Endogenous and Agonist-induced Opening of Mitochondrial Big Versus Small Ca2+-sensitive K+ Channels on Cardiac Cell and Mitochondrial Protection

Abstract

Both big (BKCa) and small (SKCa) conductance Ca2+-sensitive K+ channels are present in mammalian cardiac cell mitochondria (m). We used pharmacological agonists and antagonists of BKCa and SKCa channels to examine the importance of endogenous opening of these channels and the relative contribution of either or both of these channels to protect against contractile dysfunction and reduce infarct size after ischemia reperfusion (IR) injury through a mitochondrial protective mechanism. Following global cardiac IR injury of ex vivo perfused guinea pig hearts we found the following: both agonists NS1619 (for BKCa) and DCEB (for SKCa) improved contractility; BKCa antagonist paxilline (PAX) alone or with SKCa antagonist NS8593 worsened contractility and enhanced infarct size; both antagonists PAX and NS8593 obliterated protection by their respective agonists; BKCa and SKCa antagonists did not block protection afforded by SKCa and BKCa agonists, respectively; and all protective effects by the agonists were blocked by scavenging superoxide anions (O2•−) with TBAP. Contractile function was inversely associated with global infarct size. In in vivo rats, infusion of NS8593, PAX, or both antagonists enhanced regional infarct size while infusion of either NS1619 or DCEB reduced infarct size. In cardiac mitochondria isolated from ex vivo hearts after IR, combined SKCa and BKCa agonists improved respiratory control index (RCI) and Ca2+ retention capacity (CRC) compared to IR alone, whereas the combined antagonists did not alter RCI but worsened CRC. Although the differential protective bioenergetics effects of endogenous or exogenous BKCa and SKCa channel opening remain unclear, each channel likely responds to different sensing Ca2+ concentrations and voltage gradients over time during oxidative stress-induced injury to individually or together protect cardiac mitochondria and myocytes.

SK channel-mediated metabolic escape to glycolysis inhibits ferroptosis and supports stress resistance in C. elegans

“…Furthermore, when mitochondrial ROS initiated by CyPPA were scavenged by the antioxidant MnTBAP, the CyPPA-mediated protection was attenuated. Several studies showed that MnTBAP is neuroprotective against oxidative stress 61,62 , while it may block the protective effect of large conductance calcium-activated K + channel (BK) channels in models of ischemia-reperfusion injury 63 , which is in line with our current observations. Interestingly, when combining DCA and MnTBAP, the protection mediated by CyPPA was even further reduced, suggesting that both glycolysis and mitochondrial ROS are essential in SK channel-mediated neuroprotection.…”

Regulation of the Mitochondrial BKCa Channel by the Citrus Flavonoid Naringenin as a Potential Means of Preventing Cell Damage

“…It has been shown that mitoBK Ca channels (like the plasma membrane BK Ca channels) are sensitive to potassium channel openers (NS1619 or NS11021) and potassium channel blockers (paxilline and iberiotoxin) [18,41]. However, the majority of mitochondrial potassium channel modulators exhibit a broad spectrum of off-target effects [42,43].…”

Specific BK Channel Activator NS11021 Protects Rat Renal Proximal Tubular Cells from Cold Storage—Induced Mitochondrial Injury In Vitro

“…Regardless, 1 µM NS11021 is still within the range of concentrations reported to be highly specific for BK channels compared to other ion channels, although off-target effects unrelated to ion channels cannot be ruled out [32,65]. In contrast, most studies pharmacologically assessing mitoBK channels as therapeutic targets to protect against cardiac IR injury used an earlier BK channel activator, NS1619 [17,21,23,24,[27][28][29]. In these studies, which are comprehensively reviewed by Bentzen et al and Balderas et al, NS1619 offered partial protection against IR injury in numerous cell and animal models, ostensibly by activation of mitoBK channels [17,29].…”

“…A second limitation is that although paxilline is regarded as a specific BK channel blocker, it also can inhibit Ca 2+ uptake by sarco/endoplasmic reticulum Ca 2+ -ATPase (SERCA) at concentrations ≥5 µM, thereby potentially modifying intracellular Ca 2+ signaling, a pathway already disrupted in IR injury as well as CS injury [77][78][79][80]. While most studies have used paxilline concentrations between 1 and 5 µM to prevent pharmacological activation of mitoBK channels by NS11021, some studies have used higher concentrations of 10-50 µM for this purpose [18,22,24,27,28,[62][63][64][81][82][83][84][85][86]. Considering that few putatively selective, cell-permeable BK channel blockers are commercially available, we chose to use the highest concentration (5 µM) of paxilline reported to effectively prevent pharmacological activation of mitoBK channels while avoiding higher concentrations associated with off-target effects [66,77].…”

“…In the quest for new therapeutic targets to ameliorate CS-induced injury, our attention was drawn to the mitochondrial isoform of the big-conductance Ca 2+ -activated K + (mitoBK) channel, which resides in the inner mitochondrial membrane (for review, see Balderas et al [17]). Many studies indicate that the endogenous and pharmacological activation of the mitoBK channel protects against cardiac ischemia-reperfusion (IR) injury and other oxidative injuries by reducing mitochondrial ROS, mitochondrial respiratory dysfunction, mitochondrial depolarization, and cell death, ultimately improving cardiac function [18][19][20][21][22][23][24][25][26][27][28]. In this regard, CS-induced renal injury is now thought to be a distinct form of IR injury, which raises the possibility that mitoBK may be a relevant therapeutic target [5].…”