Zhenxing Fu

Epidermal Sensing of Oxygen Is Essential for Systemic Hypoxic Response

Skin plays an essential role, mediated in part by its remarkable vascular plasticity, in adaptation to environmental stimuli. Certain vertebrates, such as amphibians, respond to hypoxia in part through the skin; but it is unknown whether this tissue can influence mammalian systemic adaptation to low oxygen levels. We have found that epidermal deletion of the hypoxia-responsive transcription factor HIF-1alpha inhibits renal erythropoietin (EPO) synthesis in response to hypoxia. Conversely, mice with an epidermal deletion of the von Hippel-Lindau (VHL) factor, a negative regulator of HIF, have increased EPO synthesis and polycythemia. We show that nitric oxide release induced by the HIF pathway acts on cutaneous vascular flow to increase systemic erythropoietin expression. These results demonstrate that in mice the skin is a critical mediator of systemic responses to environmental oxygen.

The asparaginyl hydroxylase factor inhibiting HIF-1α is an essential regulator of metabolism

Factor inhibiting HIF-1alpha (FIH) is an asparaginyl hydroxylase. Hydroxylation of HIF-alpha proteins by FIH blocks association of HIFs with the transcriptional coactivators CBP/p300, thus inhibiting transcriptional activation. We have created mice with a null mutation in the FIH gene and found that it has little or no discernable role in mice in altering classical aspects of HIF function, e.g., angiogenesis, erythropoiesis, or development. Rather, it is an essential regulator of metabolism: mice lacking FIH exhibit reduced body weight, elevated metabolic rate, hyperventilation, and improved glucose and lipid homeostasis and are resistant to high-fat-diet-induced weight gain and hepatic steatosis. Neuron-specific loss of FIH phenocopied some of the major metabolic phenotypes of the global null animals: those mice have reduced body weight, increased metabolic rate, and enhanced insulin sensitivity and are also protected against high-fat-diet-induced weight gain. These results demonstrate that FIH acts to a significant degree through the nervous system to regulate metabolism.

Pulmonary microvascular permeability : responses to high vascular pressure after induction of pacing-induced heart failure in dogs

The pressure threshold for injury of pulmonary capillaries is approximately 50 to 55 cm H2O in the canine lung, as measured by changes in the filtration coefficient (Kf,c). Since the pulmonary endothelial basement membrane has been observed to thicken in patients with heart failure and pulmonary venous hypertension, we hypothesized that both baseline permeability and the threshold for high-vascular-pressure injury would be altered as a result. Dogs (n = 12) were chronically paced at 245 beats per minute for approximately 4 weeks, then were paced at 225 beats per minute for an additional 3 weeks. Lung lobes from anesthetized paced dogs and additional control dogs (n = 14) were then isolated, ventilated, and perfused with blood. Although vascular resistance was increased nearly threefold and vascular compliance reduced by 50% in the paced group, Kf,c referenced to 1 g blood-free dry weight was no different from control. Despite this lack of difference at normal pulmonary vascular pressures, several significant results were obtained. First, in the paced group there was a significant increase in the threshold for high-vascular-pressure injury: Kf,c measured at pulmonary vascular pressures commonly seen in heart failure (20 to 50 cm H2O) were significantly less in this group compared with control. Model predictions showed that in vivo, this difference in Kf,c would result in a 50% reduction in the amount of water and protein cleared across the pulmonary capillary endothelial barrier in the paced group.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative aspects of the strength of pulmonary capillaries in rabbit, dog, and horse.

