Energetic performance is improved by specific activation of K+ fluxes through KCa channels in heart mitochondria

Mitochondrial volume regulation depends on K+ movement across the inner membrane and a mitochondrial Ca2+-dependent K+ channel (mitoKCa) reportedly contributes to mitochondrial K+ uniporter activity. Here we utilize a novel KCa channel activator, NS11021, to examine the role of mitoKCa in regulating mitochondrial function by measuring K+ flux, membrane potential (ΔΨm), light scattering, and respiration in guinea pig heart mitochondria. K+ uptake and the influence of anions were assessed in mitochondria loaded with the K+ sensor PBFI by adding either the chloride (KCl), acetate (KAc), or phosphate (KH2PO4) salts of K+ to energized mitochondria in a sucrose-based medium. K+ fluxes saturated at ∼ 10 mM for each salt, attaining maximal rates of 172 ± 17, 54 ± 2.4, and 33 ± 3.8 nmol K+/min/mg in KCl, KAc, or KH2PO4, respectively. NS11021 (50 nM) increased the maximal K+ uptake rate by 2.5-fold in the presence of KH2PO4 or KAc and increased mitochondrial volume, with little effect on ΔΨm. In KCl, NS11021 increased K+ uptake by only 30% and did not increase volume. The effects of NS11021 on K+ uptake were inhibited by the KCa toxins charybdotoxin (200 nM) or paxilline (1 μM). Fifty nanomolar of NS11021 increased the mitochondrial respiratory control ratio (RCR) in KH2PO4, but not in KCl; however, above 1 μM, NS11021 decreased RCR and depolarized ΔΨm. A control compound lacking KCa activator properties did not increase K+ uptake or volume but had similar nonspecific (toxin-insensitive) effects at high concentrations. The results indicate that activating K+ flux through mitoKCa mediates a beneficial effect on energetics that depends on mitochondrial swelling with maintained ΔΨm.