Isolated Guinea Pig Papillary Muscles Research Articles

The Effects of Sevoflurane on Contractile and Electrophysiologic Properties in Isolated Guinea Pig Papillary Muscles

The Effects of Sevoflurane on Contractile and Electrophysiologic Properties in Isolated Guinea Pig Papillary Muscles

The effects of sevoflurane on contractile and electrophysiologic properties in isolated guinea pig papillary muscles.

We examined, in guinea pig papillary muscles, whether the negative inotropic effect of sevoflurane is due to the depression of the influx of extracellular Ca2+ or to inhibition of the availability of intracellularly stored Ca2+. Sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normally polarized guinea pig papillary muscles. Sevoflurane produced a depression of contractile force with different rates or patterns of stimulation in the rested state and at low stimulation frequencies. In a potentiated state, sevoflurane did not depress contractile forces. Although sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normal Tyrode's solution, in high K+ Tyrode's solution, it caused a depression of contractile force without a shortening of action potential duration. Sevoflurane also depressed contractile force in normal and high K+ Tyrode's solution with ryanodine 1 microM. Our results suggest that in myocardial contractile force the negative inotropic effect of sevoflurane might be caused by depression of transsarcolemmal Ca2+ influx, accompanied by shortening of the action potential duration.

Endothelin-1 partially inhibits ATP-sensitive K+ current in guinea pig ventricular cells.

To clarify the pathophysiological significance of endothelin (ET) in the ischemic myocardium, we examined the effect of endothelin-1 (ET-1) on the ATP-sensitive K+ current (IK.ATP) and compared it with that of ET-3 in guinea pig ventricular cells using conventional microelectrode and patch clamp techniques. In isolated guinea pig papillary muscles, ET-1 (30 nM) markedly increased developed tension (DT), with little influence on action potential duration (APD), whereas ET-3 at the same concentration failed to affect DT or APD. Both nicorandil (1 mM) and cromakalim (30 microM) markedly shortened APD and decreased DT in papillary muscles. ET-1, but not ET-3, partially reversed the nicorandil-induced decreases in APD and DT in a concentration-dependent manner. ET-1 also attenuated the cromakalim-induced decreases in APD and DT. In single ventricular myocytes, both nicorandil and cromakalim increased a steady-state outward current, which was sensitive to 1 microM glibenclamide, suggesting that these drugs activate IK.ATP. ET-1 (30 nM) significantly inhibited the IK.ATP, whereas ET-3 failed to affect it. The ET-1 induced inhibition of IK.ATP was abolished by BQ-485 (100 nM), an ETA receptor-selective antagonist. Neither the protein kinase C (PKC) inhibitor staurosporine (20 nM) nor the calmodulin antagonist W-7 (50 microM) affected the inhibitory action of ET-1 on the nicorandil-induced IK.ATP. In pertussis toxin (PTX)-treated cells, the inhibitory action of ET-1 on IK.ATP was augmented rather than attenuated. These results suggest that ET-1 partially inhibits the IK.ATP through the activation of ETA receptors, although the precise intracellular mechanism remains to be clarified. Because activation of the ATP-sensitive K+ channels is considered to protect the ischemic myocardium, the partial inhibition of IK.ATP by ET-1 may lead to the aggravation of myocardial injury, potentially due to an increase in transmembrane Ca2+ influx.

Comparison of Binding to Rapidly Activating Delayed Rectifier K+ Channel, IKr, and Effects on Myocardial Refractoriness for Class III Antiarrhythmic Agents

Saturation binding studies in guinea pig ventricular myocytes with 3H-dofetilide, a radioligand for the cardiac rapidly activating delayed rectifier K+ IKr channel, indicated specific high-affinity binding with a Kd of 83 nM and a Bmax of 0.18 pmol/mg cellular protein (1.36 x 10(6) sites/cell). Using displacement of high-affinity 3H-dofetilide binding as a measure of interaction with the IKr channel, potencies (Ki values) for binding to the IKr channel in guinea pig myocytes for six class III antiarrhythmic agents were characterized and compared to indices of functional electrophysiologic activity in isolated guinea pig papillary muscles [EC25 values, concentration required to increase effective refractory period (ERP) 25% above baseline]. Dofetilide, E-4031, sematilide, and d-sotalol, which have been characterized previously as selective IKr blockers, displayed good agreement between Ki values for displacement of 3H-dofetilide binding (47 +/- 7 nM, 38 +/- 8 nM, 12 +/- 5 microM, and approximately 100 microM, respectively) and EC25 values for increasing ERP in papillary muscles (45.0 nM, 76.9 nM, 20.2 microM and 63.5 microM, respectively). Ibutilide and RP58866, which have been reported to act via mechanisms other than IKr block, had Ki values for displacement of 3H-dofetilide binding (16 +/- 7 nM and 17 +/- 2 nM, respectively) that were approximately 10-fold lower than EC25 values for increasing ERP in papillary muscles (185.8 nM and 223.5 nM, respectively). The potent displacement of high-affinity 3H-dofetilide binding by ibutilide and RP58866 strongly suggest a role for interaction with IKr in their actions.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic blocking property of dl-sotalol maintains class III efficacy in guinea pig ventricular muscle after isoproterenol.

