It takes two to tango: Regulation of sarcoplasmic reticulum calcium ATPase by CaMK and PKA in a mouse cardiac myocyte

Abstract

At the cellular level, one of the most important regulators of myocardial relaxation is the sarcoplasmic reticulum calcium ATPase (SERCA), which is controlled by phosphorylation both directly and indirectly through the endogenous inhibitor phospholamban. According to experimental observations, this regulation is interplay of protein kinase A (PKA), and calcium/calmodulin dependent kinase II (CaMK). Despite the physiological importance of this modulation, the significance of the crosstalk in this dual regulation has not been thoroughly described, and the exact quantitative roles of these two enzymes have remained partly elusive. We use the approach of mathematical modeling to dissect the different aspects of the regulation of SERCA in cardiomyocytes. We present a novel model of SERCA that includes phosphorylation targets for both PKA and CaMK. To study the physiological impact of these regulatory mechanisms, we implement the SERCA model into a mathematical model of a mouse ventricular myocyte. We validate the model by comparing the simulated results to the corresponding in vivo observations, both in physiological phenomena and transgenic test cases. Our results show that under varying levels of beta-adrenergic stimulation, and thus at varying PKA activities, the frequency-dependent changes in the calcium dynamics show a clear dose-dependence. Furthermore, the in silico experiments indicate that these changes are drastically blocked with CaMK inhibitors. The results also point out the diverging time windows of regulation for PKA (∼ tens of seconds) and CaMK (∼ few minutes). Based on the results, we conclude that despite the prominent role of PKA in the beta-adrenergic modulation of calcium dynamics in a cardiac myocyte, both the direct and the indirect effects mediated by CaMK phosphorylation appear to more important for the regulation of SERCA, though the time scale of observation has a significant impact on the quantitative roles of these two enzymes. © 2009 Springer Berlin Heidelberg.