Energetic state is a strong regulator of sarcoplasmic reticulum Ca2+ loss in cardiac muscle: different efficiencies of different energy sources.
OBJECTIVE
Increased diastolic sarcoplasmic reticulum (SR) Ca(2+) loss could depress contractility in heart failure. Since the failing myocardium has impaired energetics, we investigated whether Ca(2+) loss is linked to changes in energetic pathways.


RESULTS
Leakage from SR in mouse permeabilized preparations was assessed using exogenous ATP, ATP + phosphocreatine (activation of bound creatine kinase, CK), ATP + mitochondrial substrates (mitochondrial activation), or with all of these together (optimal energetic conditions) in Ca(2+)-free solution. In ventricular fibres caffeine-induced tension transients under optimal energetic conditions were used to estimate SR [Ca(2+)]. In cardiomyocytes, intra-SR Ca(2+) was monitored by use of the fluorescent marker Mag-fluo 4. In fibres, SR Ca(2+) content after 5 min incubation strongly depended on energy supply (100%-optimal energetic conditions; 27 +/- 5%-exogenous ATP only, 52 +/- 5%-endogenous CK activation; 88 +/- 8%-mitochondrial activation, P < 0.01 vs. CK system). The significant loss with only exogenous ATP was not inhibited by the ryanodine receptor blockers tetracaine or ruthenium red. However, the SR Ca(2+)-ATPase (SERCA) inhibitors cyclopiazonic acid or 2,5-di(tert-butyl)-1,4-benzohydroquinone significantly decreased Ca(2+) loss. At 100 nM external [Ca(2+)], the SR Ca(2+) loss was also energy dependent and was not significantly inhibited by tetracaine. In cardiomyocytes, the decline in SR [Ca(2+)] at zero external [Ca(2+)] was almost two times slower under optimal energetic conditions than in the presence of exogenous ATP only.


CONCLUSIONS
At low extra-reticular [Ca(2+)], the main leak pathway is an energy-sensitive backward Ca(2+) pump, and direct mitochondrial-SERCA ATP channelling is more effective in leak prevention than local ATP generation by bound CK.
Kuum
Malle
M
Faculté de Pharmacie, U-769 INSERM, Université Paris-Sud, Châtenay-Malabry, France.
Kaasik
Allen
A
Joubert
Frederic
F
Ventura-Clapier
Renée
R
Veksler
Vladimir
V
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004734:Energy Metabolism / Q000502:physiology*; D007211:Indoles / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D023421:Models, Animal; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism*; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D015398:Signal Transduction / Q000502:physiology; D013748:Tetracaine / Q000494:pharmacology
D002121:Calcium Channel Blockers; D007211:Indoles; D019837:Ryanodine Receptor Calcium Release Channel; D013748:Tetracaine; D000255:Adenosine Triphosphate; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium; C000543:cyclopiazonic acid