Satoshi Akabane

Critical Degree of Renal Arterial Stenosis That Causes Hypertension in Dogs

The minimum degree of renal arterial stenosis needed to cause hypertension was identified by renal arterial angiography of anesthetized dogs. The effects of renal nerves and prostanoids on the critical stenosis were also examined. The left renal artery was constricted concentrically by a radiolucent con strictor device, and the stenosis of the artery was evaluated by cineangiography with the kidney either innervated or denervated. At this time, renal blood flow, renal perfusion pressure, and systemic blood pressure were serially monitored. In another group of dogs, renal venous and aortic blood samples were taken as the stenosis increased; these were assayed for prostaglandin E2 and plasma re nin activity. The same experiments were done again after treatment with a cyclooxygenase inhibitor, aspirin DL-lysine (54 mg/kg). With the kidney either innervated or denervated, systemic blood pressure began to increase when the stenosis was more than 70% of the diameter of the renal artery; the renal blood flow decreased when the stenosis was more than 75% of the diameter. Aspirin treatment attenuated the increase in blood pres sure but did not affect the autoregulation of the renal blood flow when stenosis was 70% or less. Prostaglandin E2 production increased in the stenotic kidney when the stenosis was more than 70%; aspirin inhibited prostaglandin synthesis and suppressed the stimulation of renin release. These results suggest that whether there is innervation or not, the critical degree of renal arterial stenosis that causes hypertension is more than about 70% of the diameter in the presence of renal prostaglandins; in their absence, the critical point above which hypertension occurs is 75% or more.

Aspirin lowers blood pressure in patients with renovascular hypertension.

To clarify the role of renal prostanoid in hyperreninemia and high blood pressure in human renovascular hypertension, we measured prostaglandin E2 and renin activity in renal venous and abdominal aortic plasma before and after the intravenous administration of the cyclooxygenase inhibitor, aspirin DL-lysine. Subjects were six patients with unilateral renovascular hypertension and six with essential hypertension. In patients with renovascular hypertension, prostaglandin E2 concentration in renal venous plasma from the stenotic kidney was 9.25 ±1.48 pg/ml, which was significantly higher (p < 0.01) than the concentration in the renal venous plasma from the normal kidney (4.97±1.02 pg/ml) or in the aortic plasma (2.59±0.15 pg/ml). Plasma renin activity was also higher in the renal vein of the stenotic kidney than in the other two sites. The stenotic side/normal side ratio of the renal venous prostaglandin E2 correlated significantly with a renin ratio greater than 1.5 (r=0.8211, p < 0.05). Intravenous injection of aspirin DL-lysine (18 mg/kg) 30 minutes later markedly suppressed prostaglandin E2 and renin levels at all sites and clearly lowered arterial blood pressure (mean: from 120±6 to 110±5 mm Hg, p < 0.01). The reduction in blood pressure correlated significantly with the suppression of plasma renin activity in the aorta (p < 0.05) and in the renal vein of the stenotic kidney (p < 0.01). Conversely, in patients with essential hypertension, aspirin had little effect on renin levels and increased mean blood pressure. These data indicate that renal prostaglandin plays an important role in the augmented release of renal renin and the pathogenesis of hypertension in human renovascular hypertension.

Increases in renal angiotensin II content and tubular angiotensin II receptors in prehypertensive spontaneously hypertensive rats

To examine the role of the intrarenal renin-angiotensin system in the development of hypertension in spontaneously hypertensive rats (SHR), we measured angiotensin II contents and tubular 125I-angiotensin II binding sites in the kidney of SHR and age-matched Wistar-Kyoto rats (WKY). In prehypertensive (4-week-old) SHR, not only the kidney angiotensin II content but also the angiotensin II receptor density in brush border membranes were significantly higher than in the WKY. In contrast, angiotensin II levels in the 20-week-old SHR kidneys were significantly lower than in the WKY. Acceleration of the intrarenal renin-angiotensin system and the increased density of tubular angiotensin II receptors in young SHR may therefore play an important role in the development of high blood pressure in SHR.

