Diabetes and the Microvasculature of the Bone and Marrow.

PURPOSE OF REVIEW: The purpose of this review is to highlight the evidence of microvascular dysfunction in bone and marrow and its relation to poor skeletal outcomes in diabetes mellitus. RECENT FINDINGS: Diabetes mellitus is characterized by chronic hyperglycemia, which may lead to microangiopathy and macroangiopathy. Micro- and macroangiopathy have been diagnosed in Type 1 and Type 2 diabetes, coinciding with osteopenia, osteoporosis, enhanced fracture risk and delayed fracture healing. Microangiopathy has been reported in the skeleton, correlating with reduced blood flow and perfusion, vasomotor dysfunction, microvascular rarefaction, reduced angiogenic capabilities, and augmented vascular permeability. Microangiopathy within the skeleton may be detrimental to bone and manifest as, among other clinical abnormalities, reduced mass, enhanced fracture risk, and delayed fracture healing. More investigations are required to elucidate the various mechanisms by which diabetic microvascular dysfunction impacts the skeleton.

Myogenic autoregulation in bone marrow arterioles and in vivo intramedullary pressure in femora of conscious, female Long Evans rats.

OBJECTIVES: The ability to regulate skeletal blood flow is critical for the maintenance of bone. The myogenic response is essential for regulating tissue blood flow. Myogenic responsiveness in bone marrow arterioles has not yet been determined. Furthermore, the literature is disparate regarding intramedullary pressures (IMP) within bone. The purposes of this study were to (1) determine whether bone marrow arterioles have myogenic activity and (2) assess if the autoregulatory zone corresponds with IMP. Also, this study provides detailed methodology on dissecting and isolating bone marrow arterioles for functional assessment. METHODS: Experiment 1: Femoral shafts of female Long Evans rats were catheterized to assess in vivo IMP. Experiment 2: Bone marrow arterioles from female Long Evans rats were cannulated. Active and passive myogenic responses were determined. RESULTS: In vivo intramedullary pressure averaged 32 ± 3 mmHg, intramedullary pulse pressure averaged 5.28 ± 0.03 mmHg, and the mean maximal diameter and wall thickness of the bone marrow arterioles were 96 ± 7 µm and 18 ± 2 µm, respectively. An active myogenic response was observed and differed (p < .001) from the passive curve. CONCLUSION: Bone marrow arterioles have myogenic responsiveness and the autoregulatory zone corresponded with the range of IMP (15-51 mmHg) within the femoral diaphysis of conscious animals.

Short-term intermittent parathyroid hormone (1-34) administration increased angiogenesis and matrix metalloproteinase 9 in femora of mature and middle-aged C57BL/6 mice.

NEW FINDINGS: What is the central question of this study? We sought to assess the effects of intermittent parathyroid hormone (1-34) administration on bone angiogenesis, the redistribution of bone marrow blood vessels, and matrix metalloproteinase 9 as a function of advancing age in mice. What is the main finding and its importance? Short-term (i.e. 10 days) intermittent parathyroid hormone (1-34) administration increased the number of small (≤29-µm-diameter) bone marrow blood vessels and augmented matrix metalloproteinase 9. These changes occurred before alterations in trabecular bone. Given the rapid response in bone angiogenesis, this investigation highlights the impact of intermittent parathyroid hormone (1-34) administration on the bone vascular network. ABSTRACT: Intermittent parathyroid hormone (PTH) administration augments bone, stimulates the production of matrix metalloproteinase 9 (Mmp9) and relocates bone marrow blood vessels closer to osteoid seams. Discrepancies exist, however, regarding bone angiogenesis. Given that Mmp9 participates in cellular homing and migration, it might aid in blood vessel relocation. We examined the influence of short-term intermittent PTH administration on angiogenesis, Mmp9 secretion and the distance between blood vessels and bone. Mature (6- to 8-month-old) and middle-aged (10- to 12-month-old) male and female C57BL/6 mice were divided into three groups: control (CON), and 5 (5dPTH) and 10 days (10dPTH) of intermittent PTH administration. Mice were given PBS (50 µl day-1 ) or PTH(1-34) (43 µg kg-1  day-1 ). Frontal sections (5 µm thick) of the right distal femoral metaphysis were triple-immunolabelled to identify endothelial cells (anti-CD31), vascular smooth muscle cells (anti-αSMA) and Mmp9 (anti-Mmp9). Vascular density, Mmp9 density, area and localization, and blood vessel distance from bone were analysed. Blood vessels were analysed according to diameter: 1-29, 30-100 and 101-200 µm. Trabecular bone microarchitecture and bone static and dynamic properties were assessed. No main effects of age were observed for any variable. The density of CD31-labelled blood vessels 1-29 and 30-100 µm in diameter was higher (P < 0.05) and tended (P = 0.055) to be higher, respectively, in 10dPTH versus 5dPTH and CON. Mmp9 was augmented (P < 0.05) in 10dPTH versus the other groups. Mmp9 was closer (P < 0.05) to blood vessels 1-29 µm in diameter and furthest (P < 0.05) from bone. In conclusion, bone angiogenesis occurred by day 10 of intermittent PTH administration, coinciding with augmented Mmp9 secretion near the smallest blood vessels (1-29 µm in diameter).

