Pathophysiologic role of natriuretic peptides

To evaluate the pathophysiologic role of atrial natriuretic peptide (ANP) in hypertension, hemodynamic effects of human ANP and antiserum against rat ANP were investigated in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Intravenous administration of human ANP caused greater hypotension associated with a decrease of cardiac output in SHR than in WKY, which suggests that SHR have enhanced responsiveness to exogenous ANP. The antiserum increased blood pressure and cardiac output, with the latter being significantly greater in SHR than in WKY. These results suggest that endogenous ANP counteract, in part, the maintenance of hypertension. In addition, hemodynamic and renal excretory effects of brain natriuretic peptide (BNP), a novel natriuretic peptide identified from porcine, were studied in SHR and WKY. BNP caused marked natriuresis and hypotension in a dose-dependent fashion, as observed with ANP. Not only ANP but also BNP may have a role in the regulation of blood pressure and water-electrolyte balance.     

利钾尿肽与心钠素摩尔比在老年原发性高血压中的作用

目的:探讨利钾尿肽(KP)与心钠素摩尔比变化在老年原发性高血压发病中的意义。方法:用放射免疫分析方法测定老年高血压患者血浆、老年自发性高血压大鼠(SHR)血浆及心房和心室组织的KP和心钠素(ANP)含量。结果:高血压患者血浆KP、ANP水平与对照组比较无显著差异。但是,KP与ANP的摩尔比显著低于对照组(P＜0.05),SHR血浆KP、ANP水平显著高于对照组(WKY)(P＜0.01和P＜0.05),KP/ANP摩尔比则显著低于WKY(P＜0.01)。SHR心房KP含量高于心室(P＜0.05),但与对照组WKY比较无显著差异。结论:KP含量及KP与ANP的比例关系的改变,在老年原发性高血压发病中起着一定的作用。     

Haemodynamics and plasma ANP (atrial natriuretic peptide) after acute blood volume expansion in normotensive and spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) was measured in plasma during acute volume load in conscious, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. During basal conditions immunoreactive ANP were similar in the SHR (630 +/- 56 pmoles l-1) and the WKY (657 +/- 114 pmoles l-1) groups. An acute 10% and 20% whole blood volume expansion resulted in a linear increase in immunoreactive plasma ANP in the WKY. In the SHR the increase in plasma ANP was attenuated during the 20% volume load. During the 10% and 20% volume load central venous pressure (CVP), central blood volume (CBV) and cardiac output increased relatively more in the SHR compared with the WKY group. In contrast, the increase in peripheral blood volume (PBV) and decrease in heart rate (HR) was attenuated in the SH rats. In the SHR group there was a shift of the ANP vs. CVP and ANP vs. CBV curves to the right compared with the WKY. We conclude that acute volume loading is a potent stimulus for ANP release in WKY as well as SHR. However, in the SHR, ANP release was blunted in spite of the increased centralization of the volume load in this rat strain. Thus, the decreased responsiveness of the ANP hormonal system may contribute to the development and maintenance of hypertension in this genetic form of hypertension.     

Development of immunoreactive atrial natriuretic peptide in fetal hearts of spontaneously hypertensive and Wistar-Kyoto rats

Summary The development of immunoreactive atrial natriuretic peptide (ANP) was studied in fetal hearts of spontaneously hypertensive (SHR) and compared to normotensive Wistar-Kyoto (WKY) rats. While SHR fetal hearts were noticeably less developed than those of WKY at 10 and 11 days gestation, both strains showed ANP immunoreactive cells in some but not all primitive heart tubes. At 12 days additional ANP immunoreactive cells appeared in formative trabeculae of the ventricle and atrium. ANP cells were also observed in the myogenic layer of the truncus and bulbus arteriosus and their derivatives from 11 through 16 days, but not at 18 days. In both strains, there were more ANP cells in the left ventricle than in right beginning at day 13. There were no obvious strain differences in the developmental pattern and timing of ANP producing cells. However, on the day of birth, staining was reduced in hearts from some WKY newborn pups compared with hearts from SHR newborns and ventricular staining was reduced in both strains when compared to fetal hearts. These observations indicate that ANP is one of the earliest peptide hormones produced and that the predisposition to genetic hypertension does not appear to influence the development of ANP.     

