Obesity hypertension: the emerging role of leptin in renal and cardiovascular dyshomeostasis.

PURPOSE OF REVIEW: Adipose tissue is now considered to be an active physiologic system operating in concert with multiple other organs. Leptin is a peptide hormone that is primarily synthesized and secreted by adipose tissue whose principal action is the control of appetite and energy balance. However, current information suggests that leptin exerts pleiotropic effects on several organ systems. Herein, we review the potential role of leptin in cardiovascular and renal physiological conditions as well as pathophysiological situations including obesity and hypertension. RECENT FINDINGS: Increasing evidence suggests that leptin may function as a pressure and volume-regulating factor under conditions of health; however, in situations characterized by chronic hyperleptinemia such as obesity, it may function pathophysiologically for the development of hypertension and possibly also for adverse renal, vascular and cardiac remodeling. SUMMARY: Adipose tissue should be regarded as a potentially important mediator of cardiorenal physiology. Further research awaits the characterization of additional mechanisms of action of leptin, including its interface with other important endocrine and hemodynamic sodium-volume regulatory systems, in both health and disease, particularly in obesity and related comorbidities. This information could lead to the development of leptin analogues as well as leptin receptor blockers that given specific circumstances could optimize the beneficial actions of the hormone and minimize its deleterious effects.

Magnetic resonance imaging of urea transporter knockout mice shows renal pelvic abnormalities.

Many transgenic and knockout mice with increased urine flow have structural abnormalities of the renal pelvis and inner medulla. Here, we used high resolution contrast enhanced T1-weighted magnetic resonance imaging of mice whose urea transporters UT-A1 and UT-A3 were deleted (UT-A1/3(-/-) mice) as a model for the in vivo study of such abnormalities. Three distinct variations in the appearance of the renal pelvis were found. These included normal kidneys with no accumulation of contrast agent in the renal pelvis; infrequent frank right-sided unilateral hydronephrosis with marked atrophy of the renal medulla; and a renal pelvic reflux pattern characterized by the presence of contrast agent in the renal pelvis surrounding the renal inner medulla but no substantial atrophy of the medulla. This last pattern was found in most of the advanced age UT-A1/3(-/-) mice and in aquaporin-1 knockout mice. The UT-A1/3(-/-) mice also had increased mean arterial blood pressures. Feeding the mice a low protein diet did not prevent development of their renal pelvic abnormalities. Our studies show that real time imaging of renal pelvic structure in genetically manipulated mice provides a tool for the non-destructive, temporal evaluation of kidney structure.

B-type natriuretic peptide decreases gastric emptying and absorption.

Natriuretic peptides have been shown to decrease contractility of isolated gastric smooth muscle cells. However there is a paucity of research showing whether this effect has functional significance in the whole animal. The objective of this study was to test whether intravenously administered B-type Natriuretic Peptide (BNP) has an effect on gastric emptying and/or absorption in a whole animal mouse model. C57BL/6-Wild-type (WT) and Natriuretic Peptide Receptor type A (NPR-A) knockout (KO) mice were used in these studies. Gastric contractility was examined in anesthetized mice before and after BNP vs. vehicle injection. Gastric emptying of gavage fed 70 Kilo Dalton (kDa) FITC-dextran and absorption of 4 kDa FITC-dextran were compared in BNP vs. vehicle treated conscious WT and KO mice. BNP decreased gastric contractility (measured in change in intragastric pressure) from 2.26 +/- 0.29 to 1.44 +/- 0.11 mmHg (P < 0.05), pressure returned to 2.08 +/- 0.17 after 5 BNP half-lives (P < 0.05). There was no significant change in the vehicle or KO. BNP also decreased gastric emptying in WT mice compared to vehicle, 87.8 +/- 0.8% vs. 97.3 +/- 1.04% (P < 0.05) and this effect showed a dose-response relationship. In KO mice emptying was 95.8 +/- 0.5% (BNP) vs. 91.7 +/- 0.7% (Vehicle) (P > 0.05). The absorption in WT mice was 28.2 +/- 7.8 (relative fluorescence units) for BNP vs. 91 +/- 25.9 for vehicle (P < 0.05). For KO mice absorption was 64.3 +/- 14.9 for BNP vs. 60.6 +/- 17.4 for vehicle (P > 0.05). The results show that BNP decreases intragastric pressure, emptying and absorption by acting via the NPR-A receptor. We postulate that this effect is aimed at decreasing preload through decreased water and electrolyte absorption from the GI tract and may also be responsible for the symptoms of impaired gastrointestinal function observed in heart failure patients.

