Stephen Vogel

Halothane depression of myocardial slow action potentials

Effects of halothane on myocardial electrophysiologic and contractile properties were studied by simultaneous measurement of action potentials (APs) and contractions in guinea pig papillary muscle. Muscles were stimulated by field electrodes and normal responses measured before, during, and after recovery from halothane application. Halothane was administered in 0.5 per cent to 4 per cent concentrations in 5 per cent CO2–95 per cent O2 bubbled through standard Tyrode perfusing solutions. Slow action potentials were then induced with 10−7 isoproterenol in partially depolarized muscles (typically −40 mV in 26 mM K+ media). AP characteristics and accompanying contractions were again measured before, during, and after halothane application. The maximum rate of rise (+&OV0312;max) of the normal (fast) AP was not depressed in any concentration of halothane, although amplitude and duration were decreased in 3 per cent halothane. In contrast, halothane depressed +&OV0312;max of the slow AP to 61 per cent, 28 per cent, and 14 per cent of control, in concentrations of 1, 2 and 3%, respectively. Decreased duration and decreased amplitude (85% of control of the slow AP), or loss of excitability (4 of 7 muscles) occurred in 3 per cent halothane. Initially, halothane application caused a 5 per cent enhancement of tension with both fast and slow APs. In 0.5 per cent halothane, contractions subsequently declined to steady-state levels of 66 per cent (fast AP) and 76 per cent (slow AP) of control. Contractions were depressed linearly with log dose to 18 per cent (fast AP) and 5 per cent (slow AP) of control in 3 per cent halothane. Halothane concentrations of 1 per cent and greater inhibit slow (Na+ – Ca++) channels which mediate the slow action potentials. The negative inotropic effect of halothane may be due in part to decreased Ca++ influx through the slow channel. The negative inotropic effect of 0.5 per cent halothane, in which the slow AP is unaffected, suggests that additional mechanisms, not involving the slow channel, also participate in the negative inotropic action of halothane.

Adriamycin cardiotoxicity: Possible pathogenic mechanisms

Abstract The antitumor agent, adriamycin, causes in humans a cardiomyopathy associated with elevated tissue Ca 2+ . Hence, adriamycin was tested for an ability to affect the Ca 2+ influx mediated by the slow channels in ventricular cells of isolated perfused chick hearts. The fast Na + channels were blocked by tetrodotoxin or voltage inactivated by elevated (25 m m ) K + , thus rendering the hearts inexcitable. Low concentrations of adriamycin (0.01 to 0.05 mg/ml) restored excitability in the form of slow action potentials (APs), and enhanced the maximum upstroke velocity ( + V max ) of slow APs induced by isoproterenol (10 −6 m ). Higher concentrations (0.1 to 0.5 mg/ml) did not induce slow APs, and actually depressed or blocked the isoproterenol-induced slow APs. On the contractions recorded from hearts perfused with normal Tyrode solution, adriamycin also had a dual effect: at low concentrations, it had a positive inotropic action, whereas at higher concentrations, it had a negative inotropic action. Adriamycin (0.05 mg/ml) caused the cyclic AMP level to increase by about 50% over the control within 15 min, thus suggesting that this might be responsible for its positive inotropism. Higher concentrations (0.3 mg/ml) also raised the cyclic AMP, but the ATP level was depressed. In isolated mitochondria, adriamycin (0.5 mg/ml) depressed ADP-stimulated respiration, suggesting that impaired mitochondrial function could cause the depressed ATP levels. The results indicate that low concentrations of adriamycin augment the slow current, possibly by an increase in cyclic AMP level, whereas high concentration (0.5 mg/ml) depresses the slow current, perhaps due to lowered ATP levels. The enhanced Ca 2+ influx (via stimulation of the slow channels) could be a factor in the Ca 2+ overload associated with the adriamycin-induced cardiomyopathy.

