The effect of cell size and capillary spacing on myocardial oxygen supply

O-GlcNAcase overexpression reverses coronary endothelial cell dysfunction in type 1 diabetic mice

Cardiovascular disease is the primary cause of morbidity and mortality in diabetes, and endothelial dysfunction is commonly seen in these patients. Increased O-linked N-acetylglucosamine (O-GlcNAc) protein modification is one of the central pathogenic features of diabetes. Modification of proteins by O-GlcNAc (O-GlcNAcylation) is regulated by two key enzymes: β-N-acetylglucosaminidase [O-GlcNAcase (OGA)], which catalyzes the reduction of protein O-GlcNAcylation, and O-GlcNAc transferase (OGT), which induces O-GlcNAcylation. However, it is not known whether reducing O-GlcNAcylation can improve endothelial dysfunction in diabetes. To examine the effect of endothelium-specific OGA overexpression on protein O-GlcNAcylation and coronary endothelial function in diabetic mice, we generated tetracycline-inducible, endothelium-specific OGA transgenic mice, and induced OGA by doxycycline administration in streptozotocin-induced type 1 diabetic mice. OGA protein expression was significantly decreased in mouse coronary endothelial cells (MCECs) isolated from diabetic mice compared with control MCECs, whereas OGT protein level was markedly increased. The level of protein O-GlcNAcylation was increased in diabetic compared with control mice, and OGA overexpression significantly decreased the level of protein O-GlcNAcylation in MCECs from diabetic mice. Capillary density in the left ventricle and endothelium-dependent relaxation in coronary arteries were significantly decreased in diabetes, while OGA overexpression increased capillary density to the control level and restored endothelium-dependent relaxation without changing endothelium-independent relaxation. We found that connexin 40 could be the potential target of O-GlcNAcylation that regulates the endothelial functions in diabetes. These data suggest that OGA overexpression in endothelial cells improves endothelial function and may have a beneficial effect on coronary vascular complications in diabetes.

Signaling in morphogenesis: transport cues in morphogenesis

Extracellular transport processes play critical roles in morphogenesis. While diffusive transport effects on morphogenesis are well illustrated in examples like blood capillary architecture and in cell morphogenetic responses to the local extracellular protein environment, the effects of fluid convection, although important in many developing and regenerating tissues, are not well understood. Convective forces are present whenever a hydrated tissue undergoes dynamic mechanical strain, and so convection could not only dominate the transport of large molecules like proteins, but might also serve as a mechanism for mechanosensing. The complex interdependence of mechanical forces, protein transport and extracellular morphogen gradients needs to be elucidated in a comprehensive way in order for the importance of transport on morphogenesis to be fully appreciated.

Modeling pO(2) distributions in the bone marrow hematopoietic compartment. I. Krogh's model

Human bone marrow (BM) is a tissue of complex architectural organization, which includes granulopoietic loci, erythroblastic islets, and lymphocytic nodules. Oxygen tension (pO(2)) is an important determinant of hematopoietic stem and progenitor cell proliferation and differentiation. Thus, understanding the impact of the BM architectural organization on pO(2) levels in extravascular hematopoietic tissue is an important biophysical problem. However, currently it is impossible to measure pO(2) levels and their spatial variations in the BM. Homogeneous Kroghian models were used to estimate pO(2) distribution in the BM hematopoietic compartment (BMHC) and to conservatively simulate pO(2)-limited cellular architectures. Based on biophysical data of hematopoietic cells and characteristics of BM physiology, we constructed a tissue cylinder solely occupied by granulocytic progenitors (the most metabolically active stage of the most abundant cell type) to provide a physiologically relevant limiting case. Although the number of possible cellular architectures is large, all simulated pO(2) profiles fall between two extreme cases: those of homogeneous tissues with adipocytes and granulocytic progenitors, respectively. This was illustrated by results obtained from a parametric criterion derived for pO(2) depletion in the extravascular tissue. Modeling results suggest that stem and progenitor cells experience a low pO(2) environment in the BMHC.

Modeling pO(2) distributions in the bone marrow hematopoietic compartment. II. Modified Kroghian models

Hematopoietic cells of various lineages are organized in distinct cellular architectures in the bone marrow hematopoietic compartment (BMHC). The homogeneous Kroghian model, which deals only with a single cell type, may not be sufficient to accurately describe oxygen transfer in the BMHC. Thus, for cellular architectures of physiological significance, more complex biophysical-transport models were considered and compared against simulations using the homogeneous Kroghian model. The effects of the heterogeneity of model parameters on the oxygen tension (pO(2)) distribution were examined using the multilayer Kroghian model. We have also developed two-dimensional Kroghian models to simulate several cellular architectures in which a cell cluster (erythroid cluster) or an individual cell (megakaryocyte or adipocyte) is located in the BMHC predominantly occupied by mature granulocytes. pO(2) distributions in colony-type cellular arrangements (erythroblastic islets, granulopoietic loci, and lymphocytic nodules) in the BMHC were also evaluated by modifying the multilayer Kroghian model. The simulated results indicate that most hematopoietic progenitors experience low pO(2) values, which agrees with the finding that low pO(2) promotes the expansion of various hematopoietic progenitors. These results suggest that the most primitive stem cells, which are located even further away from BM sinuses, are likely located in a very low pO(2) environment.