Mechanisms of atrial natriuretic peptide secretion from the atrium.

Since the discovery of atrial natriuretic peptide (ANP) more than 20 years ago, numerous studies have focused on the mechanisms regulating ANP secretion. From a physiological standpoint, the most important factor governing ANP secretion is mechanical stretching of the atria, which normally occurs when extracellular fluid volume or blood volume is elevated. In addition, the ability of several vasoconstrictors to increase ANP secretion can be traced to their indirect effects on atrial stretch via increases in cardiac preload or afterload. Whether vasoconstrictors such as angiotensin II and vasopressin have a direct positive or negative effect on ANP secretion has not been determined with certainty. Two paracrine factors derived from endothelial cells play important roles in modulating ANP secretion. Endothelin, a potent vasoconstrictor, stimulates ANP secretion and augments stretch induced ANP secretion. The dramatic increase in ANP release produced by cardiac ischemia appears to be mediated in part by endothelin. Nitric oxide (NO), an important vasodilator, is also produced by endothelial cells and inhibits ANP secretion acting through cyclic GMP as an intracellular messenger. Several recent studies have helped to define the cellular mechanism contributing to regulation of ANP secretion including stretch-activated ion channels, prostaglandins, cytochrome P450, G proteins and cell calcium. A number of steps in the cellular transduction of the ANP signal remain to be resolved. The release of ANP in disease states such as myocardial infarction and heart failure appears to be related to both mechanical and cellular events.

Effects of cardiac hormones on arterial pressure and sodium excretion in NPRA knockout mice.

These studies were designed to determine if the atria contains natriuretic substances that act through a non-natriuretic peptide type A (NPRA) receptor mechanism. C57BL/6 mice, either wild-type NPRA++ (WT) or NPRA-- knockout (KO), were anesthetized with pentobarbital. Catheters were placed in the trachea, carotid artery, jugular vein, and bladder. Urine was collected for six 30-min periods. Both groups received an iv injection of 100 ng of rat atrial natriuretic peptide (rANP) in 200 microl of saline after the first period (30 mins) and 200 microl of rat atrial extract after the fourth period (120 mins). ANP injection increased urine flow (UF) to 2.7 +/- 0.5 microl/min in the WT versus 1.9 +/- 0.2 in KO. Extract increased UF to 7.9 +/- 1.5 microl/min in WT versus 2.7 +/- 0.4 in KO (P < 0.01). ANP increased sodium excretion (ENa) to 0.47 +/- 0.10 micromoles/min in WT versus 0.27 +/- 0.04 in KO (P < 0.05). Extract increased ENa to 1.44 +/- 0.47 micromoles/min in WT versus 0.26 +/- 0.06 in KO (P < 0.05). Extract decreased mean arterial pressure (MAP) to 62 +/- 3 mm Hg in the WT versus 81 +/- 5 in KO (P < 0.01). ENa and MAP responses to extract in KO were not different from responses to 200 microl of saline. A constant 150-min infusion of rat atrial extract increased urine flow by 3-fold and ENa by 5-fold (both P < 0.05) in the WT mice but had no significant effect in the KO mice. Thus, acute renal and MAP responses to atrial extracts require the NPRA receptor.

Combined treatment with vessel dilator and kaliuretic hormone in persons with congestive heart failure.

Vessel dilator and kaliuretic hormone, two cardiovascular peptide hormones, enhance urine flow 2- to 13-fold and 4-fold, respectively, in persons with class III New York Heart Association congestive heart failure (CHF). The natriuresis and diuresis secondary to vessel dilator and kaliuretic hormone are not blunted as are atrial natriuretic peptide and brain natriuretic peptide effects in persons with CHF compared with healthy individuals. The present investigation determined if the two peptide hormones that do not have blunted effects in persons with CHF may have added beneficial effects when given simultaneously to individuals with class III CHF. Together with each at 100 ng/kg of body weight per minute, vessel dilator and kaliuretic hormone increased urine flow rate 3.5-fold (P < 0.05) compared with their 60-min baseline and control CHF subjects' urine flow rates. Combined, they enhanced the excretion rate of sodium a maximum of 3.6-fold (P < 0.05) with 2.5- and 2-fold enhancement 2 and 3 hrs after infusion. These data indicate that vessel dilator and kaliuretic hormone have diuretic and natriuretic effects when used in combination, but these effects are not additive over their individual effects in persons with CHF.

