Guanylyl cyclase mediates ANP-induced vasoconstriction of murine splenic vessels

Heart-Microcirculation Connection: Effects of ANP (Atrial Natriuretic Peptide) on Pericytes Participate in the Acute and Chronic Regulation of Arterial Blood Pressure

Cardiac ANP (atrial natriuretic peptide) moderates arterial blood pressure. The mechanisms mediating its hypotensive effects are complex and involve inhibition of the renin-angiotensin-aldosterone system, increased natriuresis, endothelial permeability, and vasodilatation. The contribution of the direct vasodilating effects of ANP to blood pressure homeostasis is controversial because variable levels of the ANP receptor, GC-A (guanylyl cyclase-A), are expressed among vascular beds. Here, we show that ANP stimulates GC-A/cyclic GMP signaling in cultured microvascular pericytes and thereby the phosphorylation of the regulatory subunit of myosin phosphatase 1 by cGMP-dependent protein kinase I. Moreover, ANP prevents the calcium and contractile responses of pericytes to endothelin-1 as well as microvascular constrictions. In mice with conditional inactivation (knock-out) of GC-A in microcirculatory pericytes, such vasodilating effects of ANP on precapillary arterioles and capillaries were fully abolished. Concordantly, these mice have increased blood pressure despite preserved renal excretory function. Furthermore, acute intravascular volume expansion, which caused release of cardiac ANP, did not affect blood pressure of control mice but provoked hypertensive reactions in pericyte GC-A knock-out littermates. We conclude that GC-A/cGMP-dependent modulation of pericytes and microcirculatory tone contributes to the acute and chronic moderation of arterial blood pressure by ANP. Graphic Abstract A graphic abstract is available for this article.

Intradermal Administration of Atrial Natriuretic Peptide Attenuates Cutaneous Vasodilation but Not Sweating in Young Men during Exercise in the Heat

INTRODUCTION

Prolonged exercise in the heat stimulates plasma release of atrial natriuretic peptide (ANP) in association with dehydration-induced reductions in blood volume. Elevated plasma ANP levels under these conditions may indirectly attenuate cutaneous blood flow and sweating responses due to the effects of this hormone on central blood volume and plasma osmolality and the resulting stimulation of nonthermal reflexes. However, it remains unclear whether cutaneous blood flow and sweating are directly modulated by ANP at the level of the cutaneous end organs (cutaneous microvessels and eccrine sweat glands) during prolonged exercise in the heat.

OBJECTIVE

Therefore, we evaluated the effects of local ANP administration on forearm cutaneous vascular conductance (CVC) and local sweat rate (LSR) during rest and exercise in the heat.

METHODS

In 9 habitually active young men (26 ± 6 years) CVC and LSR were evaluated at 3 intradermal microdialysis sites continuously perfused with lactated Ringer solution (control) or ANP (0.1 or 1.0 μM). Participants rested in a non-heat-stress condition (25°C) for approximately 60 min followed by 70 min in the heat (35°C). They then performed 50 min of moderate-intensity cycling (approx. 55% VO2 peak), with a 30-min recovery. Thereafter, 50 mM sodium nitroprusside was administered at all sites to elicit maximum CVC, which was subsequently used to normalize all values (CVC%max).

RESULTS

No effects of ANP on CVC%max were observed in the non-heat-stress resting condition compared to the untreated control site (both p > 0.05). Conversely during rest in the heat there was an 11% (5-17%) reduction in CVC%max at the 1.0 μM ANP site relative to the untreated control site (p < 0.05). At the end of exercise CVC%max was attenuated by 12% (1-23%) at the 0.1 μM ANP site and by 21% (7-35%) at the 1.0 μM ANP site relative to the untreated control site (all p < 0.05). Conversely, neither concentration of ANP influenced sweating at any time point (all p > 0.05).

CONCLUSION

Intradermal ANP administration directly attenuated cutaneous blood flow, but not sweating, in habitually active young men during rest and exercise in the heat.

