Frequency-dependent contractile strength in mice over- and underexpressing the sarco(endo)plasmic reticulum calcium-ATPase

Abstract

One of the prominent markers of end-stage heart failure at the molecular level is a decrease in function and/or expression of the sarcoplasmic reticulum ATPase protein [sarco(endo)plasmic reticulum calcium-ATPase, SERCA]. It has been often postulated that a decrease in SERCA pump activity can contribute in a major way to decreased cardiac function. To establish a functional relationship, we assessed how alterations in SERCA activity level affect basic contractile function in healthy myocardium devoid of other significant molecular changes. We investigated baseline contractile function, frequency-dependent activation, and beta-adrenergic response in ultrathin trabeculae isolated from hearts of mice overexpressing SERCA (transgenic, TG), underexpressing SERCA2a (heterozygous knockout, Het), and their respective wild-type (WT) littermates. At physiological temperature and frequency, compared with their respective WT littermates, SERCA1a mice displayed increased developed force at frequencies of 4-8 Hz ( approximately 90% increase at 4 Hz) and force equal to WT mice at 10-14 Hz. Force development at 4 Hz in presence of 1 muM isoproterenol was similar in TG and WT mice. In Het mice, developed force was nearly identical at the lower end of the frequency range (4-8 Hz) but slightly depressed at higher frequency (P < 0.05 at 14 Hz). In presence of 1 muM isoproterenol, developed force at 4 Hz was equal to that in WT mice. Compared with normal levels, increased SERCA activity enhanced force development only at subphysiological frequencies. A reduction in SERCA activity only showed a depression of force at the higher frequency range. Thus generalizations regarding the correlation between SERCA activity and contractility can be highly ambiguous, because this relationship is critically dependent on other factors including stimulation frequency.

Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere

“…We did not find any differences in mice with enhanced Ca 2ϩ cycling [i.e., overexpressing sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA)1a] (19,25), reduced Ca 2ϩ cycling (SERC2a heterozygous knockout mice) (19,31), or various groups of control mice (36). Likewise, in mdx mice, which suffer from muscular dystrophy due to a lack of dystrophin, or in a more severe model where the compensatory utrophin is also lacking (13,21), this contraction-relaxation balance is completely maintained.…”

“…The majority of data presented in this article are derived from muscle studies published in the past 5 yr by our laboratory with or without the help of collaborating laboratories (18,19,21,23,29,36), with the remaining experiments having been conducted for this study specifically. However, the specific data in this article were collected during those studies but were not analyzed or presented in/for those studies.…”

Determinants of frequency-dependent contraction and relaxation of mammalian myocardium

“…However, if the SR is already functioning at its maximal store capacity, an increase in frequency is not sufficient to further increase this load/release process. This interpretation is supported by the findings that a large increase in SERCA pump activity, achieved by SERCA overexpression similarly can show a negative or blunted force frequency response [4,21,22]. Thus, such muscles are already contracting at maximal rate and may not be responsive to increased frequency.…”

“…As stated above, an increase in SERCA activity well beyond the normal spectrum may also result in negative frequency-dependent behavior, because already at low frequency the baseline SR load is higher, and thus there is less room for further enhancement. In addition, if re-uptake of calcium becomes too fast, the cytoplasmic calcium concentration near the myofilaments will decline very rapidly, preventing appropriate activation of the myofilaments, hindering adequate force development [4,21,22]. Combined, these studies indicate that SERCA activity has an optimal range, rather than a direct linear correlation to contractile performance.…”

Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes

“…The basic mechanisms have been reviewed at length (Koss & Kranias 1996;Bers 2002;MacLennan & Kranias 2003;Periasamy et al 2008), including considerations of the properties of SERCA isoforms ) and the specific mode of its action, especially the role of calcium affinity (Vangheluwe et al 2006). The availability of several transgenic animal models for investigating the calcium-handling proteins (Kiriazis & Kranias 2000;Sorrentino & Rizzuto 2001;Hiranandani et al 2007) is an additional recent development, and the knowledge of those studies can also be used here. In the second part of the present work, we offer some novel theoretical considerations that may be used to understand the calcium transport function of SERCA that is so central to cardiac function.…”