaine, were examined in ischemically injured canine epicardium, 4 days after coronary artery occlusion. Experiments were performed utilizing 1) composite electrode recordings from the intact heart in the anesthetized dog and 2) intracellular and extracellular recordings from superfused canine epicardium. In intact hearts, both clofilium (2 mg/kg i.v.) and lidocaine (6 mg/kg i.v.) increased refractoriness (188 +/- 16 to 331 +/- 39 and 288 +/- 18 msec, respectively, P less than .01), and produced tachycardia-dependent conduction disorders in ischemically injured epicardium. For both drugs, slowing the sinus heart rate with vagus nerve stimulation (32 +/- 6/min) returned activation delays to predrug values. Unlike lidocaine, clofilium failed to increase maximal activation delays in ischemically injured epicardium preceding conduction block (116 +/- 14 msec vs. 71 +/- 7 msec and 147 +/- 16 msec for clofilium and lidocaine, respectively, P less than .01 for both drugs). In superfused epicardium, both clofilium (3 x 10(-7) M) and lidocaine (4 mg/l) prolonged refractoriness in ischemically injured epicardium (175 +/- 16 predrug vs. 273 +/- 33 msec, P less than .01) and (181 +/- 3 predrug vs. 216 +/- 10 msec, P less than .01), respectively, whereas only lidocaine reduced Vmax and prolonged local conduction times in the same tissue. The results demonstrate that 1) lidocaine increases refractoriness in ischemically injured tissue via a decrease in Vmax and conduction velocity and 2) clofilium increases refractoriness in ischemically injured tissue without altering action potential duration, Vmax or conduction velocity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological effects of ACC-9358, a novel class I antiarrhythmic agent, on isolated canine Purkinje fibers and ventricular muscle

Class I antiarrhythmic agents are heterogeneous with respect to their cardiac electrophysiological effects and have been subdivided into three categories: la, lb and lc. The purpose of the present study was to determine the classification and investigate the mechanism of action of ACC-9358 [4-hydroxy-N-phenyl-3,5-bis (1-pyrrolidinyl-methyl)benzamide], a novel class I antiarrhythmic agent currently under clinical investigation. The effects of ACC-9358 on action potentials from isolated canine Purkinje fibers and ventricular muscle were examined using standard microelectrode techniques. In Purkinje fibers, ACC-9358 (1-50 microM) exerted a dose-dependent reduction in maximum upstroke velocity (Vmax) and action potential duration at 50 and 90% repolarization (APD50 and APD90). The reduction of Vmax was voltage-dependent (greater at an extracellular potassium concentration of 6 mM than at 2.7 mM), frequency-dependent (greater at a basic cycle length of 500 than at 2000 msec) and very slow in onset (rate constant of 0.017 action potentials-1) and offset (recovery half-time of 66.9 sec). In Purkinje fibers, ACC-9358 attenuated the action potential shortening effects of lidocaine but not that of nicardipine or nicorandil and shortened APD50 to a greater extent at a basic cycle length of 2000 than at 500 msec. In ventricular muscle, ACC-9358 (1-50 microM) exerted a dose-dependent reduction in Vmax and prolongation of APD50 and APD90.(ABSTRACT TRUNCATED AT 250 WORDS)     

Depolarizing agents induce oscillations in canine bronchial smooth muscle membrane potential: possible mechanisms.

Tetraethylammonium (TEA) 925 mM), 4-aminopyridine (4-AP) (5 mM) and carbachol (3 X 10(-7) M) elicited membrane depolarization (approximately 20 mV) and oscillation (0.5-1.0 Hz; up to 25 mV in amplitude) in canine bronchi (3rd to 5th order). BaCl2 (1 mM) also elicited large depolarizations but not oscillations. The oscillations were antagonized by nitrendipine (NT) (10(-8) M) or atropine (10(-6) M) and were unaffected by phentolamine, propranolol or apamin (all 10(-7) M). TEA- or 4-AP-induced membrane depolarizations were partially reversed by atropine. After replacement of extracellular Ca++ with Sr++, oscillations elicited by carbachol or TEA were considerably slower (although amplitudes were similar to those seen in Ca++). Excitatory junction potentials were not altered by NT, were inhibited by 4-AP or replacement of extracellular Ca++ with Sr++, were potentiated by Ba++ and were variably affected by TEA. Contractile responses to acetylcholine were supported by Ca++ or Sr++ (with reduced efficacy), and only the latter were sensitive to NT. Our data suggest that, in canine bronchi (3rd to 5th order), 1) there are K+ channels and Ca++ channels (not L-type) on the cholinergic nerve endings, which modulate neurotransmitter release, 2) there are voltage-dependent K+ channels on the smooth muscle, which regulate membrane potential and suppress excitatory activity, and 3) membrane depolarization leads to alternating opening and closing of voltage-dependent (L-type) Ca++ channels on the smooth muscle, producing oscillations in membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of papaverine on spontaneous activity of isolated right atria from small mammals

