Yoshihiko Oishi

Cardioprotective Role of AT2 Receptor in Postinfarction Left Ventricular Remodeling

Abstract—The aim of this study was to determine the role of the AT2 receptor (AT2R) in left ventricular (LV) remodeling after myocardial infarction (MI). The left anterior descending arteries were ligated in AT2R gene knockout (Agtr2-) and wild-type (Agtr2+) mice. The LV remodeling was evaluated by echocardiography and histology over a period of 2 weeks after MI. The infarct sizes in hearts excised from Agtr2+ and Agtr2- mice on day 1 were similar. The mortality rate of Agtr2- mice (62.9%) on day 14 after MI was significantly (P <0.05) higher than that of Agtr2+ mice (39.7%). Accordingly, LV/body weight ratios (3.7±0.2 versus 3.0±0.1 on day 14) and LV end-diastolic (4.8±0.3 versus 3.9±0.4 mm on day 7) and end-systolic (4.4±0.3 versus 3.2±0.6 mm on day 7) dimensions evaluated by echocardiography were significantly greater in Agtr2- than in Agtr2+ mice. The rates of ventricular arrhythmia, rates of cardiac rupture, and blood pressures in the 2 strains were similar after MI. Myocyte cross-sectional areas were increased after MI, but the magnitudes were similar in Agtr2+ and Agtr2- mice, indicating the greater increases in LV dimensions and weight in Agtr2- mice are due to elongation of myocyte length and/or an increase in the interstitial weight (including vasculatures, infiltrated cells, and interstitial fluid). Interstitial fibrosis in remote myocardium was not evident in either strain. These results indicate AT2R plays a significant role in the protection against early development of LV dilation, thereby reducing the early mortality rate after MI.

Genetic ablation of the transcription repressor Bach1 leads to myocardial protection against ischemia/reperfusion in mice

Bach1 is a transcriptional repressor of heme oxygenase‐1 gene (Hmox‐1) and β‐globin gene. Heme oxygenase (HO)‐1 is an inducible cytoprotective enzyme that degrades pro‐oxidant heme to carbon monoxide (CO) and biliverdin/bilirubin, which are thought to mediate anti‐inflammatory and anti‐oxidant actions of HO‐1. In the present study, we investigated the role of Bach1 in tissue protection against myocardial ischemia/reperfusion (I/R) injury in vivo using mice lacking the Bach1 gene (Bach1−/−) and wild‐type (Bach1+/+) mice. In Bach1−/− mice, myocardial expression of HO‐1 protein was constitutively up‐regulated by 3.4‐fold compared to that in Bach1+/+ mice. While myocardial I/R induced HO‐1 protein in ischemic myocytes in both strains of mice, the extent of induction was significantly greater in Bach1−/− mice than in Bach1+/+ mice. Myocardial infarction was markedly reduced in size by 48.4% in Bach1−/− mice. Pretreatment of Bach1−/− mice with zinc‐protoporphyrin, an inhibitor of HO activity, abolished the infarction‐reducing effect of Bach1 disruption, indicating that reduction in the infarct size was mediated, at least in part, by HO‐1 activity. Thus, Bach1 plays a pivotal role in setting the levels of both constitutive and inducible expression of HO‐1 in the myocardium. Bach1 inactivation during I/R appears to be a key mechanism controlling the activation level of cytoprotective program involving HO‐1.

Dopamine D2 receptor modulates sodium handling via local production of dopamine in the kidney