Constriction of rat extra-splenic veins to lipopolysaccharide involves endothelin-1

The spleen has an important role in blood volume regulation and increased resistance of post-capillary hilar veins (in mesentery adjoining the spleen) can regulate this. This study investigated whether venular constriction to lipopolysaccharide (LPS) involved endothelin-1 (ET-1). Pressure myography was used to study isolated extra-splenic (hilar) vessels from male Wistar rats (n = 111). Arteries and veins were treated with LPS (50 microg ml(-1)) for 4 h. Extra-splenic veins constricted to LPS (p < 0.05), but there was no effect on arteries. Denudation did not abolish venular constriction to LPS, indicating an endothelial independent mechanism. However, the dual ET-1 receptor antagonist bosentan (10(-5) M) and specific ET(A) and ET(B) antagonists ABT-627 (atrasentan, 6.3 x 10(-6) M) and A-192621(1.45 x 10(-6) M) completely abolished constriction of LPS-treated veins. ET-1 alone also constricted the extra-splenic arteries and veins (p < 0.05), with a greater response observed in veins (p < 0.05). ELISA also confirmed that serum and spleen levels of ET-1 increased in response to LPS (p < 0.05). That LPS-induced constriction of extra-splenic veins is mediated by ET-1. Greater constriction of post- versus pre-capillary extra-splenic vessels to LPS would result in increased intra-splenic fluid extravasation and hypovolaemia in vivo.

Role of spleen in integrated control of splanchnic vascular tone: physiology and pathophysiology

Aside from its established immunologic and hematologic functions, the spleen also plays an important role in cardiovascular regulation. This occurs through changes in intrasplenic microvascular tone, as well as through splenic neurohormonal modulation of the renal and mesenteric vascular beds. Splenic regulation of blood volume occurs predominantly through fluid extravasation from the splenic circulation into lymphatic reservoirs; this is controlled by direct modulation of splenic pre- and postcapillary resistance by established physiologic agents such as atrial natriuretic peptide (ANP), nitric oxide (NO), and adrenomedullin (ADM). In addition to physiologic fluid regulation, splenic extravasation is a key factor in the inability to maintain adequate intravascular volume in septic shock. The spleen also controls renal microvascular tone through reflex activation of the splenic afferent and renal sympathetic nerves. This splenorenal reflex not only contributes to the physiologic regulation of blood pressure, but also contributes to the cardiovascular dysregulation associated with both septic shock and portal hypertension. In septic shock, the splenorenal reflex protectively limits splenic extravasation and potentially promotes renal sodium and water reabsorption and release of the vasoconstrictor angiotensin II; this function is eventually overwhelmed as shock progresses. In portal hypertension, on the other hand, the splenorenal reflex-mediated reduction in renal vascular conductance exacerbates sodium and water retention in the kidneys and may eventually contribute to renal dysfunction. Preliminary evidence suggests that the spleen also may play a role in the hemodynamic complications of portal hypertension via neurohormonal modulation of the mesenteric vascular bed. Lastly, the spleen itself may be a source of a vasoactive factor.

Interleukin-1 regulates multiple atherogenic mechanisms in response to fat feeding

Background

Atherosclerosis is an inflammatory process that develops in individuals with known risk factors that include hypertension and hyperlipidaemia, influenced by diet. However, the interplay between diet, inflammatory mechanisms and vascular risk factors requires further research. We hypothesised that interleukin-1 (IL-1) signaling in the vessel wall would raise arterial blood pressure and promote atheroma.

