Separation of early afterdepolarizations from arrhythmogenic substrate in the isolated perfused hypokalaemic murine heart through modifiers of calcium homeostasis

Aims: We resolved roles for early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in Langendorff‐perfused hypokalaemic murine hearts paced from the right ventricular epicardium. Methods: Left ventricular epicardial and endocardial monophasic action potentials (MAPs) and arrhythmogenic tendency were compared in the presence and absence of the L‐type Ca²⁺ channel blocker nifedipine (10 nm –1 μm ) and the calmodulin kinase type II inhibitor KN‐93 (2 μm ). Results: All the hypokalaemic hearts studied showed prolonged epicardial and endocardial MAPs, decreased epicardial‐endocardial APD₉₀ difference, EADs, triggered beats and ventricular tachycardia (VT) (n = 6). In all spontaneously beating hearts, 100 (but not 10) nm nifedipine reduced both the incidence of EADs and triggered beats from 66.9 ± 15.7% to 28.3 ± 8.7% and episodes of VT from 10.8 ± 6.3% to 1.2 ± 0.7% of MAPs (n = 6 hearts, P < 0.05); 1 μm nifedipine abolished all these phenomena (n = 6). In contrast programmed electrical stimulation (PES) still triggered VT in six of six hearts with 0, 10 and 100 nm but not 1 μm nifedipine. 1 μm nifedipine selectively reduced epicardial (from 66.1 ± 3.4 to 46.2 ± 2.5 ms) but not endocardial APD₉₀, thereby restoring ΔAPD₉₀ from ?5.9 ± 2.5 to 15.5 ± 3.2 ms, close to normokalaemic values. KN‐93 similarly reduced EADs, triggered beats and VT in spontaneously beating hearts to 29.6 ± 8.9% and 1.7 ± 1.1% respectively (n = 6) yet permitted PES‐induced VT (n = 6), in the presence of a persistently negative ΔAPD₉₀. Conclusions: These findings empirically implicate both EADs and triggered beats alongside arrhythmogenic substrate of ΔAPD₉₀ in VT pathogenesis at the whole heart level.     

Arrhythmogenic mechanisms in the isolated perfused hypokalaemic murine heart

Aim: Hypokalaemia is associated with a lethal form of ventricular tachycardia (VT), torsade de pointes, through pathophysiological mechanisms requiring clarification. Methods: Left ventricular endocardial and epicardial monophasic action potentials were compared in isolated mouse hearts paced from the right ventricular epicardium perfused with hypokalaemic (3 and 4 mm [K⁺]_o) solutions. Corresponding K⁺ currents were compared in whole‐cell patch‐clamped epicardial and endocardial myocytes. Results: Hypokalaemia prolonged epicardial action potential durations (APD) from mean APD₉₀s of 37.2 ± 1.7 ms (n = 7) to 58.4 ± 4.1 ms (n =7) and 66.7 ± 2.1 ms (n = 11) at 5.2, 4 and 3 mm [K⁺]_o respectively. Endocardial APD₉₀s correspondingly increased from 51.6 ± 1.9 ms (n = 7) to 62.8 ± 2.8 ms (n = 7) and 62.9 ± 5.9 ms (n = 11) giving reductions in endocardial–epicardial differences, ΔAPD₉₀, from 14.4 ± 2.6 to 4.4 ± 5.0 and ?3.4 ± 6.0 ms respectively. Early afterdepolarizations (EADs) occurred in epicardia in three of seven spontaneously beating hearts at 4 mm [K⁺]_o with triggered beats followed by episodes of non‐sustained VT in nine of 11 preparations at 3 mm . Programmed electrical stimulation never induced arrhythmic events in preparations perfused with normokalemic solutions yet induced VT in two of seven and nine of 11 preparations at 4 and 3 mm [K⁺]_o respectively. Early outward K⁺ current correspondingly fell from 73.46 ± 8.45 to 61.16±6.14 pA/pF in isolated epicardial but not endocardial myocytes (n = 9) (3 mm [K⁺]_o). Conclusions: Hypokalaemic mouse hearts recapitulate the clinical arrhythmogenic phenotype, demonstrating EADs and triggered beats that might initiate VT on the one hand and reduced transmural dispersion of repolarization reflected in ΔAPD₉₀ suggesting arrhythmogenic substrate on the other.     

