Increased SR Ca2+ cycling contributes to improved contractile performance in SERCA2a-overexpressing transgenic rats.
OBJECTIVE
Heart failure is associated with reduced function of sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) but increased function of sarcolemmal Na+/Ca2+ exchanger (NCX), leading to decreased SR Ca2+ content and loss of frequency-potentiation of contractile force. We reported that SERCA2a-overexpression in transgenic rat hearts (TG) results in improved contractility. However, it was not clear whether TG have improved contractility due to frequency-dependent improved SR Ca2+ handling.


METHODS
Therefore, we characterized TG (n=35) vs. wild-type (WT) control rats (n=39) under physiological conditions (37 degrees C, stimulation rate <8 Hz). Twitch force, intracellular Ca2+ transients ([Ca2+]i), and SR Ca2+ content were measured in isolated muscles. The contribution of transsarcolemmal Ca2+ influx (I(Ca)) through L-type Ca2+ channels (LTCC) and reverse mode NCX (I(Na/Ca)) to Ca2+ cycling were studied in isolated myocytes.


RESULTS
With increasing frequency, force increased in TG muscles by 168+/-35% (8 Hz; P<0.05) and SR Ca2+ content increased by maximally 118+/-31% (4 Hz; P<0.05). In WT, there was a flat force-frequency response without changes in SR Ca2+ content. Relaxation parameters of force and [Ca2+]i decay were accelerated at each frequency in TG vs. WT by approximately 10%. At prolonged rest intervals (<240 s), force and SR Ca2+ content increased significantly more in TG. Consequently, absolute SR Ca2+ content measured in myocytes was increased approximately 2-fold in TG. Transsarcolemmal Ca2+ fluxes estimated by I(Ca) (at 0 mV -10.2+/-1.1 vs. -16.9+/-1.3 pA/pF) and I(Na/Ca) (0.17+/-0.02 vs. 0.46+/-0.05 pA/pF) were decreased in TG vs. WT (P<0.05), whereas NCX and LTCC protein expression was only slightly reduced (P=n.s.).


CONCLUSIONS
In summary, SERCA2a-overexpression improved contractility in a frequency-dependent way due to increased SR Ca2+ loading whereas transsarcolemmal Ca2+ fluxes were decreased.
Maier
Lars S
LS
Abt. Kardiologie and Pneumologie/Herzzentrum, Georg-August-Universität Göttingen, Germany.
Wahl-Schott
Christian
C
Horn
Wiebke
W
Weichert
Stefan
S
Pagel
Christian
C
Wagner
Stefan
S
Dybkova
Nataliya
N
Müller
Oliver J
OJ
Näbauer
Michael
M
Franz
Wolfgang-M
WM
Pieske
Burkert
B
D000818:Animals; D030801:Animals, Genetically Modified; D015153:Blotting, Western / Q000379:methods; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004558:Electric Stimulation; D006333:Heart Failure / Q000378:metabolism; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D012508:Sarcolemma / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019831:Sodium-Calcium Exchanger / Q000378:metabolism
D020746:Calcium Channels, L-Type; D019831:Sodium-Calcium Exchanger; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D002118:Calcium