Progressive ossification of the bone marrow vasculature with advancing age corresponds with reduced red blood cell count and percentage of circulating lymphocytes in male Fischer-344 rats.

OBJECTIVE: Assess the link between bone marrow blood vessel ossification, Tb. and cortical bone, and hematological parameters across the lifespan in rats. METHODS: Right femora and whole blood samples were taken from male Fischer-344 rats at 1, 6, 12, 18 and 24 months. Femora were scanned by micro-computed tomography (MicroCT) to determine bone marrow blood vessel ossification (ie, ossified vessel volume [OsVV], ossified vessel thickness (OsV.Th), ossified vessel density (OsV density), and structural model index [SMI]). Bone microarchitecture (ie, bone volume [BV/TV], trabecular thickness, trabecular number, and trabecular separation), density and SMI, and cortical bone parameters (ie, cortical shell thickness, porosity, and density) were also determined by MicroCT. Complete blood counts with differentials were conducted. RESULTS: Ossified vessel volume increased throughout the lifespan, coinciding with reduced trabecular BV/TV and cortical shell thickness at 24 months. Many of the hematological parameters were unchanged (ie, white blood cells, lymphocyte number) or increased (monocyte number, percent monocyte, granulocyte number, percent granulocyte, hemoglobin, hematocrit, mean corpuscular hemoglobin concentration, red blood cell distribution width, platelet, mean platelet volume) with advancing age; however, red blood cells (RBC) and percent lymphocytes (LY%) were reduced at 24 months. In addition, OsV density was similar to trabecular bone density. CONCLUSIONS: Declines in trabecular BV/TV, cortical shell thickness, RBC, and LY% with advanced age coincided with augmented ossification of bone marrow blood vessels.

Discovery of a bone-like blood particle in the peripheral circulation of humans and rodents.

OBJECTIVE: To characterize ossified bone marrow blood vessels and confirm the presence of ossified particles (OSP) in humans and rodents. METHODS: Human bone marrow blood vessels were processed for scanning and transmission electron microscopy. Whole blood samples were collected from younger (26-39 years; n = 6) and older (55-63 years; n = 6) volunteers and male Fischer-344 rats (1 month, n = 7; 6 months, n = 7; 12 months, n = 7; 18-months, n = 6; 24 months, n = 8). OSP in the whole blood samples were sorted and imaged with microscopy to determine diameter, circularity, and solidity. Additionally, the chemical composition of OSP was determined via elemental analysis. RESULTS: SEM revealed two types of ossified bone marrow blood vessels: that is, "transitioning" and "ossified." OSP were adhered to the surface of transitioning vessels and theoretically gain access to and circulate within the blood. The majority of OSP were ≤15 µm in diameter, but many were of sufficient size to serve as emboli (ie, >15 µm).OSP were predominately oblong in shape and several had jagged tips and edges. CONCLUSIONS: We introduce a novel, bone-like blood particle that may be diagnostic of bone marrow blood vessel ossification. Further, OSP may associate with several disease states (eg, atherosclerosis).

Influence of estradiol supplementation on neuropeptide Y neurotransmission in skeletal muscle arterioles of F344 rats.

