The SR-mitochondria interaction: a new player in cardiac pathophysiology.

Mitochondria are essential for energy supply and cell signalling and may be triggers and effectors of cell death. Mitochondrial respiration is tightly controlled by the matrix Ca(2+) concentration, which is beat-to-beat regulated by uptake and release mainly through the mitochondrial Ca(2+) uniporter and Na(+)/Ca(2+) exchanger, respectively. Recent studies demonstrate that mitochondrial Ca(2+) uptake is more dependent on anatomo-functional microdomains established with the sarcoplasmic reticulum (SR) than on cytosolic Ca(2+). This privileged communication between SR and mitochondria is not restricted to Ca(2+) but may involve ATP and reactive oxygen species, which has important implications in cardiac pathophysiology. The disruption of the SR-mitochondria interaction caused by cell remodelling has been implicated in the deterioration of excitation-contraction coupling of the failing heart. The SR-mitochondria interplay has been suggested to be involved in the depressed Ca(2+) transients and mitochondrial dysfunction observed in diabetic hearts as well as in the genesis of certain arrhythmias, and it may play an important role in myocardial reperfusion injury. During reperfusion, re-energization in the presence of cytosolic Ca(2+) overload results in SR-driven Ca(2+) oscillations that may promote mitochondrial permeability transition (MPT). The relationship between MPT and Ca(2+) oscillations is bidirectional, as recent data show that the induction of MPT in Ca(2+)-overloaded cardiomyocytes may result in mitochondrial Ca(2+) release that aggravates Ca(2+) handling and favours hypercontracture. A more complete characterization of the structural arrangements responsible for SR-mitochondria interplay will allow better understanding of cardiac (patho)physiology but also, and no less important, should serve as a basis for the development of new treatments for cardiac diseases.