[Characteristics of force-frequency relations in the myocardium of the squirrel Citellus undulatus]. / Osobennosti ritmoinotropnykh otnoshenii v miokarde suslika Citellus undulatus.

The force-frequency relationship (FFR) in papillary muscles of the heart of active ground squirrel in different seasons was studied. For comparison, similar preparations from rat and rabbit were used. It was shown that the FFR of papillary muscles of active ground squirrel undergo significant seasonal changes. In summer and a part of autumn squirrels, a negative staircase (a decrease in the isometric force with increasing stimulation frequency) similar to that in adult rat was revealed. The FFR of the majority of autumn, winter and spring squirrels were polyphasic and contained both positive and negative components. Changes in the force in response to the introduction of pauses at a constant stimulation frequency were recorded. Two types of the post-rest recovery pattern were revealed in the myocardium of ground squirrels. For frequencies range with the negative direction of FFR, a typical pattern of rest-potentiation similar to that in rat papillary muscles was observed. The amplitude of the first post-rest contraction (F1) was usually higher than that of the preceding steady-state contraction. In papillary muscles of autumn animals the F1 value was greater that in summer, which suggests an enhanced release of Ca2+ from the sarcoplasmic reticulum. There was no post-rest potentiation in the range of frequencies with positive direction of FFR, and the post-rest recovery pattern in these cases was principally different from those of rat and rabbit preparations. It was proposed that seasonal differences of the FFR of active ground squirrel heart are associated with changes in the ratio of activities of the calcium-transporting system in the hibernation period.

Long-term electrophysiological effects of regional cardiac sympathetic denervation of the neonatal dog.

OBJECTIVE: In many cardiac arrhythmias, both a triggering factor and a favorable myocardial substrate are required. Whereas the sympathetic nervous system may trigger tachyarrhythmias, its function as a long-term modulator of the myocardial substrate is less well understood. Therefore, we tested the hypothesis that regional sympathetic denervation at birth would produce an abnormal myocardial substrate. The comparator was the substrate associated with inherited, lethal tachyarrhythmias at 5 months of age in German shepherd dogs with incomplete sympathetic innervation. METHODS: Mongrel dogs underwent right cardiac stellectomy (RSX) within the first day of life and were terminally studied with control littermates at 5 months of age. RESULTS: On days 1-21 of life, RSX animals manifested significant QT prolongation on ECG and sudden, asystolic death. Beyond this age, QT intervals normalized and deaths did not occur. At 5 months, action potentials (AP) were recorded from Purkinje fibers (PF) and midmyocardial preparations in anteroseptal (AS) and posterobasal (PB) left ventricle. Early afterdepolarizations occurred only in left ventricular PF from RSX dogs. Isoproterenol prolonged AP duration in AS and shortened it in PB of RSX but not control dogs. The incidence of isoproterenol-initiated triggered activity and the amplitude of delayed afterdepolarizations were greater in RSX than control dogs. CONCLUSION: Five months after RSX heterogeneous alterations of LV electrophysiological properties were similar to those previously observed in animals having inherited deficits in sympathetic innervation and sudden death. This implicates the sympathetic nerves as long-term modulators of an arrhythmogenic substrate. That 5-month-old RSX dogs did not experience tachyarrhythmias or sudden death indicates that further anomalies--beyond those explicable by the substrate change--must exist to induce sudden death.

Impact of sex and gonadal steroids on prolongation of ventricular repolarization and arrhythmias induced by I(K)-blocking drugs.

