Related Experiment Videos
Sodium/calcium exchange contributes to contraction and relaxation in failed human ventricular myocytes
J P Gaughan1, S Furukawa, V Jeevanandam
1Departments of Physiology and Cardio-Thoracic Surgery, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA. jgaughan<@pond.com>
Insights
Defects in human heart failure involve myocyte contraction and relaxation. The sodium/calcium exchanger plays a key role in both contraction and relaxation, particularly linking relaxation to the action potential repolarization phase.
Area of Science:
- Cardiology
- Molecular Biology
- Physiology
Background:
- Myocyte contraction and relaxation abnormalities are central to human heart failure.
- Understanding the specific roles of ion transporters, like the sodium/calcium exchanger, is crucial for elucidating heart failure mechanisms.
Purpose of the Study:
- To differentiate and quantify the contribution of the sodium/calcium exchanger (NCX) to human ventricular myocyte contraction and relaxation.
- To investigate the relationship between NCX activity, action potential duration, and myocyte relaxation dynamics.
Main Methods:
- Isolation of viable human ventricular myocytes from explanted hearts.
- Recording of myocyte action potentials and contractions under varying temperatures (25°C and 37°C) and frequencies.
- Utilizing voltage-clamp techniques, selective blockers (thapsigargin, Kanebo 7943), and ion-free solutions to isolate NCX-mediated currents and contractions.
Main Results:
- Myocyte contraction duration was temperature-dependent, correlating with action potential duration at 37°C, suggesting a role for NCX in relaxation.
- Voltage-clamp experiments revealed biphasic contractions with distinct SR-mediated and slower voltage-dependent NCX-mediated components.
- Pharmacological blockade confirmed the SR's role in the rapid contraction phase and NCX's role in the slower component and overall relaxation.
Conclusions:
- The sodium/calcium exchanger significantly contributes to both contraction and relaxation in human ventricular myocytes.
- NCX-mediated relaxation is linked to the repolarization phase of the action potential, especially at physiological temperatures.
- These findings highlight the NCX as a potential therapeutic target for heart failure.
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) Ca(2+)-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 degrees C and 37 degrees C. Contraction duration was dependent on action potential (AP) duration at 37 degrees C but not at 25 degrees 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.