Catecholamines antagonize the efficacy of several class III antiarrhythmic agents. To determine the role of the intrinsic beta-adrenergic blocking property of dl-sotalol in maintaining class III efficacy during a high-catecholamine state, we compared the electrophysiological properties of dl-sotalol with those of d-sotalol, which is devoid of significant beta-adrenergic blocking effect, before and after isoproterenol infusion. Action potential duration at 90% repolarization (APD90) was prolonged in isolated guinea pig papillary muscles perfused with d-sotalol and dl-sotalol 10(-4) mol/L over stimulation cycle lengths from 200 to 2000 ms. The increases in APD90 for d-sotalol and dl-sotalol over control were 10.9 +/- 2.5 to 23.7 +/- 4.8 ms and 27.9 +/- 4.0 to 39.0 +/- 5.6 ms, respectively. APD90 shortened to less than control in papillary muscles treated with d-sotalol but not dl-sotalol on addition of isoproterenol 10(-6) mol/L: -31.2 +/- 3.5 to -18.3 +/- 4.8 ms and 10.5 +/- 3.6 to 33.3 +/- 7.8 ms, respectively, P < .003. Single guinea pig ventricular myocytes were studied by the whole-cell patch clamp method. Time-dependent (Iout) and total (Itot) outward current in response to a 300-ms pulse to 20 mV and tail current (Itail) to -35 mV were measured after Ca2+ channel block and Na+ channel inactivation. Iout, Itail, and Itot were reduced in myocytes perfused with d-sotalol and dl-sotalol 10(-4) mol/L: Iout, -36.1 +/- 4.1%, -40.5 +/- 3.3%; Itail, -59.3 +/- 4.6%, -62.2 +/- 11.1%; Itot, -27.3 +/- 4.3%, -50.0 +/- 11.8%. Iout and Itot increased to a greater degree in myocytes treated with d-sotalol than dl-sotalol on addition of isoproterenol 10(-6) mol/L: Iout, 100.3 +/- 20.6%, 11.3 +/- 7.6%, P = .002; Itot, 86.8 +/- 39.2%, -41.1 +/- 20.9%, P = .01. Itail tended to increase more in myocytes treated with d-sotalol than dl-sotalol on addition of isoproterenol, but the difference was not significant (-9.1 +/- 13.5%, -28.0 +/- 9.0%). The beta-adrenergic blocking property of dl-sotalol maintains APD prolongation and repolarizing outward current block during isoproterenol infusion in guinea pig ventricular muscle. Extrapolation of these data to a clinical setting may explain the efficacy of dl-sotalol in diminishing ventricular arrhythmia recurrence.

Vasopressin V1-Receptor Stimulation Produces a Positive Inotropic Response without Affecting pHi in Guinea Pig Papillary Muscles

Effects of arginine-vasopressin (AVP) on the contractile force, action potential (AP) and intracellular pH (pHj) were studied in isolated guinea pig papillary muscles using conventional and ion-selective microelectrode techniques. AVP increased the developed tension and the resting tension, and these responses were attenuated by the V1-receptor antagonist OPC-21268 (1-{1-[4-(3-acetylaminopropoxy)benzoyl]-4-piperidyl}-3,4-dihydro-2(1H)-Quinolinone). However, AVP failed to affect AP configuration or pH^ These results suggest that AVP produces a positive inotropy by mechanism(s) other than intracellular alkalinization.

Aftereffects of high-intensity DC stimulation on the electromechanical performance of ventricular muscle.