BACKGROUND: Mechanisms for longer rate-corrected QT intervals and higher incidences of drug-induced torsade de pointes in women than in men are incompletely defined, although gonadal steroids are assumed to be important determinants of these differences. METHODS AND RESULTS: We used microelectrode techniques to study isolated rabbit right ventricular endocardium from control male and female and castrated male (ORCH) and female (OVX) rabbits. Action potential duration to 30% repolarization (APD(30)) was significantly shorter in male than female and in ORCH than OVX at a cycle length of 500 ms. The I(Ks) blocker chromanol 293B had no effect on APD in males or females. The I(Kr) blocker dofetilide prolonged APD in female and ORCH more than in male and OVX. At 10(-)(6) mol/L dofetilide (cycle length=1 second), the incidence of early afterdepolarizations was: female, 67%; ORCH, 56%; male, 40%; and OVX, 28%. Serum 17beta-estradiol levels were unrelated to the effects of dofetilide, but as testosterone levels increased, the dofetilide effect to increase APD diminished, as did early afterdepolarization incidence. CONCLUSIONS: Sex-related differences in basal right ventricular endocardial AP configuration persist in castrated rabbits, suggesting that extragonadal factors contribute to the differences in ventricular repolarization. In this model, drugs that block I(Kr) but not I(Ks) prolong repolarization in a way that suggests that protection from excess prolongation in males is attributable to testosterone, whereas the risk of excess prolongation of repolarization in females is related to sex-determined factors in addition to estrogen.

beta(1) and beta(2)-adrenergic receptor subtype effects in German shepherd dogs with inherited lethal ventricular arrhythmias.

OBJECTIVE: Delayed afterdepolarization-induced triggered activity originating in ventricular myocardium is a mechanism for some age-dependent, inherited ventricular tachycardias in a colony of German shepherd dogs. METHODS: We used standard microelectrode techniques to study beta-adrenergic receptor subtype modulation of the triggered activity in anteroseptal left ventricular myocardium from eleven of these dogs and seven unafflicted, age-matched German shepherd controls. RESULTS: During sustained stimulation at cycle lengths of 300-4000 ms, 10(-9)-10(-7) M isoproterenol concentration-dependently shortened action potential duration (APD) to 90% repolarization more in myocardium from afflicted than from unafflicted dogs. This shortening was prevented by a beta(1)-blocker CGP20712A (10(-7) M) while a beta(2)-blocker ICI118551 (10(-7) M) did not modify the effect of isoproterenol in either group. The beta(2)-agonist zinterol 10(-8)-10(-6) M had no effect on APD. Stimulation at a cycle length of 250 ms in the presence of 10(-7) M isoproterenol induced more triggered AP in myocardium from afflicted than unafflicted dogs. beta(1)-Blockade completely eliminated, while beta(2)-blockade facilitated, and the beta(2)-agonist zinterol did not induce triggered activity in the two groups. CONCLUSION: Isoproterenol effects on APD and triggered activity in the myocardium of dogs with inherited arrhythmias are due primarily to an abnormality of beta(1)-adrenoceptor mediated signaling that is subject to beta(2)-adrenergic modulation.

[Effect of insulin on the myocardium of the active, hibernating and wakening ground squirrel Citellus undulatus]. / Vliianie insulina na miokard aktivnogo, giberniruiushchego i probyzhdaemogo suslika Citellus undulatus.

The effect of insulin (0.1-100 nM) on isometric force of contraction in isolated ground squirrel papillary muscle was investigated. In summer, autumn and winter active animals, insulin had a negative inotropic effect on papillary muscles, decreasing the amplitude of contraction by about 30% of the control value. In some cases, predominantly in the summer group of animals, insulin produced different effects on contractility: low doses (0.1-0.5 nM) caused a transient activation of isometric contraction by about 10-15% of control, whereas high doses produced a negative inotropic effect by about 30% of the control level. During deep hibernation (at 5-6 degrees C of heart temperature) and during arousal from hibernation (from 3 to 20 degrees C), insulin had no significant effect on contractility. Opposite inotropic effects of insulin at concentrations of 0.1-50 nM were found during arousal: from 26 to 31 degrees C of heart temperature--a positive inotropic effect by about 20-25% of control, and from 32 to 36 degrees C--a negative one by about 30-40% of the control value.

Effects of mibefradil, a T-type calcium current antagonist, on electrophysiology of Purkinje fibers that survived in the infarcted canine heart.

