Atrial natriuretic peptide attenuates flow in an isolated lymph duct preparation

Atrial Natriuretic Peptide Inhibits Spontaneous Contractile Activity of Lymph Nodes

Atrial natriuretic peptide dose-dependently inhibited spontaneous phase and tonic activity of smooth muscle strips from the capsule of isolated bovine mesenteric lymph nodes. Pretreatment with L-NAME, diclofenac, and methylene blue had practically no effect on the peptide-induced relaxation responses. In contrast, glibenclamide significantly reduced the inhibitory effect of atrial natriuretic peptide. We suppose that the NO-dependent and cyclooxygenase signaling pathways are not involved in implementation of the inhibitory effects of atrial natriuretic peptide. ATP-sensitive K(+)-channels of the smooth muscle cell membrane are the last component in the signaling pathway leading to relaxation of smooth muscles of the lymph node capsule caused by atrial natriuretic peptide; activation of these channels leads to membrane hyperpolarization and smooth muscle relaxation.

A rational approach to fluid therapy in sepsis

Aggressive fluid resuscitation to achieve a central venous pressure (CVP) greater than 8 mm Hg has been promoted as the standard of care, in the management of patients with severe sepsis and septic shock. However recent clinical trials have demonstrated that this approach does not improve the outcome of patients with severe sepsis and septic shock. Pathophysiologically, sepsis is characterized by vasoplegia with loss of arterial tone, venodilation with sequestration of blood in the unstressed blood compartment and changes in ventricular function with reduced compliance and reduced preload responsiveness. These data suggest that sepsis is primarily not a volume-depleted state and recent evidence demonstrates that most septic patients are poorly responsive to fluids. Furthermore, almost all of the administered fluid is sequestered in the tissues, resulting in severe oedema in vital organs and, thereby, increasing the risk of organ dysfunction. These data suggest that a physiologic, haemodynamically guided conservative approach to fluid therapy in patients with sepsis would be prudent and would likely reduce the morbidity and improve the outcome of this disease.

Iatrogenic salt water drowning and the hazards of a high central venous pressure

Current teaching and guidelines suggest that aggressive fluid resuscitation is the best initial approach to the patient with hemodynamic instability. The source of this wisdom is difficult to discern, however, Early Goal Directed therapy (EGDT) as championed by Rivers et al. and the Surviving Sepsis Campaign Guidelines appears to have established this as the irrefutable truth. However, over the last decade it has become clear that aggressive fluid resuscitation leading to fluid overload is associated with increased morbidity and mortality across a diverse group of patients, including patients with severe sepsis as well as elective surgical and trauma patients and those with pancreatitis. Excessive fluid administration results in increased interstitial fluid in vital organs leading to impaired renal, hepatic and cardiac function. Increased extra-vascular lung water (EVLW) is particularly lethal, leading to iatrogenic salt water drowning. EGDT and the Surviving Sepsis Campaign Guidelines recommend targeting a central venous pressure (CVP) > 8 mmHg. A CVP > 8 mmHg has been demonstrated to decrease microcirculatory flow, as well as renal blood flow and is associated with an increased risk of renal failure and death. Normal saline (0.9% salt solution) as compared to balanced electrolyte solutions is associated with a greater risk of acute kidney injury and death. This paper reviews the adverse effects of large volume resuscitation, a high CVP and the excessive use of normal saline.

Role of caval venous manipulation in treatment of protein-losing enteropathy

Protein-losing enteropathy has been associated with the Fontan operation, restrictive cardiomyopathy, constrictive pericarditis, tricuspid valvar stenosis and insufficiency, and superior caval venous obstruction. The mechanism of development of this complication of cardiac disease likely is multifactorial. We report here our experience with 3 patients with protein-losing enteropathy. Two had superior caval venous occlusion. The third patient had undergone an extracardiac Fontan operation, with stenosis of the extracardiac conduit placed to the right pulmonary artery. In all three cases, the complication was resolved by restoring unobstructed flow of blood in the superior or inferior caval vein. Since the aetiology of protein-losing enteropathy associated with cardiac disease is multifactorial, reduction of pressure in the tributaries of the superior caval vein may, in some cases, be curative. It is more difficult to cure the problem when it is associated with the Fontan operation.

Effects of atrial natriuretic peptide on the extrasplenic microvasculature and lymphatics in the rat in vivo