The effects of estradiol on neuropeptide Y (NPY) neurotransmission in skeletal muscle resistance vessels have not been described. The purpose of this study was to determine the effects of long-term estradiol supplementation on NPY overflow, degradation, and vasoconstriction in gastrocnemius first-order arterioles of adult female rats. Female rats (4 mo; n = 34) were ovariectomized (OVX) with a subset (n = 17) receiving an estradiol pellet (OVE; 17ß-estradiol, 4 µg/day). After conclusion of the treatment phase (8 wk), arterioles were excised, placed in a physiological saline solution (PSS) bath, and cannulated with micropipettes connected to albumin reservoirs. NPY-mediated vasoconstriction via a Y(1)-agonist [Leu31Pro34]NPY decreased vessel diameter 44.54 ± 3.95% compared with baseline; however, there were no group differences in EC(50) (OVE: -8.75 ± 0.18; OVX: -8.63 ± 0.10 log M [Leu31Pro34]NPY) or slope (OVE: -1.11 ± 0.25; OVX: -1.65 ± 0.34% baseline/log M [Leu31Pro34]NPY). NPY did not potentiate norepinephrine-mediated vasoconstriction. NPY overflow experienced a slight increase following field stimulation and significantly increased (P < 0.05) over control conditions in the presence of a DPPIV inhibitor (diprotin A). Estradiol status did not affect DPPIV activity. These data suggest that NPY can induce a moderate decrease in vessel diameter in skeletal muscle first-order arterioles, and DPPIV is active in mitigating NPY overflow in young adult female rats. Long-term estradiol supplementation did not influence NPY vasoconstriction, overflow, or its enzymatic breakdown in skeletal muscle first-order arterioles.

Old and new biomarkers for volumetric muscle loss.

Volumetric muscle loss (VML) impacts skeletal muscles and causes damage to associated tissues such as blood vessels and other structural tissues. Despite progress in the VML field, current preclinical approaches are often ineffective at restoring muscle volume. Additional research is paramount to develop strategies that improve muscle mass and function, while restoring supporting tissues. We highlight mechanisms that govern normal muscle function that are also key players for VML, including intracellular calcium signaling/homeostasis, mitochondria signaling (calcium, reactiove oxidative species (ROS)/oxidative stress), and angiogenesis. We propose an integration of these processes within the context of emerging biomaterials that provide structural support for muscle regeneration. We posit that new biomarkers (i.e. myokines and lipid signaling mediators) may serve as sentinels of early muscle injury and regeneration. We conclude that as new ideas, approaches, and models come together, new treatments will emerge to allow the full rebuilding of skeletal muscles and functional recovery of skeletal muscles after VML.

Bone Marrow Microvasculature.

The skeleton is highly vascularized due to the various roles blood vessels play in the homeostasis of bone and marrow. For example, blood vessels provide nutrients, remove metabolic by-products, deliver systemic hormones, and circulate precursor cells to bone and marrow. In addition to these roles, bone blood vessels participate in a variety of other functions. This article provides an overview of the afferent, exchange and efferent vessels in bone and marrow and presents the morphological layout of these blood vessels regarding blood flow dynamics. In addition, this article discusses how bone blood vessels participate in bone development, maintenance, and repair. Further, mechanical loading-induced bone adaptation is presented regarding interstitial fluid flow and pressure, as regulated by the vascular system. The role of the sympathetic nervous system is discussed in relation to blood vessels and bone. Finally, vascular participation in bone accrual with intermittent parathyroid hormone administration, a medication prescribed to combat age-related bone loss, is described and age- and disease-related impairments in blood vessels are discussed in relation to bone and marrow dysfunction. © 2020 American Physiological Society. Compr Physiol 10:1009-1046, 2020.

An Implanted Magnetic Microfluidic Pump for In Vivo Bone Remodeling Applications.