Differential haemodynamic effects of atrial natriuretic peptide (ANP) in normotensive and spontaneously hypertensive rats

Central haemodynamic parameters and cardiac performance were measured in conscious spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) control rats after a 10-min infusion of rat ANP (103-125), 1 micrograms kg-1 min-1. Mean Arterial blood pressure (MAP) decreased by approximately 10% in both groups of rats. Heart rate (HR) increased slightly in both strains during the infusion. In the normotensive group the fall in MAP was due to a reduction in cardiac output (CO) while in the SHR there was a decrease in CO as well as in total peripheral resistance (TPR). The ANP infusion also reduced central blood volume (CBV) and stroke volume (SV) in both groups of rats. The reduction in CBV and CO was significantly more pronounced in the WKY strain. Left ventricular end diastolic pressure (LVEDP) and cardiac contractility (dP/dt) did not change while central venous pressure (CVP) was slightly decreased in the WKY group as a result of the ANP infusion. We conclude that ANP reduces MAP in normotensive animals by a reduction in CO. In the SHR a reduction in TPR also contributes to the fall in MAP. Atrial natriuretic peptide did not exert any negative inotropic effects, but the reduction of CO was due to an increased venous compliance.     

Brain natriuretic peptide stimulates particulate guanylate cyclase activity in selected areas of the rat brain

The effect of porcine brain natriuretic peptide (pBNP) on cyclic guanosine monophosphate (cGMP) production was investigated in localized rat brain areas by radioimmunoassay procedure. Porcine BNP activated particulate guanylate cyclase in the median eminence, subfornical organ, choroid plexus, olfactory bulb, paraventricular nucleus and pineal gland in a concentration-dependent fashion and its action was comparable to that of rat atrial natriuretic peptide (alpha-ANP), with ED50 values ranging from 5 to 7 x 10(-7) M for both peptides. Our results suggest that the activation of a specific receptor coupled to the guanylate cyclase system and the subsequent elevation of cGMP levels constitutes the common mechanism of the central action of BNP and ANP.     

Influence of age on L-type Ca2+ channels in the pulmonary artery and vein of spontaneously hypertensive rats

The influence of age on the density and localization of L-type Ca²⁺ channels was studied during development of hypertension in the pulmonary artery and vein of spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar–Kyoto (WKY) rats by radioligand binding assay and light microscope autoradiography. SHR were examined at 6 weeks (juvenile, pre-hypertensive stage), 12 weeks (young, developing hypertension) and 24 weeks (mature, established hypertension). The dihydropyridine-type Ca²⁺ antagonist [³H]nicardipine was used as a radioligand. It was bound specifically to sections of rat pulmonary artery and vein. Dissociation constant (K_d) values were similar in WKY rats and SHR, whereas maximum density of binding sites (B_max) values increased in SHR in comparison with WKY rats. This increase was noticeable from the pre-hypertensive phase. The pharmacological profile of [³H]nicardipine binding was similar in different age groups of either normotensive and hypertensive rats. Quantitative analysis of autoradiographs from SHR revealed a progressive increase of silver grains in smooth muscle of tunica media and to a lesser extent in the adventitia of pulmonary artery but not of pulmonary vein from pre-hypertensive stage to developing hypertension. No further changes were observed in established hypertension. The above data indicate that the density of L-type Ca²⁺ channels of pulmonary arteries is increased in SHR. This augmentation after the pre-hypertensive phase suggests the occurrence of dysregulation of Ca²⁺ handling in the pulmonary vasculature of developing SHR.     

Renal and cardiovascular effects of atrial natriuretic peptide in Wistar-Kyoto and spontaneously hypertensive rats during a chronic salt loading

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were maintained on tap water or 1.5% NaCl for 3 weeks. During the high sodium regime 24-h urinary sodium excretion increased 10-fold and the basal blood pressure increased in the SHR. After 3 weeks the rats received arterial (carotid artery), venous and bladder catheters (suprapubic). Saline was infused continuously and in conscious rats atrial natriuretic peptide (alpha-hANP) was administered as bolus injections (8 and 16 nmol kg-1) and the blood pressure and heart rate and the urinary excretions of sodium, potassium (flame photometry), noradrenaline and dopamine (HPLC) were followed at 5-min intervals. The administration of ANP caused a short-lasting blood pressure reduction, tachycardia, diuresis and increased urinary excretions of sodium, potassium, noradrenaline and dopamine. The blood pressure responses to ANP did not differ between the rat strains, irrespective of the diet. The natriuresis and diuresis to ANP was reduced in animals on a high sodium diet, especially in the SHR. This may be interpreted as a down-regulation of target organ responsiveness to ANP during a high sodium diet and the inappropriately large decrease in the responsiveness that was observed in the SHR may be related to increase in blood pressure during the high sodium diet.     