Methodology/Principal Findings

Apoe^−/− and Apoe^−/−/IL-1R1^−/− mice were fed high fat diets for 8 weeks, and their blood pressure and atherosclerosis development measured. Apoe^−/−/IL-R1^−/− mice had a reduced blood pressure and significantly less atheroma than Apoe^−/− mice. Selective loss of IL-1 signaling in the vessel wall by bone marrow transplantation also reduced plaque burden (p<0.05). This was associated with an IL-1 mediated loss of endothelium-dependent relaxation and an increase in vessel wall Nox 4. Inhibition of IL-1 restored endothelium-dependent vasodilatation and reduced levels of arterial oxidative stress.

Conclusions/Significance

The IL-1 cytokine system links atherogenic environmental stimuli with arterial inflammation, oxidative stress, increased blood pressure and atherosclerosis. This is the first demonstration that inhibition of a single cytokine can block the rise in blood pressure in response to an environmental stimulus. IL-1 inhibition may have profound beneficial effects on atherogenesis in man.

Natriuretic peptides in hormonal regulation of hypoxia responses

The possibility that natriuretic peptides' effects are important in hypoxia responses of vertebrates is reviewed. Both the transcription and release of natriuretic peptides are affected by oxygen tension. Furthermore, many of the effects observed in hypoxia, such as diuresis and a reduction of plasma volume, are also caused by treatment of the animal with natriuretic peptides. Also, several clinical observations about changes in natriuretic peptide levels in, e.g., sleep apnea and cyanotic congenital heart disease, are consistent with the idea that hypoxia is involved in the etiology of conditions, in which natriuretic peptide levels increase. Virtually all published information on the relationship between oxygen and natriuretic peptides is based on human studies. Because hypoxic conditions are more common in aquatic than terrestrial environments, future studies about the possible role of natriuretic peptides in hypoxia, as well as the role of hypoxia in the evolution of natriuretic peptides, including the different subtypes, should increasingly involve also aquatic organisms.

Bimodal effects of chronically administered neurokinin B (NKB) on in vivo and in vitro cardiovascular responses in female rats

The in vivo cardiovascular effects of acutely administered neurokinin B (NKB) have been attributed both to direct effects on vascular tone and to indirect effects on central neuroendocrine control of the circulation. We proposed: 1) that a modest long-term increase in plasma NKB levels would decrease mean arterial pressure (MAP) due to attenuated peripheral vascular tone, and 2) that chronic high-dose NKB would increase MAP, due to increased sympathetic outflow which would override the peripheral vasodilation. We examined the in vivo and in vitro cardiovascular effects of chronic peripheral NKB. Low- (1.8 nmol/h) or high- (20 nmol/h) dose NKB was infused into conscious female rats bearing telemetric pressure transducers. MAP, heart rate (HR) and the pressor responses to I.V. phenylephrine (PE, 8 microg) and angiotensin II (Ang II, 150 ng) were measured. Concentration-response curves of small mesenteric arteries were constructed to PE using wire myography. Low-dose NKB reduced basal MAP (88+/-2 mm Hg to 83+/-2 mm Hg), did not affect resting HR, reduced the pressor responses to PE, and attenuated the maximal constriction of mesenteric arteries to PE and KCl. By contrast, high-dose NKB increased basal MAP (86+/-1 mm Hg to 89+/-1 mm Hg), increased HR (350+/-3 beats/min to 371+/-3 beats/min), increased the pressor responses to Ang II and, contrary to our hypothesis, increased the maximum contractile responses of mesenteric arteries to PE and KCl. The cardiovascular effects of NKB are thus dose-dependent: whereas chronic low-dose NKB directly modulates vascular tone to reduce blood pressure, chronic high-dose NKB induces an increase in blood pressure through both central (indirect) and peripheral (direct) pathways.

