Toshihiro Hamada

Presynaptic α2 adrenoceptors inhibit glutamate release from rat spinal cord synaptosomes

Abstract: The presynaptic regulation of amino acid release from nerve terminals was investigated using synaptosomes prepared from the rat spinal cord. The basal releases of endogenous glutamate (Glu), aspartate (Asp), and γ‐amino‐butyric acid (GABA) were 34.6, 21.5, and 10.0 pmol/min/mg of protein, respectively. Exposure to a depolarizing concentration of KCl (30 mM) evoked 2.7‐, 1.5‐, and 2.9‐fold increases in Glu, Asp, and GABA release, respectively. Clonidine reduced the K+‐evoked overflow of Glu to 56% of the control overflow with a potency (IC50) of 17 nM, but it did not affect K+‐evoked overflow of Asp, GABA, and their basal releases. Similarly, noradrenaline inhibited the K+‐evoked overflow of Glu, although phenylephrine and isoproterenol showed no effect. The inhibitory effect of clonidine was counteracted by α2‐adrenoceptor antagonists, rauwolscine, yohimbine, and idazoxan, regardless of the imidazoline structures. Because Glu is considered a neurotransmitter of primary afferents that transmit both nociceptive and nonnociceptive stimuli in the spinal cord, these data suggest that part of Glu release may be regulated by the noradrenergic system through α2 adrenoceptors localized on the primary afferent terminals.

Uricosuric Action of Losartan via the Inhibition of Urate Transporter 1 (URAT 1) in Hypertensive Patients

BACKGROUND The angiotensin receptor blocker losartan inhibited urate transporter 1 (URAT1) according to in vitro experiments. However, it is still unknown whether the inhibitory effect of losartan on URAT1 contributes to its uricosuric action in humans. METHODS Thirty-two patients with hypertension and nine patients with idiopathic renal hypouricemia (five with and four without hypertension) were enrolled for this study. Hypertensive patients were prescribed oral losartan (50 mg/day, n = 16) or candesartan (8 mg/day, n = 16). Before and after 1-month treatment, the serum concentration of urate (Sur) and creatinine (Scr), and the clearance value of urate (Cur) and creatinine (Ccr) were determined. Clearance studies using the URAT1 inhibitor benzbromarone (100 mg/day) or losartan (50 mg/day) loading test were also performed in these patients. RESULTS Blood pressure (BP) significantly decreased in the patients treated with either losartan or candesartan. Losartan significantly reduced Sur, which was associated with a concomitant increase in the Cur/Ccr ratio, whereas candesartan did not alter these parameters. In hypertensive patients with loss-of-function mutation of URAT1, losartan did not alter either Sur or Cur/Ccr, nor did benzbromarone. The lack of effect of URAT1 inhibitors on renal excretion of urate was independent of the renal function of hypouricemic patients. On the other hand, both losartan and benzbromarone increased Cur/Ccr ratio in hypertensive patients harboring the wild URAT1 gene, regardless of the presence of hypouricemia. CONCLUSIONS These findings suggested that losartan inhibited URAT1 and thereby it lowered Sur levels in hypertensive patients.

Presynaptic inhibition by clonidine of neurotransmitter amino acid release in various brain regions

The release of endogenous aspartic acid (Asp), glutamic acid (Glu) and gamma-aminobutyric acid (GABA) was investigated in synaptosomes prepared from various regions of the rat brain. The basal release of Asp, Glu and GABA from various regions was 12-35, 24-107 and 15-43 pmol/min per mg protein, respectively. Exposure to a depolarizing concentration of KCl (30 mM) resulted in 1.7 to 3.6-fold increases in Asp, Glu and GABA release. When clonidine (10(-4) M) was added to the perfusion medium, the K(+)-evoked overflow of both Asp and Glu was inhibited by 50-90% in the anterior cortex, thalamus and hypothalamus. Clonidine inhibited the K(+)-evoked Glu overflow by 30-40% in the posterior cortex and hippocampus. No significant effects were observed in the other brain regions (olfactory bulb, striatum, midbrain, cerebellum, pons, medulla oblongata). The inhibitory effects of clonidine were counteracted by an alpha 2-adrenoceptor antagonist, rauwolscine. The data suggest that the basal and K(+)-evoked release of Asp, Glu and GABA from nerve terminals is different in rat brain regions and that the presynaptic alpha 2-adrenoceptors which regulate the release of excitatory amino acids are mainly distributed in the anterior cerebral cortex, thalamus and hypothalamus of the rat brain.

