Dietary sodium chloride increases blood pressure in obese Zucker rats

Impact of Mineralocorticoid Receptor and Angiotensin II Type 1 Receptor Antagonism on Blood Pressure Regulation in Obese Zucker Rats: Role of Sex Differences

BACKGROUND

Previous studies suggest that obesity-induced hypertension in females, but not males, is due to leptin-mediated stimulation of aldosterone secretion and subsequent activation of the mineralocorticoid receptor (MR). Although angiotensin II type 1 receptor (AT1R) antagonism lowers blood pressure (BP) in male obese Zucker rats (OZR), which have defective leptin signaling, the potential role of sex differences in BP responses to renin-angiotensin-aldosterone system blockade, including MR antagonism, in obesity is still unclear. We tested the cardiovascular effects of MR antagonism, alone or in combination with AT1R blockade in male and female OZR (n = 5/sex) and lean Zucker rats (n = 7/sex).

METHODS

BP and heart rate (HR) were measured by telemetry 24 hour/day. After a 6-day control period, spironolactone (40 mg/kg/day) was given for 10 days followed by a 7-day combined treatment with losartan (20 mg/kg/day), and followed by 6-day post-treatment recovery period.

RESULTS

Compared with lean rats, OZR were hypertensive (mean arterial pressure: 115 ± 4 vs. 104 ± 2 and 111 ± 1 vs. 100 ± 3 mm Hg for males and females) and had lower HR (355 ± 9 vs. 393 ± 7 and 367 ± 10 vs. 412 ± 13 bpm). MR blockade alone did not alter BP or HR in lean or obese male and female Zucker rats, whereas combined treatment reduced BP in obese and lean rats by 31 ± 3 vs. 21 ± 1 and 8 ± 1 vs. 5 ± 1 mm Hg in males and females, respectively. No changes were observed in HR.

CONCLUSIONS

These results suggest that there are important sex differences in BP responses to chronic AT1R blockade but no major involvement of MR activation in BP regulation in OZR.

Renal Protective Effects of N-Acetyl-Seryl-Aspartyl-Lysyl-Proline (Ac-SDKP) in Obese Rats on a High-Salt Diet

BACKGROUND

Obesity is a public health problem, associated with salt sensitive hypertension, kidney inflammation, and fibrosis. N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a tetra peptide with anti-inflammatory and anti-fibrotic properties. However, its effect on preventing kidney damage in obesity is unknown. We hypothesized that Zucker obese (ZO) rats on a high-salt (HS) diet develop renal damage, inflammation, fibrosis, and this is prevented with Ac-SDKP treatment.

METHODS

Zucker lean (ZL) and ZO rats (8 weeks old) were treated with Ac-SDKP (1.6 mg/kg/day) while maintained on either a normal-salt (NS; 0.4%) or HS (4%) diet for 8 weeks. Systolic blood pressure (SBP), albuminuria, renal inflammation, and fibrosis were evaluated.

RESULTS

HS diet increased macrophage infiltration in the kidneys of both ZL and ZO rats but was significantly higher in ZO rats receiving the HS diet (ZL + NS, 13.9 ± 1.3 vs. ZL + HS, 19.14 ± 1.5 and ZO + NS, 25.5 ± 1.4 vs. ZO + HS, 87.8 ± 10.8 cells/mm2; P < 0.05). Ac-SDKP prevented macrophage infiltration in ZO rats (ZO + HS + Ac-SDKP, 32.18 ± 2.4 cells/mm2; P < 0.05). Similarly, glomerulosclerosis, cortical, and medullary interstitial fibrosis were increased in ZO rats fed the HS diet, and Ac-SDKP attenuated these alterations (P < 0.05). SBP was increased in ZO rats fed the HS diet (ZO + NS, 121.3 ± 8.9 vs. ZO + HS, 164 ± 6.9 mm Hg; P < 0.05), and it was significantly decreased with Ac-SDKP treatment (ZO + HS + Ac-SDKP, 144.05 ± 14.1 mm Hg; P = 0.004). Albuminuria was higher in ZO rats than in ZL rats; however, neither HS nor Ac-SDKP treatment affected it.

