Kenichi Imahashi

Inhibition of δ-Protein Kinase C Protects Against Reperfusion Injury of the Ischemic Heart In Vivo

Background—Current treatment for acute myocardial infarction (AMI) focuses on reestablishing blood flow (reperfusion). Paradoxically, reperfusion itself may cause additional injury to the heart. We previously found that &dgr;-protein kinase C (&dgr;PKC) inhibition during simulated ischemia/reperfusion in isolated rat hearts is cardioprotective. We focus here on the role for &dgr;PKC during reperfusion only, using an in vivo porcine model of AMI. Methods and Results—An intracoronary application of a selective &dgr;PKC inhibitor to the heart at the time of reperfusion reduced infarct size, improved cardiac function, inhibited troponin T release, and reduced apoptosis. Using 31P NMR in isolated perfused mouse hearts, we found a faster recovery of ATP levels in hearts treated with the &dgr;PKC inhibitor during reperfusion only. Conclusions—Reperfusion injury after cardiac ischemia is mediated, at least in part, by &dgr;PKC activation. This study suggests that including a &dgr;PKC inhibitor at reperfusion may improve the outcome for patients with AMI.

Intracellular Sodium Accumulation During Ischemia as the Substrate for Reperfusion Injury

To elucidate the role of intracellular Na+ kinetics during ischemia and reperfusion in postischemic contractile dysfunction, intracellular Na+ concentration ([Na+]i) was measured in isolated perfused rat hearts using 23Na nuclear magnetic resonance spectroscopy. The extension of the ischemic period from 9 minutes to 15, 21, and 27 minutes (at 37 degrees C) increased [Na+]i at the end of ischemia from 270.0+/-10.4% of preischemic level (mean+/-SE, n=5) to 348.4+/-12.0% (n=5), 491.0+/-34.0% (n=7), and 505.3+/-12.1% (n=5), respectively, whereas the recovery of developed pressure worsened with the prolongation of the ischemic period (95.1+/-4.2%, 84.3+/-1. 2%, 52.8+/-13.7%, and 16.9+/-6.4% of preischemic level). The kinetics of [Na+]i recovery during reperfusion was analyzed by the fitting of a monoexponential function. When the hearts were reperfused with low-[Ca]o (0.15 mmol/L) solution, the time constants of the recovery (tau) after 15-minute (8.07+/-0.85 minutes, n=5) and 21-minute ischemia (6.44+/-0.90, n=5) were significantly extended, with better functional recovery (98.5+/-1.4% for 15-minute [P<0.05]; 98.0+/-1.0% for 21-minute [P<0.05]) compared with standard reperfusion ([Ca]o=2.0 mmol/L, tau=3.58+/-0.28 minutes for 15-minute [P<0.0001]; tau=3.02+/-0.20 for 21-minute [P<0.0001]). A selective inhibitor of Na+/Ca2+ exchanger also decelerated the [Na+]i recovery, which suggests that the recovery reflects the Na+/Ca2+ exchange activity. In contrast, high-[Ca]o reperfusion (5 mmol/L) accelerated the [Na+]i recovery after 9-minute ischemia (tau=2.48+/-0.11 minute, n=5 [P<0.0001]) and 15-minute ischemia (tau=2.10+/-0.07, n=6 [P<0. 05]), but functional recovery deteriorated only in the hearts with 15-minute ischemia (29.8+/-9.4% [P<0.05]). [Na+]i recovery after 27-minute ischemia was incomplete and decelerated by low-[Ca]o reperfusion, with limited improvement of functional recovery (42. 5+/-7.9%, n=5 [P<0.05]). These results indicate that intracellular Na+ accumulation during ischemia is the substrate for reperfusion injury and that the [Na+]i kinetics during reperfusion, which is coupled with Ca2+ influx, also determines the degree of injury.

Targeted Deletion of Thioredoxin-Interacting Protein Regulates Cardiac Dysfunction in Response to Pressure Overload

Biomechanical overload induces cardiac hypertrophy and heart failure, and reactive oxygen species (ROS) play a role in both processes. Thioredoxin-Interacting Protein (Txnip) is encoded by a mechanically-regulated gene that controls cell growth and apoptosis in part through interaction with the endogenous dithiol antioxidant thioredoxin. Here we show that Txnip is a critical regulator of the cardiac response to pressure overload. We generated inducible cardiomyocyte-specific and systemic Txnip-null mice (Txnip-KO) using Flp/frt and Cre/loxP technologies. Compared with littermate controls, Txnip-KO hearts had attenuated cardiac hypertrophy and preserved left ventricular (LV) contractile reserve through 4 weeks of pressure overload; however, the beneficial effects were not sustained and Txnip deletion ultimately led to maladaptive LV remodeling at 8 weeks of pressure overload. Interestingly, these effects of Txnip deletion on cardiac performance were not accompanied by global changes in thioredoxin activity or ROS; instead, Txnip-KO hearts had a robust increase in myocardial glucose uptake. Thus, deletion of Txnip plays an unanticipated role in myocardial energy homeostasis rather than redox regulation. These results support the emerging concept that the function of Txnip is not as a simple thioredoxin inhibitor but as a metabolic control protein.

Troglitazone enhances glucose uptake induced by alpha-adrenoceptor stimulation via phosphatidylinositol 3-kinase in rat heart.

1. Thiazolidinedione‐derived agents have been reported to act as insulin sensitizers by augmenting insulin‐dependent stimulation of phosphatidylinositol 3‐kinase (PI3K) activity in a specific manner. It has been suggested that α‐adrenoceptor stimulation mediates glucose uptake through PI3K in the heart.

Type IV phosphodiesterase inhibitor suppresses insulin-dependent myocardial glucose uptake.

1. Phosphodiesterase (PDE) IV has been localized at cardiomyocytes and the coronary vasculature and modulates cAMP, but the effect of PDE IV on myocardial glucose uptake has not been demonstrated.

Mechanisms for Ischemia/Reperfusion Injury: Application of 23Na Magnetic Resonance Spectroscopy

Intracellular sodium concentration ([Na+]i) of myocardium dramatically increases during ischemia and rapidly returns after reperfusion. [Na+]i kinetics during ischemia/reper-fusion is coupled with those of other important ions such as Ca2+ and K+. Na+ movement of intact perfused heart can be easily detected by 23Na nuclear magnetic resonance spectroscopy (MRS) combined with a shift reagent. Furthermore, the sequential [Na+]i measurement is possible due to the nucleus’ abundance in living tissue and high NMR sensitivity. Thus, 23Na-MRS has been considered to be very valuable informative method in the research of ischemia/reperfusion injury. We have applied 23Na-MRS to elucidate the mechanisms for [Na+]i kinetics during ischemia/reperfusion and its role in injury. This technique combined with quantitative method provides understanding of underlying mechanism for the alteration of ion homeostasis during ischemia/reperfusion as well as [Na+]i movement.