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim: To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3. Methods: Left ventricular endocardial and epicardial monophasic action potentials and arrhythmogenic tendency were compared in isolated wild type (WT) and Scn5a+/Δ hearts perfused with 0.1 and 1 μm propranolol and paced from the right ventricular epicardium. Results: All spontaneously beating bradycardic Scn5a+/Δ hearts displayed EADs, triggered beats and ventricular tachycardia (VT; n = 7), events never seen in WT hearts (n = 5). Perfusion with 0.1 and 1 μm propranolol suppressed all EADs, triggered beats and episodes of VT. In contrast, triggering of VT persisted following programmed electrical stimulation in 6 of 12 (50%), one of eight (12.5%), but six of eight (75%) Scn5a+/Δ hearts perfused with 0, 0.1 and 1 μm propranolol respectively in parallel with corresponding alterations in repolarization gradients, reflected in action potential duration (ΔAPD₉₀) values. Thus 0.1 μm propranolol reduced epicardial but not endocardial APD₉₀ from 54.7 ± 1.6 to 44.0 ± 2.0 ms, restoring ΔAPD₉₀ from ?3.8 ± 1.6 to 3.5 ± 2.5 ms (all n = 5), close to WT values. However, 1 μm propranolol increased epicardial APD₉₀ to 72.5 ± 1.2 ms and decreased endocardial APD₉₀ from 50.9 ± 1.0 to 24.5 ± 0.3 ms, increasing ΔAPD₉₀ to ?48.0 ± 1.2 ms. Conclusion: These findings empirically implicate EADs in potentially initiating spontaneous arrhythmogenic phenomena and transmural repolarization gradients in the re‐entrant substrate that would sustain such activity when provoked by extrasystolic activity in murine hearts modelling human LQT3 syndrome.     

A quantitative analysis of the effect of cycle length on arrhythmogenicity in hypokalaemic Langendorff-perfused murine hearts

The clinically established proarrhythmic effect of bradycardia and antiarrhythmic effect of lidocaine (10 μM) were reproduced in hypokalaemic (3.0 mM K⁺) Langendorff-perfused murine hearts paced over a range (80–180 ms) of baseline cycle lengths (BCLs). Action potential durations (at 90% repolarization, APD₉₀s), transmural conduction times and ventricular effective refractory periods (VERPs) were then determined from monophasic action potential records obtained during a programmed electrical stimulation procedure in which extrasystolic stimuli were interposed following regular stimuli at successively decreasing coupling intervals. A novel graphical analysis of epicardial and endocardial, local and transmural relationships between APD₉₀, corrected for transmural conduction time where appropriate, and VERP yielded predictions in precise agreement with the arrhythmogenic findings obtained over the entire range of BCLs studied. Thus, in normokalaemic (5.2 mM K⁺) hearts a statistical analysis confirmed that all four relationships were described by straight lines of gradients not significantly (P > 0.05) different from unity that passed through the origin and thus subtended constant critical angles, θ with the abscissa (45.8° ± 0.9°, 46.6° ± 0.5°, 47.6° ± 0.5° and 44.9° ± 0.8°, respectively). Hypokalaemia shifted all points to the left of these reference lines, significantly (P < 0.05) increasing θ at BCLs of 80–120 ms where arrhythmic activity was not observed (∼63°, ∼54°, ∼55° and ∼58°, respectively) and further significantly (P < 0.05) increasing θ at BCLs of 140–180 ms where arrhythmic activity was observed (∼68°, ∼60°, ∼61° and ∼65°, respectively). In contrast, the antiarrhythmic effect of lidocaine treatment was accompanied by a significant (P < 0.05) disruption of this linear relationship and decreases in θ in both normokalaemic (∼40°, ∼33°, ∼39° and ∼41°, respectively) and hypokalaemic (∼40°, ∼44°, ∼50° and ∼48°, respectively) hearts. This extended a previous approach that had correlated alterations in transmural repolarization gradients with arrhythmogenicity in murine models of the congenital long QT syndrome type 3 and hypokalaemia at a single BCL. Thus, the analysis in terms of APD₉₀ and VERP provided a more sensitive indication of the effect of lidocaine than one only considering transmural repolarization gradients and may be particularly applicable in physiological and pharmacological situations in which these parameters diverge.     