CONCLUSIONS

Ac-SDKP treatment in ZO rats fed a HS diet prevented renal damage by reducing inflammation, fibrosis, and SBP.

Physiological changes induced by salt intake in female Spontaneously Diabetic Torii-Lepr(fa) (SDT fatty) rat, a novel obese type 2 diabetic model

Salt plays an important role in the control of blood pressure in obesity and diabetes mellitus. In this study, we investigated physiological changes such as blood pressure and renal function in salt-loaded female Spontaneously Diabetic Torii-Lepr(fa) (SDT fatty) rats. SDT fatty rats were given 1% NaCl in drinking water for 14 weeks, from 4 to 18 weeks of age. Significant salt-sensitive hypertension was observed in the salt-loaded SDT fatty rats. Moreover, the salt-loaded rats showed a decrease of creatinine clearance and deterioration on pathological renal findings, including glomerulosclerosis and tubular and interstitial lesions. Female SDT fatty rat is a useful model for investigating the mechanisms of high salt sensitivity in obesity and diabetes mellitus.

Changes in renal vessels following the long-term administration of an angiotensin II receptor blocker in Zucker fatty rats

INTRODUCTION

The nephro-protective effects of angiotensin II receptor blockers (ARBs) are widely known; however, there are few reports of long-term effects focusing on the renal vessels. We studied afferent arteriolar changes induced by the long-term administration of an ARB.

MATERIALS AND METHODS

Thirty-two 6-week-old male Zucker fatty rats (ZFRs) were divided into following four groups (n = 8 in each): ZFR Group and ZFR+High Group fed a standard or high-salt diet, respectively; ZFR+ARB Group and ZFR+High+ARB Group fed a standard or high-salt diet with ARB (Olmesartan, 5 mg/kg/day), respectively. Blood pressure, proteinuria, morphological examinations and glomerular haemodynamics in vivo were studied.

RESULTS

Marked proliferative changes in the afferent arteriolar smooth muscle cells (SMCs) were frequently observed in the two groups given ARBs; in the ZFR+ARB group (77.3±10.3%) compared with the two groups without ARB (1.7%, p < 0.005; 1.2%, p < 0.0005) and 37.4±15.6% in the ZFR+High+ARB group. Proteinuria markedly decreased in the groups treated with ARBs, but the glomerular erythrocyte velocities showed no differences.

CONCLUSIONS

Our findings indicate that long-term ARB administration induced unusual proliferative changes in SMCs of afferent arterioles of ZFRs. These changes could narrow arteriolar lumens and reduce intraglomerular pressure, but they could cause also irreversible damage to the arterioles.

Programmed hypertension in rats treated with a NF-κB inhibitor during nephrogenesis: renal mechanisms

Suppression of the renin-angiotensin system (RAS) during murine lactation causes progressive renal injury, indicating a physiological action of angiotensin II on nephrogenesis. The nuclear factor NF-κB system is one of the main intracellular mediators of angiotensin II. We investigated whether inhibition of this system with pyrrolidine dithiocarbamate (PDTC) during rat nephrogenesis would lead to similar hypertension and renal injury as observed with RAS suppressors. Immediately after delivery, 32 Munich-Wistar dams, each nursing 6 male pups, were divided into 2 groups: C, untreated, and PDTC, receiving PDTC, 280 mg kg(-1) day(-1) orally, during 21 days. After weaning, the offspring were followed until 10 months of age without treatment. Adult rats that received neonatal PDTC exhibited stable hypertension and myocardial injury, without albuminuria. To gain additional insight into this process, the renal expression of RAS components and sodium transporters were determined by quantitative real-time PCR (qRT-PCR) at 3 and 10 months of life. Renal renin and angiotensinogen were upregulated at 3 and downregulated at 10 months of age, suggesting a role for early local RAS activation. Likewise, there was early upregulation of the proximal sodium/glucose and sodium/bicarbonate transporters, which abated later in life, suggesting that additional factors sustained hypertension in the long run. The conclusions drawn from the findings were as follows: (1) an intact NF-κB system during nephrogenesis may be essential to normal renal and cardiovascular function in adult life; (2) neonatal PDTC represents a new model of hypertension, lacking overt structural injury or functional impairment of the kidneys; and (3) hypertension in this model seems associated with early temporary activation of renal RAS and sodium transporters.

