Genetic manipulation of calcium-handling proteins in cardiac myocytes. II. Mathematical modeling studies

Abstract

We developed a mathematical model specific to rat ventricular myocytes that includes electrophysiological representation, ionic homeostasis, force production, and sarcomere movement. We used this model to interpret, analyze, and compare two genetic manipulations that have been shown to increase myocyte relaxation rates, parvalbumin (Parv) de novo expression, and sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA2a) overexpression. The model was used to seek mechanistic insights into 1) the relative contribution of two mechanisms by which SERCA2a overexpression modifies Ca 2ϩ sequestration, i.e., more pumps and an increase in the SERCA2a-tophospholamban ratio, 2) the mechanisms behind postrest potentiation and how Parv and SERCA2a influence this response, and 3) why Parv myocytes retain their fast kinetics when endogenous SERCA2a is partially impaired by thapsigargin (a condition used to mimic diastolic dysfunction). The model was also utilized to predict whether Parv metal-binding characteristics might be modified to improve diastolic and systolic functions and whether Parv or SERCA2a might affect diastolic Ca 2ϩ levels and myocyte energetics. One outcome of the model was to demonstrate a higher peak and total ATP consumption in SERCA2a myocytes and more even distribution of ATP throughout the cardiac cycle in Parv myocytes. This may have implications for failing hearts that are energetically compromised. mathematical model; rat; parvalbumin; sarco(endo)plasmic reticulum calcium-adenosinetriphosphatase 2aHEART FAILURE affects more than 5,000,000 people in the United States (25). It has been estimated that 40% of heart failure patients exhibit pure diastolic dysfunction (20), a condition characterized by slowing of Ca 2ϩ uptake at the cellular level, increase in stiffness during relaxation at the tissue level, and impaired ventricular filling at the organ level. Alterations in Ca 2ϩ -handling proteins play an important role in diastolic dysfunction (23). Several genetic manipulations have been proposed to restore myocyte Ca 2ϩ -handling function in failing hearts (12), including modification of expression or functionality of sarco(endo)plasmic Ca 2ϩ -ATPase (SERCA2a), phospholamban (PLB), Na ϩ /Ca 2ϩ exchanger (NCX), and parvalbumin (Parv), a specialized Ca 2ϩ buffer (8). In our experimental study (10), we used gene transfer in intact isolated rat adult cardiac myocytes to study, characterize, and compare the effects of SERCA2a overexpression or Parv expression on isolated cardiac myocyte function. We found that when myocytes were subjected to single unloaded contractions, both methods resulted in similar increases in myocyte relaxation rate. However, we established that the response of SERCA2a-expressing myocytes to other conditions, such as ␤-adrenergic stimulation [with isoproterenol (Iso)], diastolic dysfunction [chemically induced with thapsigargin (TG), a SERCA2a inhibitor], or changes in stimulation pacing [during postrest potentiation (PRP) tests], was different from that of Parv-expressing myocytes. The e...

Genetic manipulation of calcium-handling proteins in cardiac myocytes. I. Experimental studies

“…In summary, the PRP data suggest that although expression of Parv and overexpression of SERCA2a accelerate relaxation similarly, they modify the ability of the myocyte to handle changes in pacing rhythm differently. The quantitative analysis of these PRP data is complex and well suited for mathematical modeling interpretation (12).…”

“…A direct comparison between data without TG at 1.8 mM external Ca 2ϩ and data with 150 nM TG at 5.0 mM external Ca 2ϩ reveals a strong diastolic dysfunction with almost no diminution in systolic function (Table 1). Other strategies, such as an augmentation of Ca 2ϩ cellular entry via an increase in action potential duration or a reduced Ca 2ϩ cellular exit via a reduced NCX function, could produce similar results (12).…”

“…The overall objective was to gain mechanistic insights into Ca 2ϩ -handling properties and to understand the consequences of SERCA2a and Parv gene transfer in important aspects of cardiac myocyte mechanical function. Accordingly, we used an experimental approach of gene transfer of Parv or SERCA2a in isolated rat adult cardiac myocytes (present study) combined with a theoretical analysis using mathematical modeling (12).…”

Cardiac-specific deletion of protein phosphatase 1β promotes increased myofilament protein phosphorylation and contractile alterations

“…Ca 2+ traces from healthy myocytes sensitive to caffeine treatment were processed using a Savitzky-Golay filter and baseline Ca 2+ levels, transient amplitude, caffeine-induced Ca 2+ release, and Ca 2+ decay kinetics were analyzed using Felix 1.1 and Ion Wizard (IonOptix) software. For cell shortening measurements in isolated adult myocytes, cells were bathed in media 199 (M199) at room temperature using an Ionoptix system as described previously [34, 35] and myocytes were electrically paced at 0.5 Hz with 60V and the solution was refreshed between each measurement.…”

Parvalbumin Isoforms for Enhancing Cardiac Diastolic Function

“…For instance, higher Parv doses can lead to excessive Ca 2+ buffering during systole, thus, negatively affecting fractional shortening and Ca 2+ transient amplitude [16,22]. Experimental data as well as mathematical modeling have been carried out to determine the optimal concentration range of Parv after gene transfer in rat myocytes [21][22][23]. This optimal expression range is estimated to be *10-100 lM [22].…”