Sodium/calcium exchange contributes to contraction and relaxation in failed human ventricular myocytes

Abstract

Defects in myocyte contraction and relaxation are key features of human heart failure. Sodium/calcium exchanger-mediated contribution to contraction and relaxation were separated from other mechanisms [L-type calcium current, sarco(endo)plasmic reticulum (SR) Ca2+-ATPase] based on voltage, temperature, and selective blockers. Rod-shaped left ventricular myocytes were isolated from failed human explants ( n = 29) via perfusion with collagenase-containing Krebs solution. Action potentials using perforated patch and contractions using an edge detector were recorded at 0.5–1.5 Hz in Tyrode solution at 25°C and 37°C. Contraction duration was dependent on action potential (AP) duration at 37°C but not at 25°C, suggesting the role of the exchanger in relaxation and linking myocyte relaxation to the repolarization phase of the AP. Voltage-clamp experiments from −50 to +10 mV for 1,500 ms in Tyrode or Na+- and K+-free solutions after conditioning pulses triggered biphasic contractions that included a rapid SR-mediated component and a slower voltage-dependent exchanger-mediated component. We used thapsigargin to block the SR, which eliminated the rapid component, and we used an exchanger blocker, Kanebo 7943, which eliminated the slow component. The exchanger was shown to contribute to contraction through reverse-mode exchange, as well as to play a key role in relaxation of human ventricular myocytes.

Inhibition by KB-R7943 of the Reverse Mode of the Na+/Ca2+ Exchanger Reduces Ca2+ Overload in Ischemic-Reperfused Rat Hearts.

“…Treatment with 10 mol/L KBR did not influence heart rate or LVP in the pre-ischemic state. Although the reverse mode of the NCX has been shown to contribute to contraction, 11 blockade of the NCX did not affect the hemodynamics in nonischemic hearts. Treatment of guinea-pig papillary muscle with 10 mol/L KBR did not significantly affect the action potential parameters.…”

“…7,10,11 With normal oxygenation, pre-treatment with 10 mmol/L caffeine markedly reduced the amplitude of Ca 2+ transients and LVP, while increasing the diastolic [Ca 2+ ]i; this reflected SR Ca 2+ release and Ca 2+ depletion (Fig 2). A small decline in [Ca 2+ ]i during the initial phase of exposure to caffeine represented Ca 2+ efflux via the forward mode of the NCX.…”

“…4,[6][7][8][9] Some previous reports have suggested that inhibition of the reverse mode of the NCX could protect ischemic myocardial cells against Ca 2+ overload, [7][8][9] but exchanger-related alterations in cytoplasmic Ca 2+ ([Ca 2+ ]i) and the effects of pharmacological intervention at the level of the NCX have not been fully described. KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl] ethyl]isothiourea methanesulfonate; KBR) selectively inhibits the reverse mode of the NCX in isolated cells, 10,11 and has been shown to reduce anoxia-induced accumulation of [Ca 2+ ]i and prevent reoxygenation-induced hypercontracture in isolated rat cardiomyocytes. 7 A demonstration of an alteration of [Ca 2+ ]i in pathologic models of the whole heart remains elusive; measurements of [Ca 2+ ]i have been carried out mainly in single-cell models and the factors involved in the ischemic whole heart are more complex than in anoxic or hypoxic myocytes.…”

Myocardial Recovery Following Left Ventricular Assist Device Therapy

“…Mechanisms regulating APD involve alteration in function and expression of ion transporters with changes in depolarising and hyperpolarising currents. Detrimental effects of APD prolongation in HF on EC coupling, cellular relaxation and contraction are recognized (Gaughan et al 1999;Wickenden et al 1998), as well as positive effects such as augmentation of intracellular Ca 2+ via alteration in L-type Ca 2+ and NCX currents, presumed to be compensatory adaptive mechanisms (Bouchard et al 1995;Weber et al 2002;Wickenden et al 1998). APD reduction has been associated with functional clinical recovery during LVAD support (Terracciano et al 2004), but delineation of a precise contributory role of APD to functional recovery is difficult as the specific role of APD changes in HF itself remains unclear.…”

Maintenance Of Calcium Homeostasis in Embryonic Stem Cell-Derived Cardiomyocytes

“…In addition, NCX can positively regulate SR Ca 2+ load via the reverse mode action (Hirota et al, 2007). Interestingly, under pathophysiological conditions such as cardiac failure, NCX also operates in the reverse mode to allow additional Ca 2+ influx for contraction in order to compensate for the reduction in Ca 2+ release from SR (Gaughan et al, 1999). NCX is found to be essential for embryo development.…”