Induction of slow action potentials by microiontophoresis of cyclic AMP into heart cells

Abstract Catecholamines, methylxanthines, and histamine, agents which increase the cyclic AMP level, are well known to increase the slow inward current ( I si ) in heart muscle. These agents can restore excitability to partially depolarized heart cells (fast Na + channels voltage inactivated) in the form of slow action potentials, whose electrogenesis is due to a regenerative increase in I si . In order to further clarify the relationship between cyclic AMP and the myocardial slow inward current, cyclic AMP was applied intracellularly in an attempt to restore slow action potentials in cardiac Purkinje fibers. The Purkinje fibers were taken from canine hearts, shortened to a length of 2 to 3 mm, and superfused at 10 ml/min with Krebs-Henseleit solution. Superfusion with an elevated K + (20 m m ) solution depolarized the fibers to about −40 mV and rendered the preparations inexcitable. Intracellular microiontophoresis of cyclic AMP (0.2 to 12 μC) restored slow action potentials in 25 out of 46 cells so tested. The induced slow action potentials persisted for a transient period of 1 to 5 min following the injection. In a separate series of experiments, theophylline (0.5 m m ) was used to induce slow action potentials. Cyclic AMP injection increased the maximal upstroke velocity ( + v max ) and overshoot of these ongoing slow action potentials. The effects obtained increased with the amount of cyclic AMP injected. A similar potentiation of the slow action potential was found in ventricular muscle (guinea-pig papillary muscles). The results indicate that cyclic AMP, applied intracellularly, can mimic the well-known restorative action of catecholamines and methylxanthines in partially depolarized heart muscle. This confirms a relationship between cyclic AMP and the net slow inward current.

Antiadrenergic action of adenosine on ventricular myocardium in embryonic chick hearts

Abstract Adenosine, a physiological metabolite, is known to exert a negative inotropic effect on atrial muscle [5, 7], to impair atrioventricular conduction [2, 7, 17] and depress pacemaker activity [18]. In addition to such direct actions, adenosine is also known to modulate several of the actions of catecholamines in different tissues [9, 15, 16]. In ventricular muscle, adenosine attenuates the inotropic effect of catecholamines, but has no contractile effect in the absence of catecholamines [1, 6, 15]. Thus, adenosine appears to exert its functions in heart by direct [2, 5, 7, 18] and indirect (anti-adrenergic) [1, 11, 15] mechanisms. The isoproterenol-induced increase in ventricular contractility is associated with an increase in cyclic AMP (cAMP) [19]. Since isoproterenol, cAMP analogs, and cAMP itself (applied internally), increase the Ca2+-inward current (Isi) and slow action potentials, a causal relationship between cAMP and Isi has been proposed [10]. The ability of isoproterenol to restore excitability to partially depolarized cardiac fibers and/or enhance pre-existing slow action potentials is attributed to increase in cAMP that leads to an increase in the Isi. Adenosine decreased the cAMP levels of catecholamine-treated hearts [1, 6, 15] but unexpectedly adenosine had no inhibitory effect on slow action potentials obtained in elevated K+ (27 mM) plus 10−7 to 10−6 M isoproterenol solution [12, 13]. In the present study, we wanted to find out whether adenosine would attenuate ventricular slow action potentials enhanced by relatively low concentrations of isoproterenol in normal K+ solution. We set out to confirm in parallel experiments that adenosine also reduces the cAMP content of hearts exposed to isoproterenol.

Fluoride stimulation of slow Ca2+ current in cardiac muscle

Fluoride, a known activator of the adenylate cyclase in broken-cell preparations of heart and other tissue [21, 22] was tested for its ability to act as a positive inotropic agent in cardiac muscle and for its ability to concomitantly induce Ca2+ channels. The induction of slow Ca2+ channels was tested in myocardial cells in which the fast Na+ channels were blocked by tetrodotoxin (TTX) or inactivated by partial depolarization in 25 m m K+. In the isolated perfused embryonic chick heart and in reaggregates of cultured cells derived from the embryonic chick ventricle, addition of NaF (0.5–5.0 m m ), to myocardial cells whose excitability was abolished, rapidly (1–6 min) induced slowly-rising overshooting electrical responses accompanied by contractions. These slow responses were blocked by Mn2+ (1 m m ) or D-600 (10−6 g/ml), known inhibitors of transmembrane Ca2+ current. Consistent with this induction of slow Ca2+ channels, fluoride (2–3 m m ) showed a positive inotropic action in intact hearts perfused with normal Ringer solution. Because many positive inotropic agents, in common, elevate cyclic AMP, it is possible that the action of fluoride in inducing slow Ca2+ channels was mediated by changes in cyclic AMP levels leading to possible changes in phosphorylation of a protein constituent of the slow channels. However, the cyclic AMP content of the fluoride-treated hearts did not increase as compared to control hearts. But since fluoride also inhibits phosphatases [8, 11], the action on the membrane slow channels could be mediated by inhibition of dephosphorylation of the slow channel.

