Male and female high-fat diet-fed Dahl SS rats are largely protected from vascular dysfunctions: PVAT contributions reveal sex differences

Vascular dysfunctions are observed in the arteries from hypertensive subjects. The establishment of the Dahl salt-sensitive (SS) male and female rat models to develop a reproducible hypertension with high-fat (HF) diet feeding from weaning allows addressing the question of whether HF diet-associated hypertension results in vascular dysfunction similar to that of essential hypertension in both sexes. We hypothesized that dysfunction of three distinct vascular layers, i.e., endothelial, smooth muscle, and perivascular adipose tissue (PVAT), would be present in the aorta from HF diet-fed versus control diet-fed male and female rats. Dahl SS rats were fed a control (10% kcal of fat) or HF (60%) diet from weaning for 24 wk. Male and female Dahl SS rats became equally hypertensive when placed on a HF diet. For male and female rats, the thoracic aorta exhibited medial hypertrophy in HF diet-induced hypertension versus control, but neither displayed a hyperresponsive contraction to the α-adrenergic agonist phenylephrine nor an endothelial cell dysfunction as measured by acetylcholine-induced relaxation. A beneficial PVAT function, support of stress relaxation, was reduced in the male versus female rats fed a HF diet. PVAT in the aorta of males but not in females retained the anticontractile activity. We conclude that this HF model does not display the same vascular dysfunctions observed in essential hypertension. Moreover, both male and female show significantly different vascular dysfunctions in this HF feeding model.NEW & NOTEWORTHY Although the aorta exhibits medial hypertrophy in response to HF diet-induced hypertension, it did not exhibit hyperresponsive contraction to an α-adrenergic agonist nor endothelial cell dysfunction; this was true for both sexes. Unlike other hypertension models, PVAT around aorta from (male) rats on the HF diet retained significant anticontractile activity. PVAT around aorta of the male on a HF diet was modestly more fibrotic and lost the ability to assist in arterial stress relaxation.

Fenfluramine-induced PVAT-dependent contraction depends on norepinephrine and not serotonin

Perivascular adipose tissue (PVAT) modulates vascular tone and altered PVAT function is observed in vascular diseases such as hypertension and atherosclerosis. We discovered that the PVAT surrounding rat thoracic aorta (RA) and the superior mesenteric artery (SMA) contain significant amounts of 5-hydroxytryptamine (5-HT). We hypothesized that the 5-HT contained within the PVAT is functional and vasoactive. Isolated tissue baths were used for isometric contractility studies and high performance liquid chromatography was used to quantitatively measure amines in the PVAT and release studies. The 5-HT releaser fenfluramine (10 nM-100 μM) was tested for its ability to contract arteries with and without PVAT. Contraction was reported as a percentage of the initial contraction to 10 μM phenylephrine. The RA with PVAT contracted to fenfluramine to a greater maximum (98 ± 10%) than RA without PVAT (24 ± 4%), while no difference in contraction of SMA to maximum fenfluramine with (78 ± 2%) and without (75 ± 6%) PVAT was observed. Contradicting our hypothesis, the maximum contraction of RA with PVAT to fenfluramine was diminished by the alpha-1 adrenoreceptor antagonist prazosin (100 nM; vehicle: 71 ± 4%, prazosin: 24 ± 2%) and the norepinephrine transporter (NET) inhibitor nisoxetine (1 μM; vehicle: 71 ± 4%, nisoxetine: 25 ± 4%) but not the 5-HT_2A/2C receptor antagonist ketanserin (10 nM) or serotonin specific reuptake inhibitor fluoxetine (10 μM). To test if fenfluramine caused release of 5-HT or NE from PVAT, PVAT from RA was incubated with vehicle or fenfluramine (10 μM-10 mM), and amines released into the incubating buffer were quantified. A pronounced concentration-dependent NE-release (more than 5-HT) was observed. Collectively, this research illustrates the pharmacology of fenfluramine to primarily stimulate NE release (better than 5-HT) in a NET-dependent manner, leading to vasoconstriction. This adds additional support to PVAT as being an important reservoir of amines.

