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Different compartments of sarcoplasmic reticulum participate in the excitation-contraction coupling process in human
S N Hatem1, A Bénardeau, C Rücker-Martin
1Laboratorie de Cardiologie Moléculaire et Cellulaire, Université de Paris XI-CNRS URA 1159, Hôpital Marie Lannelongue, Le Plessis Robinson, France.
Insights
Human atrial myocytes exhibit a unique excitation-contraction coupling. L-type calcium current (Ica) partially controls ryanodine receptors (RyRs), leading to a prolonged, dome-shaped intracellular calcium transient.
Area of Science:
- Cardiology
- Cell Physiology
Background:
- Excitation-contraction coupling in human atrial myocytes is crucial for cardiac function.
- Understanding the role of intracellular calcium (Cai) transients and their regulation is vital.
Purpose of the Study:
- To investigate the excitation-contraction coupling process in human atrial myocytes.
- To elucidate the control mechanisms of intracellular calcium transients by L-type calcium current (Ica) and ryanodine receptors (RyRs).
Main Methods:
- Voltage-clamped human atrial myocytes were used.
- Intracellular calcium transients were monitored using indo 1.
- Ryanodine receptors and sarcomeric alpha-actinin were visualized via immunofluorescence microscopy.
Main Results:
- L-type calcium current (Ica) elicited a prolonged, dome-shaped Cai transient.
- Ryanodine significantly inhibited the Cai transient, indicating sarcoplasmic reticulum Ca2+ release.
- Cai transient activation preceded Ica and was independent of its voltage dependence.
- RyRs were found in subsarcolemmal and deeper intracellular locations, including transverse bands at Z lines.
Conclusions:
- Intracellular calcium transients in human atrial myocytes are primarily driven by sarcoplasmic reticulum Ca2+ release via RyRs.
- Ica only partially controls RyR activation.
- The unique Cai transient shape likely results from RyRs not directly coupled to L-type Ca2+ channels.
Abstract:
The excitation-contraction coupling process of human atrial myocytes was studied in voltage-clamped myocytes isolated from right atrial appendages obtained during cardiac surgery. Intracellular Ca2+ transients (Cai transients) were monitored with 0.1 mmol/L indo 1 added to the internal dialyzing solution. Ryanodine receptors (RyRs) and sarcomeric alpha-actinin were stained with specific antibodies and visualized using plane and confocal microscopy. L-Type Ca2+ current (Ica) elicited a prolonged Cai transient, with an initial rapidly activating phase (slope 1, 23.6 +/- 1.2 s-1) followed by a slowly activating phase (slope 2, 5.8 +/- 0.4 s-1; P < .001 versus slope 1), resulting in a dome-shaped Cai transient. Ryanodine (100 mumol/L) inhibited 79 +/- 6% of the Cai transient, indicating that it was due essentially to sarcoplasmic reticulum Ca2+ release. During step depolarizations, maximal activation of the Cai transient or tail current (Itail) (in cells dialyzed with Ca2+ buffer-free internal solution) preceded that of Ica and did not follow its voltage dependence (n = 12). Test pulses lasting from 5 to 150 milliseconds elicited a similar time course of both Cai transient and Itail (n = 5). In a given cell, the two components of the Cai transient could be dissociated by altering the intracellular Ca2+ load, by increasing the stimulation rate from 0.1 to 1 Hz, or by varying the amplitude of Ica. Immunostaining of atrial sections and isolated myocytes showed that a large number of RyRs were located not only in a subsarcolemmal position but also deeper inside the cell, in a regularly spaced transverse band pattern at the level of Z lines. Together, our results indicate that, in human atrial myocytes, Ica only partially controls the activation of RyRs, with the prolonged and dome-shaped Cai transient of these cells probably reflecting the activation of RyRs not coupled to L-type Ca2+ channels.