Abstract 552: Mitochondrial KCa and Anion Channels Influence Mitochondrial Volume and Respiratory Rate: Relevance to Cardioprotection
Ion channels in the mitochondrial inner membrane influence the response of the heart to ischemia and reperfusion - the opening of mitochondrial K+ channels (mitoKCa or mitoKATP) induce cardioprotection while the excessive activation of an inner membrane anion channel (IMAC) contributes to post-ischemic contractile and electrical dysfunction. Here we examined the effects of KCa channel openers and inhibitors on mitochondrial volume and respiration in KCl-based medium in energized isolated guinea-pig heart mitochondria. Inhibition of IMAC with a mitochondrial benzodiazepine ligand was also assessed. Mitochondrial swelling and contraction were evoked by the sequential addition of 5mM glutamate (Glu) and 5mM malate (Mal), respectively. Both the swelling and contraction responses involved energy-dependent fluxes of K+ and Cl−. The Glu-induced swelling response was suppressed by preincubation with either the KCa channel toxin paxilline, or by the IMAC blocker 4′-chlorodiazepam (4-Cl-DZP). The KCa opener NS1619, at concentrations <10μM, initiated mitochondrial swelling responses either in the absence or presence of substrates. This effect was blocked by paxilline or charybdotoxin. Massive and irreversible swelling was induced by [NS1619] >10μM. Under similar conditions, mitochondrial respiratory control (State3/State4 VO2 ratio) was maintained at >15 for [NS1619] <10μM, but was strongly suppressed above 20μM due to an increase in state4- and a decrease in state3-respiration. 4-Cl-DZP did not affect respiration and only slightly increased the K0.5 for ADP. The findings indicate that mitochondrial matrix volume is modulated by mitoKCa channel activity under normal conditions and in the presence of KCa channel openers, while IMAC channels underlie compensatory anion movements in response to K+ flux. Physiological regulation of mitochondrial volume by ion transport is in turn linked to altered respiratory flux, with important implications for cardioprotection and recovery of mitochondrial function in the post-ischemic heart.