EFFECTS OF BRAIN NATRIURETIC PEPTIDE ON RENIN SECRETION IN NORMAL AND HYPERTONIC SALINE-INFUSED KIDNEY

The effects of brain natriuretic peptide (BNP) on renin secretion were evaluated in normal and hypertonic saline-infused kidneys of anesthetized dogs. In the normal kidney (N = 5), intrarenal infusion of porcine BNP-(1-26) (pBNP) at a dose of 50 ng/kg per min attenuated the renin secretion rate significantly to 9 +/- 27% of control without exerting a significant effect on mean arterial pressure (MAP), renal blood flow (RBF) or glomerular filtration rate (GFR); urine flow (V) was significantly increased to 260 +/- 33% of control and urinary excretion of sodium (UNaV) to 480 +/- 140% of control. In the hypertonic saline infusion group (N = 6), intrarenal infusion of hypertonic saline (20% w/v) at 0.5, 0.8, and 1.0 mEq NaCl/min caused a decrease in GFR and natriuresis in a dose-dependent manner. The renin secretion rate was attenuated by hypertonic saline infusion (1 mEq NaCl/min) to 87 +/- 31% of control. In another group (N = 6), administration of pBNP at a dose of 50 ng/kg per min during hypertonic saline infusion (1 mEq NaCl/min) increased the renin secretion rate to 196 +/- 57%, increased RBF to 160 +/- 13%, increased GFR to 137 +/- 22%, increased V to 221 +/- 29%, and increased UNaV to 218 +/- 29% of the values measured during hypertonic saline infusion. Our results indicate that BNP inhibits renin secretion through sodium delivery to the macula densa and effectively inhibits the tubuloglomerular feedback response that is activated by intrarenal hypertonic saline infusion.

Effects of taurine on stress-evoked hemodynamic and plasma catecholamine changes in spontaneously hypertensive rats.

Cardiovascular hemodynamics (microspheres) and plasma norepinephrine and epinephrine levels at rest and during short-term shaker stress were investigated in conscious spontaneously hypertensive rats and Wistar-Kyoto rats, with or without oral taurine (1.5%) treatment for 8 weeks. Taurine effects were evaluated by comparing data on the taurine-treated and untreated rats. Taurine affected neither the resting hemodynamics nor the resting plasma catecholamine levels in spontaneously hypertensive and Wistar-Kyoto rats. Taurine slightly but significantly reduced the left ventricular/body weight ratio in the spontaneously hypertensive rats (p less than 0.05) and caused an insignificant 10 mm Hg decrease in the resting mean arterial pressure. Spontaneously hypertensive and Wistar-Kyoto rats responded in a qualitatively similar manner to stress, as evidenced by resistance-dominated increases in mean arterial pressure and increases in heart rate, with a blood flow redistribution from splanchnic, cutaneous, and testicular to skeletal muscle and cerebral circulations and by increases in plasma norepinephrine and epinephrine levels. These changes were more marked in the spontaneously hypertensive rats. Taurine significantly reduced the stress values of mean arterial pressure (untreated, 189 +/- 4 (SE) mm Hg; treated, 166 +/- 4 mm Hg in the spontaneously hypertensive rats; p less than 0.05), while it significantly reduced stress values of heart rate in spontaneously hypertensive and Wistar-Kyoto rats (p less than 0.05). Taurine also blunted the stress values of splanchnic, testicular, and cutaneous vascular resistance in the spontaneously hypertensive rats. There were no or only slight regional effects in the Wistar-Kyoto rats. Taurine substantially decreased plasma levels of norepinephrine (untreated, 615 +/- 76 pg/ml; treated, 383 +/- 49 pg/ml) and epinephrine (untreated, 892 +/- 187 pg/ml; treated, 232 +/- 59 pg/ml) during stress in the spontaneously hypertensive rats. These results indicate that chronic taurine treatment attenuates short-term shaker stress-induced hemodynamic and plasma catecholamine changes in spontaneously hypertensive rats.

Mechanisms of intracellular pH regulation in the hamster inner medullary collecting duct perfused in vitro

To examine the mechanisms of H+ transport in the mid-inner medullary collecting duct of hamsters, we measured the intracellular pH (pHi) in the in vitro perfused tubules by microscopic fluorometry using 2′,7′-bis(carboxyethyl)-carboxyfluorescein (BCECF) as a fluorescent probe. In the basal condition, pHi was 6.74±0.04 (n=45) in HCO3−-free modified Ringer solution. Either elimination of Na+ from the bath or addition of amiloride (1 mM) to the bath produced a reversible fall in pHi After acid loading with 25 mM NH4Cl, pHi spontaneously recovered with an initial recovery rate of 0.096±0.012 (n=23) pH unit/min. In the absence of ambient Na+, after removal of NH4+, the pHi remained low (5.95±0.10, n=8) and showed no signs of recovery. Subsequent restoration of Na+ only in the lumen had no effect on pHi. However, when Na+ in the bath was returned to the control level, pHi recovered completely. Amiloride (1 mM) in the bath completely inhibited the Na+-dependent pHi recovery. Furthermore, elimination of Na+ from the bath, but not from the lumen, decreased pHi from 6.97±0.07 to 6.44±0.05 (n=12) in the HCO3−/Ringer solution or 6.70±0.03 to 6.02±0.05 (n=8) in the HCO3−free solution. pHi spontaneously returned to 6.76±0.08 with a recovery rate of 0.017±0.5 pH unit/min in the presence of CO2/HCO3−, whereas it did not recover in the absence of CO2/HCO3−. Although elimination of ambient Na+ depolarized the basolateral membrane voltage (VB) from −78±1.2 to −72 ±0.6 mV (n=5, P<0.01), the level of VB was not sufficient to explain the pHi recovery solely by HCO3−entry driven by the voltage. These results indicate that (a) pHi of the inner medullary collecting duct is regulated mainly by a Na+/H+ exchanger in the basolateral membranes, (b) no apparent Na+-dependent H+ transport system exists in the luminal membranes and (c) Na+-independent H+ transport may also operate in the presence of CO2/HCO3−