To clarify the mechanisms underlying cardiac dysfunction after electrical defibrillation, we investigated the effects of direct current field stimulation (10 ms, 1-80 V/cm) on isolated guinea pig papillary muscles. Shocks (S2) > 15 V/cm lowered the plateau height of the S2-induced action potential and inhibited its terminal repolarization. Subsequent responses to basic stimuli (S1, 1.0 Hz) for 1-3 min were characterized by a decrease in the maximum diastolic potential, a shortening of action potential duration, and an increase of the developed tension. With S2 > 30 V/cm, a marked delay in repolarization of the S2-induced action potential was followed by oscillation of membrane potential, resulting in repetitive spontaneous activity and often refractoriness to S1 stimulation. The aftereffects were independent of the phase of S2 application. Most of the aftereffects were preserved in the presence of nifedipine (1 microM) or ryanodine (1 microM). Only sodium channel blockade by tetrodotoxin (10 microM) modified the aftereffects by depressing the generation of spontaneous activity. These findings suggest that strong shocks (> 15 V/cm) will produce abnormal arrhythmogenic responses probably through a transient rupture of sarcolemmal membrane (electroporation) leading to a disturbance of the ionic equilibrium of the myocyte.

Pharmacological actions of SDZ 218-135, a novel positive inotropic agent.

The effects of the new inotropic agent, SDZ 218-135 [(+)-(S)-4-[3-(4-diphenyl-methyl-1-piperazinyl)-2-hydroxy-propyl]- 6-(2-hydroxyethyl)-5-methyl-1,2,4-triazolo-[1,5-a]pyrimidin-7(4H)-one], were investigated using in vitro and in vivo techniques. In isolated rat atria, SDZ 218-135 elicited a dose-dependent increase in contractile force (+50% at 10 microM), which was paralleled by an increase in functional refractory period. In anesthetized rats SDZ 218-135 enhanced left ventricular (+)dP/dtmax by 100% at 10 mg/kg without influencing heart rate, arterial blood pressure, and cardiac output. In contrast to its predecessor, DPI 201-106, cardiac relaxation remained essentially unimpaired. The positive inotropic action was also maintained in a rabbit model of depressed heart function after myocardial infarction, where SDZ 218-135 increased peak acceleration of blood in the aorta. The prolongation of the effective refractory period in rat atria suggested possible antiarrhythmic effects. Indeed, SDZ 218-135 showed a dose-dependent marked reduction in reperfusion arrhythmias after coronary artery occlusion in rats. This effect was most likely due to a Class III action, since SDZ 218-135 significantly increased action potential duration (+10% at 10 microM/l) of the isolated guinea pig papillary muscle. In conclusion, SDZ 218-135 is a novel positive inotropic agent with an interesting profile of action. It does not impair cardiac relaxation and shows antiarrhythmic effects in a model of reperfusion-induced arrhythmias. The in vivo and in vitro data are consistent with a mechanism of action via sodium channel agonism.

Cellular electrophysiological effects of hyperthermia on isolated guinea pig papillary muscle. Implications for catheter ablation.

The primary mechanism of tissue injury by radiofrequency catheter ablation is presumed to be thermally mediated. However, the myocardial cellular electrophysiological effects of hyperthermia are not well characterized. We used an in vitro model of isolated guinea pig right ventricular papillary muscle to investigate the acute cellular electrophysiological effects of hyperthermia. Excised guinea pig right ventricular papillary muscles were pinned in a high-flow tissue bath and superfused with Tyrode's solution at 37.0 +/- 0.5 degrees C. The superfusate temperature was rapidly changed to 38.0 to 56.0 degrees C for 60 seconds and then returned to 37.0 degrees C. Conventional microelectrodes were used to measure membrane potential (Vm), maximum rate of rise of the action potential (dV/dtmax), and action potential (AP) amplitude and AP duration at 50% (APD50) and 90% (APD90) repolarization. Hyperthermia resulted in (1) a progressive depolarization of Vm at temperatures > or = 40.0 degrees C, which became more prominent at temperatures > or = 45.0 degrees C; (2) changes in the AP characterized by a temperature-dependent increase in dV/dtmax and a temperature-dependent decrease in AP amplitude, APD50, and APD90; (3) reversible loss of cellular excitability within a temperature range of 42.7 to 51.3 degrees C (median, 48.0 degrees C); (4) irreversible loss of cellular excitability and tissue injury at temperatures > or = 50.0 degrees C; and (5) the development of abnormal automaticity at temperatures > 45.0 degrees C. Hyperthermia causes significant changes in myocardial cellular electrophysiological properties that include membrane depolarization, reversible and irreversible loss of excitability, and abnormal automaticity. There appear to be specific temperature ranges for reversible and irreversible electrophysiological changes. These observations may have important implications for tissue temperature monitoring during radiofrequency catheter ablation.

