Myogenic Tone in Peripheral Resistance Arteries and Arterioles: The Pressure Is On!

Resistance arteries and downstream arterioles in the peripheral microcirculation contribute substantially to peripheral vascular resistance, control of blood pressure, the distribution of blood flow to and within tissues, capillary pressure, and microvascular fluid exchange. A hall-mark feature of these vessels is myogenic tone. This pressure-induced, steady-state level of vascular smooth muscle activity maintains arteriolar and resistance artery internal diameter at 50–80% of their maximum passive diameter providing these vessels with the ability to dilate, reducing vascular resistance, and increasing blood flow, or constrict to produce the opposite effect. Despite the central importance of resistance artery and arteriolar myogenic tone in cardiovascular physiology and pathophysiology, our understanding of signaling pathways underlying this key microvascular property remains incomplete. This brief review will present our current understanding of the multiple mechanisms that appear to underlie myogenic tone, including the roles played by G-protein-coupled receptors, a variety of ion channels, and several kinases that have been linked to pressure-induced, steady-state activity of vascular smooth muscle cells (VSMCs) in the wall of resistance arteries and arterioles. Emphasis will be placed on the portions of the signaling pathways underlying myogenic tone for which there is lack of consensus in the literature and areas where our understanding is clearly incomplete.

Network analysis of the left anterior descending coronary arteries in swim-trained rats by an in situ video microscopic technique

Background We aimed to identify sex differences in the network properties and to recognize the geometric alteration effects of long-term swim training in a rat model of exercise-induced left ventricular (LV) hypertrophy. Methods Thirty-eight Wistar rats were divided into four groups: male sedentary, female sedentary, male exercised and female exercised. After training sessions, LV morphology and function were checked by echocardiography. The geometry of the left coronary artery system was analysed on pressure-perfused, microsurgically prepared resistance artery networks using in situ video microscopy. All segments over > 80 μm in diameter were studied using divided 50-μm-long cylindrical ring units of the networks. Oxidative-nitrative (O-N) stress markers, adenosine A2A and estrogen receptor (ER) were investigated by immunohistochemistry. Results The LV mass index, ejection fraction and fractional shortening significantly increased in exercised animals. We found substantial sex differences in the coronary network in the control groups and in the swim-trained animals. Ring frequency spectra were significantly different between male and female animals in both the sedentary and trained groups. The thickness of the wall was higher in males as a result of training. There were elevations in the populations of 200- and 400-μm vessel units in males; the thinner ones developed farther and the thicker ones closer to the orifice. In females, a new population of 200- to 250-μm vessels appeared unusually close to the orifice. Conclusions Physical activity and LV hypertrophy were accompanied by a remodelling of coronary resistance artery network geometry that was different in both sexes.

MO069SARS-COV-2 RECEPTOR ACE-2, TMPRSS2 AND SOLUBLE ACE-2 IN PATIENTS WITH END STAGE KIDNEY DISEASE

Background and Aims Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to dramatic loss of lives due to COVID-19. Individuals with chronic conditions, including patients with kidney failure and/or kidney transplants, are affected more substantially due to multiple comorbidities and altered immune system. The first step of this infection process is the binding of SARS-CoV-2 with angiotensin-converting enzyme 2 (ACE-2) receptor, followed by its priming by transmembrane protease serine 2 (TMPRSS2). We hypothesized that the expressions of ACE-2 and TMPRSS2 are increased in microvasculature, in addition to increased circulating soluble ACE-2 levels in patients with end stage kidney disease (ESKD); i.e. prerequisites to explain to why ESKD patients are susceptible to SARS-CoV-2 infection. Therefore, we assessed if there is a difference in the expression of ACE-2 and TMPRSS2 receptors in the resistance artery and subcutaneous adipose tissue, alongside circulating soluble ACE-2 levels in ESKD patients versus controls. Method A total of 210 participants were enrolled, representing 80 ESKD and 73 healthy controls for soluble ACE-2 analysis, and 31 ESKD and 26 healthy controls for isolated subcutaneous vasculature bioassay. Immunofluorescence techniques were performed for the detection and evaluation of ACE-2 and TMPRSS2 in isolated subcutaneous resistance artery (200-300 µm of internal âCE€) and adipose tissue. Soluble ACE-2 protein concentration was detected using commercially available ELISA kits. Results Soluble ACE-2 levels were significantly higher in ESKD (3.8 ng/mL, IQR 2.4-5.5, n=80) vs control groups (2.7 ng/mL, IQR 2.1-3.7 ng/mL, n=73). There was no difference in soluble ACE-2 between females and males in either group. Soluble ACE-2 was positively correlated with IL-6 (rho=0.257, p=0.02, n=80), while it was negatively correlated with cholesterol (rho= -0.248, p=0.02, n=78) in ESKD patients. The expression of ACE-2 receptor was observed both on endothelium and vascular smooth muscle cells (VSMCs) in arteries from both groups. The expression was higher in ESKD patients (19.1%, n=23) vs controls (15.4%, n=15). Patients with ESKD on ACE-inhibitor/angiotensin receptor blocker treatment showed higher expression of ACE-2 vs. non-treatment group (treatment: 20.2%, n=12 vs non-treatment: 12.8%, n=11) in resistance artery. In subcutaneous adipose tissue the ACE-2 staining was not statistically different among the groups (ESKD: 2.9%, n=10 vs controls: 3.6%, n=10). In addition, TMPRSS2 was expressed both on endothelium and VSMCs in resistance artery, however there was no difference in the expression (ESKD: 8.4%, n=23 vs controls: 10.2%, n=15) between the groups. Conclusion Soluble ACE-2 levels and ACE-2 receptor expression in the vasculature were higher in patients with ESKD as compared to controls. The ACE-2 receptor is present both in the endothelium and VSMCs from arteries in peripheral microcirculation. This supports the suggestion that the uremic milieu induces an optimal environment for SARS-CoV-2 entrance in microcirculation with following consequence on the vasculature during the COVID-19. Similarly, TMPRSS2 expression was observed in vessels from both groups, while increased expression of ACE-2 receptor was observed in those ESKD patients receiving ACE-inhibitor/angiotensin receptor blocker treatment. Further studies are warranted to assess possible sex differences in the target receptor expressions with further elaboration on specific treatment regime(s) for different comorbidities present in patients with ESKD.

