Changes of Cytosolic $Ca^{2+}$ under Metabolic Inhibition in Isolated Rat Ventricular Myocytes

To characterize cytosolic $Ca^{2+}$ fluctuations under metabolic inhibition, rat ventricular myocytes were exposed to $200{\mu}M$ 2,4-dinitrophenol (DNP), and mitochondrial $Ca^{2+}$, mitochondrial membrane potential (${\Delta}{\Psi}m$), and cytosolic $Ca^{2+}$ were measured, using Rhod-2 AM, TMRE, and Fluo-4 AM fluorescent dyes, respectively, by Laser Scanning Confocal Microscopy (LSCM). Furthermore, the role of sarcolemmal $Na^+$/$Ca^{2+}$ exchange (NCX) in cytosolic $Ca^{2+}$ efflux was studied in KB-R7943 and $Na^+$-free normal Tyrode's solution (143 mM LiCl ). When DNP was applied to cells loaded with Fluo-4 AM, Fluo-4 AM fluorescence intensity initially increased by $70{\pm}10$% within $70{\pm}10$ s, and later by $400{\pm}200$% at $850{\pm}45$ s. Fluorescence intensity of both Rhod-2 AM and TMRE were initially decreased by DNP, coincident with the initial increase of Fluo-4 AM fluorescence intensity. When sarcoplasmic reticulum (SR) $Ca^{2+}$ was depleted by $1{\mu}M thapsigargin plus $10{\mu}M ryanodine, the initial increase of Fluo-4 AM fluorescence intensity was unaffected, however, the subsequent progressive increase was abolished. KB-R7943 delayed both the first and the second phases of cytosolic $Ca^{2+}$ overload, while $Na^+$-free solution accelerated the second. The above results suggest that: 1) the initial rise in cytosolic $Ca^{2+}$ under DNP results from mitochondrial depolarization; 2) the secondary increase is caused by progressive $Ca^{2+}$ release from SR; 3) NCX plays an important role in transient cytosolic $Ca^{2+}$ shifts under metabolic inhibition with DNP.