Mitochondrial and cytosolic calcium in rat hearts under high-K+ cardioplegia and pyruvate: mechano-energetic performance

Abstract

High-K(+)-cardioplegia (CPG) and pyruvate (Pyr) are used as cardioprotective agents. Considering that mitochondria play a critical role in cardiac dysfunction, we investigated the effect of CPG on mitochondrial Ca(2+) uptake and sarcorreticular (SR) calcium handling. Cytosolic and mitochondrial Ca(2+), as well as mitochondrial membrane potential (ΔΨm) were assessed in rat cardiomyocytes by confocal microscopy. Mechano-calorimetrical correlation was studied in perfused hearts. CPG did not modify JC-1 (ΔΨm), but transiently increased, by up to 1.8 times, the Fura-2 (intracellular Ca concentration, [Ca(2+)]i) and Rhod-2 (mitochondrial free Ca concentration [Ca(2+)]m) fluorescence of resting cells, with exponential decays. The addition of 5µmol·L(-1) thapsigargin (Tpg) increased the Rhod-2 fluorescence in a group of cells without any effect on the Fura-2 signal. In rat hearts perfused with CPG, 1µmol·L(-1) Tpg decreased resting heat rate (ΔH(r):-0.44± 0.07mW·g(-1)), while the addition of 5µmol·L(-1) KB-R7943 increased resting pressure (ΔrLVP by+5.26± 1.10mm Hg; 1mm Hg= 133.322 Pa). The addition of 10mmol·L(-1) Pyr to CPG increased H(r) (+3.30± 0.24mW·g(-1)) and ΔrLVP (+2.2± 0.4mm Hg), which are effects potentiated by KB-R7943. The results suggest that under CPG, (i) there was an increase in [Ca(2+)]i and [Ca(2+)]m (without changing ΔΨm) that decayed by exothermic removal mechanisms; (ii) mitochondrial Ca(2+) uptake contributed to the removal of cytosolic Ca(2+), in a process that was potentiated by inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), and reduced by KB-R7943; (iii) under these conditions, SERCA represents the main energetic consumer; (iv) Pyr increased the energetic performance of hearts,mainly by inducing mitochondrial metabolism.