The chemerin knockout rat reveals chemerin dependence in female, but not male, experimental hypertension

Measures of the adipokine chemerin are elevated in multiple cardiovascular diseases, including hypertension, but little mechanistic work has been done to implicate chemerin as being causative in such diseases. The chemerin knockout (KO) rat was created to test the hypothesis that removal of chemerin would reduce pressure in the normal and hypertensive state. Western analyses confirmed loss of chemerin in the plasma and tissues of the KO vs. wild-type (WT) rats. Chemerin concentration in plasma and tissues was lower in WT females than in WT males, as determined by Western analysis. Conscious male and female KO rats had modest differences in baseline measures vs. the WT that included systolic, diastolic, mean arterial and pulse pressures, and heart rate, all measured telemetrically. The mineralocorticoid deoxycorticosterone acetate (DOCA) and salt water, combined with uninephrectomy as a hypertensive stimulus, elevated mean and systolic blood pressures of the male KO higher than the male WT. By contrast, all pressures in the female KO were lower than their WT throughout DOCA-salt treatment. These results revealed an unexpected sex difference in chemerin expression and the ability of chemerin to modify blood pressure in response to a hypertensive challenge.-Watts, S. W., Darios, E. S., Mullick, A. E., Garver, H., Saunders, T. L., Hughes, E. D., Filipiak, W. E., Zeidler, M. G., McMullen, N., Sinal, C. J., Kumar, R. K., Ferland, D. J., Fink, G. D. The chemerin knockout rat reveals chemerin dependence in female, but not male, experimental hypertension.

Whole-Body but Not Hepatic Knockdown of Chemerin by Antisense Oligonucleotide Decreases Blood Pressure in Rats

Chemerin is an inflammatory adipokine positively associated with hypertension and obesity. The majority of chemerin derives from the liver and adipose tissue, however, their individual contributions to blood pressure are unknown. We began studying chemerin in the normal rat using antisense oligonucleotides (ASO) with whole-body activity (Gen 2.5 chemerin ASO) or liver-restricted activity (GalNAc chemerin ASO). We hypothesized that in normotensive male Sprague-Dawley rats, circulating chemerin is predominately liver-derived and regulates blood pressure. A dosing study of the Gen 2.5 chemerin ASO (with a scrambled control ASO) supported 25 mg/kg as the appropriate dose. GalNAc chemerin ASO was also assessed and used at 10 mg/kg. Radiotelemetry monitored mean arterial pressure (MAP) for a 1-week baseline and weekly subcutaneous ASO injections for 4 weeks. Two days after the final injection, animals were euthanized for tissue reverse transcription-polymerase chain reaction and chemerin Western analysis. Gen 2.5 chemerin ASO treatments reduced chemerin mRNA and protein in liver, retroperitoneal fat (RP), and mesenteric perivascular adipose tissue (mPVAT), as well as reducing protein in plasma. GalNAc chemerin ASO treatments reduced chemerin mRNA and protein in liver and chemerin protein in plasma but had no effect on expression in RP fat or mPVAT. Gen 2.5 chemerin ASO treatment reduced MAP compared with control ASO but was unchanged in animals receiving the GalNAc chemerin ASO. Although circulating chemerin is liver-derived, it does not play a major role in blood pressure regulation. Local effects of chemerin from fat may explain this discrepancy and support chemerin's association with hypertension and obesity.

Abstract P102: Liver Acquitted, Fat Indicted: Hepatic Chemerin Knockdown Does Not Reduce Blood Pressure While Whole-body Knockdown Does