Preventing increased blood pressure in the obese Zucker rat improves severity of stroke

Obesity is a risk factor for stroke, but the determinants of increased stroke risk in obesity are unknown. We have previously reported that obese Zucker rats (OZRs) have a worse stroke outcome and display evidence of remodeling of the middle cerebral artery (MCA), in parallel with hypertension, compared with lean controls. This study tested the hypothesis that hypertension is an essential determinant of cerebral vascular remodeling and increased stroke damage in OZRs. Blood pressure was measured by telemetry in lean and obese rats with and without hydrochlorthiazide (HCT; 2 mg.kg(-1).day(-1)) from 8 to 15 wk of age. A separate group of rats was also chronically fed a low-sodium (LS) diet. Vessel structure was assessed in isolated, pressurized MCAs. Cerebral ischemia was induced for 60 min using an intralumenal suture technique, followed by 24 h of reperfusion. HCT treatment effectively prevented the increase in blood pressure in obese rats; however, the LS diet did not lower pressure. Importantly, infarct size was normalized by HCT after ischemia-reperfusion injury. Additionally, HCT improved the changes in MCA structure observed in untreated OZRs. There were no benefits of the LS diet on stroke injury or vessel structure. These results indicate that increased pressure is essential for driving the changes in infarct size in OZRs.

A moderately high fat diet promotes salt-sensitive hypertension in obese zucker rats by impairing nitric oxide production

The objective of this research was to examine the contribution of a moderately high fat (MHF) diet to the development of salt-sensitive hypertension in obese Zucker rats. Lean and obese Zucker rats were fed either a MHF diet or a diet of standard rat chow (control diet) for 10 weeks. From week 4 through week 10, the drinking water was supplemented with 1% NaCl. Blood pressure was measured weekly, and urinary excretion of nitric oxide metabolites (NO(x)) was determined at weeks 4 and 10. At week 10, renal nitric oxide synthase (NOS) activity was assessed in kidney homogenates. Blood pressures of obese, but not lean, rats on the MHF fat diet were significantly increased by salt-supplementation, whereas blood pressures of rats on the control diet were not appreciably affected. NO(x) excretion was increased in response to salt-supplementation in rats on the control diet, with the effect being particularly dramatic in obese rats. After salt-supplementation, NO(x) excretion by rats on the MHF diet was lower than rats on the control diet. In obese rats on the MHF diet, this decrease in NO production was accompanied by a reduction in renal NOS activity. These results indicate that obese rats are more inclined than lean rats to develop diet-induced hypertension in response to a moderately high fat, salt-supplemented diet. Furthermore, they suggest that MHF diet-induced defects in NO production may promote the salt-sensitivity of blood pressure in obese Zucker rats, which appear to require more NO to maintain blood pressure during a salt challenge.

Insulin's impact on renal sodium transport and blood pressure in health, obesity, and diabetes

Insulin has been shown to have antinatriuretic actions in humans and animal models. Moreover, endogenous hyperinsulinemia and insulin infusion have been correlated to increased blood pressure in some models. In this review, we present the current state of understanding with regard to the regulation of the major renal sodium transporters by insulin in the kidney. Several groups, using primarily cell culture, have demonstrated that insulin can directly increase activity of the epithelial sodium channel, the sodium-phosphate cotransporter, the sodium-hydrogen exchanger type III, and Na-K-ATPase. We and others have demonstrated alterations in the expression at the protein level of many of these same proteins with insulin infusion or in hyperinsulinemic models. We also discuss how this regulation is perturbed in type I and type II diabetes mellitus. Finally, we discuss a potential role for regulation of insulin receptor signaling in the kidney in contributing to sodium balance and blood pressure.

