Vila Petroff Martín G, Palomeque Julieta, Mattiazzi Alicia R
Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, Argentina.
J Physiol. 2003 Aug 1;550(Pt 3):801-17. doi: 10.1113/jphysiol.2003.044321.
In most mammalian species, an increase in stimulation frequency (ISF) produces an increase in contractility (treppe phenomenon), which results from larger Ca2+ transients at higher frequencies, due to an increase in sarcoplasmic reticulum Ca2+ load and release. The present study attempts to elucidate the contribution of the Na(+)-Ca2+ exchanger (NCX) to this phenomenon. Isolated cat ventricular myocytes, loaded with [Ca2+]i- and [Na+]i-sensitive probes, were used to determine whether the contribution of the NCX to the positive inotropic effect of ISF is due to an increase in Ca2+ influx (reverse mode) and/or a decrease in Ca2+ efflux (forward mode) via the NCX, due to frequency-induced [Na+]i elevation, or whether it was due to the reduced time for the NCX to extrude Ca2+. The results showed that the positive intropic effect produced by ISF was temporally dissociated from the increase in [Na+]i and was not modified by KB-R7943 (1 or 5 microM), a specific blocker of the reverse mode of the NCX. Whereas the ISF from 10 to 30 beats min(-1) (bpm) did not affect the forward mode of the NCX (assessed by the time to half-relaxation of the caffeine-induced Ca2+ transient), the ISF to 50 bpm produced a significant reduction of the activity of the forward mode of the NCX, which occurred in association with an increase in [Na+]i (from 4.33+/-0.40 to 7.25+/-0.50 mM). However, both changes became significant well after the maximal positive inotropic effect had been reached. In contrast, the positive inotropic effect produced by ISF from 10 to 50 bpm was associated with an increase in diastolic [Ca2+]i, which occurred in spite of a significant increase in the relaxation rate and at a time at which no increases in [Na+]i were detected. The contribution of the NCX to stimulus frequency inotropy would therefore depend on a decrease in NCX-mediated Ca2+ efflux due to the reduced diastolic interval between beats and not on [Na+]i-dependent mechanisms.
在大多数哺乳动物物种中，刺激频率增加（ISF）会导致收缩性增强（阶梯现象），这是由于较高频率下更大的Ca2+瞬变所致，这是由于肌浆网Ca2+负荷和释放增加。本研究试图阐明钠钙交换体（NCX）对这一现象的贡献。使用负载有[Ca2+]i和[Na+]i敏感探针的分离猫心室肌细胞，以确定NCX对ISF正性肌力作用的贡献是由于Ca2+内流增加（反向模式）和/或通过NCX的Ca2+外流减少（正向模式），这是由于频率诱导的[Na+]i升高，还是由于NCX挤出Ca2+的时间减少。结果表明，ISF产生的正性肌力作用在时间上与[Na+]i的增加分离，并且未被NCX反向模式的特异性阻滞剂KB-R7943（1或5 microM）改变。虽然从10到30次/分钟（bpm）的ISF不影响NCX的正向模式（通过咖啡因诱导的Ca2+瞬变的半松弛时间评估），但到50 bpm的ISF会导致NCX正向模式的活性显著降低，这与[Na+]i增加（从4.33±0.40到7.25±0.50 mM）相关。然而，这两种变化在达到最大正性肌力作用后很久才变得显著。相反，从10到50 bpm的ISF产生的正性肌力作用与舒张期[Ca2+]i增加相关，尽管舒张速率显著增加，且在未检测到[Na+]i增加的时间点出现。因此，NCX对刺激频率性肌力作用的贡献将取决于由于心动周期之间舒张期间隔缩短导致的NCX介导的Ca2+外流减少，而不是取决于[Na+]i依赖性机制。