Effects of dietary protein on food and water intake in spontaneously hypertensive rats

We have been studying the development of hypertension in spontaneously hypertensive rats (SHR) fed a low protein diet. The effects of a low protein diet upon food and water intake were examined. Body weight gain, food and water intake were measured in three to twenty-three week-old SHR and Wistar Kyoto rats (WKY) fed diets containing 8%, 15% or 25% casein. Body weights of SHR and WKY fed an 8% casein diet were significantly lower at 23 weeks than rats on the higher protein diets, although both groups on the 8% diet consumed more food and water per g of body weight. In addition, SHR fed an 8% casein diet drank less water per gram of food than WKY or SHR fed 15% and 25% casein diets. These results indicate that changes in food and water intake, as a consequence of low protein diets, should be an additional consideration when examining the effects of dietary protein on the development of hypertension.     

Pre- and postsynaptic plasticity underlying augmented glutamatergic inputs to hypothalamic presympathetic neurons in spontaneously hypertensive rats

Increased sympathetic outflow plays an important role in the pathogenesis of hypertension. Glutamatergic inputs in the paraventricular nucleus (PVN) of the hypothalamus maintain resting sympathetic vasomotor tone in spontaneously hypertensive rats (SHR). In this study, we determined the synaptic and cellular mechanisms of increased glutamatergic inputs to PVN presympathetic neurons in SHR. The spinally projecting PVN neurons were retrogradely labelled by fluorescent microspheres injected into the intermediolateral cell column of the spinal cord. Blockade of NMDA and non-NMDA receptors significantly decreased the firing activity of labelled PVN neurons in brain slices in SHR, but not in normotensive Wistar–Kyoto rats (WKY). The basal frequency of glutamatergic spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs, respectively) of labelled PVN neurons was significantly greater in SHR than in WKY. But the frequency of neither sEPSCs nor mEPSCs stimulated by 4-aminopyridine or capsaicin differed significantly between WKY and SHR. Furthermore, the amplitude of postsynaptic NMDA currents elicited by either electrical stimulation or puff application in labelled PVN neurons was significantly higher in SHRs than in WKY. However, the evoked AMPA current amplitude in PVN neurons was similar in WKY and SHR. This study provides new evidence of how the glutamatergic synaptic inputs to PVN presympathetic neurons are increased and how they contribute to the elevated firing activity of these neurons in SHR. The augmented glutamatergic tone in the PVN is maintained by an increase in presynaptic glutamate release and an up-regulation of postsynaptic NMDA receptor function in SHR.     

Lithium chloride stabilizes systolic blood pressure and increases adrenal catecholamines in the spontaneously hypertensive rat

The effects of daily, intraperitoneal injections of LiCl (3 mEq/kg) on systolic blood pressure (SBP) and adrenal catecholamine levels were measured in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto rats (WKY). Control animals from each strain were injected with equivalent volumes (0.1 ml/100 g b.wt.) of 0.9% saline (0.15 mEq/kg). SBP in LiCl-treated SHR was significantly lower (p less than 0.05) than that of saline-treated SHR (177 +/- 7 vs. 196 +/- 4 mm Hg, respectively) after one week. After two weeks SBP was lower in LiCl SHR than in saline controls, but this difference was not significant. While SBP of both LiCl and saline treated WKY was not significantly different (146 +/- 4 vs. 147 +/- 8 mm Hg, respectively), SBP in both WKY groups remained lower than the SBP for either group of SHR. LiCl induced a significant weight loss in the SHR, but not in the WKY. Adrenal norepinephrine and epinephrine were significantly (p less than 0.05) higher in LiCl-treated rats of both strains; dopamine was also higher in LiCl-treated rats of both strains, but significant only between SHR-LiCl and SHR controls. It appears that LiCl's effect in slowing the development of hypertension is independent of its action on adrenal catecholamines. The SHR's increased sensitivity to LiCl, relative to weight loss and SBP, may reflect differences in genetic or physiological status of the animal compared to WKY. These differences may be associated with alterations in membrane ion transport systems.     