Chemerin is an inflammatory adipokine positively associated with hypertension and obesity with the majority of chemerin thought to derive from the liver and adipose tissue. The contributions of liver-derived chemerin to plasma chemerin levels and blood pressure regulation are unknown. We compared whole-body vs liver chemerin inhibition using antisense oligonucleotides (ASO) with liver-restricted activity (GalNAc) or liver and fat activity (Gen 2.5). We hypothesized that in normotensive male SD rats, circulating chemerin is predominately liver-derived and regulates blood pressure. A scrambled ASO control and phosphate-buffered saline (PBS) were used as controls and radiotelemetry was used to monitor blood pressure. Baseline mean arterial blood pressure (MAP) was recorded for one week, beginning 5 days after surgery to establish a baseline. ASOs were given weekly by subcutaneous injections for four weeks. Two days after the final injection, animals were sacrificed for tissue RT-PCR and plasma chemerin measurements using Western analysis. Gen 2.5 chemerin ASO treatments (compared to PBS control) reduced chemerin mRNA in liver, retroperitoneal fat, and mesenteric perivascular adipose tissue (mPVAT) by 99.5% ± 0.1%, 95.2% ± 0.3%, and 69% ± 2%, respectively, and plasma chemerin was reduced to undetectable levels. GalNAc chemerin ASO treatments (compared to PBS control) reduced chemerin mRNA in liver by 97.9% but had no effect on chemerin expression in retroperitoneal fat and mPVAT; plasma chemerin was reduced by 90% ± 5%. Gen 2.5 chemerin ASO treatment reduced MAP, which reached a nadir of 7 ± 2.1 mmHg 48 – 72 hours after each dose compared to the scrambled ASO controls. By contrast, MAP was unchanged in animals receiving the GalNAc chemerin ASO. Thus, although most circulating chemerin is liver-derived, plasma chemerin does not play a role in blood pressure regulation. This study suggests that while chemerin is still generally associated with increased blood pressure, circulating chemerin levels cannot directly predict this effect. In addition, local effects of chemerin from fat may explain this discrepancy and support chemerin’s association with both hypertension and obesity.

5-HT causes splanchnic venodilation

Serotonin [5-hydroxytryptamine (5-HT)] causes relaxation of the isolated superior mesenteric vein, a splanchnic blood vessel, through activation of the 5-HT₇ receptor. As part of studies designed to identify the mechanism(s) through which chronic (≥24 h) infusion of 5-HT lowers blood pressure, we tested the hypothesis that 5-HT causes in vitro and in vivo splanchnic venodilation that is 5-HT₇ receptor dependent. In tissue baths for measurement of isometric contraction, the portal vein and abdominal inferior vena cava relaxed to 5-HT and the 5-HT_1/7 receptor agonist 5-carboxamidotryptamine; relaxation was abolished by the 5-HT₇ receptor antagonist SB-269970. Western blot analyses showed that the abdominal inferior vena cava and portal vein express 5-HT₇ receptor protein. In contrast, the thoracic vena cava, outside the splanchnic circulation, did not relax to serotonergic agonists and exhibited minimal expression of the 5-HT₇ receptor. Male Sprague-Dawley rats with chronically implanted radiotelemetry transmitters underwent repeated ultrasound imaging of abdominal vessels. After baseline imaging, minipumps containing vehicle (saline) or 5-HT (25 μg·kg^-1·min^-1) were implanted. Twenty-four hours later, venous diameters were increased in rats with 5-HT-infusion (percent increase from baseline: superior mesenteric vein, 17.5 ± 1.9; portal vein, 17.7 ± 1.8; and abdominal inferior vena cava, 46.9 ± 8.0) while arterial pressure was decreased (~13 mmHg). Measures returned to baseline after infusion termination. In a separate group of animals, treatment with SB-269970 (3 mg/kg iv) prevented the splanchnic venodilation and fall in blood pressure during 24 h of 5-HT infusion. Thus, 5-HT causes 5-HT₇ receptor-dependent splanchnic venous dilation associated with a fall in blood pressure.NEW & NOTEWORTHY This research is noteworthy because it combines and links, through the 5-HT₇ receptor, an in vitro observation (venorelaxation) with in vivo events (venodilation and fall in blood pressure). This supports the idea that splanchnic venodilation plays a role in blood pressure regulation.

