Weisser-Thomas Jutta, Piacentino Valentino, Gaughan John P, Margulies Kenneth, Houser Steven R
Cardiovascular Research Group, Department of Physiology, Temple University School of Medicine, 3400 North Broad Street, Philadelphia, PA 19140, USA.
Cardiovasc Res. 2003 Mar 15;57(4):974-85. doi: 10.1016/s0008-6363(02)00732-0.
Prolongation of the Ca2+ transient and action potential (AP) durations are two characteristic changes in myocyte physiology in the failing human heart. The hypothesis of this study is that Ca2+ influx via reverse mode Na+/Ca2+ exchanger (NCX) or via L-type Ca2+ channels directly activates contraction in failing human myocytes while in normal myocytes this Ca2+ is transported into the sarcoplasmic reticulum (SR) to regulate SR Ca2+ stores.
Myocytes were isolated from failing human (n=6), nonfailing human (n=3) and normal feline hearts (n=9) and whole cell current and voltage clamp techniques were used to evoke and increase the duration of APs (0.5 Hz, 37 degrees C). Cyclopiazonic acid (CPA 10(-6) M), nifedipine (NIF;10(-6) M) and KB-R 7943 (KB-R; 3x10(-6) M) were used to reduce SR Ca2+ uptake, Ca2+ influx via the L-type Ca2+ current and reverse mode NCX, respectively. [Na+)i was changed by dialyzing myocytes with 0, 10 and 20 mM Na(+) pipette solutions.
Prolongation of the AP duration caused an immediate prolongation of contraction and Ca2+ transient durations in failing myocytes. The first beat after the prolonged AP was potentiated by 21+/-5 and 27+/-5% in nonfailing human and normal feline myocytes, respectively (P<0.05), but there was no significant effect in failing human myocytes (+5+/-4% vs. steady state). CPA blunted the potentiation of the first beat after AP prolongation in normal feline and nonfailing human myocytes, mimicking the failing phenotype. NIF reduced steady state contraction in feline myocytes but the potentiation of the first beat after AP prolongation was unaltered (21+/-3% vs. base, P<0.05). KB-R reduced basal contractility and abolished the potentiation of the first beat after AP prolongation (2+/-1% vs. steady state). Increasing [Na+]i shortened AP, Ca2+ transient and contraction durations and increased steady state and post AP prolongation contractions. Dialysis with 0 Na+ eliminated these effects.
Ca2+ enters both normal and failing cardiac myocytes during the late portion of the AP plateau via reverse mode NCX. In (normal) myocytes with good SR function, this Ca(2+) influx helps maintain and regulate SR Ca2+ load. In (failing) human myocytes with poor SR function this Ca2+ influx directly contributes to contraction. These studies suggest that the Ca2+ transient of the failing human ventricular myocytes has a higher than normal reliance on Ca2+ influx via the reverse mode of the NCX during the terminal phases of the AP.
Ca2+瞬变和动作电位（AP）时程延长是衰竭人类心脏中肌细胞生理学的两个特征性变化。本研究的假设是，通过反向模式钠钙交换体（NCX）或L型钙通道的Ca2+内流直接激活衰竭人类肌细胞的收缩，而在正常肌细胞中，这种Ca2+被转运到肌浆网（SR）中以调节SR Ca2+储存。
从衰竭人类心脏（n = 6）、非衰竭人类心脏（n = 3）和正常猫心脏（n = 9）中分离出肌细胞，并使用全细胞电流和电压钳技术诱发并增加AP的时程（0.5 Hz，37℃）。使用环匹阿尼酸（CPA 10(-6) M）、硝苯地平（NIF；10(-6) M）和KB-R 7943（KB-R；3×10(-6) M）分别减少SR Ca2+摄取、通过L型钙电流的Ca2+内流和反向模式NCX。通过用0、10和20 mM Na(+)移液管溶液透析肌细胞来改变[Na+)i。
AP时程延长导致衰竭肌细胞的收缩和Ca2+瞬变时程立即延长。在非衰竭人类和正常猫肌细胞中，延长AP后的第一个搏动分别增强了21±5%和27±5%（P<0.05），但在衰竭人类肌细胞中没有显著影响（与稳态相比增加5±4%）。CPA减弱了正常猫和非衰竭人类肌细胞中AP延长后第一个搏动的增强，模拟了衰竭表型。NIF降低了猫肌细胞的稳态收缩，但AP延长后第一个搏动的增强未改变（与基础值相比为21±3%，P<0.05）。KB-R降低了基础收缩力并消除了AP延长后第一个搏动的增强（与稳态相比为2±1%）。增加[Na+]i缩短了AP、Ca2+瞬变和收缩时程，并增加了稳态和AP延长后的收缩。用0 Na+透析消除了这些影响。
在AP平台期后期，Ca2+通过反向模式NCX进入正常和衰竭的心肌细胞。在SR功能良好的（正常）肌细胞中，这种Ca(2+)内流有助于维持和调节SR Ca2+负荷。在SR功能较差的（衰竭）人类肌细胞中，这种Ca2+内流直接促进收缩。这些研究表明，在AP的终末期，衰竭人类心室肌细胞的Ca2+瞬变对通过NCX反向模式的Ca2+内流的依赖性高于正常。