Restitution analysis of alternans and its relationship to arrhythmogenicity in hypokalaemic Langendorff-perfused murine hearts

Alternans and arrhythmogenicity were studied in hypokalaemic (3.0 mM K⁺) Langendorff-perfused murine hearts paced at high rates. Epicardial and endocardial monophasic action potentials were recorded and durations quantified at 90% repolarization. Alternans and arrhythmia occurred in hypokalaemic, but not normokalaemic (5.2 mM K⁺) hearts (P < 0.01): this was prevented by treatment with lidocaine (10 μM, P < 0.01). Fourier analysis then confirmed transition from monomorphic to polymorphic waveforms for the first time in the murine heart. Alternans and arrhythmia were associated with increases in the slopes of restitution curves, obtained for the first time in the murine heart, while the anti-arrhythmic effect of lidocaine was associated with decreased slopes. Thus, hypokalaemia significantly increased (P < 0.05) maximal gradients (from 0.55 ± 0.14 to 2.35 ± 0.67 in the epicardium and from 0.67 ± 0.13 to 1.87 ± 0.28 in the endocardium) and critical diastolic intervals (DIs) at which gradients equalled unity (from −2.14 ± 0.52 ms to 50.93 ± 14.45 ms in the epicardium and from 8.14 ± 1.49 ms to 44.64 ± 5 ms in the endocardium). While treatment of normokalaemic hearts with lidocaine had no significant effect (P > 0.05) on either maximal gradients (0.78 ± 0.27 in the epicardium and 0.83 ± 0.45 in the endocardium) or critical DIs (6.06 ± 2.10 ms and 7.04 ± 3.82 ms in the endocardium), treatment of hypokalaemic hearts with lidocaine reduced (P < 0.05) both these parameters (1.05 ± 0.30 in the epicardium and 0.89 ± 0.36 in the endocardium and 30.38 ± 8.88 ms in the epicardium and 31.65 ± 4.78 ms in the endocardium, respectively). We thus demonstrate that alternans contributes a dynamic component to arrhythmic substrate during hypokalaemia, that restitution may furnish an underlying mechanism and that these phenomena are abolished by lidocaine, both recapitulating and clarifying clinical findings.     

The contribution of refractoriness to arrhythmic substrate in hypokalemic Langendorff-perfused murine hearts

The clinical effects of hypokalemia including action potential prolongation and arrhythmogenicity suppressible by lidocaine were reproduced in hypokalemic (3.0 mM K⁺) Langendorff-perfused murine hearts before and after exposure to lidocaine (10 μM). Novel limiting criteria for local and transmural, epicardial, and endocardial re-excitation involving action potential duration (at 90% repolarization, APD₉₀), ventricular effective refractory period (VERP), and transmural conduction time (Δlatency), where appropriate, were applied to normokalemic (5.2 mM K⁺) and hypokalemic hearts. Hypokalemia increased epicardial APD₉₀ from 46.6 ± 1.2 to 53.1 ± 0.7 ms yet decreased epicardial VERP from 41 ± 4 to 29 ± 1 ms, left endocardial APD₉₀ unchanged (58.2 ± 3.7 to 56.9 ± 4.0 ms) yet decreased endocardial VERP from 48 ± 4 to 29 ± 2 ms, and left Δlatency unchanged (1.6 ± 1.4 to 1.1 ± 1.1 ms; eight normokalemic and five hypokalemic hearts). These findings precisely matched computational predictions based on previous reports of altered ion channel gating and membrane hyperpolarization. Hypokalemia thus shifted all re-excitation criteria in the positive direction. In contrast, hypokalemia spared epicardial APD₉₀ (54.8 ± 2.7 to 60.6 ± 2.7 ms), epicardial VERP (84 ± 5 to 81 ± 7 ms), endocardial APD₉₀ (56.6 ± 4.2 to 63.7 ± 6.4 ms), endocardial VERP (80 ± 2 to 84 ± 4 ms), and Δlatency (12.5 ± 6.2 to 7.6 ± 3.4 ms; five hearts in each case) in lidocaine-treated hearts. Exposure to lidocaine thus consistently shifted all re-excitation criteria in the negative direction, again precisely agreeing with the arrhythmogenic findings. In contrast, established analyses invoking transmural dispersion of repolarization failed to account for any of these findings. We thus establish novel, more general, criteria predictive of arrhythmogenicity that may be particularly useful where APD₉₀ might diverge sharply from VERP.     

Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart

The recently described exchange protein directly activated by cAMP (Epac) has been implicated in distinct protein kinase A-independent cellular signalling pathways. We investigated the role of Epac activation in adrenergically mediated ventricular arrhythmogenesis. In contrast to observations in control conditions (n = 20), monophasic action potentials recorded in 2 of 10 intrinsically beating and 5 of 20 extrinsically paced Langendorff-perfused wild-type murine hearts perfused with the Epac activator 8-pCPT-2′-O-Me-cAMP (8-CPT, 1 μM) showed spontaneous triggered activity. Three of 20 such extrinsically paced hearts showed spontaneous ventricular tachycardia (VT). Programmed electrical stimulation provoked VT in 10 of 20 similarly treated hearts (P < 0.001; n = 20). However, there were no statistically significant accompanying changes (P > 0.05) in left ventricular epicardial (40.7 ± 1.2 versus 44.0 ± 1.7 ms; n = 10) or endocardial action potential durations (APD₉₀; 51.8 ± 2.3 versus 51.9 ± 2.2 ms; n = 10), transmural (ΔAPD₉₀) (11.1 ± 2.6 versus 7.9 ± 2.8 ms; n = 10) or apico-basal repolarisation gradients, ventricular effective refractory periods (29.1 ± 1.7 versus 31.2 ± 2.4 ms in control and 8-CPT-treated hearts, respectively; n = 10) and APD₉₀ restitution characteristics. Nevertheless, fluorescence imaging of cytosolic Ca²⁺ levels demonstrated abnormal Ca²⁺ homeostasis in paced and resting isolated ventricular myocytes. Epac activation using isoproterenol in the presence of H-89 was also arrhythmogenic and similarly altered cellular Ca²⁺ homeostasis. Epac-dependent effects were reduced by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibition with 1 μM KN-93. These findings associate VT in an intact cardiac preparation with altered cellular Ca²⁺ homeostasis and Epac activation for the first time, in the absence of altered repolarisation gradients previously implicated in reentrant arrhythmias through a mechanism dependent on CaMKII activity.     

Empirical correlation of triggered activity and spatial and temporal re-entrant substrates with arrhythmogenicity in a murine model for Jervell and Lange-Nielsen syndrome

KCNE1 encodes the β-subunit of the slow component of the delayed rectifier K⁺ current. The Jervell and Lange-Nielsen syndrome is characterized by sensorineural deafness, prolonged QT intervals, and ventricular arrhythmogenicity. Loss-of-function mutations in KCNE1 are implicated in the JLN2 subtype. We recorded left ventricular epicardial and endocardial monophasic action potentials (MAPs) in intact, Langendorff-perfused mouse hearts. KCNE1 ^−/− but not wild-type (WT) hearts showed not only triggered activity and spontaneous ventricular tachycardia (VT), but also VT provoked by programmed electrical stimulation. The presence or absence of VT was related to the following set of criteria for re-entrant excitation for the first time in KCNE1 ^−/− hearts: Quantification of APD₉₀, the MAP duration at 90% repolarization, demonstrated alterations in (1) the difference, ∆APD₉₀, between endocardial and epicardial APD₉₀ and (2) critical intervals for local re-excitation, given by differences between APD₉₀ and ventricular effective refractory period, reflecting spatial re-entrant substrate. Temporal re-entrant substrate was reflected in (3) increased APD₉₀ alternans, through a range of pacing rates, and (4) steeper epicardial and endocardial APD₉₀ restitution curves determined with a dynamic pacing protocol. (5) Nicorandil (20 μM) rescued spontaneous and provoked arrhythmogenic phenomena in KCNE1 ^−/− hearts. WTs remained nonarrhythmogenic. Nicorandil correspondingly restored parameters representing re-entrant criteria in KCNE1 ^−/− hearts toward values found in untreated WTs. It shifted such values in WT hearts in similar directions. Together, these findings directly implicate triggered electrical activity and spatial and temporal re-entrant mechanisms in the arrhythmogenesis observed in KCNE1 ^−/− hearts.     

Electrophysiological determinants of hypokalaemia‐induced arrhythmogenicity in the guinea‐pig heart

Aim: Hypokalaemia is an independent risk factor contributing to arrhythmic death in cardiac patients. In the present study, we explored the mechanisms of hypokalaemia‐induced tachyarrhythmias by measuring ventricular refractoriness, spatial repolarization gradients, and ventricular conduction time in isolated, perfused guinea‐pig heart preparations. Methods: Epicardial and endocardial monophasic action potentials from distinct left ventricular (LV) and right ventricular (RV) recording sites were monitored simultaneously with volume‐conducted electrocardiogram (ECG) during steady‐state pacing and following a premature extrastimulus application at progressively reducing coupling stimulation intervals in normokalaemic and hypokalaemic conditions. Results: Hypokalaemic perfusion (2.5 mm K⁺ for 30 min) markedly increased the inducibility of tachyarrhythmias by programmed ventricular stimulation and rapid pacing, prolonged ventricular repolarization and shortened LV epicardial and endocardial effective refractory periods, thereby increasing the critical interval for LV re‐excitation. Hypokalaemia increased the RV‐to‐LV transepicardial repolarization gradients but had no effect on transmural dispersion of APD₉₀ and refractoriness across the LV wall. As determined by local activation time recordings, the LV‐to‐RV transepicardial conduction and the LV transmural (epicardial‐to‐endocardial) conduction were slowed in hypokalaemic heart preparations. This change was attributed to depressed diastolic excitability as evidenced by increased ventricular pacing thresholds. Conclusion: These findings suggest that hypokalaemia‐induced arrhythmogenicity is attributed to shortened LV refractoriness, increased critical intervals for LV re‐excitation, amplified RV‐to‐LV transepicardial repolarization gradients and slowed ventricular conduction in the guinea‐pig heart.     