Effects of elastase on contractility and morphology of elastic tissue in isolated guinea pig papillary muscles.

Elastase (ELA) is an enzyme catalyzing the digestion of elastin, an essential constituent of elastic fibers. Using isolated guinea pig papillary muscles, we examined the effect of ELA (3 x 10(-7) -3 x 10(-4) g/ml) on resting tension (RT) and twitch tension (TT). The effects of ELA on elastic fibers located in the subendocardium were examined histologically. A relatively high concentration of ELA (3 x 10(-4) g/ml) increased TT transiently, with progressive decreases in RT. In contrast, a relatively low concentration (3 x 10(-5) g/ml) decreased both TT and RT straightforwardly. Much lower concentrations (3 x 10(-6) -3 x 10(-7) g/ml) did not reveal significant effects. The ELA-induced increases in TT were unaffected in the presence of atenolol (10(-5) g/ml), ouabain (10(-7) M) or ryanodine (10(-6)M). ELA did not increase the maximum rate of rise of slow action potentials recorded using standard microelectrodes. ELA (3 x 10(-5) g/ml) decreased the maximum TT obtained at optimal RT or Lmax, and decreased the slope of the ascending and descending limbs of the TT-RT relation curve (Frank-Starling's). Electron-microscopic findings revealed that subendocardial elastin was mostly digested at ELA concentrations of 3 x 10(-5) -3 x 10(-4) g/ml. These findings suggest that the decrease of RT by ELA may be, at least in part, caused by a decomposition of the elastic fibers. On the other hand, the increase of TT by ELA could not be attributed to a release of endogenous catecholamine, an inhibition of Na+, K(+)-pump, a release of Ca2+ from sarcoplasmic reticulum, or an increase of slow inward current.

High Selectivity for Inhibition of Phosphodiesterase III and Positive Inotropic Effects of MCI-154 in Guinea Pig Myocardium

MCI-154 (0.3-100 microM) exerted a concentration-dependent positive inotropic effect in isolated guinea pig papillary muscles (EC50 0.8 microM). The efficacy of MCI-154 (253% of predrug value) was 1.7-fold higher than that of saterinone but comparable to that of milrinone. Carbachol markedly reduced the increase in force of contraction (FOC) of MCI-154. In intact contracting papillary muscles, the positive inotropic effect was accompanied by an increase in cyclic AMP content to 0.78 +/- 0.09 pmol/mg wet weight (n = 10), corresponding to 150% of the basal value (0.51 +/- 0.05 pmol/mg wet weight, n = 21) in the presence of submaximal cyclic AMP phosphodiesterase (PDE) isoenzyme III inhibiting concentrations of MCI-154 (30 microM). MCI-154 (1-1,000 microM) concentration-dependently inhibited the activity of PDE III from homogenates of guinea pig myocardium. The IC50 was 3.8 microM. PDE I, II, and IV were not significantly affected up to 100 microM (PDE I and IV) and up to 1,000 microM (PDE II). In comparison, milrinone and saterinone were PDE III/IV-selective PDE inhibitors. Rolipram inhibited PDE IV only. IBMX and theophylline were nonselective PDE inhibitors. MCI-154 had only a marginal positive chronotropic effect. The frequency of spontaneously beating right auricles from guinea pig heart was increased by 8.7% at most (n = 5). MCI-154 increased Ca2+ sensitivity in chemically skinned porcine ventricular muscle fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological Effects of the Combination of Imipramine and Desipramine in Guinea Pig Papillary Muscles