Effects of Salt Rich Diet in the Obese Zucker Rats: Studies on Renal Function During Isotonic Volume Expansion

Obese Zucker rats (OZR) are hyperinsulenemic, hyperglycemic and dyslipidemic and develop salt dependent hypertension. Since salt sensitivity is considered to be due to impaired handling of renal sodium excretion, these studies were conducted in the obese and lean Zucker rats (LZR) anesthetized with Inactin to evaluate renal function under basal conditions and during acute isotonic fluid volume expansion (VE). Mean Arterial blood pressure (MBP), heart rate (HR), renal blood flow(RBF) and glomerular filtration rate (GFR) were not significantly different between the lean Zucker rats fed normal diet or that fed salt rich diet(8% NaCI). However, basal UV and UNaV were significantly greater in the LZR fed high salt. During VE essentially identical increases occurred in GFR, UV and UNaV in both the lean groups. In the OZR fed salt rich diet also, there were no significant changes in the heart rate, RBF and GFR. However, arterial blood pressure of the OZR fed salt rich diet was significantly greater than that of the OZR on the normal diet as well as that of both the lean groups. Also, as in the LZR, basal UV and UNaV were significantly greater in the salt fed obese rats. During volume expansion there were no impairments in the ability of the obese groups fed normal or salt rich diet to eliminate sodium and water during volume load. In fact, the net sodium and water excretions during and 60 min after VE in both the obese groups were significantly greater than that of corresponding lean groups. Furthermore, these values in the OZR fed salt rich diet were significantly greater than that of the obese rats on normal salt diet perhaps due to the contribution of pressure natriuretic mechanisms'. These data demonstrate that although OZR are salt sensitive, the renal mechanisms that would collectively respond to acute isotonic VE were fully functional. An unexpected and a novel finding in these studies is that the salt rich diet, in addition to increasing arterial blood pressure also significantly lowered plasma of insulin levels and enhanced glucose and cholesterol levels in the obese Zucker rats.

Influence of plasma insulin levels on antinatriuretic and vasoconstrictor actions of angiotensin-II

The objective of the present study is to investigate whether plasma insulin levels play a role in the antinatriuretic and vasoconstrictor actions of angiotensin-II (Ang-II). We evaluated antinatriuretic function of endogenous Ang-II using an AT1 receptor antagonist, candesartan in anesthetized Sprague-Dawley rats. In control rats, candesartan produced significant increases in natriuresis and diuresis and these effects were abolished in streptozocin (STZ, 55 mg/kg i.p.) treated rats. Replacement of insulin restored these renal effects of candesartan. In a separate group of rats pretreated with an autonomic ganglionic blocker, pressor responses to Ang-II and norepinephrine (NE) before or after L-NNA, a nitric oxide synthase inhibitor were not affected by STZ treatment. However, insulin replacement greatly augmented these responses. These data provide evidence in vivo showing that insulin can enhance both antinatriuretic and vasoconstrictor actions of Ang-II. Hence exaggerated renal and vascular effects of Ang-II in the obese Zucker rats observed in our previous studies may be related to hyperinsulimemia and this phenomena could contribute to salt-sensitivity and development of sustained hypertension.

Intestinal dopaminergic activity in obese and lean Zucker rats: response to high salt intake

