Shear fluid-induced Ca2+ release and the role of mitochondria in rat cardiac myocytes.
Cardiac myocyte contraction occurs when Ca2+ influx through voltage-gated L-type Ca2+ channels causes Ca2+ release from ryanodine receptors of the sarcoplasmic reticulum (SR). Although mitochondria occupy about 35% of the cell volume in rat cardiac myocytes, and are thought to be located <300 nm from the junctional SR, their role in the beat-to-beat regulation of cardiac Ca2+ signaling remains unclear. We have recently shown that rapid ( approximately 20 ms) application of shear fluid forces ( approximately 25 dynes/cm2) to rat cardiac myocytes triggers slowly ( approximately 300 ms) developing Cai transients that were independent of activation of all transmembrane Ca2+ transporting pathways, but were suppressed by FCCP, CCCP, and Ru360, all of which are known to disrupt mitochondrial function. We have here used rapid 2-D confocal microscopy to monitor fluctuations in mitochondrial Ca2+ levels ([Ca2+]m) and mitochondrial membrane potential (Delta Psi m) in rat cardiac myocytes loaded either with rhod-2 AM or tetramethylrhodamine methyl ester (TMRM), respectively. Freshly isolated intact rat cardiac myocytes were plated on glass coverslips and incubated in 5 mM Ca2+ containing Tyrode's solution and 40 mM 2,3-butanedione monoxime (BDM) to inhibit cell contraction. Alternatively, myocytes were permeabilized with 10 microM digitonin and perfused with an "intracellular" solution containing 10 microM free [Ca2+], 5 mM EGTA, and 15 mM BDM. Direct [Ca2+]m measurements showed transient mitochondrial Ca2+ accumulation after exposure to 10 mM caffeine, as revealed by a 66% increase in the rhod-2 fluorescence intensity. Shear fluid forces, however, produced a 12% decrease in signal, suggesting that application of a mechanical force releases Ca2+ from the mitochondria. In addition, caffeine and CCCP or FCCP strongly reduced Delta Psi m, while application of a pressurized solution produced a transient Delta Psi m hyperpolarization in intact ventricular myocytes loaded with TMRM. The close proximity of mitochondria to ryanodine receptors and large [Ca2+] that develop in microdomains following calcium release are likely to play a critical role in regulating cytosolic Ca2+ signaling. We suggest that mitochondria may accumulate and release Ca2+ in response to mechanical forces generated by blood flow, independent of surface membrane-regulated CICR. The extent to which such a signaling mechanism contributes to stretch-induced increase in myocardial force and pathogenesis of arrhythmias remains to be assessed.
Belmonte
Steve
S
Department of Pharmacology, Georgetown University, 3900 Reservoir Road, Washington, DC 20007, USA.
Morad
Martin
M
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D016572:Cyclosporine / Q000494:pharmacology; D008929:Mitochondria, Heart / Q000187:drug effects / Q000502:physiology*; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology; D051381:Rats; D012402:Rotenone / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D016276:Ventricular Function
D012402:Rotenone; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D002110:Caffeine; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D016572:Cyclosporine; D002118:Calcium