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 Ca2+ uptake and sarcorreticular (SR) calcium handling. Cytosolic and mitochondrial Ca2+, 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, [Ca2+]i) and Rhod-2 (mitochondrial free Ca concentration [Ca2+]m) fluorescence of resting cells, with exponential decays. The addition of 5 µmol·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, 1 µmol·L–1 Tpg decreased resting heat rate (ΔHr: –0.44 ± 0.07 mW·g–1), while the addition of 5 µmol·L–1 KB-R7943 increased resting pressure (ΔrLVP by +5.26 ± 1.10 mm Hg; 1 mm Hg = 133.322 Pa). The addition of 10 mmol·L–1 Pyr to CPG increased Hr (+3.30 ± 0.24 mW·g–1) and ΔrLVP (+2.2 ± 0.4 mm Hg), which are effects potentiated by KB-R7943. The results suggest that under CPG, (i) there was an increase in [Ca2+]i and [Ca2+]m (without changing ΔΨm) that decayed by exothermic removal mechanisms; (ii) mitochondrial Ca2+ uptake contributed to the removal of cytosolic Ca2+, in a process that was potentiated by inhibition of sarco–endoplasmic reticulum Ca2+-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.