The effects of papaverine and verapamil were studied in spontaneously beating right atria of guinea pigs, rabbits and rats and on segments of sinoatrial node tissue from rabbits. Papaverine (10(-4)M) produced a significant negative chronotropic effect in guinea pigs and rats, with a lesser effect in rabbits. Papaverine antagonized the positive chronotropic response to transmural nerve stimulation (TNS) of the sinoatrial node of rabbit atria in a concentration-dependent manner. The chronotropic response to 5 X 10(-7)M norepinephrine was enhanced in the presence of 10(-6)M papaverine, but was noncompetitively antagonized at concentrations above 10(-5)M. The negative chronotropic response to TNS was unaltered by papaverine except at concentrations at or above 5 X 10(-5)M; however, the negative chronotropic response to 5 X 10(-6)M methacholine was not altered by 10(-4)M papaverine. A local anesthetic (lidocaine) inhibited both the positive and negative chronotropic TNS response at a concentration that had no effect on the response to exogenous norepinephrine. Verapamil (10(-7) to 10(-6)M) had no significant effect on either the positive or negative chronotropic response to TNS. The effects of papaverine on TNS are attributed to the influence of at least three actions: 1) phosphodiesterase inhibition, 2) local anesthetic activity and 3) physiological antagonism of norepinephrine. The effects of 2 X 10(-4)M papaverine on action potentials of rabbit sinoatrial node cells were studied. Papaverine decreased the maximum diastolic potential, increased the action potential overshoot and transformed the normal ramp configuration of slow diastolic depolarization into a concave shape.     

Calcium and calmodulin antagonists binding to calmodulin and relaxation of coronary segments

Ca++ antagonist drugs (also known as Ca++ channel blockers) and the calmodulin antagonists trifluoperazine (TFP) and W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide] were capable of half-maximally relaxing porcine coronary segments at 1.5 X 10(-10) M for felodipine, 6.5 X 10(-8) M for verapamil, 2.6 X 10(-7) M for diltiazem, 7 X 10(-7) M for prenylamine, 7 X 10(-6) M for TFP and 45 X 10(-6) M for W-7. Their correspondent binding to calmodulin was half-maximal at 2.8 X 10(-6), 30 X 10(-6), 80 X 10(-6), 5 X 10(-7), 5.0 X 10(-6) and 11 X 10(-6) M, respectively. Only prenylamine, TFP and W-7 were capable of relaxing coronaries over the same concentration range in which they bind to calmodulin. The relaxations produced by these calmodulin antagonists and prenylamine could not be overcome by contractile agonists which release Ca++ from internal stores (histamine and serotonin), whereas the relaxations produced by felodipine, verapamil and diltiazem were readily reversed by either of these agonists. This is consistent with TFP and W-7 and to some degree prenylamine-inducing vasodilation by calmodulin antagonism and with felodipine, verapamil and diltiazem vasodilating through Ca++ antagonism of Ca++ channels at the level of the cell membrane.     

Differential Electrophysiologic Effects of Chronically Administered Amiodarone on Canine Purkinje Fibers versus Ventricular Muscle

BACKGROUND: Acute and chronic treatment with amiodarone has been reported to cause different electrocardiographic changes in patients. The cellular electrophysiologic effects of chronic administration (50 mg/kg/day orally for 6 weeks) and acute superfusion (5 μM in the tissue bath) of amiodarone were therefore studied in dog cardiac ventricular muscle and Purkinje fibers using conventional microelectrode techniques. METHODS AND RESULTS: During stimulation at 1 Hz, chronic amiodarone treatment lengthened the ventricular muscle action potential duration (APD) from 227.8 +/- 6.3 ms (n = 20) to 262.3 +/- 5.2 ms (n = 21; P <.01), but shortened that of Purkinje fibers from 337.6 +/- 9.2 (n = 21) to 308.3 +/- 7.1 (n = 19; P <.05). Acute superfusion of 5 μM amiodarone in cardiac tissue obtained from chronically treated dogs did not change ventricular muscle APD but shortened Purkinje fiber AP from 309.7 +/- 13.6 ms to 281.9 +/- 11.9 ms (n = 8; P <.05). Neither the chronic nor the acute amiodarone exposure prevented the APD shortening in ventricular muscle evoked by 10 μM pinacidil, suggesting that amiodarone does not interfere with the ATP-dependent potassium channels. The normal difference in APD between ventricular muscle and Purkinje fibers in untreated, control preparations was 110 ms but decreased to 46 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in the presence of 30 μM sotalol, a class III antiarrhythmic drug used for comparison. Amiodarone (5 μM) applied directly abolished early afterdepolarizations (EADs) (induced by 1 μM dofetilide + 20 μM BaCl(2) + 2 mM CsCl) in 5 of 6 experiments and caused strong use-dependent V(max) block with relatively fast onset kinetics (rate constant = 1.23 +/- 0.13 AP(-1), n = 5) and offset (time constant = 364 +/- 62.5 ms, n = 5). After chronic amiodarone treatment, in contrast with acute sotalol application (30 μM), no reverse use-dependent effect was observed on the APD in Purkinje fibers. CONCLUSIONS: These results provide further evidence that amiodarone differs from other recognized class III antiarrhythmic drugs (ie, it is a mixed type agent with acute fast kinetic class I [type B] and a unique class III antiarrhythmic action characterized by decreased dispersion of APDs between ventricular muscle and Purkinje fibers). Amiodarone can abolish EADs unlike other class III agents that are usually associated to induction of EADs. These features might be responsible not only for the antiarrhythmic efficacy, but also for the relative safety (low incidence of torsade de pointes) of amiodarone in clinical settings.     