Chemerin-induced arterial contraction is Gi- and calcium-dependent

Chemerin is an adipokine associated with increased blood pressure, and may link obesity with hypertension. We tested the hypothesis that chemerin-induced contraction of the vasculature occurs via calcium flux in smooth muscle cells. Isometric contraction of rat aortic rings was performed in parallel with calcium kinetics of rat aortic smooth muscle cells to assess the possible signaling pathway. Chemerin-9 (nonapeptide of the chemerin S¹⁵⁷ isoform) caused a concentration-dependent contraction of isolated aorta (EC₅₀ 100nM) and elicited a concentration-dependent intracellular calcium response (EC₅₀ 10nM). Pertussis toxin (G_i inhibitor), verapamil (L-type Ca²⁺ channel inhibitor), PP1 (Src inhibitor), and Y27632 (Rho kinase inhibitor) reduced both calcium influx and isometric contraction to chemerin-9 but PD098059 (Erk MAPK inhibitor) and U73122 (PLC inhibitor) had little to no effect on either measure of chemerin signaling. Although our primary aim was to examine chemerin signaling, we also highlight differences in the mechanisms of chemerin-9 and recombinant chemerin S¹⁵⁷. These data support a chemerin-induced contractile mechanism in vascular smooth muscle that functions through G_i proteins to activate L-type Ca²⁺ channels, Src, and Rho kinase. There is mounting evidence linking chemerin to hypertension and this mechanism brings us closer to targeting chemerin as a form of therapy.

Abstract P182: The Novel Perivascular Adipose Tissue Adipokine, Chemerin, Signals Through G i - and Calcium-Dependent Mechanisms

Chemerin is an adipokine associated with inflammation, increased blood pressure, and may be a link between the pathologies of obesity and hypertension. We tested the hypothesis that chemerin-induced contraction of the vasculature occurs via the chemerin receptor and calcium flux in smooth muscle cells. Known mediators of the amplified arterial responsiveness seen in hypertension (L-type Ca 2+ channels, Src, and Rho kinase) were interrogated by isometric contraction of rat aortic rings in parallel with calcium kinetics of rat aortic smooth muscle cells. Western blots were also used to observe phosphorylation of Erk/MAPK. Chemerin-9 (nonapeptide of the chemerin S 157 isoform) caused a concentration-dependent contraction of isolated aorta (EC 50 100 nM) and elicited a concentration-dependent intracellular calcium response (EC 50 10 nM). Both calcium influx and isometric contraction, respectively, were reduced (units of “% of vehicle response”) by Pertussis toxin (G i inhibitor; 0±3% and 23±9%), verapamil (L-type Ca 2+ channel inhibitor; 38±20% and 23±4%), PP1 (Src inhibitor; 43±23% and 15±4%), and Y27632 (Rho Kinase inhibitor; 58±23% and 22±4%) but U73122 (PLC inhibitor) had little to no effect (71±31% and 71±12%). PD098059 (Erk/MAPK inhibitor) did not inhibit chemerin-9 induced contraction (117±19%) and phosphorylation did not change after chemerin-9 stimulation [1.12±0.14 (44 kDa) and 1.11±0.29 (42 kDa) fold-increase with chemerin-9 contraction compared to vehicle, p>0.05]. The chemerin receptor-selective antagonist CCX832 inhibited chemerin-9-induced calcium flux and aortic contraction and calcium flux (0.1±10.3% and 10±7%). These data support a chemerin-induced contractile mechanism in vascular smooth muscle that functions through the G i -linked chemerin receptor to activate L-type Ca 2+ channels, Src, and Rho kinase. There is mounting evidence linking chemerin to hypertension and this mechanism brings us one step closer to targeting chemerin as a unique form of therapy.

The adipokine chemerin amplifies electrical field-stimulated contraction in the isolated rat superior mesenteric artery

