Increased Ca2+ leak and spatiotemporal coherence of Ca2+ release in cardiomyocytes during β‐adrenergic stimulation

J Ogrodnik, E Niggli - The Journal of physiology, 2010 - Wiley Online Library
J Ogrodnik, E Niggli
The Journal of physiology, 2010Wiley Online Library
β‐Adrenergic receptor (β‐AR) stimulation of cardiac muscle has been proposed to enhance
Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs).
However, the anticipated increase in RyR Ca2+ sensitivity has proven difficult to study in
intact cardiomyocytes, due to accompanying alterations in SR Ca2+ content, inward Ca2+
current (ICa) and diastolic cytosolic Ca2+ concentration ([Ca2+] i). Here, we studied whole‐
cell Ca2+ release and spontaneous Ca2+ leak (Ca2+ sparks) in guinea‐pig ventricular …
β‐Adrenergic receptor (β‐AR) stimulation of cardiac muscle has been proposed to enhance Ca2+ release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs). However, the anticipated increase in RyR Ca2+ sensitivity has proven difficult to study in intact cardiomyocytes, due to accompanying alterations in SR Ca2+ content, inward Ca2+ current (ICa) and diastolic cytosolic Ca2+ concentration ([Ca2+]i). Here, we studied whole‐cell Ca2+ release and spontaneous Ca2+ leak (Ca2+ sparks) in guinea‐pig ventricular myocytes with confocal Ca2+ imaging before and during β‐AR stimulation by isoproterenol (Iso), but under otherwise nearly identical experimental conditions. The extent of SR Ca2+ loading was controlled under whole‐cell voltage‐clamp conditions. UV flash‐induced uncaging of Ca2+ from DM‐nitrophen was employed as an invariant trigger for whole‐cell Ca2+ release. At matched SR Ca2+ content, we found that Iso enhanced the spatiotemporal coherence of whole‐cell Ca2+ release, evident from spatially intercorrelated release and accelerated release kinetics that resulted in moderately (∼20%) increased release amplitude. This may arise from higher RyR Ca2+ sensitivity, and was also reflected in spontaneous SR Ca2+ leak. At comparable SR Ca2+ content and cytosolic [Ca2+]i, we observed a ∼4‐fold increase in Ca2+ spark frequency in Iso that also appeared in quiescent cells within 2 min without increased SR Ca2+ content. This was likely to have been mediated by Ca2+/calmodulin‐dependent protein kinase (CaMKII), rather than cAMP dependent protein kinase (PKA). We conclude that Iso increases the propensity of RyRs to open, both in response to rapid elevations of [Ca2+]i and at diastolic [Ca2+]i. While this could be beneficial in enhancing and synchronizing systolic whole‐cell SR Ca2+ release, the same behaviour could also be proarrhythmogenic during diastole.
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