Intracellular Ca alternans: coordinated regulation by sarcoplasmic reticulum release, uptake, and leak.
Beat-to-beat alternation in the cardiac intracellular Ca (Ca(i)) transient can drive action potential (AP) duration alternans, creating a highly arrhythmogenic substrate. Although a steep dependence of fractional sarcoplasmic reticulum (SR) Ca release on SR Ca load has been shown experimentally to promote Ca(i) alternans, theoretical studies predict that other factors are also important. Here we present an iterated map analysis of the coordinated effects of SR Ca release, uptake, and leak on the onset of Ca(i) alternans. Predictions were compared to numerical simulations using a physiologically realistic AP model as well as to AP clamp experiments in isolated patch-clamped rabbit ventricular myocytes exposed to 1), the Ca channel agonist BayK8644 (100 nM) to increase SR Ca load and release fraction, 2), overexpression of an adenoviral SERCA2a construct to increase SR Ca uptake, and 3), low-dose FK506 (20 microM) or ryanodine (1 microM) to increase SR Ca leak. Our findings show that SR Ca release, uptake, and leak all have independent direct effects that promote (release and leak) or suppress (uptake) Ca(i) alternans. However, since each factor affects the other by altering SR Ca load, the net balance of their direct and indirect effects determines whether they promote or suppress alternans. Thus, BayK8644 promotes, whereas Ad-SERCA2a overexpression, ryanodine, and FK506 suppress, Ca(i) alternans under AP clamp conditions.
Cardiovascular Research Laboratory, Department of Medicine (Cardiology), David Geffen School of Medicine at the University of California, Los Angeles, California 90095, USA.
Lai-Hua
LH
Xie
Daisuke
D
Sato
Alan
A
Garfinkel
Zhilin
Z
Qu
James N
JN
Weiss
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005936:Glucans / Q000378:metabolism*; D006352:Heart Ventricles / Q000166:cytology; D042541:Intracellular Space / Q000378:metabolism*; D008954:Models, Biological; D032342:Muscle Cells / Q000166:cytology / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011817:Rabbits; D015203:Reproducibility of Results; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013696:Temperature
D005936:Glucans; C439358:alternan; D002118:Calcium