Involvement Of Veratridine-Induced Increase Of Reversed I_NCX In Intracellular Ca²⁺ Overload And Extension Of Action Potential Duration In Rabbit Ventricular Myocytes

Objective:The objectives of this study were to investigate the effects of veratridine （VER） onpersistent sodium current (I_Na.P), Na⁺/Ca²⁺exchange current (I_NCX), calcium transients and theaction potential （AP） in rabbit ventricular myocytes, and to explore the mechanism inintracellular calcium overload and myocardial contraction enhancement.Methods:Used whole cell patch clamp recording technique, visual motion edge detection system,intracellular calcium measurement system and multi-channel physiological signal acquisitionand processing system to record I_Na.P、 I_NCX、calcium transients andAP，and used tetrodotoxinand renolazine to intervene.Results:The results showed that I_Na.Pand reversed I_NCXin ventricular myocytes obviouslyincreased after given10,20μmol/L VER, with the current density of I_Na.Pincreasing from-0.221±0.1248to-0.610±0.1293and-2.151±0.1360pA/pF （P<0.01, n=10）, and that ofreversed I_NCXincreasing from1.623±0.1235to2.189±0.0901and2.576±0.1134pA/pF （P<0.05,n=10）. After adding4μmol/L tetrodotoxin （TTX）, current density of I_Na.Pand reversed I_NCXreturned to-0.074±0.1405and1.688±0.1508pA/pF （P<0.05, n=10）. Another specific blockerof I_Na.P, renolazine （RAN）, could obviously inhibit VER-increased I_Na.Pand reversed I_NCX. Aftergiving2.5μmol/L VER, the maximal contraction rate of ventricular myocytes increased from-0.91±0.29to-1.53±0.29μm/s （P<0.01, n=7）, the amplitude of contraction increased from0.10±0.04to0.16±0.04μm （P<0.05, n=7）, the baseline of calcium transients （diastolic calciumconcentration） increased from1.21±0.08to1.37±0.12（P<0.05, n=7）. After adding2μmol/LTTX, the maximal contraction rate and amplitude of ventricular myocytes decreased to -0.86±0.24μm/s and0.09±0.03μm （P<0.01, n=7） respectively. And the baseline of calciumtransients reduced to1.17±0.09（P<0.05, n=7）. VER （20μmol/L） could extend action potentialduration at50%repolarization (APD₅₀) and at90%repolarization (APD₉₀) in ventricularmyocytes from123.177±23.7016to271.896±32.8141and from146.938±24.1505to429.793±32.0430ms （P<0.01, n=6） respectively. Early afterdepolarizations （EADs） appeared in3out of the6cases. After adding4μmol/L TTX, APD₅₀and APD₉₀were reduced to99.072±22.8141and163.838±26.0647ms （P<0.01, n=6） respectively, and EADs disappearedaccordingly in3cases. It could be suggested that:1) As a specific agonist of the I_Na.P, VERcould result in I_Na.Pincrease and intracellular Na⁺overload, and subsequently intracellular Ca²⁺overload with the increase of reversed I_NCX.2) The VER-increased I_Na.Pcould further extend theaction potential duration （APD） and induce EADs.3) TTX could restrain the abnormalVER-induced changes of the above-mentioned indexes, indicating that these abnormal changeswere caused by the increase of I_Na.P.Conclusion:Based on this study, it could be concluded that as the I_Na.Pagonist, VER could enhancereversed I_NCXby increasing I_Na.P, leading to intracellular Ca²⁺overload and APD abnormalextension. Objective:The objectives of this study were to investigate the effects of veratridine （VER） onpersistent sodium current （INa.P）, Na+/Ca2+exchange current （INCX）, calcium transients and theaction potential （AP） in rabbit ventricular myocytes, and to explore the mechanism inintracellular calcium overload and myocardial contraction enhancement.Methods:Used whole cell patch clamp recording technique, visual motion edge detection system,intracellular calcium measurement system and multi-channel physiological signal acquisitionand processing system to record INa.P、 INCX、calcium transients andAP，and used tetrodotoxinand renolazine to intervene.Results:The results showed that INa.Pand reversed INCXin ventricular myocytes obviouslyincreased after given10,20μmol/L VER, with the current density of INa.Pincreasing from-0.221±0.1248to-0.610±0.1293and-2.151±0.1360pA/pF （P<0.01, n=10）, and that ofreversed INCXincreasing from1.623±0.1235to2.189±0.0901and2.576±0.1134pA/pF （P<0.05,n=10）. After adding4μmol/L tetrodotoxin （TTX）, current density of INa.Pand reversed INCXreturned to-0.074±0.1405and1.688±0.1508pA/pF （P<0.05, n=10）. Another specific blockerof INa.P, renolazine （RAN）, could obviously inhibit VER-increased INa.Pand reversed INCX. Aftergiving2.5μmol/L VER, the maximal contraction rate of ventricular myocytes increased from-0.91±0.29to-1.53±0.29μm/s （P<0.01, n=7）, the amplitude of contraction increased from 0.10±0.04to0.16±0.04μm （P<0.05, n=7）, the baseline of calcium transients （diastolic calciumconcentration） increased from1.21±0.08to1.37±0.12（P<0.05, n=7）. After adding2μmol/LTTX, the maximal contraction rate and amplitude of ventricular myocytes decreased to-0.86±0.24μm/s and0.09±0.03μm （P<0.01, n=7） respectively. And the baseline of calciumtransients reduced to1.17±0.09（P<0.05, n=7）. VER （20μmol/L） could extend action potentialduration at50%repolarization （APD50） and at90%repolarization （APD90） in ventricularmyocytes from123.177±23.7016to271.896±32.8141and from146.938±24.1505to429.793±32.0430ms （P<0.01, n=6） respectively. Early afterdepolarizations （EADs） appeared in3out of the6cases. After adding4μmol/L TTX, APD50and APD90were reduced to99.072±22.8141and163.838±26.0647ms （P<0.01, n=6） respectively, and EADs disappearedaccordingly in3cases. It could be suggested that:1) As a specific agonist of the INa.P, VERcould result in INa.Pincrease and intracellular Na+overload, and subsequently intracellular Ca2+overload with the increase of reversed INCX.2) The VER-increased INa.Pcould further extend theaction potential duration （APD） and induce EADs.3) TTX could restrain the abnormalVER-induced changes of the above-mentioned indexes, indicating that these abnormal changeswere caused by the increase of INa.P.Conclusion:Based on this study, it could be concluded that as the INa.Pagonist, VER could enhancereversed INCXby increasing INa.P, leading to intracellular Ca2+overload and APD abnormalextension.