Expression of SERCA and phospholamban in rainbow trout (Oncorhynchus mykiss) heart: comparison of atrial and ventricular tissue and effects of thermal acclimation

Abstract

2+uptake in cardiac homogenates of the cold-acclimated (CA) trout (Aho and Vornanen, 1998;Aho and Vornanen, 1999). These findings strongly suggest that temperature-induced changes in cardiac contractility in the CA trout are partly due to increases in the activity of the SR Ca 2+ pump. However, the molecular basis of these changes is still poorly elucidated.The interplay between cardiac SERCA2 and phospholamban (PLN) is crucial for Ca 2+ cycling through the SR and therefore for Accepted 5 December 2011 SUMMARY In the heart of rainbow trout (Oncorhynchus mykiss), the rate of contraction and Ca 2+ uptake into the sarcoplasmic reticulum (SR) are faster in atrial than ventricular muscle, and contraction force relies more on SR Ca 2+ stores after acclimation to cold. This study tested the hypothesis that differences in contractile properties and Ca 2+ regulation between atrial and ventricular muscle, and between warm-(WA) and cold-acclimated (CA) trout hearts, are associated with differences in expression of sarco(endo)plasmic reticulum Ca 2+ ATPase (SERCA) and/or phospholamban (PLN), an inhibitor of the cardiac SERCA. Quantitative PCR (SERCA only) and antibodies raised against SERCA and PLN were used to determine abundances of SERCA2 transcripts and SERCA and PLN proteins, respectively, in atrium and ventricle of trout acclimated to cold (+4°C, CA) and warm (+18°C, WA) temperatures. Expression of SERCA2 transcripts was 1.6 and 2.1 times higher in atrium than ventricle of WA and CA trout, respectively (P<0.05). At the protein level, differences in SERCA expression between atrium and ventricle were 6.1-and 23-fold for WA and CA trout, respectively (P<0.001). Acclimation to cold increased SERCA2 transcripts 2.6-and 2.0-fold in atrial and ventricular muscle, respectively (P<0.05). At the protein level, cold-induced elevation of SERCA (4.6-fold) was noted only in atrial (P<0.05) but not in ventricular tissue (P>0.05). The expression pattern of PLN was similar to that of the SERCA protein, but chamber-specific and temperature-induced differences were much smaller than in the case of SERCA. In the ventricle, PLN/SERCA ratio was 2.1 and 7.0 times higher than in the atrium for WA and CA fish, respectively. These findings are consistent with the hypothesis that low PLN/SERCA ratio in atrial tissue enables faster SR Ca 2+ reuptake and thus contributes to faster kinetics of contraction in comparison with ventricular muscle. Similarly, cold-induced decrease in PLN/SERCA ratio may be associated with faster contraction kinetics of the CA trout heart, in particular in the atrial muscle.

Cold acclimation increases cardiac myofilament function and ventricular pressure generation in trout

“…Evidence for our hypothesis is that cold acclimation increases Ca 2+ release and uptake from the sarcoplasmic reticulum (SR), as well as the twitch kinetics of intact trout cardiac fibers (Aho and Vornanen, 1998;Aho and Vornanen, 1999). These changes in function have been shown to occur in conjunction with a proliferation of sarcoplasmic reticulum (SR) content and an increase in the amount of SERCA in the heart with cold acclimation in cold-active species (Keen et al, 1994;Tiitu and Vornanen, 2002;Shiels et al, 2011;Korajoki and Vornanen, 2012;Korajoki and Vornanen, 2013). This increased roll of the SR with cold acclimation would also complement the increase in the Ca 2+ sensitivity of the myofilament characterized in the current study.…”

“…Previous studies have demonstrated that cold acclimation of rainbow trout results in an increase in the maximal rate of cardiac actomyosin ATPase (AM-ATPase) (Yang et al, 2000;Klaiman et al, 2011) and the speed of twitch kinetics of intact fibers (Aho and Vornanen, 1999), indicating changes to the contractile machinery of the heart. Cold acclimation of trout also affects transcript levels of troponin I (Alderman et al, 2012), troponin C (Genge et al, 2013) and sarcoplasmic reticulum Ca 2+ -ATPase (SERCA) (Korajoki and Vornanen, 2012), all of which are key proteins involved in contraction. Korajoki and Vornanen (Korajoki and Vornanen, 2013) have also found that cold acclimation of burbot causes a fourfold increase in the levels of SERCA in the heart, a modification thought to help maintain Ca 2+ cycling at low temperatures.…”

Thyroid hormone regulates cardiac performance during cold acclimation in Zebrafish (Danio rerio)

“…Hence, thyroid hormone would affect thermal performance breadth of the heart, particularly at warmer temperatures. In the cold-acclimated fish, increased maximum heart rate was paralleled by increases in SERCA activity and sympathetic outflow, both of which are typical cold acclimation responses in fish (Aho and Vornanen, 1998;Aho and Vornanen, 1999;Shiels et al, 2002;Klaiman et al, 2011;Shiels et al, 2011;Korajoki and Vornanen, 2012). The SERCA activity levels reported in our fish are comparable to those previously measured in cold-acclimated carp and trout (Aho and Vornanen, 1998), but both our zebrafish and the carp and trout activities are approximately two orders of magnitude larger than those measured in a recent multispecies comparison (Landeira-Fernandez et al, 2012).…”

“…Many fish species alter calcium cycling, heart rate, heart morphology and sympathetic outflow to optimize oxygen delivery across a temperature range (Aho and Vornanen, 1998;Aho and Vornanen, 1999;Shiels et al, 2002;Klaiman et al, 2011;Shiels et al, 2011;Korajoki and Vornanen, 2013;Korajoki and Vornanen, 2012). For instance, sarcoplasmic reticulum Ca 2+ uptake via the sarco-endoplasmic reticulum Ca 2+ -ATPase (SERCA), which plays an important role in muscle force production and fatigue resistance, increases in the heart of coldactive rainbow trout during cold exposure (Aho and Vornanen, 1999).…”

Cardiac function in an endothermic fish: cellular mechanisms for overcoming acute thermal challenges during diving

“…The cellular proteins that cycle Ca 2þ and develop force during excitation-contraction coupling are all known to be acutely temperature-sensitive (e.g. the L-type Ca 2þ current (I Ca ) [24,25], the ryanodine receptor [26][27][28][29], the SR Ca 2þ ATPase (SERCA) [30][31][32], the Na þ -Ca 2þ exchanger [33,34] and the myofilaments [35,36]). However, changes in the shape of the rainbow trout AP have been shown to offset the acute effect of temperature on I Ca [22], suggesting compensatory interplay between thermal sensitivity of the different components of excitation-contraction coupling in fish.…”