自发性高血压大鼠血浆一氧化氮浓度的变化

目的　观察自发性高血压大鼠 (SHR)血浆NO浓度的变化 ,探讨NO与高血压发生发展的关系。方法　颈总动脉插管测定大鼠血压 ,硝酸银还原法测定SHR血浆中NO的含量。结果　 (1)血压变化 :各时期WKY大鼠血压无显著性差异 ;高血压组大鼠血压随着月龄增加逐渐升高 ,均明显高于WKY大鼠 (P <0 0 1)。 (2 )血浆NO浓度的变化 :WKY大鼠血浆NO浓度各时期无显著性差异 ;高血压组大鼠血浆NO浓度在 3m、6m时间点明显高于WKY大鼠 ,但在 12m时间点却较WKY大鼠明显降低 (P<0 0 1) ,同组内与 3m、6m时间点比较亦显著减少 (P <0 0 1)。相关分析显示 ,在 3m、6m时间点高血压大鼠血浆NO浓度与血压之间呈正相关 (P <0 0 1) ,在 12m时间点与血压呈负相关 (P <0 0 1)。结论　随着高血压发生和发展 ,血浆NO升高 ,而在高血压后期 ,NO的变化不明显 ,这种变化可能与高血压的发生发展有关。     

Brain dopamine D2 receptor mRNA levels are elevated in young spontaneously hypertensive rats

Levels of brain dopamine D2 receptor expression were compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) controls by quantitative in situ hybridisation, using a complementary RNA probe for D2 receptor mRNA. In SHR which were 6 weeks of age, significantly higher levels of D2 receptor mRNA were found in the caudate-putamen (42%), nucleus accumbens (23%), olfactory tubercle (17%) and substantia nigra (38%) compared to age-matched WKY controls. D2 receptor mRNA levels were also higher in the substantia nigra (27%) of 12-14-week old SHR compared to WKY. The increased levels of dopamine D2 receptor gene expression displayed in young prehypertensive SHR could implicate altered central dopaminergic activity in the pathogenesis of hypertension.     

Alterations in tissue concentration and gene expression of adrenomedullin in spontaneously hypertensive rats

In order to elucidate the role of adrenomedullin in hypertension, we have compared concentrations of immunoreactive rat adrenomedullin and adrenomedullin messenger RNA levels in tissues of 8-week-old spontaneously hypertensive rats (SHR) with those of age-matched Wistar-Kyoto rats (WKY). The adrenomedullin immunoreactivity concentrations in adrenal gland and cardiac atrium were significantly higher in SHR than in WKY. The adrenomedullin content of cardiac ventricle was also significantly higher in SHR than in WKY. The rat adrenomedullin messenger RNA levels in adrenal gland and heart of SHR were also higher than those of WKY. These results suggest that adrenomedullin participates in the mechanism to counteract the blood pressure elevation in SHR.     

Elevated arteriolar adenosine 3',5'-cyclic monophosphate production by SHR

Adenosine 3',5'-cyclic monophosphate (cAMP) metabolism was studied in the microcirculation (100- to 150-micrometers arterioles) of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) at different stages of hypertension. Mesenteric arterioles from animals 4, 6, 12, and 18 wk old were incubated in Krebs-Ringer bicarbonate buffer for 30 min at 37 degrees C, pH 7.4, with and without the phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX). cAMP was assayed by radioimmunoassay. Arteriolar production of cAMP was age related in both WKY and SHR rats although the temporal patterns were different. At 6 wk (developmental stage of hypertension in SHR) cAMP accumulation in the presence or absence of MIX by SHR arterioles was higher than in the WKY before falling to normotensive levels at 12 wk. Salbutamol (a beta 2-agonist) stimulated dose-dependent increases in cAMP in both WKY and SHR at 6 wk. Stimulation of cAMP by salbutamol or by isoproterenol was blocked by propranolol. Neither agonist increased guanosine 3',5'-cyclic monophosphate. These data indicate that differences in cAMP metabolism are evident at the arteriolar level during the developmental stage of SHR hypertension. These differences may contribute to the morphological and physiological changes occurring at this time.     

自发性高血压大鼠蓝斑核超微结构的研究

用5只雄性自发性高血压大鼠作为实验组，另2只正常血压雄性大鼠作为对照组。常规电镜技术处理后，观察、拍照并计数每个电镜视野内的有关结构，比较两组蓝斑核神经元超微结构的特点。结果：在两组的蓝斑核神经元内均可见到大量的细胞器和胞质内含物：高尔基复合体、线粒体、粗面内质网、神经分泌颗粒、膜下囊泡和棘器，实验组比对照组多且发达，其中后三者尤为显著；神经毯内两组的突触类型均以GrayⅠ型（不对称型）为主，但实验组的突触前膨大内圆形清亮突触小泡（S型）和致密核芯颗粒突触小泡（g型）较多，平行性突触、连续性突触及嵴突触等特殊形态的突触实验组亦多于对照组。结果提示：自发性高血压的形成与蓝斑核神经元超微结构的功能活动和结构变化密切相关，在高血压时，蓝斑核有较多的神经无处于兴奋状态，而正常血压的调节和维持则有赖于蓝斑核神经元兴奋和抑制状态的平衡。     