The present study examined intestinal dopaminergic activity and its response to high salt (HS, 1% NaCl over a period of 24 hours) intake in obese (OZR) and lean Zucker rats (LZR). The basal Na+,K+-ATPase activity (nmol Pi/mg protein/min) in the jejunum of OZR was higher than in LZR on normal salt (NS) (OZR-NS = 111.3 +/- 6.0 vs. LZR-NS = 88.0 +/- 8.3). With the increase in salt intake, the basal Na+,K+-ATPase activity significantly increased in both animals (OZR-HS = 145.9 +/- 11.8; LZR-HS = 108.8 +/- 6.7). SKF 38393 (10 nM), a specific D1-like dopamine receptor agonist, inhibited the jejunal Na+,K+-ATPase activity in OZR on HS intake, but failed to inhibit enzyme activity in OZR on NS intake and LZR on NS and HS intakes. The aromatic L-amino acid decarboxylase (AADC) activity in OZR was lower than in LZR on NS intake. The HS intake increased AADC activity in OZR, but not in LZR. During the NS intake the jejunal monoamine oxidase (MAO) activity in OZR was similar to that in LZR. The HS intake significantly decreased MAO activity in both OZR and LZR. The jejunal COMT activity in OZR was higher than in LZR on NS intake. The HS intake reduced COMT activity in OZR but not LZR. It is concluded that inhibition of jejunal Na+,K+-ATPase activity through D1 dopamine receptors is dependent on salt intake in OZR, whereas in LZR, the enzyme failed to respond to the activation of D1 dopamine receptors irrespective of their salt intake.

Supplementation of plant sterols and minerals benefits obese Zucker rats fed an atherogenic diet

In most hypertensive rat models, serum total cholesterol is typically low and the cholesterol is primarily in the HDL rather than the LDL fraction. This difference from humans usually makes these animals unsuitable for experimental atherosclerosis studies. In the present study, we induced severe hypercholesterolemia including a 10-fold increase in serum LDL cholesterol, endothelial dysfunction and hypertension as well as vascular and renal damage in obese Zucker rats by feeding a human-type high fat, high cholesterol and high salt diet (butter 18, cholesterol 1 and NaCl 6 g/100 g dry weight). Supplementation of this atherogenic diet with plant sterols (1 g/100 g) and replacing the NaCl partially by calcium, magnesium and potassium effectively prevented the diet-induced increases in total and LDL cholesterols and 24-h systolic and mean blood pressures, and markedly improved endothelial function. Plant sterols and the minerals also protected against vascular and renal damage and extended the life span of the obese Zucker rats by 60% compared with the rats fed the atherogenic diet. Our findings suggest that human-type cardiovascular disorders can be induced in obese Zucker rats by feeding a human-type atherogenic diet. This seems to be a suitable animal model for experimental studies on atherosclerosis and hypertension as well as for evaluating new dietary approaches to reducing cardiovascular risk.

Salt-loading elevates blood pressure and aggravates insulin resistance in Wistar fatty rats: a possible role for enhanced Na+ -H+ exchanger activity

OBJECTIVE

Increased Na+-H+ exchanger activity (NHE) has been reported as an intermediate phenotype in hypertensive subjects, particularly those with insulin resistance. To investigate whether NHE abnormality plays a role in hypertension, Wistar fatty rat (WFR) with overt obesity, hyperglycemia and marked hyperinsulinemia was examined.

METHODS

WFR and Wistar lean rats (WLR) as a control (n = 12, each) were fed either with normal (0.38%) or high sodium (4% NaCl) diet for 12 weeks and then sacrificed to examine platelets NHE activity.

RESULTS

Mean arterial pressure (MAP) was higher in WFR than in WLR (113 +/- 4 versus 96 +/- 7 mmHg, P < 0.05) under a normal chow. Vmax values of NHE activity were significantly higher in WFR than in WLR. WFR fed with a high sodium diet showed higher MAP than those with a normal chow (128 +/- 3 versus 113 +/- 4 mmHg, P < 0.05). Though Km values were not different between WFR and WLR under a normal chow, both maximal transport rate (Vmax) and half maximal transport (Km) values were significantly higher in WFR with a high salt diet than those with a control diet. Vmax showed significant correlation with MAP, whereas Km values correlated with immunoreactive insulin (IRI) levels. Significant interaction between dietary sodium intake and the strain differences was observed both on blood pressure and on IRI levels by two-way analysis of variance (ANOVA).