Evaluation of changes in sympathetic nerve activity and heart rate in essential hypertensive patients induced by amlodipine and nifedipine

Objective To compare the effects of amlodipine and nifedipine on heart rate and parameters of sympathetic nerve activity during the acute and chronic treatment periods in order to elucidate their influence on cardiovascular outcome. Design A randomized and single-blind study. Methods We performed 24 h ambulatory electrocardiography and blood pressure monitoring of 45 essential hypertensive inpatients. Plasma and urinary catecholamine levels were measured during the control (pretreatment) period, on the first day (acute period) and after 4 weeks (chronic period) of administration of amlodipine and of short-acting nifedipine or its slow-releasing formulation. The low-frequency and high-frequency power spectral densities and low-frequency: high-frequency ratio were obtained by heart rate power spectral analysis. Results Blood pressure was significantly and similarly reduced by administrations of amlodipine, short-acting nifedipine and slow-releasing nifedipine during the chronic period. The total QRS count per 24 h, which remained constant during the chronic period of administration of slow-releasing nifedipine and was increased by administration of nifedipine, was decreased by 2.8% by administration of amlodipine. Administration of amlodipine decreased the plasma and urinary norepinephrine levels during the chronic period, whereas the levels were significantly increased by administration of short-acting nifedipine and not changed by administration of slow-release nifedipine. Although low-frequency: high-frequency ratio was increased significantly by administration of short-acting nifedipine and slightly by administration of slow-releasing nifedipine, administration of amlodipine reduced it during the acute and chronic periods. Conclusions Administration of amlodipine did not induce an increase in sympathetic nerve activity in essential hypertensive patients during the chronic period, suggesting that beneficial effects on essential hypertension can be expected after its long-term administration. Administration of slow-releasing nifedipine induces milder reflex sympathetic activation than does that of short-acting nifedipine.

Treatment of Xerostomia with the Bile Secretion-Stimulating Drug Anethole Trithione: A Clinical Trial

BACKGROUND Saliva protects the oral mucosa, inhibiting microbial overgrowth. Hyposalivation, therefore, induces multiple oral disorders, although treatment of hyposalivation is very difficult. METHODS A cholagogue, anethole trithione (AT) was administered to patients with symptomatic hyposalivation (xerostomia) caused by senile hypofunction (4 men and 17 women; senile group), medications (6 men and 17 women; drug group), and oral cancer therapy (two men and three women; cancer group). For control groups, an artificial saliva was administered to 45 patients consisting of senile hypofunction (10 men and 16 women), drug-induced xerostomia (3 men and 10 women) and oral cancer therapy-induced xerostomia (four men and two women). RESULTS Two weeks after administration of AT (6 tablets per day), both nonstimulated salivary flow rate (SFR) and stimulated SFR increased in a statistically significantly manner from 0.76 +/- 0.41 and 5.18 +/- 3.02 to 1.54 +/- 1.33 (P<0.05) and 9.07 +/- 4.10 mL/10 min (P<0.05), respectively. Of the three groups, the drug group showed the largest increases in both SFRs, from 0.90 +/- 0.54 and 6.29 +/- 4.12 to 1.69 +/- 1.65 and 12.09 +/- 5.10 mL/10 min (P<0.05 and P<0.02, respectively). Patients in the control group had almost constant SFRs. After AT administration, the salivary viscosity was, however, mildly decreased and concentrations of secretory-immunoglobulin A, lactoferrin, potassium, and chloride in nonstimulated saliva were almost constant. Corresponding with the increase of salivation, oral discomfort and inflammation were improved or resolved in 41 patients of the AT group within about 4 weeks, whereas improvement was observed in only nine patients of the control group. CONCLUSIONS These results indicate that AT sufficiently stimulates salivation and improves xerostomia.

Allopurinol Reduces Neointimal Hyperplasia in the Carotid Artery Ligation Model in Spontaneously Hypertensive Rats