The adipokine chemerin causes arterial contraction and is implicated in blood pressure regulation, especially in obese subjects with elevated levels of circulating chemerin. Because chemerin is expressed in the perivascular adipose tissue (PVAT) that surrounds the sympathetic innervation of the blood vessel, we tested the hypothesis that chemerin (endogenous and exogenous) amplifies the sympathetic nervous system in mediating electrical field-stimulated (EFS) contraction. The superior mesenteric artery, with or without PVAT and with endothelium and sympathetic nerve intact, was mounted into isolated tissue baths and used for isometric contraction and stimulation. Immunohistochemistry validated a robust expression of chemerin in the PVAT surrounding the superior mesenteric artery. EFS (0.3-20 Hz) caused a frequency-dependent contraction in isolated arteries that was reduced by the chemerin receptor ChemR23 antagonist CCX832 alone (100 nM; with, but not without, PVAT), but not by the inactive congener CCX826 (100 nM). Exogenous chemerin-9 (1 μM)-amplified EFS-induced contraction in arteries (with and without PVAT) was blocked by CCX832 and the α-adrenergic receptor antagonist prazosin. CCX832 did not directly inhibit, nor did chemerin directly amplify, norepinephrine-induced contraction. Whole mount immunohistochemical experiments support colocalization of ChemR23 with the sympathetic nerve marker tyrosine hydroxylase in superior mesenteric PVAT and, to a lesser extent, in arteries and veins. These studies support the idea that exogenous chemerin modifies sympathetic nerve-mediated contraction through ChemR23 and that ChemR23 may be endogenously activated. This is significant because of the well-appreciated role of the sympathetic nervous system in blood pressure control.

An immunohistochemical analysis of SERT in the blood-brain barrier of the male rat brain

5-Hydroxytryptamine (5-HT) was originally discovered as a vasoconstrictor. 5-HT lowers blood pressure when administered peripherally to both normotensive and hypertensive male rats. Because the serotonin transporter (SERT) can function bidirectionally, we must consider whether 5-HT can be transported from the bloodstream to the central nervous system (CNS) in facilitating the fall in blood pressure. The blood-brain barrier (BBB) is a highly selective barrier that restricts movement of substances from the bloodstream to the CNS and vice versa, but the rat BBB has not been investigated in terms of SERT expression. This requires us to determine whether the BBB of the rat, the species in which we first observed a fall in blood pressure to infused 5-HT, expresses SERT. We hypothesized that SERT is present in the BBB of the male rat. To test this hypothesis, over 500 blood vessels were sampled from coronal slices of six male rat brains. Immunofluorescence of these coronal slices was used to determine whether SERT and RecA-1 (an endothelial cell marker) colocalized to the BBB. Blood vessels were considered to be capillaries if they were between 1.5 and 23 µm (intraluminal diameter). SERT was identified in the largest pial vessels of the BBB (mean ± SEM = 228.70 ± 18.71 µm, N = 9) and the smallest capillaries (mean ± SEM = 2.75 ± 0.12 µm, N = 369). SERT was not identified in the endothelium of blood vessels ranging from 20 to 135 µm (N = 45). The expression of SERT in the rat BBB means that 5-HT entry into the CNS must be considered a potential mechanism when investigating 5-HT-induced fall in blood pressure.

5-Hydroxytryptamine does not reduce sympathetic nerve activity or neuroeffector function in the splanchnic circulation

Infusion of 5-hydroxytryptamine (5-HT) in conscious rats results in a sustained (up to 30 days) fall in blood pressure. This is accompanied by an increase in splanchnic blood flow. Because the splanchnic circulation is regulated by the sympathetic nervous system, we hypothesized that 5-HT would: 1) directly reduce sympathetic nerve activity in the splanchnic region; and/or 2) inhibit sympathetic neuroeffector function in splanchnic blood vessels. Moreover, removal of the sympathetic innervation of the splanchnic circulation (celiac ganglionectomy) would reduce 5-HT-induced hypotension. In anaesthetized Sprague-Dawley rats, mean blood pressure was reduced from 101±4 to 63±3mm Hg during slow infusion of 5-HT (25μg/kg/min, i.v.). Pre- and postganglionic splanchnic sympathetic nerve activity were unaffected during 5-HT infusion. In superior mesenteric arterial rings prepared for electrical field stimulation, neither 5-HT (3, 10, 30nM), the 5-HT1B receptor agonist CP 93129 nor 5-HT1/7 receptor agonist 5-carboxamidotryptamine inhibited neurogenic contraction compared to vehicle. 5-HT did not inhibit neurogenic contraction in superior mesenteric venous rings. Finally, celiac ganglionectomy did not modify the magnitude of fall or time course of 5-HT-induced hypotension when compared to animals receiving sham ganglionectomy. We conclude it is unlikely 5-HT interacts with the sympathetic nervous system at the level of the splanchnic preganglionic or postganglionic nerve, as well as at the neuroeffector junction, to reduce blood pressure. These important studies allow us to rule out a direct interaction of 5-HT with the splanchnic sympathetic nervous system as a cause of the 5-HT-induced fall in blood pressure.