TNF‐α hyperpolarizes membrane potential and potentiates the response to nicotinic receptor stimulation in cultured rat myenteric neurones

Aim: Mechanically induced early afterdepolarization (EAD) is morphologically similar but different in the mechanisms with drug‐induced EAD, which lead to arrhythmia. Pacing suppresses the drug‐induced EAD and arrhythmia, however the effect of pacing on mechanically induced EAD and arrhythmia is not clear. This study addressed this issue in right ventricle (RV) of anaesthetized lambs. Methods: Six lambs were anaesthetized, and their hearts exposed. Nine monophasic action potential (MAP) electrodes were placed on RV apex, outflow and inflow regions, and recorded before, during, and after a 10 s occlusion of pulmonary artery at a number of pacing rates. Results: Pacing significantly reduced the baseline MAP duration at 90% repolarization (MAPD₉₀), decreased the reduction of MAPD at early repolarization at the peak of occlusion. Nonetheless, the percentage of reduction was not significantly different among them. Pacing was able to reduce the frequencies, size of mechanically induced EADs. MAPD₉₀ at the peak of occlusion was all shortened during pacing rather than some lengthened at intrinsic rate. Therefore, the dispersion of MAPD₉₀ at the peak of occlusion reduced from 86 ± 6 ms at intrinsic rate to 42 ± 4 ms at 120 beats min⁻¹ , 38 ± 3 ms at 150 beats min⁻¹ and 26 ± 3 ms at 170 beats min⁻¹. Ultimately, pacing reduced/suppressed mechanically induced premature ventricular beats. These alterations were inversely related to heart rates. Conclusion: Pacing reduces/suppresses both stretch‐induced EADs and arrhythmia. These modulations are remarkably similar to those on other EADs by the pacing.     

Insulin modulation of ATP-sensitive K+ channel of rat skeletal muscle is impaired in the hypokalaemic state

 In the present work, we examined the effects of in vivo administration of insulin to rats made hypokalaemic by feeding a K⁺-free diet. The i.p. injection of insulin in the hypokalaemic rats provoked muscle paralysis within 3–5 h. Consistent with this observation, the skeletal muscle fibres of the paralysed rats were depolarized. In contrast, in the normokalaemic animals, insulin neither provoked paralysis nor produced significant fibre hyperpolarization. In the hypokalaemic rats, insulin almost completely abolished the sarcolemma adenosine triphosphate (ATP)-sensitive K⁺ currents without altering the sensitivity of the channels to ATP or glibenclamide. In contrast, in the normokalaemic rats, insulin enhanced ATP-sensitive K⁺ currents that became also resistant to ATP and glibenclamide. Our experiments indicate that the modulation of the sarcolemma ATP-sensitive K⁺ channels by insulin is impaired in the hypokalaemic state. This phenomenon appears to be related to the fibre depolarization and paralysis observed in the same animals. Received: 21 July 1998 / Received after revision: 17 September 1998 / Accepted: 25 September 1998     

Influence of postnatal changes in action potential duration on Na-Ca exchange in rabbit ventricular myocytes

 Cardiac Na-Ca exchanger (NCX) expression and current density are significantly greater in newborn rabbit hearts compared with adults. However, the relatively short action potential (AP) at birth may limit the impact of increased NCX expression by diminishing Ca²⁺ entry via Na-Ca exchange current (I _NaCa). To address the interdependence of AP duration and NCX activity, we voltage-clamped newborn (NB, 1–5 day), juvenile (JV, 10–14 day) and adult (AD) rabbit myocytes with a series of APs of progressively increasing duration (APD₉₀: 108–378 ms) under nominally chloride-free conditions. In each age group we quantified an increase in outward (Q _Exout) and inward (Q _Exin) Ni²⁺-sensitive charge movement in response to AP prolongation. Q _Exout and Q _Exin measured during age-appropriate APs declined postnatally [Q _EXout: NB (2 day) 0.19 ± 0.02, JV (10 day) 0.10 ± 0.01, AD 0.04 ± 0.002; Q _EXin: NB –0.2 ± 0.01, JV –0.11 ± 0.02; AD –0.04 ± 0.003 pC/pF] despite the significantly shorter APD₉₀ of newborn myocytes (NB 122 ± 10; AD 268 ± 22 ms). When Ca²⁺ fluxes by other transport pathways were blocked with nifedipine, ryanodine and thapsigargin, age-appropriate APs elicited contractions in NB and JV but not AD myocytes (NB 4.8 ± 0.5, JV 1.2 ± 0.3% resting length). These data demonstrate that a shorter AP does not negate the impact of increased NCX expression at birth. Received: 23 September 1997 / Received after revision: 2 January 1998 / Accepted: 5 January 1998     