Modulations of fluid flow inside the bone intramedullary cavity has been found to stimulate bone cellular activities and augment bone growth. However, study on the efficacy of the fluid modulation has been limited to external syringe pumps connected to the bone intramedullary cavity through the skin tubing. We report an implantable magnetic microfluidic pump which is suitable for in vivo studies in rodents. A compact microfluidic pump (22 mm diameter, 5 mm in thickness) with NdFeB magnets was fabricated in polydimethylsiloxane (PDMS) using a set of stainless-steel molds. An external actuator with a larger magnet was used to wirelessly actuate the magnetic microfluidic pump. The characterization of the static pressure of the microfluidic pump as a function of size of magnets was assessed. The dynamic pressure of the pump was also characterized to estimate the output of the pump. The magnetic microfluidic pump was implanted into the back of a Fischer-344 rat and connected to the intramedullary cavity of the femur using a tube. On-demand wireless magnetic operation using an actuator outside of the body was found to induce pressure modulation of up to 38 mmHg inside the femoral intramedullary cavity of the rat.

The cardiovascular disease burden of non-traumatic fractures for adults with and without cerebral palsy.

BACKGROUND: Individuals with cerebral palsy (CP) are vulnerable to non-trauma fracture (NTFx) and have an elevated burden of cardiovascular disease (CVD) related morbidity and mortality. However, very little is known about the contribution of NTFx to CVD risk among adults with CP. The purpose of this study was to determine if NTFx is a risk factor for CVD among adults with CP and if NTFx exacerbates CVD risk compared to adults without CP. METHODS: Data from 2011 to 2016 Optum Clinformatics® Data Mart and a random 20% sample Medicare fee-for-service were used for this retrospective cohort study. Diagnosis codes were used to identify adults (18+ years) with and without CP, NTFx, incident CVD up to 2 years (i.e., ischemic heart disease, heart failure, cerebrovascular disease), and pre-NTFx comorbidities. Crude incidence rates per 100 person years of CVD measures were estimated. Cox regression estimated hazard ratios (HR and 95% confidence interval [CI]) for CVD measures, comparing: (1) CP and NTFx (CP + NTFx; n = 1012); (2) CP without NTFx (CP w/o NTFx; n = 8345); (3) without CP and with NTFx (w/o CP + NTFx; n = 257,355); and (4) without CP and without NTFx (w/o CP w/o NTFx; n = 4.8 M) after adjusting for demographics and pre-NTFx comorbidities. RESULTS: The crude incidence rate was elevated for CP + NTFx vs. CP w/o NTFx and w/o CP + NTFx for any CVD and for each CVD subtype. After adjustments, the HR was elevated for CP + NTFx vs. CP w/o NTFx for any CVD (HR = 1.16; 95%CI = 0.98-1.38), heart failure (HR = 1.31; 95%CI = 1.01-1.70), and cerebrovascular disease (HR = 1.23; 95%CI = 0.98-1.55); although, only heart failure was statistically significant. The adjusted HR was elevated for CP + NTFx vs. w/o CP + NTFx for any CVD and for each CVD subtype (all P < .05). Stratified analyses showed a higher CVD risk by NTFx location, <65 year olds, and men when comparing CP + NTFx vs. CP w/o NTFx and w/o CP + NTFx. CONCLUSIONS: NTFx increases 2-year CVD risk among adults with CP and compared to adults without CP. Findings suggest that NTFx is a risk factor for CVD among adults with CP.

Low-Trauma Fracture Increases 12-Month Incidence of Cardiovascular Disease for Adults With Cerebral Palsy.

Individuals with cerebral palsy (CP) have poor skeletal and cardiovascular health. However, no studies have examined if skeletal fragility enhances cardiovascular disease (CVD) risk for this population. The purpose of this study was to determine whether adults with CP have higher 12-month CVD incidence following a low-trauma fracture compared with adults without CP. Data, from the Optum Clinformatics® Data Mart, were extracted from adults (18+ years) that sustained a low-trauma fracture between 01/01/2012 and 12/31/2016. The primary outcome measure was incident CVD within 12 months following a low-trauma fracture. Cox proportional hazards regression models were used to compare 12-month incident CVD with adjustment for sociodemographics and chronic disease comorbidities. Mean age (SD) at baseline was 54.7 (18.9) for adults with CP (n = 1,025, 43.3% men) and 60.4 (19.7) for adults without CP (n = 460,504, 33.7% men). During the follow-up, 121 adults with CP (11.8%, mean age [SD] = 63.9 [16.3]) and 45,330 adults without CP (9.8%, mean age [SD] = 74.5 [11.9]) developed CVD. In the fully adjusted model, adults with CP had higher 12-month post-fracture CVD incidence (hazard ratio [HR] = 1.63; 95% confidence interval [CI] = 1.37-1.95). When the outcome was stratified by CVD subtype, adults with CP had higher 12-month post-fracture incidence of ischemic heart disease (HR = 1.45; 95% CI = 1.09-1.92), heart failure (HR = 1.68; 95% CI = 1.22-2.31), and cerebrovascular disease (HR = 1.96; 95% CI = 1.54-2.50). Study findings suggest that among adults with CP, low-trauma fracture may enhance 12-month CVD incidence compared with adults without CP. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:803-810, 2020.