The electrophysiological effects of imipramine and its active metabolite desipramine, alone or in combination, were studied in isolated guinea pig papillary muscles. The maximum upstroke velocity (Vmax) of the action potential was used as an indirect index of the magnitude of the sodium inward current. In muscles driven at 0.02 Hz, imipramine alone (5 x 10(-6) M), desipramine alone (5 x 10(-6) M), or their combination had no effect on action potential characteristics. However, in the presence of imipramine or desipramine, trains of stimuli at rates between 0.5 and 3 Hz led to a frequency-dependent Vmax block that was augmented at higher stimulation rates. At each driving rate, the Vmax block produced by desipramine was significantly greater than that produced by imipramine. The combination of imipramine and desipramine increased the onset rate of the frequency-dependent Vmax block, but the total amount of Vmax block was similar to that produced by desipramine alone. In the presence of imipramine alone, the recovery of Vmax was a monoexponential process, the time constant (tau re) being 2.3 +/- 0.4 s, whereas in the presence of desipramine, it was better defined by a biexponential function (tau re = 1.5 +/- 0.4 and 15.8 +/- 2.8 s). In the presence of the combination, the recovery process was also defined by a biexponential function and the tau re values were similar to those found in the presence of desipramine alone. Thus, according to their onset and offset kinetics of the frequency-dependent Vmax block, imipramine and desipramine can be classified as sodium channel blockers with intermediate and slow kinetics, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and subacute effects of doxorubicin on postextrasystolic potentiation in guinea pig papillary muscles.

To investigate the effects of doxorubicin on postextrasystolic potentiation (PESP), isolated guinea pig papillary muscles were field-stimulated and the resulting isometric force was recorded. Postextrasystolic contraction was evoked following trains of 37 regular stimulations. The effects of acute doses of doxorubicin (0.2 mM) on regular contractions and postextrasystolic contractions were examined for 2 hr. The effects of subacute doses of doxorubicin (total dose 5 mg/kg intraperitoneally) on the relationship between %PESP (postextrasystolic/regular contraction) and both the extra-stimulus coupling interval and the postextrasystolic interval were examined. Acute administration of doxorubicin decreased the amplitude of postextrasystolic contractions more than that of regular contractions. Thus, %PESP in the doxorubicin-treated group decreased significantly over time. There was no similar decrease in the control papillary muscles. Both the extra-stimulus coupling interval and the postextrasystolic interval had less of an effect on %PESP in doxorubicin-treated animals than in control animals. Since PESP depends upon sarcoplasmic reticulum activity, our results indicate that acute and subacute exposure to doxorubicin impairs the activity of the sarcoplasmic reticulum of guinea pig papillary muscles.

Influence of KATP channel modulation on net potassium efflux from ischaemic mammalian cardiac tissue.

The aim was to investigate the effect of pre-exposure to inhibitors or activators of the ATP regulated K+ (KATP) channels on the ischaemia induced early changes of the extracellular K+ concentration, the extracellular pH, and the action potential in mammalian cardiac tissue. An in vitro model simulating ischaemia was applied to isolated guinea pig papillary muscles while simultaneous microelectrode measurements of the transmembrane potentials and of the pH and K+ activity at the surface of the preparation (pHs and asK respectively) were made. During conditions of superfusion arrest, when accumulation of metabolic acids causes acidification of pHs, the simultaneous development of a true hypoxic state of the muscle is required in order to induce shortening of the action potential duration and accumulation of extracellular K+ with associated membrane depolarisation. Glibenclamide (10-50 microM) slowed the decrease of the action potential duration in 50% of the muscles, while the increase of asK was only moderately (approximately 20%) influenced. In the presence of 200 microM of the KATP channel inhibitor, the mean K+ accumulation was not significantly different from the control. The presence of tolbutamide (1 mM) had no effect on the decrease of action potential duration, but moderately slowed the increase of asK. Superfusion with lemakalim (BRL 38227) decreased action potential duration dose dependently. In papillary muscles in which action potential duration was shortened to approximately 60% of the control duration by presuperfusion with lemakalim, a subsequent ischaemic episode did not increase but rather delayed the rise in asK. During simulated ischaemia in the presence of Ba2+ ions (1 mM), asK showed a transient decrease followed by a rise at a rate similar to that in the absence of Ba2+. Early ischaemic K+ accumulation and surface acidification are relatively insensitive to KATP channel inhibition or activation prior to the ischaemic insult. The rather loose coupling of the increase of asK and the decrease of action potential duration, as well as the limited effect of sulphonylureas on the K+ increase, suggests that mechanisms other than KATP channel opening could possibly contribute to the initial phase of ischaemic K+ accumulation.