In previous studies of rabbit and dog lung, we demonstrated stress failure of pulmonary capillaries at high transmural pressures (Ptm). The Ptm necessary to elicit stress failure was 40 cmH2O higher in dog than rabbit, and the total blood-gas barrier (BGB) thickness was greater in dog than rabbit. This suggests that stress failure may be related to BGB thickness, and is consistent with the Laplace relationship which states that wall stress is proportional to capillary radius but inversely proportional to wall thickness. In the present studies, we compared BGB thickness and an index of capillary radius in lungs from 3 rabbits, 3 dogs, and 2 horses perfusion fixed at a Ptm of approximately 30 cmH2O. Thicknesses of the BGB were measured at right angles to the barrier at random points on the capillary wall determined by test line intersections. Capillary radius was determined from the mean of major and minor axes measured on electron micrographs. Capillary pressure for failure in the horse was taken to be the mean of pulmonary arterial and left atrial pressures observed in galloping thoroughbreds known to develop exercise-induced pulmonary hemorrhage, although the actual pressure required for failure may be less than this. Average capillary radii were 3.6, 3.4, and 3.2 microns for rabbits, dogs, and horses, respectively. We found that the BGB was thinnest in the rabbit, intermediate in the dog, and thickest in the horse. Calculated capillary wall stress values for the median total BGB thickness at a nominal Ptm of 30 cmH2O were 2.5 x 10(4), 1.7 x 10(4), and 1.5 x 10(4) N.m-2 for rabbits, dogs, and horses, respectively. This species ranking fits with the pressures required to cause stress failure which are approximately 50, 90, and 130 cmH2O in rabbit, dog, and horse, respectively. We conclude that the differences in capillary radius of curvature and BGB thickness account for some of the observed differences in Ptm necessary to cause stress failure. However, other factors may also be important in determining the strength of the BGB.

Structure-function studies of blood and air capillaries in chicken lung using 3D electron microscopy.

Avian pulmonary capillaries differ from those of mammals in three important ways. The blood-gas barrier is much thinner, it is more uniform in thickness, and the capillaries are far more rigid when their transmural pressure is altered. The thinness of the barrier is surprising because it predisposes the capillaries to stress failure. A possible mechanism for these differences is that avian pulmonary capillaries, unlike mammalian, are supported from the outside by air capillaries, but the details of the support are poorly understood. To clarify this we studied the blood and air capillaries in chicken lung using transmission electron microscopy (EM) and two relatively new techniques that allow 3D visualization: electron tomography and serial block-face scanning EM. These studies show that the pulmonary capillaries are flanked by epithelial bridges composed of two extremely thin epithelial cells with large surface areas. The junctions of the bridges with the capillary walls show thickening of the epithelial cells and an accumulation of extracellular matrix. Collapse of the pulmonary capillaries when the pressure outside them is increased is apparently prevented by the guy wire-like action of the epithelial bridges. The enlarged junctions between the bridges and the walls could provide a mechanism that limits the hoop stress in the capillary walls when the pressure inside them is increased. The support of the pulmonary capillaries may also be explained by an interdependence mechanism whereby the capillaries are linked to a rigid assemblage of air capillaries. These EM studies show the supporting structures in greater detail than has previously been possible, particularly in 3D, and they allow a more complete analysis of the mechanical forces affecting avian pulmonary capillaries.

HIF-1 and ventilatory acclimatization to chronic hypoxia

Ventilatory acclimatization to hypoxia (VAH) is a time-dependent increase in ventilation and ventilatory O2-sensitivity that involves plasticity in carotid body chemoreceptors and CNS respiratory centers. Hypoxia inducible factor-1alpha (HIF-1alpha) controls the expression of several genes that increase physiological O2 supply. Studies using transgenic mice show HIF-1alpha expression in the carotid bodies and CNS with chronic sustained and intermittent hypoxia is important for VAH. Other O2-sensitive transcription factors such as HIF-2alpha may be important for VAH by reducing metabolic O2 demands also. Specific gene targets of HIF-1alpha shown to be involved in VAH include erythropoietin, endothelin-1, neuronal nitric oxide synthase and tyrosine hydroxylase. Other HIF-1alpha targets that may be involved in VAH include vascular endothelial growth factor, heme oxygenase 1 and cytoglobin. Interactions between these multiple pathways and feedback control of HIF-1alpha expression from some of the targets support a complex and powerful role for HIF-1alpha in neural plasticity of physiological control circuits with chronic hypoxia.