INTRODUCTION: We studied the effects of mibefradil (MIB), a nondihydropyridine T-type Ca2+ channel antagonist, on T- and L-type Ca2+ (I(CaT), I(CaL)) currents in Purkinje myocytes dispersed from the subendocardium of the left ventricle of normal (NZPC) and 48-hour infarcted (IZPC) hearts. METHODS AND RESULTS: Currents were recorded with Cs+- and EGTA-rich pipettes and in Na+-K+-free external solutions to eliminate overlapping currents. In all cells, I(Ca) was reduced by MIB (0.1 to 10 microM). No change in the time course of decay of peak I(Ca) was noted. Average peak T/L ratio decreased in NZPCs but not IZPCs with 1 microM MIB. Steady-state availability of I(CaL) was altered with 1 microM MIB in both cell types (mean +/- SEM) (V0.5 = -22 +/- 4 mV for NZPC and -25 +/- 5 mV for IZPC before drug; -63 +/- 9 mV for NZPC and -67 +/- 6 mV for IZPC after drug; P < 0.05). For I(CaT), V0.5 (-50 +/- 3 mV for NZPC and -52 +/- 1 mV for IZPC before drug) shifted to -60 +/- 2 mV (NZPC) and -62 +/- 3 mV (IZPC) (P < 0.05) after drug. We also determined the effects of MIB on spontaneously beating Purkinje normal fibers and on depolarized abnormally automatic fibers from the infarcted heart using standard microelectrode techniques. When NZPC and IZPC fibers were superfused with [K+]o = 2.7 mM, MIB 3 microM and 10 microM had no effect on rate or the maximum diastolic potential, but action potential plateau shifted to more negative values, the slope of repolarization phase 3 decreased, and action potential duration increased. CONCLUSION: MIB blocks L- and T-type Ca2+ currents in Purkinje myocytes but lacks an effect on either normal or abnormal automaticity in Purkinje fibers.

Electrophysiological effects of LU111995 on canine hearts: in vivo and in vitro studies.

We studied the electrophysiological effects of LU111995 (1-15 mg/kg p.o.) in conscious dogs with chronic atrioventricular block and ventricular pacing at 50 to 130 beats/min. LU111995 had no effects on idioventricular rhythm, QRS duration, and ventricular conduction time. It significantly prolonged Q-T interval (by 5-8%) and effective refractory period (ERP) (by 5-12%) with the maximal effect at 4 h after a 10 mg/kg dose. At 10 and 15 mg/kg, it increased the ERP/Q-T ratio. In vitro, the effects of LU111995 (1 x 10(-7) to 1 x 10(-5) M) on action potentials of Purkinje fibers (PFs) and M cells were studied at cycle lengths (CL) of 300 to 2000 ms. It had no effects on maximum diastolic potential and action potential amplitude in either tissue. High concentrations induced a moderate, rate-independent decrease of Vmax in M cells. In PFs and M cells, it produced reverse use-dependent lengthening of action potential duration (APD). In PFs at long CL, the drug exhibited a biphasic concentration-dependent effect on APD: maximum prolongation (by 26% at a CL of 2000 ms) was attained at 1 x 10(-6) M, and a decrease of APD occurred at higher concentrations. In M cells, the maximum effect on APD occurred at 3 x 10(-6) M. Early afterdepolarizations were seen in 50% of M cell preparations but only at CL of 2000 ms. Triggered activity did not occur. In summary, LU111995 prolongs the Q-T interval to a limited degree and is not arrhythmogenic over the physiological range of CLs.

The controversial M cell.

A recent publication by us has been interpreted by some as arguing against the existence and importance of M cells. We suppose this is the reason we have been asked to write this "controversy." Regrettably for the controversy, neither our work nor we deny the existence of M cells. Rather, we have confirmed, conceivably ad nauseum, that M cells do exist and contribute importantly to the expression of electrical activity in the intact myocardium. What controversy there is relates to (1) whether there is an inhomogeneous transmural gradient for ventricular repolarization in normal hearts, and (2) why the electrophysiologic properties of different myocardial sites differ so markedly at the level of the isolated tissue and single cell and yet become so much more homogenous in the intact ventricle. These issues are addressed on the following pages.

