Electrophysiological mechanisms of sophocarpine as a potential antiarrhythmic agent.

AIM: To examine the electrophysiological effects of sophocarpine on action potentials (AP) and ionic currents of cardiac myocytes and to compare some of these effects with those of amiodarone. METHODS: Langendorff perfusion set-up was used in isolated guinea pig heart, and responses to sophocarpine were monitored using electrocardiograph. Conventional microelectrode, voltage clamp technique and perforated patch were employed to record fast response AP (fAP), slow response AP (sAP) and ionic currents in guinea pig papillary muscle or rabbit sinus node cells. RESULTS: Tachyarrhythmia produced by isoprenaline (15 µmol/L) could be reversed by sophocarpine (300 µmol/L). Sophocarpine (10 µmol/L) decreased the amplitude by 4.0%, maximal depolarization velocity (V(max)) of the fAP by 24.4%, and Na(+) current (I(Na)) by 18.0%, while it prolonged the effective refractory period (ERP) by 21.1%. The same concentration of sophocarpine could also decrease the amplitude and V(max) of the sAP, by 26.8% and 25.7%, respectively, and attenuated the Ca(2+) current (I(CaL)) and the K(+) tail current substantially. Comparison of sophocarpine with amiodarone demonstrated that both prolonged the duration and the ERP of fAP and sAP, both decreased the amplitude and V(max) of the fAP and sAP, and both slowed the automatic heart rate. CONCLUSION: Sophocarpine could reverse isoprenaline-induced arrhythmia and inhibit I(Na), I(CaL), and I(Kr) currents. The electrophysiological effects of sophocarpine are similar to those of amiodarone, which might be regarded as a prospective antiarrhythmic agent.

Beta-receptor activation increases sodium current in guinea pig heart.

AIM: To study the influence of beta-receptor activation on sodium channel current and the physiological significance of increased sodium current with regard to the increased cardiac output caused by sympathetic excitation. METHODS: Multiple experimental approaches, including ECG, action potential recording with conventional microelectrodes, whole-cell current measurements, single-channel recordings, and pumping-force measurements, were applied to guinea pig hearts and isolated ventricular myocytes. RESULTS: Isoprenaline was found to dose-dependently shorten QRS waves, increase the amplitude and the V(max) of action potentials, augment the fast sodium current, and increase the occurrence frequencies and open time constants of the long-open and burst modes of the sodium channel. Increased levels of membrane-permeable cAMP have similar effects. In the presence of a calcium channel blocker, TTX reversed the increased pumping force produced by isoprenaline. CONCLUSION: Beta-adrenergic modulation increases the inward sodium current and accelerates the conduction velocity within the ventricles by changing the sodium channel modes, which might both be conducive to the synchronous contraction of the heart and enhance its pumping function.

Possible arrhythmiogenic mechanism produced by ibuprofen.

AIM: The aim of the present study was to investigate the electrophysiological effect of ibuprofen on the cardiac action potentials (AP) and electrocardiograms (ECG), and to identify its arrhythmiogenic mechanism. METHODS: The intracellular microelectrode recording technique was employed to record the fast- and slowresponse AP in guinea pig papillary muscles. The cardiac responses of ibuprofen were monitored by ECG, both in in vivo and in vitro studies. RESULTS: The ECG recording revealed that ibuprofen could induce arrhythmias, both in vitro and in vivo. Fatal ventricular fibrillations are readily produced in in vitro experiments by ibuprofen. Our results show that ibuprofen could dose dependently shorten the duration of AP and the effective refractory period (ERP), and it could also decrease the maximum depolarization velocity of phase 0 (V(max)) in both the fast- and slow-response AP. The duration of the QRS complex wave (QRS duration) in ECG was prolonged. Although the heart rate was depressed by ibuprofen, the corrected QT interval duration (QTc) decreased. CONCLUSION: Ibuprofen could inhibit cardiac Na+ and Ca2+ channels as it slows V(max) in both fast- and slowresponse AP. Furthermore, ibuprofen shortens the ERP and decreases the excitation propagation within the heart, which might provide a substrate for an arrhythmiogenic re-entry circuit. Taken together, we conclude that ibuprofen, when used improperly, may impose a potential hazard in inducing cardiac arrhythmias in patients with existing heart diseases.

Reduced sinoatrial cAMP content plays a role in postnatal heart rate slowing in the rabbit.

1. Decreasing heart rate during development is known to be the result of parasympathetic nervous system maturation that depresses the pacemaker current (If) by acetylcholine (ACh). However, a direct effect of ACh on If has been ruled out and the involvement of other secondary messengers, such as cAMP, was verified in previous studies. Therefore, we hypothesized that reduced basal cAMP production in sinoatrial (SA) nodal cells may contribute to the slowing of heart rate after birth. 2. The electrocardiogram and heart rate variability (HRV) were documented and measured in vivo and in vitro (in isolated perfused Langendorff preparations) for rabbits aged 2, 4, 6, 8 and 12 weeks. Sinoatrial node action potential (AP) recording and perforated patch-clamp analyses were used to investigate the spontaneous depolarization rate and pacemaker If currents. Concentrations of cAMP in SA nodal tissues were determined by radioimmunoassay. Relative expression of adenylate cyclases (ADCY1, 5) and phosphodiesterases (PDE1A, 4A and 8A) were quantified by real-time reverse transcription-polymerase chain reaction. 3. Significantly reduced heart rate, but unchanged HRV, was observed in perfused hearts in the older age groups, accompanied with a slowed phase 4 spontaneous depolarization rate (90.5 +/- 4.7 vs 49.6 +/- 2.6 mV/s for 2 week vs 4 week hearts, respectively; n = 5; P < 0.05), a negative shift of the If threshold potential (-45.5 +/- 3.0 vs -51.1 +/- 6.0 mV for 2 week vs 4 week hearts, respectively; n = 9; P < 0.05) and decreasing basal levels of SA nodal cAMP (0.31 +/- 0.05 vs 0.025 +/- 0.002 micromol/L for 2 week vs 4 week hearts, respectively; n = 6; P < 0.05). Gene expression levels of PDE1A, 4A and 8A were increased in the 12 week group compared with the 2 week group 1.5-, 2- and 1.8-fold, respectively (P < 0.05), with little change in ADCY1 and 5. 4. These data suggest that, in addition to autonomic innervation, slowing of heart rate during postnatal maturation can be attributed to a negative shift of the If activation caused by diminished baseline cAMP content in SA nodal cells.