Short-term intermittent PTH 1-34 administration and bone marrow blood vessel ossification in Mature and Middle-Aged C57BL/6 mice.

Intermittent parathyroid hormone (PTH) administration augments bone and progressive bone marrow blood vessel (BMBV) ossification occurs with advancing age. Since intermittent PTH administration augments bone, it may also serve to increase BMBV ossification. We assessed the influence of 5- and 10-days of intermittent PTH 1-34 administration on trabecular and cortical bone and BMBV ossification in mature (6-8 mon; n = 30) and middle-aged (10-12 mon; n = 30) male and female C57BL/6 mice. Mice were divided accordingly: control (CON) and 5-days (5dPTH) and 10-days (10dPTH) of PTH. Mice were given PBS (50 µl) or PTH 1-34 (43 µg/kg/d) for 5- and 10-consecutive days. Trabecular bone microarchitecture (i.e., BV/TV [%], Tb.Th [µm], Tb.N [/mm], and Tb.Sp [µm]) was assessed in the distal femoral metaphysis and cortical bone parameters (i.e., Ct.Th [µm] and CSMI [mm4]) at the femoral mid-shaft. BMBV ossification (i.e., ossified vessel volume [OsVV, %] and ossified vessel thickness [OsV.Th, µm]) was assessed in the medullary cavity of the femoral shaft. All parameters were determined by µCT. At this sample size, no gender-related differences were observed so female and male data were pooled. There were no main effects nor interactions for trabecular microarchitecture and Ct.Th. However, CSMI was larger (p < 0.05) in Middle-Age vs. Mature and larger (p < 0.05) in CON and 10dPTH vs. 5dPTH. OsVV tended (p = 0.057) to be higher (0.18 ±â€¯0.04% vs. 0.09 ±â€¯0.02%, respectively) and OsV.Th was higher (p < 0.05; 17.4 ±â€¯1.6 µm vs. 12.1 ±â€¯1.4 µm, respectively) in Middle-Aged vs. Mature mice. OsVV was not altered, but ossified vessels tended (p = 0.08) to be thicker in 10dPTH (17.6 ±â€¯2.0 µm) vs. CON (12.5 ±â€¯1.7 µm). No interactions were observed for OsVV and OsV.Th. In conclusion, this is the first report of ossified BMBV in C57BL/6 mice. The increased OsV.Th in Middle-Aged mice coincides with previous reports of increased OsVV in aged rats. The tendency of augmented OsV.Th in 10dPTH suggests that this treatment may ultimately impair the patency of bone marrow blood vessels.

Age, gender, and hormonal status modulate the vascular toxicity of the diesel exhaust extract phenanthraquinone.

Inhaled airborne pollutants such as particulate matter increase the susceptibility to adverse health consequences and cardiovascular events. Diesel exhaust contributes significantly to the ambient particle pollution burden. The purpose of this investigation was to determine if exposure to a common component of diesel exhaust, phenanthraquinone (PQ), impairs endothelium-dependent vasodilation of the femoral principal nutrient artery and to determine whether age, gender, and/or hormonal status alter the putative effects of PQ on vasodilation. Vasodilation to acetylcholine (ACh) was assessed in vitro in intact control (age 6, 14, and 24 mo) and ovariectomized (age 6, 14, and 24 mo) female rats and intact (age 6 and 24 mo) male rats. Gender did not influence vasodilator capacity of the femoral principal nutrient artery, and there was an age-related decline in endothelium-dependent vasodilation in both female and male 24-mo-old rats. Exposure to PQ elicited a gender-specific affect in 6-mo-old rats; i.e., vasodilation was impaired 63% in male rats but had no effect in female rats. Exposure to PQ abolished vasodilation in 14- and 24-mo-old rats of both genders, and ovariectomy compromised vasodilator responsiveness to ACh in all age groups. The data demonstrate a vasoprotective mechanism in young female rats that may be related to endogenous ovarian hormones and provides evidence that suggests certain subsets of the population (e.g., elderly, males, and postmenopausal women) may be more susceptible to the adverse consequences of airborne pollutants.