[The action of insulin on cardiac contractility in active, hibernating and arousing susliks Citellus undulatus]. / Vliianie insulina na izmenenie napravleniia inotropii sokrashcheniia miokarda giberniruiushchego suslika Citellus undulatus na razlichnykh stadiiakh protsessa probuzhdeniia.

During the deep hibernation (at 5-6 degrees C of the heart temperature) and during arousal from hibernation (at 15-16 degrees C) insulin have no effect on contractility. Two opposite inotropic effects of insulin at concentrations 0.1-50 nM were found at higher temperature of arousing: a transient positive inotropic effect between 21-28 degrees C, and a negative one (about 20-30% from the control value) above 28 degrees C. In active summer and winter animals insulin produced mainly the negative inotropic effect.

[An electrophysiological study of the anti-arrhythmia effect of antioxidant ionol]. / Elektrofiziologicheskoe issledovanie antiaritmicheskogo deistviia antioksidanta ionola.

The paper describes the study of anti-arrhythmia effects of ionol. In isolated rabbit papillary muscle, ionol (a) had no effect on the depolarization-induced automaticity; (b) did not influence early afterdepolarizations: (c) delayed the onset of post-depolarizations initiated by Ca-overload and therefore inhibited the ectopic focal activity in myocardium. In isolated left auricles of rabbit, ionol suppressed the shortening of the excitation wavelength induced with adrenaline and thus protected the heart of reentry and consequent rhythm disturbances.

Positive inotropic effect of ryanodine on rabbit ventricular muscle: dependence on the intracellular calcium load.

Two types of electrical and mechanical responses to 1 mumol/l ryanodine, depending on the intracellular calcium load, were observed in rabbit papillary muscles. In a normal calcium solution, ryanodine induced a transient decline followed by a stable increase in the developed force (by 20 +/- 5% of the pretreatment level; n = 30) and prolonged the action potential (AP). The positive ryanodine response showed an increased time-to-peak force and was completely suppressed by 2 mumol/l nifedipine, partially blocked by 50 mumol/l tetracaine (Ca2+ release blocker), but greatly potentiated by 20 mmol/l CsCl or (-) Bay R 5414 which prolonged the AP. The prolonged time-to-peak force of the positive ryanodine response was shortened by procedures raising the content of Ca2+ in the sarcoplasmic reticulum (SR). It is suggested that the initial decline in the force amplitude results from Ca2+ leakage from the SR which is further compensated for by an elevation of both the transmembrane Ca2+ entry and intracellular Ca2+ release. In calcium overloaded myocardium, 1 mumol/l ryanodine caused irreversible contracture and dramatic AP shortening, explained by a massive Ca2+ release from the overloaded SR into the cytoplasm. It is concluded that the calcium content in the SR is the main modulator of the electrical and mechanical effects of ryanodine in ventricular myocardium.

The "pacemaker" function of the transient outward current in the rabbit myocardium.

The single sucrose gap technique was employed to study the electrically induced automaticity in rabbit papillary muscles. When the potential was clamped at the level of the "maximum diastolic potential" following the first spike of automaticity an initial decline of the outward ionic current with subsequent activation of the delayed potassium current was observed. The initial decline was potential-sensitive with a maximum at approximately -2 mV; it diminished when the rate of stimulation increased and was abolished with 4-aminopyridine plus Sr2+. It is suggested that the transient outward current determines the development of the "pacemaker potential" after the first spike of electrically induced automaticity in rabbit papillary muscles.

Transient outward current and rate dependence of action potential duration in rabbit cardiac ventricular muscle.