Uric acid and oxidative stress promote cardiovascular diseases, including atherosclerosis and hypertension. Xanthine oxidase, through which uric acid is generated, is a free-radical generating enzyme. The aim of the current study was to investigate whether allopurinol, an inhibitor of xanthine oxidase activity, affects vascular remodeling and vascular smooth muscle cell (VSMC) proliferation. In the carotid artery ligation model using spontaneously hypertensive rats (SHR), treatment with allopurinol induced a reduction in the neointima/media ratio by 27% (38.5±34.3% in the control group and 28.1±20.8% in the allopurinol-treated group, respectively, p<0.01) without alterations in vascular circumference at 3 weeks after ligation when compared to the control. Allopurinol lowered the serum uric acid concentration (147.0±3.6 μmol/l in the control group and 16.1±3.6 μmol/l in the allopurinol-treated group, respectively p<0.01) and xanthine oxidase activity, but not the blood pressure. In an in vitro study, high concentrations of uric acid (100 and 200 μmol/l) stimulated VSMC growth, but there was no stimulation of these cells by a low concentration of uric acid (50 μmol/l) or by any of three concentrations of xanthine (50, 100 and 200 μmol/l). In addition, allopurinol (5 μmol/l) had no effect on the cell growth. In conclusion, uric acid is a potent stimulator of VSMC proliferation, and allopurinol prevented vascular remodeling in SHR at least in part by inhibiting uric acid concentration.

Depletion of Uric Acid Due to SLC22A12 (URAT1) Loss-of-Function Mutation Causes Endothelial Dysfunction in Hypouricemia

BACKGROUND Uric acid (UA) serves as an antioxidant in vascular endothelial cells. UA transporter 1 (URAT1) encoded by SLC22A12 is expressed in the kidney and vessels and its loss of function causes hypouricemia. The purpose of this study was to examine whether there is any endothelial dysfunction in patients with hypouricemia. METHODS AND RESULTS Twenty-six patients with hypouricemia (<2.5 mg/dl) and 13 healthy control subjects were enrolled. Endothelial function was evaluated using flow-mediated dilation (FMD). mRNA of UA transporters expressed in cultured human umbilical endothelial cells (HUVEC) was detected on RT-PCR. There was a positive correlation between FMD and serum UA in the hypouricemia group. URAT1 loss-of-function mutations were found in the genome of 21 of 26 patients with hypouricemia, and not in the other 5. In the hypouricemia groups, serum UA in homozygous and compound heterozygous patients was significantly lower than in other groups, suggesting that severity of URAT1 dysfunction may influence the severity of hypouricemia. Thirteen of 16 hypouricemia subjects with homozygous and compound heterozygote mutations had SUA <0.8 mg/dl and their FMD was lower than in other groups. HUVEC do not express mRNA of URAT1, suggesting the null role of URAT1 in endothelial function. CONCLUSIONS Depletion of UA due to SLC22A12/URAT1 loss-of-function mutations causes endothelial dysfunction in hypouricemia patients.

The release of the substrate for xanthine oxidase in hypertensive patients was suppressed by angiotensin converting enzyme inhibitors and alpha1-blockers.

Objective Hyperuricemia is associated with the vascular injury of hypertension, and purine oxidation may play a pivotal role in this association, but the pathophysiology is not fully understood. We tested the hypothesis that in hypertensive patients, the excess amount of the purine metabolite, hypoxanthine, derived from skeletal muscles, would be oxidized by xanthine oxidase, leading to myogenic hyperuricemia as well as to impaired vascular resistance caused by oxygen radicals. Methods We investigated the production of hypoxanthione, the precursor of uric acid and substrate for xanthine oxidase, in hypertensive patients and found that skeletal muscles produced hypoxanthine in excess. We used the semi-ischemic forearm test to examine the release of hypoxanthine (ΔHX), ammonium (ΔAmm) and lactate (ΔLAC) from skeletal muscles in essential hypertensive patients before (UHT:n = 88) and after treatment with antihypertensive agents (THT:n = 37) in comparison to normotensive subjects (NT:n = 14). Results ΔHX, as well as ΔAmm and ΔLAC, were significantly higher in UHT and THT (P < 0.01) than in NT. This release of ΔHX from exercising skeletal muscles correlated significantly with the elevation of lactate in NT, UHT and THT (y = 0.209 + 0.031 x;R2 = 0.222, n = 139:P < 0.01). Administration of doxazosin (n = 4), bevantolol (n = 5) and alacepil (n = 8) for 1 month significantly suppressed the ratio of percentage changes in ΔHX by − 38.4 ± 55.3%, − 51.3 ± 47.3% and − 76.3 ± 52.2%, respectively (P < 0.05) but losartan (n = 3), atenolol (n = 7) and manidipine (n = 10) did not reduce the ratio of changes; on the contrary, they increased it in ΔHX by +188.2 ± 331%, +96.2 ± 192.2% and +42.6 ± 137.3%, respectively. The elevation of ΔHX after exercise correlated significantly with the serum concentration of uric acid at rest in untreated hypertensive patients (y = 0.194 − 0.255x;R2 = 0.185, n = 30:P < 0.05). The prevalence of reduction of both ΔHX and serum uric acid was significantly higher in the patients treated with alacepril, bevantolol and doxazosin (67%:P < 0.02) than in the patients treated with losartan, atenolol and manidipine (12%). Conclusions It is concluded that the skeletal muscles of hypertensive patients released ΔHX in excess by activation of muscle-type adenosine monophosphate (AMP) deaminase, depending on the degree of hypoxia. The modification of ΔHX by angiotensin-converting enzyme inhibitors and α1-blockers influenced the level of serum uric acid, suggesting that the skeletal muscles may be an important source of uric acid as well as of the substrate of xanthine oxidase in hypertension.