Restoration of slow responses in hypoxic heart muscle by alkaline pH

Abstract Hypoxia, metabolic poisons, and acid pH were shown to depress the inward slow current in myocardial cells. In the present study, the rate of inhibition of the slow responses (dependent on Ca 2+ and Na + ) during hypoxia was determined at different pH values in isolated guinea pig papillary muscles. To study the slow responses, the fast Na + current was first voltage-inactivated by partial depolarization to about −45 mV using an elevated (25 m m ) K + solution. Under these conditions, the muscles became inexcitable and did not contract. Isoproterenol (10 −6 m ) or elevated Ca 2+ (7 m m ) allowed slowly-rising overshooting electrical responses accompanied by contractions to be elicited by electrical stimulation. In normoxia ( P o 2 of 576 ± 18 mmHg), the maximal upstroke velocity ( + V max ) and peak overshoot of the control slow responses recorded at pH 7.4 did not change significantly when the pH was varied to 6.8 or 8.0. Although hypoxia ( P o 2 of 23 ± 1 mmHg) caused slow response blockade irrespective of the pH value used, the rate of blockade was slower at more alkaline pH levels. The times to 90% inhibition were 15 min, 25 min, and 65 min at pH 6.8, 7.4, and 8.0, respectively. The developed tension declined concomitantly with + V max of the slow response. Raising the pH of the hypoxic solution to 8.0 allowed partial (and temporary) restoration of slow responses and contractions that had been blocked at pH 7.4 or 6.8. This effect may be mediated by reversal of the acidosis normally produced by hypoxia. The results suggest that the slow current is depressed only gradually at low P o 2 values if the cells are exposed to alkaline pH, and may explain the well-known fact that alkaline pH improves mechanical performance of ventricular muscle during hypoxia. These findings are consistent with a role of acidosis in inhibition of metabolically-dependent myocardial slow channels under adverse conditions such as hypoxia. However, an additional factor could be a change in K + conductance during hypoxia and acidosis.

Effect of Bupivacaine and Levobupivacaine on Exocytotic Norepinephrine Release from Rat Atria

Background: The cardiotoxic mechanism of local anesthetics may include interruption of cardiac sympathetic reflexes. The authors undertook this investigation to determine if clinically relevant concentrations of bupivacaine and levobupivacaine interfere with exocytotic norepinephrine release from cardiac sympathetic nerve endings. Methods: Rat atria were prepared for measurements of twitch contractile force and 3[H]-norepinephrine release. After nerve endings were loaded with 3[H]-norepinephrine, the tissue was electrically stimulated in 5-min episodes during 10 10-min sampling periods. After each period, a sample of bath fluid was analyzed for radioactivity and 3[H]-norepinephrine release was expressed as a fraction of tissue counts. Atria were exposed to buffer alone during sampling periods 1 and 2 (S1 and S2). Control atria received saline (100 &mgr;l each, n = 6 atria) in S3-S10. Experimental groups (n = 6 per group) received either bupivacaine or levobupivacaine at concentrations (in &mgr;M) of 5 (S3-S4), 10 (S5-S6), 30 (S7-S8), and 100 (S9-S10). Results: Bupivacaine and levobupivacaine decreased stimulation-evoked fractional 3[H]-norepinephrine release with inhibitory concentration 50% values of 5.1 ± 0.5 and 6.1 ± 1.3 &mgr;m. The inhibitory effect of both local anesthetics (∼70%) approached that of tetrodotoxin. Local anesthetics abolished the twitch contractions of atria with inhibitory concentration 50% values of 12.6 ± 5.0 &mgr;m (bupivacaine) and 15.7 ± 3.9 &mgr;m (levobupivacaine). In separate experiments, tetrodotoxin inhibited twitch contractile force by only 30%. Conclusions: The results indicate that clinically relevant cardiotoxic concentrations of bupivacaine and levobupivacaine markedly depress cardiac sympathetic neurotransmission. A possible mechanism of local anesthetics in reducing evoked norepinephrine release from sympathetic endings is blockade of tetrodotoxin-sensitive fast sodium channels.

Analysis of halothane effects on myocardial force-interval relationships at anesthetic concentrations depressing twitches but not tetanic contractions

Background Tetanic contractions in rat myocardium depend solely on cellular Calcium2+ uptake, whereas twitches depend on Calcium2+ release from the sarcoplasmic reticulum. Because halothane may cause loss of sequestered Calcium2+, the anesthetic was tested for its differential effects on twitch and tetanic forces. The in vitro effects of halothane on the twitch force‐interval relationship were then evaluated, using a mathematical model that relates twitch contractile force to the Calcium2+ content of intracellular compartments.