A comparative study of alkaline phosphatase in calcifying cartilage, odontoblasts and the enamel organ.

The enzyme alkaline phosphatase (AP) (EC 3.1.3.1) in three different calcification areas was studied by means of a spectrophotometric micro method using p-nitrophenylphosphate as a substrate. Rat maxillary incisor odontoblasts and enamel organ from the zones of matrix formation and maturation and tissue from rabbit metatarsal cartilage were allowed to react with the substrate in glycine-NaOH buffer at room temperature. The reaction was found to be linear for a minimum of 20 min. The pH optima for AP from these tissues were in the pH range of 10.0-10.3. In order to compare AP from the four calcification areas different parameters were studied. Heating at 56 degrees C or 60 degrees C for varying times revealed that the enzymes were almost completely inactivated after 10 min. Mg2+ ions activated the enzymes by about 25% at concentrations of 2.5 mM (enamel organ 1.25 mM); while only higher concentrations of Mg2+ had an inactivating effect, Ca2+ and PO3-4 ions were inactivating at varying concentrations. F- ions showed no effect on AP activity at concentrations below 250 mM (enamel organ 125 mM) but caused inactivation of the enzymes at about 50% at 1 M. EDTA was found to be a very effective AP inactivator at concentrations above 0.06 mM, whereas urea did not noticeably affect the enzyme reactions at concentrations below 1 M. At higher concentrations, inactivation was observed. In order to determine AP localization in the epiphyseal plate successive 40-mum-thick, freeze-sectioned slices were analyzed. The activity was highest nearest the zone of cartilage calcification and decreased towards the reserve cell zone. It was concluded that the same AP isoenzyme is present in these quite different calcification loci.     

Depression by isoflurane of the action potential and underlying voltage-gated ion currents in isolated rat neurohypophysial nerve terminals

We characterized the effects of the volatile anesthetic isoflurane on the ion currents that contribute to the action potential (AP) in isolated rat neurohypophysial (NHP) nerve terminals using patch-clamp electrophysiology. Mean resting membrane potential and AP amplitude were -62.3 +/- 4.1 and 69.2 +/- 2.9 mV, respectively, in NHP terminals. Two components of outward K(+) current (I(K)) were identified in voltage-clamp recordings: a transient I(K) and a sustained I(K) with minimal inactivation. Some terminals displayed a slowly activating I(K), probably the big Ca(2+)-activated K(+) current (BK). Isoflurane reversibly inhibited AP amplitude and increased AP half-width in normal extracellular Ca(2+) (2.2 mM). In high extracellular Ca(2+) (10 mM), isoflurane also reduced the afterhypolarization peak amplitude. A transient tetrodotoxin-sensitive Na(+) current (I(Na)) was the principal current mediating the depolarizing phase of the AP. A slowly inactivating Cd(2+)-sensitive current (probably a voltagegated Ca(2+) current; I(Ca)) followed the initial I(Na). Isoflurane reversibly inhibited both I(Na) and I(Ca) elicited by a voltage-stimulus based on an averaged AP waveform. The isoflurane IC(50) for AP waveform-evoked I(Na) was 0.36 mM. Isoflurane (0.84 +/- 0.04 mM) inhibited AP waveform-evoked I(Ca) by 37.5 +/- 0.16% (p < 0.05). The isoflurane IC(50) for peak I(K) was 0.83 mM and for sustained I(K) was 0.73 mM, with no effect on the voltage dependence of activation. The results indicate that multiple voltage-gated ion channels (Na(+) > K(+) > Ca(2+)) in NHP terminals, although not typical central nervous system terminals, are inhibited by the volatile general anesthetic isoflurane. The net inhibitory effects of volatile anesthetics on nerve terminal action potentials and excitability result from integrated actions on multiple voltage-gated currents.