Naoyuki Otani

Roles of organic anion transporters (OATs) in renal proximal tubules and their localization

Organic anions (OAs) are secreted in renal proximal tubules in two steps. In the first step, OAs are transported from the blood through basolateral membranes into proximal tubular cells. The prototypical substrate for renal organic anion transport systems, para-aminohippurate (PAH), is transported across basolateral membranes of proximal tubular cells via OAT1 (SLC22A6) and OAT3 (SLC22A8) against an electrochemical gradient in exchange for intracellular dicarboxylates. In the second step, OAs exit into urine through apical membranes of proximal tubules. This step is thought to be performed by multidrug efflux transporters and a voltage-driven organic anion transporter. However, the molecular nature and precise functional properties of these efflux systems are largely unknown. Recently, we characterized an orphan transporter known as human type I sodium-phosphate transporter 4, hNPT4 (SLC17A3), using the Xenopus oocyte expression system. hNPT4 acts as a voltage-driven efflux transporter (“human OATv1”) for several OAs such as PAH, estrone sulfate, diuretic drugs, and urate. Here, we describe a model for an OA secretory pathway in renal tubular cells in which OAs exit cells and enter the tubular lumen via hOATv1 (hNPT4). Additionally, hOATv1 functions as a common renal secretory pathway for both urate and drugs, indicating that hOATv1 may be a leak pathway for excess urate that is reabsorbed via apical URAT1 to control the intracellular urate levels. Therefore, we propose a molecular mechanism for the induction of hyperuricemia by diuretics: the diuretics enter proximal tubular cells via basolateral OAT1 and/or OAT3 and may then interfere with the NPT4-mediated apical urate efflux in the renal proximal tubule.

Left ventricular function in pulmonary hypertension

To elucidate left ventricular function in pulmonary hypertension, we measured parameters of left ventricular as well as right ventricular function by echocardiography in 11 patients with pulmonary hypertension (idiopathic pulmonary artery hypertension in 4, chronic thromboembolic pulmonary hypertension in 5, and other pulmonary hypertension in 2). The percent change in these parameters 6 months after treatment with pulmonary artery vasodilators (beraprost in 8 and sildenafil in 3) was assessed. There was a correlation between the relative change in right ventricular systolic pressure (RVSP) and the relative changes in left ventricular outflow tract velocity–time integral (r = −0.730, P = 0.011) and mitral valve velocity–time integral (r = −0.621, P = 0.041). However, there was no correlation between the relative change in RVSP and the relative changes in left ventricular ejection fraction, left ventricular diastolic dimension, and systolic blood pressure. The relative change in RVSP was also correlated with the relative change in early diastolic myocardial velocity at the medial mitral annulus (r = −0.675, P = 0.023). Reduction of RVSP by pulmonary artery vasodilators might increase left ventricular preload, leading to an increase in stroke volume. Right ventricular load reduction might improve left ventricular diastolic function in patients with pulmonary hypertension, possibly through altered interventricular septal performance.

Protective Effect of K201 on Isoproterenol-Induced and Ischemic–Reperfusion-Induced Ventricular Arrhythmias in the Rat Comparison With Diltiazem

Aim: Ventricular arrhythmia (VA) is a risk for sudden death. Polymorphic ventricular tachycardia (VT) degenerating to ventricular fibrillation occurs subsequent to the prolongation of the QT interval following administration of catecholamines under Ca2+ loading. Fatal VA also occurs in ischemia and ischemic–reperfusion. We compared the suppressive effect of K201 (JTV519), a multiple-channel blocker and cardiac ryanodine receptor-calcium release channel (RyR2) stabilizer, with that of diltiazem, a Ca2+ channel blocker, in 2 studies of isoproterenol-induced (n = 30) and ischemic–reperfusion-induced VAs (n = 38) in rats. Methods: Adult male Wistar rats were administered 12 mg/kg/min calcium chloride (CaCl2) for 20 minutes and then 6 μg/kg/min isoproterenol was infused with CaCl2 for a further 20 minutes. In other rats, the left coronary artery was ligated for 5 minutes followed by reperfusion for 20 minutes. K201 or diltiazem (both 1 mg/kg) was administered before infusion of the isoproterenol or induction of ischemia. Results: After administration of isoproterenol under Ca2+ loading, fatal VA frequently occurred in the vehicle (9 of 10 animals, 90%) and diltiazem (8 of 10, 80%) groups, and K201 significantly suppressed the incidences of arrhythmia and mortality (2 of 10, 20%). In the reperfusion study, the incidence and the time until occurrence of reperfusion-induced VA and mortality were significantly suppressed in the K201 (2 of 15 animals, 13%) and diltiazem (1 of 9 animals, 11%) groups compared to the vehicle group (8 of 14 animals, 57%). Significance: Induction of VA in an experimental model was achieved with a low dose of isoproterenol under Ca2+ loading. K201 markedly suppressed both the isoproterenol-induced and the reperfusion-induced VAs, whereas diltiazem did not suppress the isoproterenol-induced VA. The results suggest that both VAs are related to early after depolarization (EAD) and indicate that K201 has the potential to suppress EAD by stabilizing RyR2 to mediate Ca2+ release from the sarcoplasmic reticulum and acting as a multiple-channel blocker.

Effects of Uric Acid on the NO Production of HUVECs and its Restoration by Urate Lowering Agents

BACKGROUND Although urate impaired the endothelial function, its underlying mechanism remains unknown. We hypothesized that urate impaired nitric oxide (NO) production in human umbilical vein endothelial cells (HUVECs) via activation of uric acid transporters (UATs). PURPOSE AND METHOD In the present study, we studied effects of urate on NO production and eNOS protein expression in HUVEC cells in the presence and absence of urate lowering agents using molecular biological and biochemical assays. RESULTS HUVECs expressed the 4 kinds of UATs, URATv1, ABCG2, MRP4 and MCT9. Exposure to urate at 7 mg/dl for 24 h significantly reduced production of NO. Pretreatment with benzbromarone, losartan or irbesartan normalized NO production. The same exposure resulted in dephosphorylation of endothelial NO synthase (eNOS) in HUVECs. Again pretreatment with benzbromarone, losartan or irbesartan abolished this effect. CONCLUSION Urate reduced NO production by impaired phosphorylation of eNOS in HUVEC via activation of UATs, which could be normalized by urate lowering agents.

Cardiac paraganglioma with a novel germline mutation of succinate dehydrogenase gene D

A 58-year-old woman with a past medical history of a carotid body tumor, resected 4 months prior to presentation, was admitted to our hospital for treatment of a cardiac tumor that was identified on post-operative echocardiography and chest computed tomography. The cardiac tumor was surgically removed and identified pathologically as a paraganglioma, similarly to the carotid body tumor. Genetic analysis of both tumors identified a non-synonymous mutation in the succinate dehydrogenase (SDH) gene D, Exon4, c.320T>C, p.Leu107Pro showing co-segregation with paternal transmission and maternal imprinting among family members. This novel mutation appears to be the cause of familial paraganglioma in this patient.