Intermittent PTH 1-34 administration improves the marrow microenvironment and endothelium-dependent vasodilation in bone arteries of aged rats.

Inflammation coincides with diminished marrow function, vasodilation of blood vessels, and bone mass. Intermittent parathyroid hormone (PTH) administration independently improves marrow and vascular function, potentially impacting bone accrual. Currently, the influence of marrow and intermittent PTH administration on aged bone blood vessels has not been examined. Vasodilation of the femoral principal nutrient artery (PNA) was assessed in the presence and absence of marrow. Furthermore, we determined the influence of PTH 1-34 on 1) endothelium-dependent vasodilation and signaling pathways [i.e., nitric oxide (NO) and prostacyclin (PGI2)], 2) endothelium-independent vasodilation, 3) cytokine production by marrow cells, and 4) bone microarchitecture and bone static and dynamic properties. Young (4-6 mo) and old (22-24 mo) male Fischer-344 rats were treated with PTH 1-34 or a vehicle for 2 wk. In the absence and presence of marrow, femoral PNAs were given cumulative doses of acetylcholine, with and without the NO and PGI2 blockers, and diethylamine NONOate. Marrow-derived cytokines and bone parameters in the distal femur were assessed. Exposure to marrow diminished endothelium-dependent vasodilation in young rats. Reduced bone volume and NO-mediated vasodilation occurred with old age and were partially reversed with PTH. Additionally, PTH treatment in old rats restored endothelium-dependent vasodilation in the presence of marrow and augmented IL-10, an anti-inflammatory cytokine. Endothelium-independent vasodilation was unaltered, and PTH treatment reduced osteoid surfaces in old rats. In conclusion, the marrow microenvironment reduced vascular function in young rats, and PTH treatment improved the marrow microenvironment and vasodilation with age. NEW & NOTEWORTHY This study investigated the influence of the marrow microenvironment on bone vascular function in young and old rats. An inflamed marrow microenvironment may reduce vasodilator capacity of bone blood vessels, diminishing delivery of blood flow to the skeleton. In young rats, the presence of the marrow reduced vasodilation in the femoral principal nutrient artery (PNA). However, intermittent parathyroid hormone administration (i.e., a treatment for osteoporosis) improved the marrow microenvironment and vasodilator capacity in old PNAs.

Aging reduces skeletal blood flow, endothelium-dependent vasodilation, and NO bioavailability in rats.

UNLABELLED: We determined whether aging diminishes bone blood flow and impairs endothelium-dependent vasodilation. Femoral perfusion was lower in old animals, as well as endothelium-dependent vasodilation and NO bioavailability. These effects could contribute to old age-related bone loss and the increased risk of fracture. INTRODUCTION: Aging has been shown to diminish bone blood flow in rats and humans. The purpose of this study was to determine whether blood flow to regions of the femur perfused primarily through the principal nutrient artery (PNA) are diminished with aging and whether this putative reduction in flow is associated with impaired endothelium-dependent vasodilation. MATERIALS AND METHODS: Blood flow was measured in conscious young adult (4-6 mo old) and aged (24-26 mo old) male Fischer-344 rats using radiolabeled microspheres. Endothelium-dependent vasodilation of the PNA was assessed in vitro using acetylcholine (ACh), whereas the contribution of the NO synthase (NOS) and cyclooxygenase (COX) signaling pathways to endothelium-dependent vasodilation was determined using the NOS and COX inhibitors L-NAME and indomethacin, respectively. RESULTS: Femoral blood flow in the aged rats was 21% and 28% lower in the proximal and distal metaphyses, respectively, and 45% lower in the diaphyseal marrow. Endothelium-dependent vasodilation was reduced with old age (young: 83 +/- 6% maximal relaxation; aged: 62 +/- 5% maximal relaxation), whereas endothelium-independent vasodilation (sodium nitroprusside) was unaffected by age. The reduction in endothelium-dependent vasodilation was mediated through impairment of the NOS signaling pathway, which resulted in lower NO bioavailability (young: 168 +/- 56 nM; aged: 50 +/- 7 nM). CONCLUSIONS: These data show that reductions in metaphyseal bone and diaphyseal marrow perfusion with old age are associated with diminished endothelium-dependent vasodilation through an impairment of the NOS mechanism. Such age-related changes in bone perfusion and vascular NO signaling could impact clinical bone loss, increase risk of fracture, and impair fracture healing in the elderly.