Major differences in the pulmonary circulation between birds and mammals

The lungs of domestic chickens were perfused with blood or dextran/saline and the pulmonary artery pressure (P(a)) and venous pressure (P(v)) were varied in relation to air capillary pressure (P(A)). In Zone 3 conditions, pulmonary vascular resistance (PVR) was virtually unchanged with increases in either P(a) or P(v). This is very different behavior from mammals where the same interventions greatly reduce PVR. In Zone 2 conditions blood flow was essentially independent of P(v) as in mammalian lungs but all the capillaries appeared to be open, apparently incompatible with a Starling resistor mechanism. In Zone 1 the capillaries were open even when P(A) exceeded P(a) by over 30 cm H(2)O which is very different behavior from that of the mammalian lung. We conclude that the air capillaries that surround the blood capillaries provide rigid support in both compression and expansion of the vessels. The work suggests a pathogenesis for pulmonary hypertension syndrome in chickens which costs the broiler industry

Regional differences in expression of VEGF mRNA in rat gastrocnemius following 1 hr exercise or electrical stimulation

1 billion per year.

The honeycomb-like structure of the bird lung allows a uniquely thin blood-gas barrier.

BackgroundVascular endothelial growth factor (VEGF) mRNA levels increase in rat skeletal muscle after a single bout of acute exercise. We assessed regional differences in VEGF165 mRNA levels in rat gastrocnemius muscle using in situ hybridization after inducing upregulation of VEGF by treadmill running (1 hr) or electrical stimulation (1 hr). Muscle functional regions were defined as oxidative (primarily oxidative fibers, I and IIa), or glycolytic (entirely IIb or IId/x fibers). Functional regions were visualized on muscle cross sections that were matched in series to slides processed through in situ hybridization with a VEGF165 probe. A greater upregulation in oxidative regions was hypothesized.ResultsTotal muscle VEGF mRNA (via Northern blot) was upregulated 3.5-fold with both exercise and with electrical stimulation (P = 0.015). Quantitative densitometry of the VEGF mRNA signal via in situ hybridization reveals significant regional differences (P ≤ 0.01) and protocol differences (treadmill, electrical stimulation, and control, P ≤ 0.05). Mean VEGF mRNA signal was higher in the oxidative region in both treadmill run (~7%, N = 4 muscles, P ≤ 0.05) and electrically stimulated muscles (~60%, N = 4, P ≤ 0.05). These regional differences were not significantly different from control muscle (non-exercised, non-stimulated, N = 2 muscles), although nearly so for electrically stimulated muscle (P = 0.056).ConclusionsModerately higher VEGF mRNA signal in oxidative muscle regions is consistent with regional differences in capillary density. However, it is not possible to determine if the VEGF mRNA signal difference is important in either the maintenance of regional capillarity differences or exercise induced angiogenesis.

The human lung: did evolution get it wrong?

Flying requires enormous energy and some birds have higher mass-specific maximal oxygen consumptions than any mammal. The bird lung is very efficient partly because of an extremely thin blood-gas barrier so that some birds have thinner barriers than any mammals. We show here that in addition to the total barrier being very thin, the interstitium which is responsible for the barriers strength is extraordinarily thin. This observation is paradoxical because intense exercise raises the pressure in pulmonary capillaries and results in large stresses in the capillary walls thus predisposing them to structural failure. For example, all galloping racehorses break their pulmonary capillaries. We propose that the explanation for how the bird can be so highly energetic yet also have such apparently fragile capillaries is the mechanical support provided by the dense packing of rigid air capillaries around the blood capillaries in the gas exchanging region of the lung. This architecture is very different from that in the mammalian lung.