CONCLUSION

WFR presented salt-sensitive blood pressure elevation. NHE activity was enhanced in WFR in correlation with the blood pressure. These results suggest that augmented NHE activity contributes to the development of salt-sensitive blood pressure elevation in WFR.

Significance of exaggerated natriuresis after angiotensin AT1 receptor blockade or angiotensin- converting enzyme inhibition in obese Zucker rats

1. Obese Zucker rats (OZR) were shown to be salt-sensitive in that they develop hypertension when placed on a high-salt diet. Because angiotensin (Ang) II is a major antinatriuretic factor, the present studies were undertaken to determine whether the characteristic of salt-sensitivity of OZR is associated with an enhanced antinatriuretic function of endogenous AngII. 2. The extent of AngII-mediated antinatriuresis was investigated in OZR and lean Zucker rats (LZR) using candesartan (100 microg/kg, i.v.), a selective angiotensin AT1 receptor antagonist, and ramipril (1 mg/kg, i.v.), an angiotensin-converting enzyme (ACE) inhibitor. The total number of AngII binding sites and their affinity were also assessed in renal cortical tubular membrane preparations of OZR and LZR using a specific radioligand-binding assay. Plasma renin activity was determined using a standard radioimmunoassay. 3. Both candesartan and ramipril produced substantially greater increases in urinary sodium excretion and urine flow in OZR and these effects were significantly greater than those observed in LZR. These observations suggest that basal antinatriuretic function of endogenous AngII is exaggerated in OZR. 4. The functional overexpression of AngII was not due to any alterations in the affinity or the total number of AngII binding sites in renal cortical tubular membranes. Higher plasma renin values in the OZR could have contributed to the phenomenon. 5. In conclusion, marked diuresis and natriuresis after AT1 receptor blockade and/or ACE inhibition suggest that the extent of endogenous AngII-mediated sodium transport under basal conditions is greatly augmented in OZR. It is proposed this phenomenon may be a contributing factor for the salt- sensitivity in the OZR.

Elevated sympathetic activity contributes to hypertension and salt sensitivity in diabetic obese Zucker rats

Zucker rats are a useful model in which to define the mechanisms that link obesity to diabetes and associated cardiovascular disease. The present study tests the hypothesis that diabetic obese (compared with nondiabetic lean) Zucker rats are hypertensive and display a further increase in arterial pressure when fed a high salt diet. Male, nondiabetic lean and diabetic obese Zucker rats were chronically instrumented with telemetry probes and fed a basal salt diet for 3 weeks followed by exposure to a high salt diet for 11 days. On the basal diet, obese (vs lean) rats had significantly higher arterial pressures ( approximately 13 mm Hg), and the high salt diet significantly elevated mean arterial pressure (MAP) in obese (but not lean) Zucker rats ( approximately 12 mm Hg). Blockade of the sympathetic nervous system with hexamethonium caused a significantly larger decrease in MAP in obese (vs lean) Zucker rats fed the basal diet (51 vs 33 mm Hg), but the high salt diet did not increase the hexamethonium-induced reduction in arterial pressure in obese rats. Acute blockade of angiotensin receptors with losartan resulted in similar decreases in MAP in both groups on either diet. Acetylcholine-induced vasodilatory capacity of the carotid artery was significantly less in the obese (vs lean) Zucker rats. Together these data indicate that increased sympathetic nervous system activity and decreased vascular reactivity may contribute to elevated arterial pressure in type 2 diabetic, obese Zucker rats, but the sympathetic nervous system does not appear to contribute to the dietary salt-sensitive hypertension in this model.