血小板活化因子对豚鼠心室肌细胞钾电流及动作电位的影响

 目的：研究血小板活化因子(PAF)对豚鼠心室肌细胞钾电流及动作电位的影响。 方法:应用全细胞膜片钳技术，记录豚鼠心室肌细胞动作电位及钾电流(I_K 与I_K1)。 结果:当电极内液ATP浓度为5 mmol/L，1 μmol/L PAF使APD₉₀由对照的(225.8±23.3)ms延长至(352.8±29.8)ms(n=5, P<0.05)；使I_K尾电流在指令电压 +30 mV 时由对照的(173.5±16.7)pA降为(152.1±11.5)pA(P<0.05, n=4)；使I_K1在指令电压 -120 mV 时从(-6.1±1.3)nA降为(-5.6±1.1)nA(P<0.05, n=5)；当电极内液ATP 为0 mmol/L，APD₉₀明显缩短，1 μmol/L PAF使APD₉₀由对照的(153.0±24.6)ms缩短为(88.2±19.4)ms (n=5, P<0.01)，而用1 μmol/L格列本脲 ( I_KATP特异性阻滞剂)预处理后，恢复了PAF可显著延长动作电位时程的作用。 结论: PAF使缺血区K_ATP开放，动作电位时程缩短，却可抑制正常区I_K 与I_K1，使动作电位延长，从而放大了缺血区与正常区的不均一性，这可能与缺血时心律失常的发生有关。     

Atrial arrhythmogenesis in wild-type and Scn5a+/Δ murine hearts modelling LQT3 syndrome

Long QT(3) (LQT3) syndrome is associated with abnormal repolarisation kinetics, prolonged action potential durations (APD) and QT intervals and may lead to life-threatening ventricular arrhythmias. However, there have been few physiological studies of its effects on atrial electrophysiology. Programmed electrical stimulation and burst pacing induced atrial arrhythmic episodes in 16 out of 16 (16/16) wild-type (WT) and 7/16 genetically modified Scn5a+/Δ (KPQ) Langendorff-perfused murine hearts modelling LQT3 (P < 0.001 for both), and in 14/16 WT and 1/16 KPQ hearts (P < 0.001 for both; Fisher’s exact test), respectively. The arrhythmogenic WT hearts had significantly larger positive critical intervals (CI), given by the difference between atrial effective refractory periods (AERPs) and action potential durations at 90% recovery (APD₉₀), compared to KPQ hearts (8.1 and 3.2 ms, respectively, P < 0.001). Flecainide prevented atrial arrhythmias in all arrhythmogenic WT (P < 0.001) and KPQ hearts (P < 0.05). It prolonged the AERP to a larger extent than it did the APD₉₀ in both WT and KPQ groups, giving negative CIs. Quinidine similarly exerted anti-arrhythmic effects, prolonged AERP over corresponding APD₉₀ in both WT and KPQ groups. These findings, thus, demonstrate, for the first time, inhibitory effects of the KPQ mutation on atrial arrhythmogenesis and its modification by flecainide and quinidine. They attribute these findings to differences in the CI between WT and mutant hearts, in the presence or absence of these drugs. Thus, prolongation of APD₉₀ over AERP gave positive CI values and increased atrial arrhythmogenicity whereas lengthening of AERP over APD₉₀ reduced such CI values and produced the opposite effect.     

Effect of catecholamines on the ventricular myocyte action potential in raised extracellular potassium