In vivo and in vitro effects of the novel antiarrhythmic ipazilide on cardiac and vascular smooth muscle function

AbstractIpazilide fumarate is a novel antiarrhythmic currently undergoing clinical evaluation. Since most antiarrhythmics have adverse cardiovascular side effects, we examined the potential effects of ipazilide on cardiac muscle and vascular hemodynamic function utilizing in vivo and in vitro models. The cardiovascular effects of ipazilide and selected reference antiarhythmics, disopyramide and sotalol, at relevant antiarrhythmic concentrations were compared. All agents decreased cardiac contractility, as evident by reductions in dP/dt and cardiac output in anesthetized dogs, and by decreases in contractile force in isolated guinea pig papillary muscles. In vivo, ipazilide was approximately 3–10 × less potent than disopyramide or sotalol while in vitro, ipazilide produced a greater decrease in contractility at a relatively high concentration (30 μM). All three agents increased cardiac preload as reflected by increases in left ventricular end diastolic pressure and right atrial pressure, with ipazilide 2–3 × less potent than disopyramide or sotalol. Differential effects on vascular function were also evident with the three agents. Disopyramide and sotalol significantly increased systemic vascular resistance, and either pulmonary vascular resistance (disopyramide) or left coronary arterial vascular resistance (sotalol); ipazilide did not significantly affect any of these parameters. In isolated rat arterial or canine coronary arterial smooth muscle, ipazilide produced concentration‐related vasorelaxation, with EC50 values ranging from 11–300 μM. Efficacy of ipazilide was higher in depolarized vascular tissue. In conclusion, since previous preclinical studies have indicated equal or greater antiarrhythmic activity of ipazilide relative to disopyramide or sotalol, the current data indicate the possibility for lessened in vivo hemodynamic effects of ipazilide at efficacious antiarrhythmic dosages. © 1992 Wiley‐Liss, Inc.

Comparative Frequency-Dependent Effects of Three Class Ic Agents, Org 7797, F]ecanide, and Proprafenone, on ventricular Action Potential Duration

The frequency-dependent electrophysiological effects of three class Ic agents, Org 7797 (0.5-5 microM), flecainide (1-20 microM), and propafenone (1-10 microM), were investigated using isolated guinea pig papillary muscle. Transmembrane cellular action potentials were recorded using conventional microelectrode techniques at driving frequencies of 0.3, 1.0, and 2.0 Hz. In concentrations inducing similar frequency-dependent decreases in Vmax, both flecainide and propafenone shortened action potential duration (APD) at the two lower stimulation frequencies, whereas increasing the driving frequency to 2 Hz attenuated drug-induced APD shortening. In contrast, Org 7797 did not shorten APD at any stimulation frequency, and indeed APD lengthening was observed at 2.0 Hz. Increases in the effective refractory period (ERP) were seen at 1.0 and 2.0 Hz in the presence of Org 7797 and at 2.0 Hz in the presence of flecainide, but ERP was either unchanged (2.0 Hz) or shortened by propafenone. The rate of onset of sodium channel block was faster in response to propafenone compared to Org 7797 and flecainide. It was concluded that all three drugs may inhibit outward repolarising potassium currents, especially at higher stimulation frequencies. However, the concentrations of Org 7797 necessary to influence repolarisation may be closer to those required to induce sodium channel block compared to flecainide and especially propafenone. These differences may influence drug-induced effects on wavelength resulting in differences in antifibrillatory efficacy.

Electrophysiologic effects of exogenous phosphocreatine in cardiac tissue: Potential antiarrhythmic actions

The cellular electrophysiologic effects of exogenous phosphocreatine (PCr) were analyzed to ascertain its purported antiarrhythmic properties during myocardial ischemia and reperfusion. Transmembrane potentials were recorded from isolated guinea pig papillary muscles and Purkinje fibers studied in vitro. Under control, normoxic conditions, 10 mmol/L PCr significantly increased the action potential duration (measured at 90% of repolarization) in ventricular muscle by 14.6 ± 3.3 msec and the effective refractory period by 11.5 ± 3.8 msec (both p < 0.01). Under ischemic-like conditions (hypoxia, lactic acidosis, elevated [K +] o, zero substrate) PCr had no effect. Phosphocreatinine, a related compound that is not a direct substrate in the creatine kinase reaction, acted similarly to PCr suggesting that siterations induced by PCr did not involve a change in the energy state of cells. However, PCr reduced free [Ca 2+] o by nearly 20%, and its electrical effects under normoxic conditions could be largely reversed by a concomitant 20% increase in [Ca 2+] o. In Purkinje fibers superfused with low [K +] o-Tyrode's solution to elicit conditions of Ca 2+ overload, delayed afterdepolarizations and triggered responses were reversibly inhibited by PCr. These data suggest that the antiarrhythmic effects of PCr in situ may involve prolongation of the effective refractory period in nonischemic tissue or attenuation of membrane changes elicited by Ca 2+ overload in ischemic cells. The mechanism by which PCr produces these effects may be related in part to changes in extracellular Ca 2+ composition.

