Mitochondrial Ca2+ flux through Na+/Ca2+ exchange.

Area of Science:

• Cardiovascular Physiology
• Mitochondrial Biology
• Ion Transport

Background:

• Mitochondrial calcium handling is crucial for cardiac function.
• The role of mitochondrial sodium-calcium exchange (mNCX) in situ remains incompletely understood.
• Understanding mNCX is vital for elucidating cellular calcium homeostasis.

Purpose of the Study:

• To characterize the functional properties of mitochondrial Na+/Ca2+ exchange in permeabilized rat ventricular myocytes.
• To determine the influence of cytoplasmic sodium concentration on mitochondrial calcium uptake and release.

Main Methods:

• Mitochondrial calcium levels were measured using the fluorescent indicator Rhod-2.
• Experiments were conducted in permeabilized rat ventricular myocytes.
• Varying concentrations of cytoplasmic calcium (Ca2+(c)) and sodium (Na+(c)) were applied.

Main Results:

• Cytoplasmic Ca2+ (300 nM) increased mitochondrial Ca2+ approximately ninefold in the absence of Na+(c).
• Elevated Na+(c) attenuated peak mitochondrial Ca2+ levels, indicating activation of the forward mode of mNCX.
• Rhod-2 fluorescence decay upon Ca2+ removal was Na+(c)-dependent (K(1/2) ≈ 1 mM) and inhibited by CGP-37157.

Conclusions:

• Mitochondrial Na+/Ca2+ exchange is active in regulating mitochondrial calcium in cardiac myocytes.
• The Na+ binding site of mNCX appears saturated at physiological Na+(c) concentrations.
• These findings provide in situ evidence for the role of mNCX in cardiac calcium homeostasis.