Endothelial alpha globin is a nitrite reductase

Small artery vasodilation in response to hypoxia is essential for matching oxygen supply to tissue oxygen demand. One source of hypoxic dilation via nitric oxide (NO) signaling is nitrite reduction by erythrocytic hemoglobin (α2β2). However, the alpha subunit of hemoglobin is also expressed in resistance artery endothelium and localized to myoendothelial junctions, a subcellular domain that contacts underlying vascular smooth muscle cells. We hypothesized that nitrite reduction mediated by endothelial alpha globin may occur at myoendothelial junctions to regulate hypoxic vasodilation. To test this concept, we created two novel mouse strains: one lacking alpha globin specifically in endothelium (EC Hba1Δ/Δ) and one where alpha globin is mutated such that its inhibitory association with endothelial NO synthase (eNOS) is prevented (Hba1WT/Δ36-39). In EC Hba1Δ/Δ or Hba1WT/Δ36-39 mice hemoglobin levels, hematocrit and erythrocyte counts were unchanged from littermate controls. Loss of the full alpha globin protein from the endothelium in the EC Hba1Δ/Δ model was associated with decreased exercise capacity and decreased intracellular nitrite utilization in hypoxic conditions. These effects were not seen in Hba1WT/Δ36-39 animals. Hypoxia induced vasodilation was decreased by 60% in isolated thoracodorsal arteries from EC Hba1Δ/Δ, while infusion of erythrocytes only partially rescued the dilatory response. Lastly, unlike other models where blood pressure is decreased, EC Hba1Δ/Δ blood pressure was not altered in response to hypoxia. Overall, we conclude that alpha globin in the resistance artery endothelium can act as a nitrite reductase to provide a local vasodilatory response to hypoxia.

Fetal Undernutrition Induces Resistance Artery Remodeling and Stiffness in Male and Female Rats Independent of Hypertension

Fetal undernutrition programs hypertension and cardiovascular diseases, and resistance artery remodeling may be a contributing factor. We aimed to assess if fetal undernutrition induces resistance artery remodeling and the relationship with hypertension. Sprague–Dawley dams were fed ad libitum (Control) or with 50% of control intake between days 11 and 21 of gestation (maternal undernutrition, MUN). In six-month-old male and female offspring we assessed blood pressure (anesthetized and tail-cuff); mesenteric resistance artery (MRA) structure and mechanics (pressure myography), cellular and internal elastic lamina (IEL) organization (confocal microscopy) and plasma MMP-2 and MMP-9 activity (zymography). Systolic blood pressure (SBP, tail-cuff) and plasma MMP activity were assessed in 18-month-old rats. At the age of six months MUN males exhibited significantly higher blood pressure (anesthetized or tail-cuff) and plasma MMP-9 activity, while MUN females did not exhibit significant differences, compared to sex-matched controls. MRA from 6-month-old MUN males and females showed a smaller diameter, reduced adventitial, smooth muscle cell density and IEL fenestra area, and a leftward shift of stress-strain curves. At the age of eighteen months SBP and MMP-9 activity were higher in both MUN males and females, compared to sex-matched controls. These data suggest that fetal undernutrition induces MRA inward eutrophic remodeling and stiffness in both sexes, independent of blood pressure level. Resistance artery structural and mechanical alterations can participate in the development of hypertension in aged females and may contribute to adverse cardiovascular events associated with low birth weight in both sexes.