Rate dependence and supernormality in excitability of guinea pig papillary muscle

It is well known that in most cardiac tissues an increase in rate results in a decrease of excitability and, eventually, conduction block. We used microelectrode techniques to evaluate the rate and time dependence of excitation latency in 27 isolated guinea pig papillary muscles (GPPM). Latency was measured as the interval between the stimulus onset and action potential upstroke. When the intensity of current was just suprathreshold, prolongation of the basic cycle length (BCL) from 300 to 1,000 ms produced an increase in latency or failure of excitation. Such behavior was observed with extracellular as well as intracellular stimulation. Rate-dependent changes in latency were maximal during the first 10-20 s following the rate change and reached a steady state in approximately 200 s. Application of premature beats revealed the presence of a "supernormal phase" in which latency abbreviated. Strength-interval and strength-duration curves demonstrated that changes in excitability accurately paralleled those observed in latency. Hence, supernormal excitability at the end of the phase 3 repolarization was consistently observed in all ventricular muscle experiments. Deceleration-induced decrease of excitability was attended by hyperpolarization, increase of action potential upstroke velocity (Vmax) and action potential amplitude, and decrease in membrane resistance. Our data suggest that paradoxical rate-related changes of excitability in GPPM are the result of changes in the passive membrane properties. Under conditions of depressed conductivity, this particular behavior may account for the occurrence of bradycardia-dependent block.

Changes in the ionic environment may alter the kind of antagonism of some histamine H 2-receptor blockers in the guinea pig papillary muscle

The effect of changes in the composition of the bathing medium on the effect of histamine and histamine H 2-receptor antagonists was investigated in the isolated guinea pig papillary muscle. Ringer or Krebs-Henseleit solutions were used as nutrient fluids. They mainly differed with respect to pH and to Mg 2+ and H 2 PO 4 − content. Whereas the effect of histamine was not altered by ionic changes, the antagonism by some H 2 blockers was different in the two nutrient solutions. The insurmountable antagonism elicited by high concentrations (≥10 −6M) of famotidine, oxmetidine and mifentidine in Ringer solution was converted to surmountable when these drugs were tested in Krebs-Henseleit solution. Conversely, the antagonism induced by ranitidine was surmountable in both solutions, and that induced by high amounts of loxtidine was insurmountable in both nutrient fluids. Results obtained in Ringer solution were not modified by pH adjustments or by the addition of ions present in Krebs-Henseleit medium. These results suggest that the interaction of histamine with H 2 receptors in the guinea pig papillary muscle was not influenced by alterations in the ionic composition of the nutrient fluid, whereas the antagonism may be critically dependent on the ionic environment.

Acidification and intracellular sodium ion activity during stimulated myocardial ischemia

With the use of microelectrodes, intracellular pH (pHi), surface pH (pHs), and intracellular Na+ activity (aiNa) were measured in isolated guinea pig papillary muscles during normal superfusion and during a reversible condition of simulated ischemia. Acid loading by NH+4 prepulse or by CO2-HCO3- addition during superfusion with pH 7.4 solutions caused internal acidification followed by a recovery of pHi, which could be inhibited by amiloride. pHi recovery was associated with an amiloride-sensitive peak rise of aiNa and membrane hyperpolarization, indicative of Na(+)-H+ exchange. Peak increase of aiNa was absent if the pH of the superfusion solution was concomitantly lowered. Imposed ischemia after control superfusion caused membrane depolarization and acidification of pHi and pHs. The change of pHs consistently was larger than that of pHi. aiNa decreased from 5.5 to 4.6 mM after 10-min ischemia. Enlarging the pHi (and pHs) decrease in ischemia by prior reduction of the tissue buffer capacity (CO2-HCO3(-)-free superfusion) was unable to induce a rise of aiNa during the subsequent ischemic period. Amiloride had no significant effect on aiNa during ischemia. It is concluded that the important acidification of pHs reduces the rate of pHi regulatory Na(+)-H+ exchange and thereby contributes to a longer maintenance of the Na+ electrochemical gradient in ischemic cardiac muscle.