Effects of aging on adipose resistance artery vasoconstriction: possible implications for orthostatic blood pressure regulation.

The purpose of this investigation was to determine mean arterial pressure (MAP) and regional vascular conductance responses in young and aged Fisher-344 rats during orthostatic stress, i.e., 70 degrees head-up tilt (HUT). Both groups demonstrated directionally different changes in MAP during HUT (young, 7% increase; aged, 7% decrease). Vascular conductance during HUT in young rats decreased in most tissues but largely remained unchanged in the aged animals. Based on the higher vascular conductance of white adipose tissue from aged rats during HUT, resistance arteries from white visceral fat were isolated and studied in vitro. There was diminished maximal vasoconstriction to phenylephrine and norepinephrine (NE: young, 42 +/- 5%; old, 18 +/- 6%) in adipose resistance arteries from aged rats. These results demonstrate that aging reduces the ability to maintain MAP during orthostatic stress, and this is associated with a diminished vasoconstriction of adipose resistance arteries.

Mechanical, hormonal and metabolic influences on blood vessels, blood flow and bone.

Bone tissue is highly vascularized due to the various roles bone blood vessels play in bone and bone marrow function. For example, the vascular system is critical for bone development, maintenance and repair and provides O2, nutrients, waste elimination, systemic hormones and precursor cells for bone remodeling. Further, bone blood vessels serve as egress and ingress routes for blood and immune cells to and from the bone marrow. It is becoming increasingly clear that the vascular and skeletal systems are intimately linked in metabolic regulation and physiological and pathological processes. This review examines how agents such as mechanical loading, parathyroid hormone, estrogen, vitamin D and calcitonin, all considered anabolic for bone, have tremendous impacts on the bone vasculature. In fact, these agents influence bone blood vessels prior to influencing bone. Further, data reveal strong associations between vasodilator capacity of bone blood vessels and trabecular bone volume, and poor associations between estrogen status and uterine mass and trabecular bone volume. Additionally, this review highlights the importance of the bone microcirculation, particularly the vascular endothelium and NO-mediated signaling, in the regulation of bone blood flow, bone interstitial fluid flow and pressure and the paracrine signaling of bone cells. Finally, the vascular endothelium as a mediator of bone health and disease is considered.

Intermittent parathyroid hormone administration attenuates endothelial dysfunction in old rats.

Aging is an independent risk factor for cardiovascular disease and is characterized by a decline in endothelial function. Parathyroid hormone (PTH) administration has been shown to increase endothelial nitric oxide synthase (eNOS) expression. The purpose of this investigation was to determine the effect of intermittent PTH administration on aortic endothelial function in old rodents. We hypothesized that intermittent PTH administration would improve endothelial function in older rodents. Old (24-mo-old) and young (4-mo-old) Fischer-344 rats were given 10 injections of PTH 1-34 (43 µg·kg-1·day-1) or phosphate-buffered saline (100 µl/day) over 15 days. Endothelium-dependent relaxation of aortic rings in response to acetylcholine (10-9 to 10-5 M) was significantly impaired in old control (OC) compared with young control (YC) as indicated by a reduced area under the curve (AUC, 100 ± 6.28 vs. 54.08 ± 8.3%; P < 0.05) and impaired maximal relaxation (Emax, 70.1 ± 4.48 vs. 92.9 ± 4.38%; P < 0.05). Emax was improved in old animals treated with PTH (OPTH) (OC, 70.1 ± 4.48 vs. OPTH, 85 ± 7.48%; P < 0.05) as well as AUC (OC, 54.08 ± 8.3 vs. OPTH, 82.5 ± 5.7%; P < 0.05) while logEC50 was not different. Endothelial-independent relaxation in response to sodium nitroprusside was not different among groups. Aortic eNOS protein expression was significantly decreased in OC compared with YC (P < 0.05). PTH treatment restored eNOS expression in OPTH animals (P < 0.05). These data suggest that PTH may play a role in attenuating age-related impairments in aortic endothelial function. NEW & NOTEWORTHY: We have demonstrated that intermittent parathyroid hormone administration can rescue age-related vascular dysfunction by improving endothelial-dependent dilation in the aorta of older rodents. This demonstrates a novel potential benefit of parathyroid hormone administration in aging.

