Different compartments of sarcoplasmic reticulum participate in the excitation-contraction coupling process in human atrial myocytes.
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.
Hatem
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Laboratorie de Cardiologie Moléculaire et Cellulaire, Université de Paris XI-CNRS URA 1159, Hôpital Marie Lannelongue, Le Plessis Robinson, France.
Bénardeau
A
A
Rücker-Martin
C
C
Marty
I
I
de Chamisso
P
P
Villaz
M
M
Mercadier
J J
JJ
D016275:Atrial Function; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism; D002148:Calmodulin-Binding Proteins / Q000378:metabolism; D002478:Cells, Cultured; D006325:Heart Atria / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D018613:Microscopy, Confocal; D009124:Muscle Proteins / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism / Q000648:ultrastructure; D018408:Patch-Clamp Techniques; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000648:ultrastructure
D015220:Calcium Channels; D002148:Calmodulin-Binding Proteins; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002118:Calcium