Interaction between norepinephrine release and intrarenal angiotensin II formation during renal nerve stimulation in dogs

Metabolic syndrome and the hepatorenal reflex

Insufficient hepatic O₂ in animal and human studies has been shown to elicit a hepatorenal reflex in response to increased hepatic adenosine, resulting in the stimulation of renal as well as muscle sympathetic nerve activity and activating the renin angiotensin system. Low hepatic ATP, hyperuricemia, and hepatic lipid accumulation reported in metabolic syndrome (MetS) patients may reflect insufficient hepatic O₂ delivery, potentially accounting for the sympathetic overdrive associated with MetS. This theoretical concept is supported by experimental results in animals fed a high fructose diet to induce MetS. Hepatic fructose metabolism rapidly consumes ATP resulting in increased adenosine production and hyperuricemia as well as elevated renin release and sympathetic activity. This review makes the case for the hepatorenal reflex causing sympathetic overdrive and metabolic syndrome in response to exaggerated splanchnic oxygen consumption from excessive eating. This is strongly reinforced by the fact that MetS is cured in a matter of days in a significant percentage of patients by diet, bariatric surgery, or endoluminal sleeve, all of which would decrease splanchnic oxygen demand by limiting nutrient contact with the mucosa and reducing the nutrient load due to loss of appetite or dietary restriction.

Metabolic syndrome and the hepatorenal reflex

Insufficient hepatic O₂ in animal and human studies has been shown to elicit a hepatorenal reflex in response to increased hepatic adenosine, resulting in stimulation of renal as well as muscle sympathetic nerve activity and activating the renin angiotensin system. Low hepatic ATP, hyperuricemia, and hepatic lipid accumulation reported in metabolic syndrome (MetS) patients may reflect insufficient hepatic O₂ delivery, potentially accounting for the sympathetic overdrive associated with MetS. This theoretical concept is supported by experimental results in animals fed a high fructose diet to induce MetS. Hepatic fructose metabolism rapidly consumes ATP resulting in increased adenosine production and hyperuricemia as well as elevated renin release and sympathetic activity. This review makes the case for the hepatorenal reflex causing sympathetic overdrive and metabolic syndrome in response to exaggerated splanchnic oxygen consumption from excessive eating. This is strongly reinforced by the fact that MetS is cured in a matter of days in a significant percentage of patients by diet, bariatric surgery, or endoluminal sleeve, all of which would decrease splanchnic oxygen demand by limiting nutrient contact with the mucosa and reducing the nutrient load due to the loss of appetite or dietary restriction.

Prejunctional AT(1) receptor subtype-dependent modification of neurotransmitter releases in canine isolated splenic arteries

1. The regulation by angiotensin II (Ang II) formed locally on nerve-stimulated purinergic and adrenergic components of double-peaked vasoconstrictions in the canine splenic artery and Ang II receptor subtypes involved were investigated. 2. The perfusion of the precursor angiotensin I (Ang I, 0.1-1 nm) did not affect the vasoconstrictor responses to noradrenaline (NA, 0.03-1 nmol) and adenosine 5'-triphosphate (ATP, 0.03-1 micromol). The second component vasoconstrictor response to nerve stimulation was dose dependently potentiated by Ang I (0.1-1 nm). The first peaked constriction was slightly, but insignificantly increased. The potentiating effects of Ang I were abolished by KRH-594 (10 nm), a selective AT(1) receptor antagonist, but not by PD 123319 (1-10 nm), an AT(2) receptor antagonist. KRH-594 (10 nm) or PD 123319 (10 nm) never affected the vasoconstrictions to either NA or ATP. 3. The treatment with KRH-594 (1-10 nm) produced a greater inhibition on the second peaked response than the first one, although both of them were dose dependently inhibited. PD 123319 (1-10 nm) did not affect the vasoconstrictor responses induced by nerve stimulation. 4. Inhibition of angiotensin-converting enzyme with 10 nm enalaprilat reduced the second peaked response, having no significant inhibition on the first peaked response. A higher dose of enalaprilat (100 nm) produced a greater inhibition of the second peak than the first one. It reduced the second peak by approximately 65%, while the first peak was decreased approximately 35%. After treatment with enalaprilat, Ang I (1 nm) failed to enhance the neuronal vascular response. Enalaprilat at doses used did not affect the vasoconstrictions to either NA or ATP. 5. The present results indicate that endogenously generated Ang II may produce a more marked potentiation of adrenergic transmission than purinergic transmission via activation of prejunctional AT(1) receptors.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Role of angiotensin II and reactive oxygen species in cyclosporine A-dependent hypertension

