Intracellular calcium increasing at the beginning of reperfusion assists the early recovery of myocardial contractility after diltiazem cardioplegia

Na+ overload-induced mitochondrial damage in the ischemic heart

Ischemia induces a decrease in myocardial contractility that may lead more or less to contractile dysfunction in the heart. When the duration of ischemia is relatively short, myocardial contractility is immediately reversed to control levels upon reperfusion. In contrast, reperfusion induces myocardial cell death when the heart is exposed to a prolonged period of ischemia. This phenomenon is the so-called "reperfusion injury". Numerous investigators have reported the mechanisms underlying myocardial reperfusion injury such as generation of free radicals, disturbance in the intracellular ion homeostasis, and lack of energy for contraction. Despite a variety of investigations concerning the mechanisms for ischemia and ischemia–reperfusion injury, ionic disturbances have been proposed to play an important role in the genesis of the ischemia–reperfusion injury. In this present study, we focused on the contribution of Na+ overload and mitochondrial dysfunction during ischemia to the genesis of this ischemia–reperfusion injury.Key words: mitochondria, myocardial ischemia, Na+ channels, Na+/H+ exchanger, Na+ overload.

Preservation of mitochondrial function during ischemia as a possible mechanism for cardioprotection of diltiazem against ischemia/reperfusion injury

A possible mechanism for D-cis-diltiazem (diltiazem)-mediated improvement of the contractile function of ischemic/reperfused hearts was examined. Thirty-five-min ischemia/60-min reperfusion recovered little the left ventricular developed pressure (LVDP) and decreased myocardial high-energy phosphates (HEPs). Ischemia induced an accumulation of tissue Na+ content, an increase in cytochrome c in the cytosolic fraction, and a decrease in the oxygen consumption rate (OCR) in perfused hearts. Treatment of the heart with 1 microM diltiazem for the last 3-min of pre-ischemia did not affect the decrease in HEPs during ischemia, whereas that with 3 microM partially attenuated the decrease in ATP, suggesting that 3 microM diltiazem exerted energy-sparing effect. Treatment with 1 microM diltiazem enhanced the post-ischemic recovery of LVDP associated with attenuation of the ischemia-induced accumulation of tissue Na+, increase in cytochrome c in the cytosolic fraction, and decrease in myocardial OCR, and restoration of the myocardial HEPs during reperfusion. Combined treatment with diltiazem and a Na+/H+ exchange inhibitor, but not a Na+ channel blocker, facilitated the attenuation of Na+ accumulation in the ischemic heart and the enhancement of the post-ischemic recovery of LVDP. Sodium lactate, a possible metabolite in ischemic hearts, and sodium chloride increased the Na+ concentration in mitochondria, released cytochrome c into incubation medium, and reduced the mitochondrial respiration. Treatment of isolated mitochondria with diltiazem failed to attenuate the sodium lactate- and sodium chloride-induced alterations. These results suggest that the cardioprotection of diltiazem may be exerted via attenuating cytosolic Na+ overload through Na+ channels in the ischemic heart, leading to preservation of mitochondrial functional ability during ischemia, followed by improvement of post-ischemic energy production and contractile recovery.