We developed a novel model using fluorescent intravital microscopy to study the effect of atrial natriuretic peptide (ANP) on the extrasplenic microcirculation. Continuous infusion of ANP into the splenic artery (10 ng min(-1) for 60 min) of male Long-Evans rats (220-250 g, n = 24) induced constriction of the splenic arterioles after 15 min (-7.2 +/- 6.6% from baseline diameter of 96 +/- 18.3 microm, mean +/- S.E.M.) and venules (-14.4 +/- 4.0% from 249 +/- 25.8 microm; P < 0.05). At the same time flow did not change in the arterioles (from 1.58 +/- 0.34 to 1.27 +/- 0.27 ml min(-1)), although it decreased in venules (from 1.67 +/- 0.23 to 1.15 +/- 0.20 ml min(-1)) and increased in the lymphatics (from 0.007 +/- 0.001 to 0.034 +/- 0.008 ml min(-1); P < 0.05). There was no significant change in mean arterial pressure (from 118 +/- 5 to 112 +/- 5 mmHg). After continuous ANP infusion for 60 min, the arterioles were dilated (108 +/- 16 microm, P < 0.05) but the venules remained constricted (223 +/- 24 microm). Blood flow decreased in both arterioles (0.76 +/- 0.12 ml min(-1)) and venules (1.03 +/- 0.18 ml min(-1); P < 0.05), but was now unchanged from baseline in the lymphatics (0.01 +/- 0.001 ml min(-1)). This was accompanied by a significant decrease in MAP (104 +/- 5 mmHg; P < 0.05). At 60 min, there was macromolecular leak from the lymphatics, as indicated by increased interstitial fluorescein isothiocyanate-bovine serum albumin fluorescence (grey level: 0 = black; 255 = white; from 55.8 +/- 7.6 to 71.8 +/- 5.9, P < 0.05). This study confirms our previous proposition that, in the extrasplenic microcirculation, ANP causes greater increases in post- than precapillary resistance, thus increasing intrasplenic capillary hydrostatic pressure (P(c)) and fluid efflux into the lymphatic system. Longer-term infusion of ANP also increases Pc, but this is accompanied by increased 'permeability' of the extrasplenic lymphatics, such that fluid is lost to perivascular third spaces.

Intraperitoneal atrial natriuretic peptide increases peritoneal fluid and solute removal

BACKGROUND

Atrial natriuretic peptide (ANP) is a hormone with well-known diuretic and vasodilating properties. Recently it was reported that ANP could increase the peritoneal fluid formation and increase peritoneal solute clearance. This study investigated the effect of ANP on peritoneal fluid and solute transport characteristics.

METHODS

Eighteen male Sprague-Dawley rats were divided into three groups. A four-hour dwell study using 25 mL 2.27% glucose dialysis solution with 50 microg/kg ANP (N = 6, H-ANP) or 5 microg/kg ANP (N = 6, L-ANP) or without ANP (N = 8, control) and frequent dialysate and blood sampling was done in each rat. Radiolabeled human albumin (RISA) was added to the solution as an intraperitoneal volume marker.

RESULTS

The intraperitoneal volume was significantly higher in the H-ANP group as compared with the control group and the L-ANP group. The drainage volume was 26.2 +/- 1.1, 25.5 +/- 0.7, and 29.8 +/- 0.8 mL for the control, L-ANP, and H-ANP groups, respectively (P < 0.01). This was related to significant differences in the peritoneal fluid absorption rates (K(E); estimated as the RISA elimination coefficient): 39 +/- 3, 38 +/- 3, and 19 +/- 4 microL/min, and in the direct lymphatic absorption rate (K(EB); estimated as the clearance of RISA from dialysate to blood): 7 +/- 1, 6 +/- 1, and 4 +/- 1 microL/min for the control, L-ANP, and H-ANP groups, respectively (all P < 0.01). No differences were found in the intraperitoneal volume, K(E), and K(EB) between the control group and the L-ANP group. The diffusive mass transport coefficient (K(BD)) for urea, sodium, potassium, and total protein did not differ among the three groups. However, the glucose D/D(0) was significantly higher, and the K(BD) for glucose was significantly lower in the H-ANP group as compared with the other two groups. Solute clearances (+175% for sodium and +26% for potassium) were significantly increased in the H-ANP group, mainly as a result of the increased fluid removal in this group.

CONCLUSIONS

Our results suggest that ANP may decrease peritoneal fluid absorption (by 51%, partially because of decreasing the direct lymphatic absorption), resulting in a significant increase in peritoneal fluid removal and small solute clearances. While the basic diffusive permeability of the peritoneal membrane was not changed, the peritoneal glucose absorption was retarded by adding ANP to peritoneal dialysate, perhaps through interaction of ANP with glucose metabolism.

ANP in regulation of arterial pressure and fluid-electrolyte balance: lessons from genetic mouse models

The recent development of genetic mouse models presenting life-long alterations in expression of the genes for atrial natriuretic peptide (ANP) or its receptors (NPR-A, NPR-C) has uncovered a physiological role of this hormone in chronic blood pressure homeostasis. Transgenic mice overexpressing a transthyretin-ANP fusion gene are hypotensive relative to the nontransgenic littermates, whereas mice harboring functional disruptions of the ANP or NPR-A genes are hypertensive compared with their respective wild-type counterparts. The chronic hypotensive action of ANP is determined by vasodilation of the resistance vasculature, which is probably mediated by attenuation of vascular sympathetic tone at one or several prejunctional sites. Under conditions of normal dietary salt consumption, the hypotensive action of ANP is dissociated from the natriuretic activity of the hormone. However, during elevated dietary salt intake, ANP-mediated antagonism of the renin-angiotensin system is essential for maintenance of blood pressure constancy, inasmuch as the ANP gene "knockout" mice (ANP -/-) develop a salt-sensitive component of hypertension in association with failure to adequately downregulate plasma renin activity. These findings imply that genetic deficiencies in ANP or natriuretic receptor activity may be underlying causative factors in the etiology of salt-sensitive variants of hypertensive disease and other sodium-retaining disorders, such as congestive heart failure and cirrhosis.