Angiopoietin-1 increases arteriolar vasoconstriction to phenylephrine during sepsis

Sepsis leads to a reduction in vascular tone and a loss of vasoconstriction in response to catecholamines. We propose that angiopoietin-1 (Ang-1), which is known to modulate vascular inflammation and nitric oxide (NO), could improve responsiveness to vasopressor agents during sepsis. Mesenteric arterioles (300-400 microm) from rats (n=19) were mounted in a pressurized myograph and incubated with lipopolysaccharide (LPS, 50 microg/mL) for up to 4 h to model sepsis. Vasoconstriction (mean+/-SD) to phenylephrine (10(-8)-10(-3) M) was reduced in the presence of LPS (4 h, pD2: 5.8+/-0.2 (controls, n=6), 1.4+/-2.2 (LPS, n=6); maximal constriction: 48.2+/-4.8% (controls), 2.6+/-5.8% (LPS), P<0.05). However, in the presence of Ang-1 (250 ng/mL) phenylephrine caused greater vasoconstriction compared to LPS alone (4 h, pD2: 4.5+/-2.1; maximal constriction: 12.6+/-4.0% (n=7), P<0.05). In conclusion, Ang-1 increases vasoconstriction to phenylephrine in the presence of LPS. During sepsis therefore, Ang-1 increases vascular reactivity and has the potential to increase blood pressure and decrease vasopressor requirements in vivo.

Vascular endothelium is critically involved in the hypotensive and hypovolemic actions of atrial natriuretic peptide

Atrial natriuretic peptide (ANP), via its vasodilating and diuretic effects, has an important physiological role in the maintenance of arterial blood pressure and volume. Its guanylyl cyclase-A (GC-A) receptor is highly expressed in vascular endothelium, but the functional relevance of this is controversial. To dissect the endothelium-mediated actions of ANP in vivo, we inactivated the GC-A gene selectively in endothelial cells by homologous loxP/Tie2-Cre-mediated recombination. Notably, despite full preservation of the direct vasodilating effects of ANP, mice with endothelium-restricted deletion of the GC-A gene (EC GC-A KO) exhibited significant arterial hypertension and cardiac hypertrophy. Echocardiographic and Doppler flow evaluations together with the Evan's blue dilution technique showed that the total plasma volume of EC GC-A KO mice was increased by 11-13%, even under conditions of normal dietary salt intake. Infusion of ANP caused immediate increases in hematocrit in control but not in EC GC-A KO mice, which indicated that ablation of endothelial GC-A completely prevented the acute contraction of intravascular volume produced by ANP. Furthermore, intravenous ANP acutely enhanced the rate of clearance of radio-iodinated albumin from the circulatory system in control but not in EC GC-A KO mice. We conclude that GC-A-mediated increases in endothelial permeability are critically involved in the hypovolemic, hypotensive actions of ANP.

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.

Patterning of renal cGMP production by the natriuretic peptide receptor type A and blood pressure in spontaneously hypertensive rats

Although important advances have been made over past decades in studying the mechanisms of hypertension, the nature of cellular signaling patterns involved and their relationship remain unclear. High cGMP production rates in isolated renal glomeruli have been presented as a characteristic of spontaneously hypertensive rat (SHR) even before the development of hypertension, which suggests that this event might be a cause of the increase in blood pressure. Using cross-breeding between SHR and WKY parental strains to obtain F1 and F2 hybrids, we have investigated the patterning of high blood pressure and cGMP production rates. We have found that, in the F2 population, the mean blood pressure and both basal and ANP(1-28)-stimulated cGMP production are similar to the parental SHR. In addition, we have found a positive correlation between blood pressure and high cGMP production rates in the F2 population. The higher cGMP production was not a consequence of hypertension, since in DOCA-salt hypertensive rats cGMP production was similar to that observed in normotensive WKY rats. These observations suggest that high cGMP production is a characteristic linked to hypertension. Finally, reciprocal crosses between the SHR and WKY parental strains showed that in the F1 population blood pressure but not cGMP production are associated with the Y chromosome.