Heart failure mice exhibit decreased gastric emptying and intestinal absorption.

Our recent study showed that intravenously administered B-type natriuretic peptide (BNP) decreases gastric emptying and intestinal absorption in mice. We aimed to test whether acute myocardial injury and heart failure have similar effects. Wild-type (WT) and natriuretic peptide receptor type A (NPR-A) knockout (KO) mice underwent cryoinfarction (myocardial infarction [MI]) of the left ventricle (LV) versus sham. LV dysfunction was confirmed by echocardiography. Percent gastric emptying and intestinal absorption were measured and analyzed one and two weeks after infarction, by gavage feeding the mice with fluorescein-isothiocyanate-dextran. Ejection fraction was 48 ± 3% versus 64 ± 2% (P < 0.05) and fractional shortening was 24 ± 2% versus 35 ± 2% (P < 0.01), MI versus sham, respectively. BNP levels (pg/mL) were 4292 ± 276 one week after MI versus 105 ± 11 in sham (n = 5, P < 0.05) and 1964 ± 755 two weeks after MI (n = 5, P < 0.05). Gastric emptying was significantly decreased, 68 ± 6% in MI versus 89 ± 3% in sham (n = 5, P < 0.05) one week after MI and 82 ± 0.5% versus 98 ± 0.4%, MI versus sham (n = 5, P < 0.05), two weeks post-MI. Absorption, measured in relative plasma fluorescence units in WT mice, was 350 ± 79 in MI versus 632 ± 121 in sham (n = 6, P < 0.05). KO mice did not show a significant difference in emptying or absorption compared with sham. These findings suggest that MI and LV dysfunction decrease gastric emptying and absorption in mice through a mechanism that involves NPR-A.

Hypotension in NKCC1 null mice: role of the kidneys.

NKCC1 null mice are hypotensive, in part, from the absence of NKCC1-mediated vasoconstriction. Whether these mice have renal defects in NaCl and water handling which contribute to the hypotension is unexplored. Therefore, we asked 1) whether NKCC1 (-/-) mice have a defect in the regulation of NaCl and water balance, which might contribute to the observed hypotension and 2) whether the hypotension observed in these mice is accompanied by endocrine abnormalities and/or downregulation of renal Na+ transporter expression. Thus we performed balance studies, semiquantitative immunoblotting, and immunohistochemistry of kidney tissue from NKCC1 (+/+) and NKCC1 (-/-) mice which consumed either a high (2.8% NaCl)- or a low-NaCl (0.01% NaCl) diet for 7 days. Blood pressure was lower in NKCC1 (-/-) than NKCC1 (+/+) mice following either high or low dietary NaCl intake. Relative to wild-type mice, NKCC1 null mice had a lower plasma ANP concentration, a higher plasma renin and a higher serum K+ concentration with inappropriately low urinary K+ excretion, although serum aldosterone was either the same or only slightly increased in the mutant mice. Expression of NHE3, the alpha-subunit of the Na-K-ATPase, NCC, and NKCC2 were higher in NKCC1 null than in wild-type mice, although differences were generally greater during NaCl restriction. NKCC1 null mice had a reduced capacity to excrete free water than wild-type mice, which resulted in hypochloremia following the NaCl-deficient diet. Hypochloremia did not occur from increased aquaporin-1 (AQP1) or 2 protein expression or from redistribution of AQP2 to the apical regions of principal cells. Instead, NKCC1 null mice had a blunted increase in urinary osmolality following vasopressin administration, which should increase free water excretion and attenuate the hypochloremia. In conclusion, aldosterone release is inappropriately low in NKCC1 null mice. Moreover, the action of aldosterone and vasopressin is altered within kidneys of NKCC1 null mice, which likely contributes to their hypotension. Increased Na+ transporter expression, increased plasma renin, and reduced plasma ANP, as observed in NKCC1 null mice, should increase vascular volume and blood pressure, thus minimizing hypotension.