Treatment with cyclosporine A (CysA), a potent immunosuppressive agent, is associated with systemic and renal vasoconstriction, leading to hypertension. The present study was conducted to elucidate the contribution of angiotensin II (Ang II) to CysA-induced hypertension and reactive oxygen species (ROS) generation. CysA (30 mg/kg per day SC), given for 3 weeks in rats, increased systolic blood pressure (SBP) from 119+/-2 to 145+/-3 mm Hg (n=7). Plasma and kidney Ang II levels were significantly higher in CysA-treated rats (136+/-10 fmol/mL and 516+/-70 fmol/g) than in vehicle-treated (1 mL olive oil) rats (76+/-10 fmol/mL and 222+/-21 fmol/g, n=7). CysA treatment increased AT1 receptor protein expression in the aorta (by 251+/-35%), whereas it was reduced in the kidney (by -32+/-4%). Superoxide anion production in aortic segments and kidney thiobarbituric acid-reactive substance (TBARS) contents were higher in CysA-treated rats (26+/-2 counts/min per milligram and 37+/-3 nmol/g) than in vehicle-treated rats (17+/-1 counts/min per milligram and 24+/-3 nmol/g). Concurrent administration of an AT1 receptor antagonist, valsartan (30 mg/kg per day, in drinking water), to CysA-treated rats (n=7) significantly decreased SBP (113+/-4 mm Hg) and prevented increases in vascular superoxide (16+/-2 counts/min per milligram) and kidney TBARS contents (21+/-3 nmol/g). Similarly, treatment with a superoxide dismutase mimetic, 4-hydroxy-2,2,6,6,-tetramethylpiperidine-N-oxyl (Tempol; 3 mmol/L in drinking water, n=7), prevented CysA-induced increases in SBP (115+/-3 mm Hg), vascular superoxide (16+/-1 counts/min per milligram), and kidney TBARS contents (19+/-2 nmol/g). These data suggest that ROS generation induced by augmented Ang II levels contributes to the development of CysA-induced hypertension.

Renal effects of adenosine A1-receptor antagonists in congestive heart failure

Renal function is a very important prognostic indicator in patients with congestive heart failure. While some of the prognostic importance of poor renal function is related to the worse physiology associated with it, there are suggestions that the dysfunction itself is detrimental. Recently, it has been shown that adenosine may mediate much kidney activity. In addition to vasoconstrictive and vasodilatory effects, adenosine is intrinsic to the tubuloglomerular feedback which occurs when an acute increase in sodium levels in the proximal tubule feeds back to decrease glomerular filtration. Adenosine works via both adenosine A1 and A2 receptors. A1-receptor antagonists decrease afferent arteriolar pressure, and increase urine flow and sodium excretion. Studies suggest that A1-receptor antagonists cause a diuretic effect not by a change in the renal haemodynamics, but by the inhibition of water and sodium reabsorption in tubular sites secondary to direct tubuloglomerular feedback. Less consistent has been the occasional finding of increased glomerular filtration rate despite the lack of improved renal plasma flow. Clinically important questions are: what role adenosine plays in causing the poor renal function associated with heart failure and what A1-receptor antagonists do in such situations? If an A1-receptor antagonist could cause diuresis while maintaining or improving glomerular filtration, it would be a useful adjunct in the treatment of severe heart failure. We evaluated the effects of the A1-receptor antagonist CVT-124 (BG-9719) in heart failure patients. CVT-124 increased sodium excretion without decreasing glomerular filtration rate. These data suggest that adenosine might be an important determinant of renal function in patients with heart failure.