Functional and stereologic estimations of myocardial capillary exchange capacity in treated and untreated spontaneously hypertensive rats

Myocardial capillary exchange capacity was investigated by stereologic and functional techniques in parallel during pressure-overload cardiac hypertrophy and after long-term antihypertensive therapy with the vasodilator felodipine. In 26-week-old female spontaneously hypertensive rats (SHR) blood pressure increased by 25% and left ventricular weight (LVW/BW) increased by 18% compared to Wistar-Kyoto rats (WKY). Myocardial capillary surface and volume densities normalized for organ weight were similar in both ventricles for both strains. Moreover, capillary surface density was higher sub-epicardially (EPI) than in the subendocardium (ENDO) in the left ventricle of SHR. Thirteen weeks of felodipine-therapy (SHR-Felo) normalized blood pressure without affecting LVW/BW although a transition from concentric to eccentric hypertrophy is known to occur. Myocardial capillary surface and volume densities and the left ventricular ENDO-EPI-gradient in surface density were similar to untreated SHR. However, felodipine-treatment increased right ventricular weight and capillary volume density. Functional capillary exchange was estimated in terms of permeability surface area products (PS) for Cr-EDTA and vitamin B12 and normalized for organ weight. PSCr-EDTA, PSB12 and the ratio PSCr-EDTA/PSB12 (an index of capillary permeability) were similar in SHR and WKY. Furthermore, the relation between functional and stereological indices of exchange capacity was investigated in a multiple linear regression analysis. However, no significant correlation between PS and neither capillary surface nor volume density was found. In conclusion, myocardial capillary exchange capacity was well adapted to the pressure overload cardiac hypertrophy present in female SHR. Despite induction of right ventricular hypertrophy, felodipine-treatment did not affect capillary exchange capacity. Furthermore, when functional and stereologic estimates were performed in parallel, the importance of dynamic factors for myocardial capillary exchange capacity (e.g. heterogeneity) was illustrated.

Diffusion limitation of O2 supply to tissue in homogeneous and heterogeneous models

The role of diffusion limitation in O2 supply was studied in cross-sectional elements of the Krogh cylinder model (with O2 supply from a central capillary) and of the solid cylinder model (with O2 supply from the outer surface). The effect of diffusion limitation was quantified in terms of the ratio O2 uptake/O2 requirement (= fraction of cross-sectional area supplied with O2), assuming local O2 requirement per unit volume to be constant and independent of PO2 at PO2 greater than 0. Calculations were performed for single cylinders of varied radius and O2 requirement (homogeneous models). Unequal distribution of diffusion conditions was represented by a model composed of three sorts of Krogh or solid cylinders, with radii in relation 3: square root of 3:1, but of equal cross-sectional area, i.e. number of cylinders of each sort in relation 1:3:9 (heterogeneous models). The results revealed the following main features. (1) At the same outer radius, diffusion limitation sets in at a smaller O2 requirement, and increases more steeply with increasing O2 requirement, in the homogeneous Krogh cylinder model compared with the homogeneous solid cylinder model. A similar behavior is observed when the radius of the cylinder section is increased at constant O2 requirement. (2) Diffusion limitation in the heterogeneous model sets in at a lower O2 requirement value, and increases more gradually with increasing O2 requirement, than in the corresponding homogeneous models with the same average cylinder diameter. This behavior is due to sequential onset, in the heterogeneous model, of anoxia in the cylinder sections of different radii. We conclude that diffusion heterogeneity has to be taken into account when the role of diffusion limitation in tissue O2 supply is investigated.

Differing patterns of capillary distribution in fish and mammalian skeletal muscle

The heterogeneity of capillary supply to muscles of different metabolic capacity and fibre size was assessed in slow and fast muscles from a fish and a mammal. The area surrounding each capillary delineated by equidistant boundaries from adjacent vessels, the capillary domain, was derived from morphometric analysis of histological sections. This 2-D integration of intercapillary distances may reveal heterogeneity of supply that is hidden by a global approach, especially when compared with the more usual 0- and 1-D indices of capillarisation. Mean radii of the equivalent Kroghian tissue cylinders (R) and heterogeneity of their lognormal distribution, represented by the logarithmic standard deviation (LogSD), were calculated. In eel slow muscle there was a 35-fold greater capillary density (CD) than fast muscle (698 vs 20 mm-2) although heterogeneity of capillary spacing was similar (LogSD congruent to 0.06). The difference in CD between slow and fast muscles of rat was less pronounced, but there was significantly lower heterogeneity in the aerobic tissue (LogSD = 0.08 vs 0.10) corresponding to a range in domain area of around 350-2300 microns 2 and 400-2900 microns 2, respectively. The overall capillary to fibre ratio (C:F) is inappropriate for sparse networks where many fibres lack direct capillary contact. The cumulative fraction of individual domains overlapping a muscle fibre (local capillary to fibre ratio, LCFR) plotted against fibre area showed the best correlation of any index in all tissue and was strongest in both fish muscles (r = 0.9), indicating a functionally homologous spatial distribution of capillaries with respect to muscle fibres in tissue of widely differing oxidative capacity. These data suggest that maximal oxygen supply to, or metabolite removal from, muscle fibres is not restricted to contiguous capillaries but also involves those remote from the fibre surface.