We describe the relationship between catecholamines and raised extracellular potassium ([K⁺]_o) on action potential parameters and calcium currents in isolated ventricular myocytes of the guinea-pig and relate these findings to the problem of understanding how the heart is protected from exercise-induced hyperkalaemia ([K⁺]_a up to 8.5 mm ). Action potential duration (APD₉₀), amplitude and upstroke velocity were recorded in stimulated (2Hz) guinea-pig ventricular myocytes using whole-cell patch electrode recordings (37 ±C). Cells were superfused with normal K⁺Tyrode and with raised K⁺Tyrode in the presence of either noradrenaline, adrenaline or raised calcium. Inward calcium current was measured using voltage clamp. Raised K⁺(8, 12, 16 mm K⁺Tyrode) caused a significant (P < 0.01) depolarisation, shortened the APD₉₀ and decreased the action potential amplitude and upstroke velocity. In raised K⁺Tyrode addition of noradrenaline (0.08–0.1 μm ) or adrenaline (0.1–0.2 μm ) increased action potential amplitude (P < 0.01), APD₉₀ (P < 0.01) and upstroke velocity (P < 0.01) (measured only in 16 mm K⁺Tyrode). In 12 mm K⁺Tyrode raised Ca²⁺(5–6 mm ) increased action potential amplitude (P < 0.05) and shortened APD₉₀ (P < 0.05). Addition of NA (0.08–0.1 μm ) increased the inward Ca²⁺current. All effects were fully reversible. In raised [K⁺]_o increases in catecholamines and [Ca²⁺]_o cause changes in action potential parameters that would be expected to maintain propagation of the cardiac action potential in the whole heart. Thus, in the ventricular myocyte the increase in conductance to Ca²⁺caused by catecholamines may be one factor that is important in minimising the potentially adverse effects of exercise-induced hyperkalaemia.     

Adenosine induces prolonged anti‐β‐adrenergic effects in guinea‐pig papillary muscle

A sustained anti‐β‐adrenergic effect of adenosine has been reported. This study was initiated to investigate this topic and especially elucidate the role of protein kinase C (PKC). Contractile force amplitude and action potential duration at 90% repolarization (APD₉₀) were measured in guinea‐pig papillary muscles before and after 5 min challenge with 5 nm isoproterenol. Protocols contained 30 min exposure to the test agents adenosine 33 μm (ado), adenosine + PKC‐inhibitor bisindolylmaleimide 20 nM (ado + BIM), PKC‐activator 1,2‐dioctanoyl‐sn‐glycerol 10 μm (DOG) and α‐agonist phenylephrine 5 μm (phe). Isoproterenol was given at the end of test exposure and after 15 min washout. Results are mean ± SEM of percentage‐change, P ≤ 0.05 considered significant and labelled *. The first isoproterenol challenge significantly increased contractile force (27 ± 7%*) in the control group. Responses in the test groups were 2 ± 4 (ado), 1 ± 5 (ado + BIM), 14 ± 4* (DOG), 0 ± 2% (phe). After washout of adenosine, DOG and phenylephrine, isoproterenol induced 3 ± 8 (ado), 23 ± 5* (ado + BIM), 13 ± 5* (DOG), 15 ± 7% (phe) increase in test groups compared with 22 ± 5%* increase in contractile force in the control group. After 45 min washout of adenosine the inotropic response was still significantly reduced compared with control (29 ± 4 vs. 79 ± 8%*). Isoproterenol stimulation shortened APD₉₀ in controls at both time points (5 ± 1%* and 4 ± 1%*), with no significant shortening in test groups. Adenosine induces sustained anti‐β‐adrenergic effects on contractile force as well as APD₉₀. A role for PKC in signal transduction is supported with respect to contractile force.     

The thermodynamics of polymerisation of aldehydes and cyclic ethers. Part II. Tetrahydrofuran and 1.3 dioxolan

The equilibrium between gaseous monomer and amorphous polymer has been studied for tetrahydrofuran and 1.3-dioxolan between 40 and 80°C. The approach to equilibrium catalysed by cationic initiators was extremely slow for tetrahydrofuran but rapid for 1.3-dioxolan. From the equilibrium pressures of monomer the ΔH⁰_gc and ΔS⁰_gc values have been calculated. By means of thermodynamic data for the vaporisation of the two monomers, obtained from vapour pressure measurements, we have also calculated ΔH⁰_1c and ΔS⁰_1c. For tetrahydrofuran:

• ΔH⁰_gc (298°K) = ?39.7 ± 2.3 kJ · mole^?1; ΔH⁰_1c (298°K) = ?7.4 ± 2.3 kJ · mole^?1;
• ΔS⁰_gc (298°K) = ?111.6 ± 6.9 J ·°K^?1 mole^?1; ΔS⁰_1c (298°K) = ?16.2 ± 7.0 J ·°K^?1 mole^?1;

and for 1.3-dioxolan:

• ΔH⁰_gc (298°K) = ?50.1 ± 0.8 kJ · mole^?1; ΔH⁰_1c (298°K) = ?14.6 ± 1.0 kJ · mole^?1;
• ΔS⁰_gc (298°K) = ?139.2 ± 2.2 J ·°K^?1 mole^?1; ΔS⁰_1c (298°K) = ?36.7 ± 3.0 J ·°K^?1 mole^?1;

