Preveleged Ca2+ Signaling Pathway between Membrane NCX and Mitochondria in Cardiac Myocytes

Abstract

Accumulating evidence suggests that mitochondrial Ca2+ signaling plays a critical role in cardiac myocytes E-C coupling and that this may involve Ca2+ entering the mitochondria via the mitochondrial Ca2+ uniporter (MCU) and exiting on the mitochondrial Na+-Ca2+ exchanger (mNCX). Here we investigate whether there are privileged Ca2+ transport pathways between surface membrane, mitochondria and SR. To address this issue, we used genetically engineered mitochondrial Ca2+ probes based on GCamP3 and GCamP6 along with synthetic fluorescent dye, Fura-2, to image mitochondrial and cytosolic Ca2+ movements simultaneously. Mitochondrial Ca2+ probes were virally introduced in either adult or neonatal cardiomyocytes. Whole-cell patch clamping and high-resolution TIRF microscopy were used to measure membrane current, mitochondrial Ca2+ and sub-sarcolemmal cytosolic Ca2+ simultaneously. In whole cell clamped myocytes dialyzed with 2mM EGTA, Na+ withdrawal- (substituting Na+ by tetra-ethylammonium, TEA) generated an outward NCX-mediated current reflecting Ca2+ entry and caused a rapid increase of mitochondrial Ca2+, without significant rise of cytosolic Cai, because of Ca2+-buffering by 2mM EGTA. When Na+ was readmitted, causing a rapid fall (release) of mitochondrial Ca2+ it triggered a significant transient rise of cytosolic Ca2+ despite the presence of buffering by 2mM EGTA. Interestingly, 5mM caffeine puffs while triggering rapid release of Ca2+ from the SR caused little or no significant rise in the mitochondrial Ca2+ signal. Blockers of MCU or mitochondrial NCX failed to suppress this Na+ withdrawal-induced mitochondrial Ca2+ rise. Our results suggest that Ca2+ buffers may block transfer of Ca2+ from the SR via the cytosol to mitochondria without blocking the privileged Ca2+ communication between sarcolemma and mitochondria. Supported by NIH grant R0-1,HL15162