Summary: We have recently demonstrated that a deletion of the dopamine D2 receptor gene caused suppression of urinary sodium excretion and salt‐sensitive elevation of blood pressure in mice. In order to understand the mechanisms underlying this impaired sodium excretion, we studied renal dopamine production and dopamine‐induced sodium excretion in 20‐ to 30‐week‐old male D2‐ receptor knockout (D2KO) mice and age‐ and sex‐matched wildtype (WT) mice. Renal local dopamine synthesis, examined by 24‐h urine free dopamine excretion (UDAV), was significantly (p < 0.05) reduced in D2KO mice compared to that in WT mice (D2KO versus WT: 1.06 ± 0.2 versus 1.5 ± 0.3 ng/mg creatinine). Such a difference between D2KO and WT mice was also observed after oral administration of 3,4‐dihydroxyphenylalanine (L‐DOPA), a precursor of dopamine, at 5 mg/kg per day for 24 h. Furthermore, activity of aromatic l‐amino acid decarboxylase, a dopamine synthetase, was significantly suppressed in D2KO mice. Next, we examined changes in 24‐h urine flow (UV) and 24‐h sodium excretion (UNaV) during chronic infusion of dopamine at sub‐pressor doses (3‐4 μg/kg per min, sq.) or a vehicle via an osmotic pump. Urine flow in 24 h and UNaV were significantly (p < 0.05) smaller in D2KO mice infused with vehicle than in WT mice infused with vehicle (UV: 210 ± 43 versus 650 ± 163 μl/day; UNaV: 20.6 ± 13.2 versus 44.4 ± 21.6 μEq/day). After administration of dopamine, UV and UNaV in D2KO mice were restored to a level similar to that in WT mice. These results indicate that D2‐dopamine receptors play a significant role in renal local dopamine synthesis and that a shortage of dopamine was, at least in part, responsible for the suppression of UV and UNaV in D2KO mice. However, we could not conclude from the present study whether renal tubular sodium reabsorption is intact in D2KO mice because the baseline dopamine contents in kidneys of D2KO mice and WT mice may be different.

Guidewire Bias in Rotational Atherectomy in the Angled Lesion

The effect of guidewire bias on angled-lesion ablation by rotational atherectomy (RA) was assessed by measuring the changes in vertical lumen diameter, horizontal lumen diameter and the intima-media thickness of the coronary artery, using intravascular ultrasound in 10 lesions with an angle greater than 10 degrees. The vertical and horizontal diameters significantly increased after RA. The intima-media thickness at the 4 orthogonal sites significantly decreased. There was a significant positive correlation between vertical diameter change and angle (r=0.642, p=0.045), but none between horizontal diameter change and angle. There was no correlation between intima-media thickness change at 0 degrees and angle; however, at 180 degrees there was a tendency to correlation with angle (r=0.602, p=0.066). These data suggest that in cases of angled lesions, the increase in vertical lumen diameter is caused more by ablation of the 180 degrees wall than by that of the 0 degrees wall, which is brought about by guidewire bias toward the vascular wall at 180 degrees.

Immunohistochemical distributions of the tissue kallikrein-kinin system in ischemic and non-ischemic mouse heart.

Summary: Kinins have been shown to play a cardioprotective role during myocardial ischemia. However, the localization of each of the components of the kallikrein‐kinin system in the heart has not been determined in a cell type‐specific manner. Recently, mK1 has been identified as the major tissue kallikrein with the strongest bradykinin‐forming activity among the products of the mouse tissue kallikrein gene superfamily. In the study presented here, we investigated the localizations of mK1, kininogen and bradykinin B2 receptors (B2Rs) in ischemic and non‐ischemic left ventricles by immunohistochemistry. Kininogen, which contains bradykinin as a surface epitope, was detected by an anti‐bradykinin antibody. Changes in the amounts of mK1 and B2R were evaluated by Western blot analysis. Myocardial ischemia was induced by ligation of the left anterior descending coronary artery for 60 min followed by reperfusion for 24 h. mK1 and B2Rs were most abundantly expressed in the vascular endothelium and, to a lesser extent, in fibroblasts. No immunohistochemical signal of these molecules was detected in myocytes. Kininogen was localized in the vascular endothelium and the smooth muscle layer. Myocardial ischemia, although it had no effect on the localization of these molecules, increased the amounts of mK1 and B2R. We have obtained immunohistochemical evidence that all components of the tissue kallikrein‐kinin system are present in the mouse heart. The coronary artery is the major site of kallikrein‐kinin activity both in ischemic and non‐ischemic hearts.