A conventional (single sucrose gap) voltage clamp technique was employed to investigate the rate dependence of ionic currents activated in the plateau range of potential in the rabbit ventricular muscle. A transient outward current of increasing amplitude was observed when the period of rest preceding the test voltage clamp pulse was increased from 0.7-60 s. The action potential duration was short when the transient outward current peak (100-150 ms after the voltage clamp pulse beginning) was high under the studied conditions of stimulation (interbeat intervals 0.7-60 s). The rate dependent transient outward current was small at low levels of depolarization above the resting potential (40 mV), had a maximum at some 90-100 mV and decreased at more positive potentials. This current was sensitive to the simultaneous application of 4-aminopyridine and calcium substitution with strontium in the Tyrode solution. It is suggested that the transient outward current is probably responsible for the changes of the action potential duration in rabbit papillary muscles when the interbeat interval varies from some 0.7-60 s.

The prolongation of the action potential in mammalian ventricular muscle at rest. Is it due to intracellular calcium content changes.

The conventional microelectrode technique was used to investigate the effect of long periods of rest (greater than 10s) on the action potential duration of mammalian ventricular muscle. The action potential duration increased as the rest period increased. This prolongation of the action potential was the greatest and the slowest in rabbit ventricle, was smaller and more rapid in guinea-pig ventricle, and was practically absent in rat ventricle. The prolongation of the action potential at rest was suppressed with aminazine and strophanthin. Low sodium concentration, lanthanum ions, ruthenium red and acidosis failed to suppress the slow prolongation. It is concluded that the slow prolongation of the action potential at rest is related to changes in the intracellular calcium content induced by a mechanism different from Na/Ca exchange.

Action potentials of the right ventricular working myocardium of newborn and adult rabbits under paired stimulation conditions.

The effect of paired stimulation on the action potentials of the working ventricular myocardium of newborn and adult rabbits was studied with the aim of elucidating the postnatal development of electrogenesis in cardiac cells. After a 2 minutes' rest pause we applied two identical rectangular electric pulses (duration 0.1-1 ms, voltage double the stimulation threshold) with an exactly defined interval, T (100 ms to 20 s). In the recordings we evaluated the duration of the plateau phase of the first (D1) and the second (D2) action potential for given stimulation pairs. The area formed by the course of the first (A1) and second (A2) action potential above the resting membrane potential value was determined planimetrically. Graphs of the correlations of D2/D1 to T and of A2/A1 to T were constructed for the evaluation. The D2/D1 curve for adult animals rose from minimum T values, attained the maximum at T=260 ms and then gradually fell. In newborn rabbits, the corresponding curve, in the majority of cases, displayed a value of T=1. The correlations of A2/A1 to T in adult animals were similar to the course of the D2/D1 curve, and the same applied to newborn animals. The results show that the working right ventricular myocardium of the adult rabbit is capable of reacting to altered stimulation intervals by a change in the duration of the plateau phase, while the myocardium of newborn animals is frequency-insensitive. The rest pause did not affect the duration of action potentials in newborn rabbits, but in adult animals it caused marked shortening of the action potential. The differences found between the neonatal and adult myocardium are probably related to rhe characteristics of the channel systems of the slow Ca-dependent inward current.

[Effect of preliminary stimulation on the repetitive firing of rabbit papillary muscle fibers]. / Effekt predvaritel'noi stimuliatsii na povtornye otvety volokon papilliarnykh myshts krolika.

It has been shown that the preliminary stimulation inhibits the initiation of repetitive firings (RF), increases the threshold current of RF and decreases membrane depolarization, induced by the action of constant current. It has been shown, that the higher the stimulation frequency, the higher the threshold current of RF and the lower the level of depolarization. It has been established that the time necessary for the restitution of the control firings of the tissue to the constant current after the action of the high-frequency stimulation does not exceed 2 minutes. It has been shown that the behavior of membrane potential under the constant current is characterized by two particularities: a) dissociation of the first response to switching on the constant current' b) a strong decay of the membrane potential to the resting potential.