Nitric oxide control of the dorsal aorta and the intestinal vein of the Australian short-finned eel Anguilla australis

This study investigated the mechanisms by which nitric oxide (NO) regulates the dorsal aorta and the intestinal vein of the Australian short-finned eel Anguilla australis. NADPH diaphorase histochemistry and immunohistochemistry using a mammalian endothelial nitric oxide synthase (NOS)antibody could not demonstrate NOS in the endothelium of either blood vessel;however, NOS could be readily demonstrated in the endothelium of the rat aorta that was used as a control. Both blood vessels contained NADPH diaphorase positive nerve fibres and nerve bundles, and immunohistochemistry using a neural NOS antibody showed a similar distribution of neural NOS immunoreactivity in the perivascular nerves. In vitro organ bath physiology showed that a NO/soluble guanylyl cyclase (GC) system is present in the dorsal aorta and the intestinal vein, since the soluble GC inhibitor oxadiazole quinoxalin-1 (ODQ; 10–5 mol l–1)completely abolished the vasodilatory effect of the NO donor, sodium nitroprusside (SNP; 10–4 mol l–1). In addition, nicotine (3×10–4 mol l–1)mediated a vasodilation that was not affected by removal of the endothelium. The nicotine-mediated dilation was blocked by the NOS inhibitor, Nω-nitro-l-arginine (l-NNA;10–4 mol l–1), and ODQ(10–5 mol l–1). More specifically, the neural NOS inhibitor, Nω-propyl-l-arginine(10–5 mol l–1), significantly decreased the dilation induced by nicotine (3×10–4 mol l–1). Furthermore, indomethacin (10–5 mol l–1) did not affect the nicotine-mediated dilation,suggesting that prostaglandins are not involved in the response. Finally, the calcium ionophore A23187 (3×10–6 mol l–1) caused an endothelium-dependent dilation that was abolished in the presence of indomethacin. We propose the absence of an endothelial NO system in eel vasculature and suggest that neurally derived NO contributes to the maintenance of vascular tone in this species. In addition,we suggest that prostaglandins may act as endothelially derived relaxing factors in A. australis.

Vasodilator mechanisms in the dorsal aorta of the giant shovelnose ray, Rhinobatus typus (Rajiformes; Rhinobatidae)

This study investigated the nature of vasodilator mechanisms in the dorsal aorta of the giant shovelnose ray, Rhinobatus typus. Anatomical techniques found no evidence for an endothelial nitric oxide synthase, but neural nitric oxide synthase was found to be present in the perivascular nerve fibres of the dorsal aorta and other arteries and veins using both NADPH-diaphorase staining and immunohistochemistry with a specific neural NOS antibody. Arteries and veins both contained large nNOS-positive nerve trunks from which smaller nNOS-positive bundles branched and formed a plexus in the vessel wall. Single, varicose nNOS-positive nerve fibres were present in both arteries and veins. Within the large bundles of both arteries and veins, groups of nNOS-positive cell bodies forming microganglia were observed. Double-labelling immunohistochemistry using an antibody to tyrosine hydroxylase showed that nearly all the NOS nerves were not sympathetic. Acetylcholine always caused constriction of isolated rings of the dorsal aorta and the nitric oxide donor, sodium nitroprusside, did not mediate any dilation. Addition of nicotine (3 x 10(-4) M) to preconstricted rings caused a vasodilation that was not affected by the nitric oxide synthase inhibitor, L-NNA (10(-4) M), nor the soluble guanylyl cyclase inhibitor, ODQ (10(-5) M). This nicotine-mediated vasodilation was, therefore, not due to the synthesis and release of NO. Disruption of the endothelium significantly reduced or eliminated the nicotine-mediated vasodilation. In addition, indomethacin (10(-5) M), an inhibitor of cyclooxygenases, significantly increased the time period to maximal dilation and reduced, but did not completely inhibit the nicotine-mediated vasodilation. These data support the hypothesis that a prostaglandin is released from the vascular endothelium of a batoid ray, as has been described previously in other groups of fishes. The function of the nitrergic innervation of the blood vessels is not known because nitric oxide does not appear to regulate vascular tone.