Renal effects of leptin in normotensive, hypertensive, and obese rats

The hemodynamic, hormonal, and renal excretory effects of intravenous bolus administration of synthetic murine leptin were examined in groups of anesthetized normotensive (Sprague-Dawley), hypertensive (spontaneously hypertensive), and both lean and obese Zucker rats. In the normotensive animals (n = 8) an intravenous bolus of 400 microgram/kg of leptin produced a significant six- to sevenfold elevation in sodium excretion compared with controls (n = 8). The onset of natriuresis was delayed for approximately 30-45 min. Mean arterial pressure (MAP), creatinine clearance, plasma renin activity (PRA), and plasma aldosterone concentration (PAC) remained unchanged. In contrast, the hypertensive rats were refractory to the natriuretic effects of leptin when infused either with 400 (n = 8) or 1,600 (n = 8) microgram/kg. Also in these animals MAP, creatinine clearance, PRA, and PAC were unmodified. Finally, whereas lean Zucker rats (n = 8) responded very similarly to the Sprague-Dawley animals, the natriuretic effect of the hormone was attenuated in the obese Zucker groups. At 400 microgram/kg (n = 8) no natriuresis was elicited, but at 1,600 microgram/kg (n = 8) a modest but significant two- to threefold increment in sodium excretion was observed in the obese rats. In both Zucker groups, MAP, creatinine clearance, PRA, and PAC were unchanged. Collectively, these results demonstrate a significant natriuretic effect of exogenous leptin in the normal rat and a blunted saluretic response in hypertension and obesity. It is suggested that leptin may be a potential salt-excretory factor in normal rats and may function pathophysiologically in obesity and hypertension.

Cardiovascular consequences of obesity: role of leptin

1. Several mechanisms have been implicated in the association between obesity and hypertension, including salt-sensitivity, insulin resistance and sympathetic activation. Obese animals and humans exhibit exaggerated blood pressure responses to increases in salt intake. 2. Although insulin resistance is common in obesity, it is clear that abnormal insulin action is not the sole or sufficient cause of hypertension in obesity. Obesity is associated with increased activity of the sympathetic nervous system. Sympathetic blockade has been reported to attenuate sodium retention and hypertension in experimental models of obesity. 3. The mediators responsible for salt sensitivity, insulin resistance and sympathetic activation in obesity remain unclear. 4. The novel protein hormone leptin is produced almost exclusively by adipose tissue and acts in the central nervous system through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. 5. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, renal and endocrine function. We have shown that leptin increases sympathetic nerve activity to kidney, hindlimb and adrenal gland, in addition to brown adipose tissue. 6. Despite this sympathoexcitatory action, acute systemic administration of leptin does not acutely increase arterial pressure or heart rate in anaesthetized animals. This may reflect opposing antihypertensive actions of leptin. For example, leptin increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin increases systemic insulin sensitivity, even in the absence of weight loss. 7. In conclusion, leptin may act as a mediator linking body adiposity with changes in insulin action, sympathetic neural outflow and renal sodium excretion. Alterations in leptin generation or action may, in part, underlie the sympathetic, endocrine and renal consequences of obesity.

Renal sympathetic nerve activity is increased in obese Zucker rats

A low level of sympathetic nerve activity (SNA) to brown adipose tissue has been found in genetically obese Zucker rats and may promote obesity through decreased thermogenesis. In contrast, acquired obesity is reportedly associated with increased SNA. To determine whether low SNA levels in obese Zucker rats extend to the kidney, we compared baseline levels of renal SNA in obese and lean conscious unrestrained Zucker rats fed for 2 weeks on low salt (0.4% NaCl) and high salt (8.0% NaCl) diets. Baseline renal SNA was calculated from multifiber recordings obtained before death under conscious, resting conditions and after death. Body weight averaged 490 +/- 12 g (mean +/- SEM) in obese rats (n = 17) and 339 +/- 7 g in lean rats (n = 19). Mean arterial pressure did not differ in obese and lean Zucker rats fed the low salt diet. However, on the high salt diet, mean arterial pressure was significantly higher in obese rats (n = 8) than in lean rats (n = 9) (113 +/- 3 and 101 +/- 3 mm Hg, respectively; P < .05). Baseline renal SNA was approximately 2 to 2.5 times higher (P < .05) in obese rats than in lean rats in all groups. These studies suggest that obese Zucker rats have heightened levels of SNA to the kidney in contrast to reduced SNA to brown adipose tissue.