Cytochrome P-450 metabolites in endothelin-stimulated cardiac hormone secretion.

We examined the role of cytochrome P-450-arachidonate (CYP450-AA) metabolites in endothelin-1 (ET-1)-stimulated atrial natriuretic peptide (ANP) and pro-ANP-(1-30) secretion from the heart. 17-Octadecynoic acid (17-ODYA, 10(-5) M) significantly inhibited ANP secretion stimulated by ET-1 (10(-8) M) in the isolated perfused rat atria and inhibited pro-ANP-(1-30) secretion stimulated by ET-1 (10(-8) M) or 20-hydroxyeicosatetraenoic acid in cultured neonatal rat ventricular myocytes (NRVM). In NRVM, 17-ODYA significantly (P < 0.05) increased secretion of cAMP but had no significant effect on the secretion of cGMP from NRVM. Staurosporine, an inhibitor of protein kinase C, completely blocked the inhibitory action of 17-ODYA, whereas a protein kinase A inhibitor, H-89 (5 x 10(-5) M), did not significantly attenuate the effects of 17-ODYA. The results show that the inhibitory action of 17-ODYA on ET-1-augmented ANP secretion is mediated through cAMP and suggest that CYP450-AA may play an important role in ET-1-induced cardiac hormone secretion.

Regulation of atrial natriuretic peptide secretion by cholinergic and PACAP neurons of the gastric antrum.

Atrial natriuretic peptide (ANP) released from enterochromaffin cells helps regulate antral somatostatin secretion, but the mechanisms regulating ANP secretion are not known. We superfused rat antral segments with selective neural agonists/antagonists to identify the neural pathways regulating ANP secretion. The nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) stimulated ANP secretion; the effect was abolished by hexamethonium but doubled by atropine. Atropine's effect implied that DMPP activated concomitantly cholinergic neurons that inhibit and noncholinergic neurons that stimulate ANP secretion, the latter effect predominating. Methacholine inhibited ANP secretion. Neither bombesin nor vasoactive intestinal polypeptide stimulated ANP secretion, whereas pituitary adenylate cyclase-activating polypeptide (PACAP)-27, PACAP-38, and maxadilan [PACAP type 1 (PAC1) agonist] each stimulated ANP secretion. The PAC1 antagonist M65 1) abolished PACAP-27/38-stimulated ANP secretion; 2) inhibited basal ANP secretion by 28 +/- 5%, implying that endogenous PACAP stimulates ANP secretion; and 3) converted the ANP response to DMPP from 109 +/- 21% above to 40 +/- 5% below basal, unmasking the cholinergic component and indicating that DMPP activated PACAP neurons that stimulate ANP secretion. Combined atropine and M65 restored DMPP-stimulated ANP secretion to basal levels. ANP secretion in the antrum is thus regulated by intramural cholinergic and PACAP neurons; cholinergic neurons inhibit and PACAP neurons stimulate ANP secretion.

Neutralization of proANP (1-30) exacerbates hypertension in the spontaneously hypertensive rat.

1. The aim of the present studies was to determine the role of proANP (1-30) in the regulation of arterial pressure. It was hypothesized that blocking endogenous proANP (1-30) would exacerbate the hypertension in susceptible animal models. 2. Pentobarbital-anaesthetized spontaneously hypertensive rats (SHR) were pretreated i.v. with 1.2 mL rabbit serum containing an antibody directed specifically against rat proANP (1-30) (SHR-AB group; n = 7) or an equal volume of normal rabbit serum as a control (SHR-NRS group; n = 5). 3. Following a 1 h equilibration period and two 30 min baseline periods, rats were volume expanded with 3 mL of 6% albumin in Krebs' solution and observed for an additional 3 h to determine the effects of the anti-proANP on arterial pressure. 4. Arterial pressure increased in both groups compared with their own baselines with volume expansion, but was significantly greater in the anti-proANP SHR group compared with the SHR-NRS group throughout the volume expansion period. A maximum difference of 21 mmHg between the anti-proANP SHR group and the NRS-SHR group was observed at 150 min of the study (183 +/- 5 vs 162 +/- 3 mmHg, respectively; P < 0.005. 5. These results suggest a protective role for proANP (1-30) in the SHR model of hypertension.