     

Cell swelling causes the action potential duration to shorten in guinea-pig ventricular myocytes by activating I KATP

 In guinea-pig ventricular myocytes, cell swelling by incubation in hypotonic solution caused a pronounced shortening of the action potential duration (APD₉₀: 15.5±14.6% compared to control; mean ± SD) after a latency of 12 min when the intracellular ATP concentration was 2 mM. This shortening was partially reversible within 10 min after reperfusion with isotonic solution (APD₉₀: 80.5±12.1% compared to control). With 5 mM intracellular ATP in the pipette electrode, the effect of cell swelling on the action potential was significantly reduced. Incubation with 1 μM glibenclamide, a blocker of the ATP-dependent K⁺ current (I _KATP), abolished the swelling-induced shortening of the action potential duration, whereas incubation with 0.5 mM 4,4’-diisothiocyanatostilbene-2,2’-disulphonic acid (DIDS), a blocker of the swelling-induced Cl^– current (I _Cl,swell), had no effect on the action potential duration in hypotonic solution. Simultaneous measurements of membrane currents substantiate that I _KATP is the current that underlies this effect. These results suggest that in the ischaemic myocardium I _KATP may be partially activated by cell swelling, resulting in a shortening of the action potential duration before the intracellular ATP concentration has fallen below 2 mM. Received: 30 March 1998 Received after revision: 7 July 1998 Accepted: 25 July 1998     

Positive inotropic and sustained anti‐β‐adrenergic effect of diadenosine pentaphosphate in human and guinea pig hearts. Role of dinucleotide receptors and adenosine receptors

Aim: Diadenosine polyphosphates are present intracellularly and in extracellular fluid due to release from secretory vesicles in platelets, chromaffin cells and other cells. This study investigates effects of diadenosine pentaphosphate (AP₅A) on heart muscle function. Methods: Contractile force amplitude and action potential duration at 90% repolarization (APD₉₀) were measured after challenge with AP₅A 50 μm or isoproterenol 50–70 nM in guinea pig papillary muscles. Isoproterenol was given immediately after AP₅A‐exposure or after 45 min washout. AP₅A was combined with antagonists to the purinergic P2 receptor (suramin 100 μm ), the dinucleotide receptor [diinosine pentaphosphate 30 μm (IP₅I)] or adenosine receptors [8‐(P‐sulfophenyl) theophylline 50 μm (8‐SPT)]. Results: Results are %‐change (mean ± SEM) from value before exposure. AP₅A increased contractile force by 22 ± 3%* (*P < 0.05), and IP₅I abolished this. AP₅A prolonged APD₉₀ by 7 ± 2%*. AP₅A significantly reduced response to isoproterenol acutely from 31 ± 4* (controls) to 9 ± 4% and after 45 min washout from 61 ± 14* (controls) to 16 ± 5%. 8‐SPT abolished the sustained effect. Increase in contractile force by AP₅A was confirmed in human atria trabecula preparations. Conclusion: AP₅A increased contractile force and prolonged APD₉₀. Contractile force increased by stimulation of the dinucleotide receptor in guinea pig myocardium. The sustained anti‐β‐adrenergic effect of AP₅A was due to adenosine receptor stimulation.     

High salt alone does not influence the kinetics of the Na+-H+antiporter

During a high-salt diet, tubular sodium reabsorption is decreased. This study concerns the effect of a high-salt diet on the proximal tubular (PT) Na⁺influx pathways. Brush-border membrane vesicles (BBMV) were prepared from rats on normal-salt (NS) and rats on high-salt (HS) diets. The initial uptake rates of Na⁺were the same in NS and HS rats, both in the absence and the presence of 1 mm amiloride. V_max and K_m for the amiloride-sensitive Na⁺/H⁺antiporter were also the same in the NS (V_max 3.69 ± 0.31 nmol mg prot^-110 s^-1, K_m 6.13 ± 0.58 mm ) and HS groups (V_max 3.54 ± 0.28 nmol mg prot^-110 s^-1, K_m 6.18 ± 0.64 mm ). There was no difference in the initial uptake rates of the Na⁺-glucose and the Na⁺-alanine symporters in NS and HS. V_max and K_m for the l -dopa-Na⁺symporter were also the same in NS (V_max 72 ± 2.5 pmol mg prot^-120 s^-1, K_m 98 ± 14 μm ) and HS groups (V_max 78 ± 6.0 pmol mg prot^-120 s^-1, K_m 106 ± 4 μm ). In summary, HS diet does not change the kinetics of the Na⁺transporters in the brush-border membrane of PT cells.