Mechanism of reactive oxygen species generation after opening of mitochondrial K_ATP channels

Abstract

OPENING of the ATP-sensitive K ϩ (K ATP ) channel mediates the cardioprotective effect induced by pathophysiological stressors such as ischemic preconditioning (IPC) (15,29), heat shock (19), and pharmacological agents, including adenosine (5), ACh (26), opioids (12), monophosphoryl lipid A (31), phosphodiesterase 5A (PDE5A) inhibitors (24,27), and mTOR inhibitor, rapamycin (20). In addition, direct opening of the mitochondrial K ATP (mitoK ATP ) channels with diazoxide induces early and delayed preconditioning effects that are abolished by the channel inhibitor 5-hydroxydecanoate (5-HD) (23). The mitoK ATP channels are activated when intracellular ATP levels drop. Within 1-3 min of ischemia, there is a pronounced shortening of the action potential duration (APD) secondary to activation of the K ATP channels (6). Activation of K ATP channels has been reported to be partially responsible for the increase in outward K ϩ currents, shortening of APD, and the increase in extracellular K ϩ concentration during anoxic or globally ischemic conditions (4). Because there was a lack of correlation between the APD shortening and cardioprotection with the pharmacological openers of K ATP channels (17), Garlid et al. (14) first proposed that mitoK ATP channels were involved in the cardioprotective effect. By using reconstituted mitochondrial vesicles or isolated mitochondria and measuring potassium flux, these investigators demonstrated that heart and liver mitoK ATP channels shared pharmacological properties with the channels found in sarcolemma while possessing a distinct profile. Furthermore, it was shown that opening of mitoK ATP channels leads to the generation of reactive oxygen species (ROS) by the mitochondria and represents an important mechanism that is required for the cardioprotective effect of IPC (25,30). Opening of these channels allowed potassium to enter the mitochondrial inner matrix, which causes generation and release of ROS from the respiratory chain (21). ROS then act as second messengers to activate a downstream pathway of protective kinases, including protein kinase C (PKC), that finally converge on the cardioprotective end effector as shown in Fig. 1. One of the principal downstream effectors of ROS is PKC⑀ (2), which is translocated to the mitochondria (3). Cardioprotection conferred by IPC and pharmacological agents is blocked by ROS scavengers before the index ischemia (11,25). In most earlier studies, mitoK ATP channels were proposed to be end effectors, and the channels were assumed to open during the index ischemia (13, 15). Recent studies, however, suggest that opening of mitoK ATP channels is also the initial trigger of the cardioprotective effect, promoting the generation of ROS and inducing the activation of PKC⑀ (11,25). Other studies have suggested that mitoK ATP channels act both as a trigger as well as an end effector of IPC (16). In contrast to these studies, Hanley and coworkers (10) recently challenged that diazoxide does not evoke superoxide (which dismutates to H 2 O 2 ) from the r...

Sildenafil stimulates the expression of gaseous monoxide-generating enzymes in vascular smooth muscle cells via distinct signaling pathways

“…While sildenafil has been shown to exert antioxidant effects in some experimental systems [30,31], pro-oxidant effects of sildenafil have also been reported in monocytes and tumor cells [32,33]. Although the mechanism by which sildenafil stimulates oxidative stress is not known, a mitochondrial pathway has been proposed [34]. Consistent with this notion, we found that sildenafil stimulates mitochondrial superoxide production in vascular SMCs.…”

“…Moreover, the sildenafil-mediated increase in HO-1 expression is blocked by the mitochondrial electron transport chain complex I inhibitor rotenone, further implicating this organelle with the induction of HO-1 by sildenafil. The mechanism by which sildenafil activates Nrf2 is unclear but it likely involves the oxidation of cysteine residues in Keap1 since several cysteine residues undergo redox-dependent alterations that result in the release and/or blockade of Keap1-dependent ubiquitination and catabolism of Nrf2 [23,34]. Indeed, we found that N-acetyl-L-cysteine inhibits the expression of Nrf2 by sildenafil.…”

Molecular Basis of Cardioprotection

“…339 The molecular identity of mitochondrial K ATP channels is not clear in detail, but there seems to be a multiprotein complex consisting of a sulphonylurea receptor and a potassium channel unit; 340-342 the potassium channel unit was recently characterized in detail and shares homology with the renal outer medullary potassium channel. 343 The mitochondrial K ATP channel is a target of NO, 344 PKC, 167,[345][346][347][348][349] and PKG 350 and on its activation releases ROS which, in turn, activate the PKCε in a positive feedback loop. 184,351 There is also a close interaction of mitochondrial K ATP with mitochondrial Cx 43 in the release of ROS.…”

Diethyldithiocarbamate potentiates the effects of protamine sulphate in the isolated rat uterus

“…4 Various steps of the contractile process have proven to be susceptible to redox modulation. First, the probability of isolated sarcoplasmic reticulum (SR) calcium ion release channels opening increases upon oxidation of accessible protein thiols.…”