Electrophysiological effect of fluoxetine on Xenopus oocytes heterologously expressing human serotonin transporter.

AIM: To investigate the electrophysiological effect of fluoxetine on serotonin transporter. METHODS: A heterologous expression system was used to introduce human serotonin transporter (hSERT) into Xenopus oocytes. A 2-electrode voltage clamp technique was used to study the pharmacological properties of fluoxetine. RESULTS: hSERT-expressing oocytes were perfused with 10 micromol/L serotonin (5-HT) to induce hSERT-current. The 5-HT-induced hSERT currents were dose-dependently reversed by fluoxetine. The RC50 (concentration that achieved a 50% reversal) was approximately 3.12 micromol/L. Fluoxetine took more time to combine with hSERT than 5-HT did, and it was also slow to dissociate from hSERT. This long-lasting effect of fluoxetine affected normal 5-HT transport. Fluoxetine significantly prolonged the time constant for 5-HT-induced hSERT current. These results might be used to explain the long-lasting anti-anxiety effect of fluoxetine in clinical practice, because it increases the concentration of 5-HT in the synaptic cleft by its enduring suppression of the function of 5-HT transporters. CONCLUSION: Fluoxetine inhibits 5-HT reuptake by competing with 5-HT and changing the normal dynamics of hSERT.

[Establishment of heterologous expression model of hSERT in Xenopus laevis oocytes].

AIM: To determine the feasibility of establishing the heterologous expression model of human- serotonin transporter(hSERT or 5-HTT). METHODS: cRNA of SERT was transcribed from cDNA, which was cloned in the pOTV vector. Each oocyte of mature xenopus laevis was injected with transcribed cRNA in vivo and incubated at room temperature for 4-9 days. Recording the current induced by 5-HT with voltage clamp technique tested the function of the expressed 5-HT transporter. RESULTS: The transporter current could be observed in Ringer's solution containing 5-HT, and the 5-HT induced current were concentration-dependent. Norepinephrine and dopamine could not induce the transporter current while the 5-HT induced current could be specifically inhibited by 5-HTT blocker, desipramine. CONCLUSION: The results demonstrate that the heterologous expression product in xenopus laevis oocytes is human 5-HT transporter.

Effect of mexiletine on long QT syndrome model.

AIM: To make a LQT3 model (one form of the long QT syndromes) and to investigate the effect of mexiletine on LQT3. METHODS: Sea anemone toxin (ATX II) was used to produce the LQT3 model. The Effect of mexiletine on LQT3 was performed on single Na channel, action potential, and electrocardiography in guinea pigs. RESULTS: With the binding of ATX II to the Na+ channels, the probability of being in the open state and the open time constant of single Na+ channel with long opening mode increased significantly. Action potential duration APD50, APD90, and the maximal upstroke velocity of phase 0 were increased by 25.8 %, 26.1 %, and 12 %, respectively. The QT interval and QTc, a rectified QT interval, increased by 12.8 % and 16.9 %. On the contrary, after application of mexiletine, the open probability of single Na+ channel was reduced greatly. In the presence of ATX II (40 nmol/L), mexiletine (1, 5, 15, 45, 70 micromol/L) shortened the APD50 by 0.5 %, 6.7 %, 14.4 %, 19.4 %, and 18.8 %, respectively, and decreased the APD90 and Vmax accordingly. In the experiments with ECG, mexiletine reversed the ATX II-produced prolongation effects on QTc in a dose-dependent manner. CONCLUSION: Mexiletine may be an effective drug in the treatment of LQT3.

[The sinus node itself also plays a role in heart rate slowing down during postnatal development].

The slowing down mechanism of heart rate during growth of the body after birth was studied in isolated rabbit heart and sinus node (SN) preparation with Langendorff perfusion method, conventional microelectrode recording and perforated patch for recording pacemaker current I(f). The radioimmunoassay was also used to measure the concentration of cAMP within SN cells. The results indicate that without the influence of nervous and humoral factors, the spontaneous heart rate would also become slower as the rabbit grew older, which is due to the decrease of spontaneous depolarized rate of phase 4 in SN cells. The negative directed shift of the threshold potential of I(f) and the decrease in cAMP concentration within SN cells may be responsible for the phenomenon. The results obtained suggest that besides the nervous and humoral factors which influence the heart rate, the changes in automaticity of SN cell itself may take part in the slowing down process as the body grows up.