Cardiac and Plasma Catecholamine Responses to Exercise in Patients with Type 2 Diabetes: Prognostic Implications for Cardiac-Cerebrovascular Events

BACKGROUND Patients with diabetes mellitus have an altered exercise plasma catecholamine response, which may be related to the abnormal sympathoadrenal function and autonomic neuropathy. Presence of autonomic neuropathy is associated with poor prognosis, but relationship between exercise plasma catecholamine and prognosis has not been investigated. This study determined if altered plasma catecholamine response to exercise was associated with cardiac-cerebrovascular events. METHODS Forty patients with type 2 diabetes without apparent macrovascular complications and 30 control subjects performed treadmill exercise with serial measurements of plasma norepinephrine and epinephrine. Clinical, exercise, and catecholaminergic variables considered relevant to the cardiac-cerebrovascular events were examined by Cox regression model. Analysis of 24-hour heart rate variability was performed in a subgroup of patients. RESULTS During 7.2 years, 8 patients, but no control subjects, had events (3 myocardial and 5 cerebral infarctions). Compared with Event(-) patients, Event(+) patients had: (1) orthostatic hypotension; (2) lower peak exercise heart rate; (3) lower plasma norepinephrine immediately after exercise; and (4) lower plasma epinephrine at peak exercise. High frequency components in heart rate variability analysis were diminished in Event(+) patients. Multivariate analysis showed that peak heart rate (P = 0.04) and plasma epinephrine at peak exercise (P = 0.03) were independent predictors of subsequent events. CONCLUSIONS These data suggest that chronotropic incompetence and lower plasma epinephrine response to exercise are associated with high risk of cardiac-cerebrovascular events in patients with type 2 diabetes.

Effects of a low-dose antihypertensive diuretic in combination with losartan, telmisartan, or candesartan on serum urate levels in hypertensive patients

BACKGROUND A combination therapy of a low-dose antihypertensive diuretic with an angiotensin II receptor blocker (ARB) may have unfavorable effects on serum urate levels. METHODS Forty-two hypertensive patients without hyperuricemia (18 men and 24 women, mean age 65 years) were randomly divided into three groups. Each of the group was allocated to a combination therapy with losartan (LOS; CAS 124750-99-8; 50 mg/day)/hydrochlorothiazide (HCTZ; CAS 58-93-5; 12.5 mg/day) (LOS/HCTZ group), telmisartan (TEL; CAS 144701-48-4; 40 mg/day)/HCTZ (12.5 mg/day) (TEL/HCTZ group), or candesartan (CND; CAS 145040-37-5; 8 mg/day)/HCTZ (12.5 mg/day) (CND/HCTZ group), respectively. Before and after the treatment, blood pressure and biochemical parameters of blood and urine were evaluated. RESULTS Both systolic and diastolic blood pressures significantly decreased in all groups (p < 0.01) without any statistical differences. The LOS/HCTZ group showed no changes in serum urate levels (5.8 +/- 1.0 mg/dl to 5.8 +/- 1.4 mg/dl) and in % fractional excretion of urate (FEUA). In the TEL/HCTZ group, the serum urate level was significantly increased, from 5.5 +/- 0.9 mg/dl to 6.5 +/- 1.2 mg/dl (p < 0.01), whereas FEUA significantly decreased (p < 0.01). Similarly, the CND/HCTZ group showed a significant increase in the serum urate level from 5.4 +/- 0.9 mg/dl to 6.0 +/- 1.2 mg/dl (p < 0.01) and a significant decrease in FEUA (p < 0.01). No significant differences were found in fasting plasma glucose and electrolytes levels in any of the groups. CONCLUSIONS A combination therapy with a low-dose HCTZ and ARBs resulted in reduced urate excretion and elevated serum urate levels. A combination therapy with the ARB losartan was not accompanied with these effects, likely because of its inhibitory action on urate transporter 1. The study limitations deserve mention in consideration of ethic restrictions, small size, short term examination and uncontrolled design.