一氧化氮合酶和一些神经递质（神经肽）在自发性高血压大鼠孤束核的分布

用ＮＡＤＰＨ－ｄｉａｐｈｏｒａｓｅ组织化学和免疫细胞化学方法，观察和比较了一氧化氮合酶、酪氨酸羟化酶、５－羟色胺、Ｐ物质和亮氨酸脑啡肽在自发性高血压大鼠和正常对照大鼠孤束核的分布。结果显示，孤束核内一氧化氮合酶阳性细胞在高血压组少于对照组，有显著性差异（Ｐ＜０．０５）。孤束核Ｐ物质和亮氨酸脑啡肽免疫阳性物在高血压组多于对照组；酪氨酸羟化酶免疫阳性物高血压组少于对照组；５－羟色胺免疫阳性物在两组的分布无明显差异。提示一氧化氮在孤束核是一种降压性局部神经调质，与其在心血管系统的降压作用一致。中枢Ｐ物质含量的异常增加可能是高血压发病的原因之一。孤束核脑啡肽含量的异常增加与高血压的形成有关，痛觉和高血压之间可能存在一些共同的发生机制。酪氨酸羟化酶在高血压组降低提示孤束核内酪氨酸羟化酶对维持正常血压发挥重要作用．孤束核内５－羟色胺与血压调节无关。     

Lack of effect of insulin on glucose utilization of the hypothalamus in normotensive and hypertensive rats

Hypertension is frequently associated with insulin resistance and enhanced sympathetic activity supposedly mediated by an effect of the hormone on the hypothalamus. In this study we sought to determine whether insulin modifies the functional activity of the hypothalamus and other brain areas of spontaneously hypertensive (SHR) and normotensive WKY rats. The study was carried out in control and hyperinsulinemic, normoglycemic rats. Insulin plasma levels were increased to 198 +/- 10 (WKY) or 220 +/- 10 microunits/ml (SHR). Brain functional activity was evaluated by the 2-[14C]deoxyglucose method for measuring local rates of glucose utilization. The results show that insulin has no effect on any of the brain areas examined including the hypothalamus, of both WKY and SHR rats. The two strains of rats have comparable cerebral metabolic rates also under basal conditions.     

WKY大鼠与自发性高血压模型大鼠(SHR)大脑中动脉拉伸力学特性的对比分析

对比分析WKY大鼠和SHR大鼠大脑中动脉的拉伸力学特性,为预防高血压提供生物力学基础。取5月龄正常WKY雄性大鼠大脑中动脉20个,5月龄SHR雄性大鼠大脑中动脉20个,试样以0.5mm/Min的实验速度进行纵向拉伸实验。WKY雄性大鼠大脑中动脉组拉伸最大应力、最大应变、弹性限度应变大于SHR雄性大鼠大脑中动脉组(P0.05),SHR雄性大鼠大脑中动脉的弹性模量值大于WKY雄性大鼠大脑中动脉(P0.05)。WKY大鼠大脑中动脉和自发性高血压模型大鼠(SHR)大脑中动脉具有不同的拉伸力学特性,自发性高血压模型大鼠(SHR)大脑中动脉拉伸力学特性发生了改变。     

Morphometrical and biochemical differences of endocrine pancreata between spontaneously hypertensive and normotensive rats with or without neonatal streptozotocin-induced diabetes

We studied the morphometrical and biochemical changes of endocrine pancreata in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) with or without noninsulin-dependent diabetes mellitus induced by neonatal streptozotocin (STZ) treatment at 4 months of age. Female (2-day-old) neonates were intraperitoneally injected with 62.5 or 75.0 mg/kg of STZ for SHR, 87.5 or 100.0 mg/kg of STZ for WKY, and vehicle for control. In STZ-treated groups, overt hyperglycemia developed in SHR with significantly decreased serum immunoreactive insulin (IRI), whereas in WKY, hyperglycemia was very mild and serum IRI was not lowered. The number and mean size of pancreatic islets did not differ between SHR and WKY, although mean islet size was reduced by half in both compared with that in the corresponding control, respectively. Percentage distribution of insulin-positive B cells in the islet was significantly reduced more in SHR than in WKY (34% of control versus 64% of control, p less than 0.05). Furthermore, pancreatic IRI content was far more reduced in SHR than in WKY (3% of control versus 43% of control, p less than 0.001). In vehicle-treated groups, the glycemic levels and the morphometrical islets did not differ between SHR and WKY. However, serum IRI was significantly lower but pancreatic IRI content was higher in SHR than in WKY. The mechanisms of strain differences between SHR and WKY seen in the present study were discussed.