Endothelium-dependent vasodilation of cerebral arteries is altered with simulated microgravity through nitric oxide synthase and EDHF mechanisms.

Cephalic elevations in arterial pressure associated with microgravity and prolonged bed rest alter cerebrovascular autoregulation in humans. Using the head-down tail-suspended (HDT) rat to chronically induce headward fluid shifts and elevate cerebral artery pressure, previous work has likewise shown cerebral perfusion to be diminished. The purpose of this study was to test the hypothesis that 2 wk of HDT reduces cerebral artery vasodilation. To test this hypothesis, dose-response relations for endothelium-dependent (2-methylthioadenosine triphosphate and bradykinin) and endothelium-independent (nitroprusside) vasodilation were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (10(-5) M) and cyclooxygenase inhibitor indomethacin (10(-5) M). MCA caveolin-1 protein content was measured by immunoblot analysis. Endothelium-dependent vasodilation to 2-methylthioadenosine triphosphate and bradykinin were both lower in MCAs from HDT rats. These lower vasodilator responses were abolished with N(G)-nitro-L-arginine methyl ester but were unaffected by indomethacin. In addition, HDT was associated with lower levels of MCA caveolin-1 protein. Endothelium-independent vasodilation was not altered by HDT. These results indicate that chronic cephalic fluid shifts diminish endothelium-dependent vasodilation through alterations in the endothelial nitric oxide synthase signaling mechanism. Such decrements in endothelium-dependent vasodilation of cerebral arteries could contribute to the elevations in cerebral vascular resistance and reductions in cerebral perfusion that occur after conditions of simulated microgravity in HDT rats.

Effects of hindlimb unloading and ionizing radiation on skeletal muscle resistance artery vasodilation and its relation to cancellous bone in mice.

Spaceflight has profound effects on vascular function as a result of weightlessness that may be further compounded by radiation exposure. The purpose of the present study was to assess the individual and combined effects of hindlimb unloading (HU) and radiation (Rad) on vasodilator responses in the skeletal muscle vasculature. Adult male C57BL/6J mice were randomized to one of four groups: control (Con), HU (tail suspension for 15 days), Rad (200 cGy of (137)Cs), and HU-Rad (15-day tail suspension and 200 cGy of (137)Cs). Endothelium-dependent vasodilation of gastrocnemius feed arteries was assessed in vitro using acetylcholine (ACh, 10(-9)-10(-4) M) and inhibitors of nitric oxide synthase (NOS) and cyclooxygenase (COX). Endothelium-independent vasodilation was assessed using Dea-NONOate (10(-9)-10(-4) M). Endothelium-dependent and -independent vasodilator responses were impaired relative to Con responses in all treatment groups; however, there was no further impairment from the combination of treatments (HU-Rad) relative to that in the HU and Rad groups. The NOS-mediated contribution to endothelium-dependent vasodilation was depressed with HU and Rad. This impairment in NOS signaling may have been partially compensated for by an enhancement of PGI2-mediated dilation. Changes in endothelium-dependent vasodilation were also associated with decrements in trabecular bone volume in the proximal tibia metaphysis. These data demonstrate that the simulated space environment (i.e., radiation exposure and unloading of muscle and bone) significantly impairs skeletal muscle artery vasodilation, mediated through endothelium-dependent reductions in NOS signaling and decrements in vascular smooth muscle cell responsiveness to NO.