The persistence of active smooth muscle in the female rat cervix through pregnancy

OBJECTIVE

A controversy exists as to whether functional smooth muscle exists in the cervix before and during pregnancy, potentially continuous with the uterus. We hypothesized that cervical smooth muscle persists through pregnancy and is functional.

STUDY DESIGN

Uteri and cervices were taken from female virgin, 11 day, and 20 day (near labor) pregnant rats. All experiments used the uterus as a positive control. Three different smooth muscle proteins (smooth muscle α-actin, SM-22α, and calponin-1) allowed immunohistochemical detection of the continuous nature of the smooth muscle from the vagina, cervix, and uterus. Tissues were also hung in isolated tissue baths for the measurement of isometric smooth muscle contraction. Uterine and cervical homogenates were also used in Western analyses to measure protein expression.

RESULTS

Immunohistochemistry revealed there to be smooth muscle as validated by an expression of all 3 markers in the cervix. This smooth muscle was continuous with that of the vagina and uterus. Smooth muscle α-actin was detected in virgin tissue (291.3 ± 32.2 arbitrary densitometry units/β-actin), day 11 (416.8 ± 19.5), and day 20 pregnant tissue (293.0 ± 34.4). The virgin, day 11, and day 20 cervices contracted 2.18 ± 0.24 g, 1.46 ± 0.08 g, and 3.88 ± 0.49 g (respectively) to depolarizing KCl. Cervices contracted at day 20 to the cholinergic muscarinic agonist carbamylcholine (maximum, 133% ± 18.2% KCl contraction, n = 4).

CONCLUSION

These findings strongly support that smooth muscle is present in the cervix through pregnancy and continuous with the uterus.

5-HT is a potent relaxant in rat superior mesenteric veins

Serotonin (5-HT, 5-hydroxytryptamine) reduces blood pressure of the conscious rat when administered chronically (1 week). 5-HT does not directly relax isolated arteries, and microsphere experiments in 5-HT-infused rats suggested that 5-HT increased flow to the splanchnic bed. We hypothesized that 5-HT increased splanchnic flow because of direct venous relaxation; our focus was thus on the superior mesenteric vein (SMV) as an important vein in splanchnic circulation. Real-time RT-PCR, immunohistochemistry and Western analyses supported the predominant expression of the 5-HT_2B and 5-HT₇ receptor in the SMV. The SMV was mounted in tissue baths for measurement of isometric contraction. 5-HT caused a concentration-dependent relaxation of the endothelin-1 (ET-1)-contracted vein. The threshold of 5-HT-induced venous relaxation was significantly lower than for 5-HT-induced venous contraction (∼2 vs. 700 nmol/L, respectively). A series of serotonergic agonists established in their use of receptor characterization was tested, and the following rank order of potency found for agonist-induced relaxation (receptor selectivity): 5-CT (5-HT₁/5-HT₇)>5-HT = LP-44 (5-HT₇)>PNU109291 (5-HT_1D) = BW723C86 (5-HT_2B). 8-OH-DPAT (5-HT_1A/7), CP93129 (5-HT_1B), mCPBG (5-HT_3/4), AS19 (5-HT₇) and TCB-2 (5-HT_2A) did not relax the isolated vein. Consistent with these findings, two different 5-HT₇ receptor antagonists SB 269970 and LY215840 but not the 5-HT_2B receptor antagonist LY272015 nor the nitric oxide synthase inhibitor LNNA abolished 5-CT-induced relaxation of the isolated SMV. 5-CT (1 μg kg⁻¹ min⁻¹, sc) also reduced blood pressure over 7 days. These findings suggest that 5-HT directly relaxes the SMV primarily through activation of the 5-HT₇ receptor.