Plasma Atrial Natriuretic Polypeptide and Polyuria during Paroxysmal Tachycardia in Wolff-Parkinson-White Syndrome Patients

The human atrial natriuretic polypeptide (hANP) concentration in plasma was measured during paroxysmal supraventricular tachycardia provoked in 2 patients with Wolff-Parkinson-White syndrome. A 10- to 20-fold increase in plasma hANP concentration was observed during the tachycardia: from 12 to 291 pg/ml in 1 case and from 14 to 174 pg/ml in the other. Although polyuria was associated with the tachycardia, urinary sodium excretion as well as urinary osmolality were decreased. The urinary arginine vasopressin was appreciably decreased during the tachycardia. These results suggest that hANP released by paroxysmal tachycardia might not act as a natriuretic factor in this range of plasma concentration. Polyuria during paroxysmal tachycardia was attributed mainly to the inhibition of arginine vasopressin release.

Characterization of α1- and α2-adrenoceptors directly associated with basolateral membranes from rat kidney proximal tubules

Abstract We have used 2-(β-(3- 125 iodo-4-hydroxyphenyl)-ethylaminoethyl)-tetralone ([ 125 I]HEAT or BE2254), an α 1 -selective antagonist, and [ 3 H]yohimbine, an α 2 -selective antagonist, to demonstrate and characterize binding sites in basolateral membranes from rat kidney cortex. Parathyroid hormone (PTH) stimulated the adenylate cyclase activity of the basolateral membranes, whereas thyrocalcitonin, arginine vasopressin (AVP) and isoproterenol did not. Therefore, the basolateral membranes were probably derived from the proximal tubules. The specific binding of [ 125 I]HEAT and [ 3 Hyohimbine to basolateral membranes was rapid, reversible, saturable and of high affinity. The maximum densities of α 1 - and α 2 receptors were 364 and 1130 fmoles/mg protein, indicating that the ratio of α 1 - to α 2 -adrenoceptors was about 1:3. The specific binding of [ 125 I]HEAT and [ 3 H]yohimbine to the basolateral membranes was displaced by various adrenergic agents in a manner that suggests that the labeled sites probably represent α 1 - and α 2 -adrenoceptors respectively. These results suggest that the binding sites of [ 125 I]HEAT and [ 3 H]yohimbine, which appear to be α 1 - and α 2 -adrenoceptors, exist in the basolateral membranes of the proximal tubules.

A Sensitive Method for Precise Measurement of Endogenous Angiotensins I, II&III in Human Plasma

We measured endogenous angiotensins (ANGs) I, II&III using a system of extraction by Sep-Pak column followed by high performance liquid chromatography (HPLC) combined with radioimmunoassay (RIA). An excellent separation of ANGs was obtained by HPLC. The recovery of ANGs I, II&III was 80-84%, when these authentic peptides were added to 6 ml of plasma. The coefficient of variation of the ANGs was 0.04-0.09 for intra-assay and 0.08-0.13 for inter-assay, thereby indicating a good reproducibility. Plasma ANGs I, II&III measured by this method in 5 normal volunteers were 51,4.5 and 1.2 pg/ml. In the presence of captopril, ANGs II&III decreased by 84% and 77%, respectively, while ANG I increased 5.1 times. This method is therefore useful to assess the precise levels of plasma ANGs.

Immunoreactive endothelin-1 contents in brain regions from spontaneously hypertensive rats.

To investigate the possible role of brain endothelin-1 (ET-1) in hypertension of spontaneously hypertensive rats (SHRs), we measured immunoreactive (ir) ET-1 contents in brain regions as well as plasma ir-ET-1 levels in SHRs aged 4-5 and 12-14 weeks and age-matched Wistar-Kyoto rats (WKY) with a radioimmunoassay for ET-1. Systolic blood pressures of SHR aged 4-5 and 12-14 weeks were significantly higher than those of corresponding WKY. Significant amounts of ir-ET-1 were detectable throughout the discrete brain regions analyzed in both strains; higher ir-ET-1 contents in structures such as thalamus, hypothalamus, midbrain, pons, medulla, and cerebellum, with the lowest in cerebral cortex, were observed. A reverse-phase high-performance liquid chromatography of the brain extracts revealed the presence of both a major component identical to the elution position of synthetic ET-1 and a minor component possibly corresponding to its oxidized form. When compared, ir-ET-1 contents in all brain regions analyzed were lower in SHRs than in WKY rats. This strain-related change of ir-ET-1 contents was significant in the medulla at 4-5 weeks of age, and in all brain regions except hypothalamus at 12-14 weeks of age. Plasma ir-ET-1 levels, in contrast, were comparable between SHRs and WKY rats. These results suggest that brain ET-1 may be involved in the development and the maintenance of hypertension in SHRs.