A computational model of the effect of capillary density variability on oxygen transport, glucose uptake, and insulin sensitivity in prediabetes

A two-layer continuously distributed capillary O2 transport model applied to blood flow regulation in resting skeletal muscle

The microcirculation is the site of direct oxygen transfer from blood to tissue, and also of oxygen delivery control via regulation of local blood flow. In addition, a number of diseases including type II diabetes mellitus (DMII) and sepsis are known to produce microcirculatory dysfunction in their early phases. Given the complexity of microvascular structure and physiology, and the difficulty of measuring tissue oxygenation at the micro-scale, mathematical modelling has been necessary for understanding the physiology and pathophysiology of O₂ transport in the microcirculation and for interpreting in vivo experiments. To advance this area, a model of blood-tissue O₂ transport in skeletal muscle was recently developed which uses continuously distributed capillaries and includes O₂ diffusion, convection, and consumption. The present work extends this model to two adjacent layers of skeletal muscle with different blood flow rates and applies it to study steady-state O₂ transport when flow regulation is stimulated using an O₂ exchange chamber. To generate a model which may be validated through in vivo experiments, an overlying O₂ permeable membrane is included. The model is solved using traditional methods including separation of variables and Fourier decomposition, and to ensure smooth profiles at the muscle-muscle and muscle-membrane interfaces matching conditions are developed. The study presents qualitative verification for the model, using visualizations of tissue PO₂ distributions for varying capillary density (CD), and presents capillary velocity response values in the near layer for varying chamber PO₂ under the assumption that outlet capillary O₂ saturation is equalized between adjacent layers. These compensatory velocity profiles, along with effective 'no-flux' chamber PO₂ values, are presented for varying CD and tissue O₂ consumption values. Insights gained from the two-layer model provide guidance for interpreting and planning future in-vivo experiments, and also provide motivation for further development of the model to improve understanding of the interaction between O₂ transport and blood flow regulation.

Aerobic-interval exercise with blood flow restriction potentiates early markers of metabolic health in man

AIM

This study examined whether aerobic-interval exercise with blood flow restriction (BFR) potentiates early markers of metabolic health compared to exercise with systemic hypoxia or normoxia in man.

METHODS

In a randomized-crossover fashion, eight healthy men completed nine 2-minute running bouts at 105% of their lactate threshold on three occasions separated by one week, either with BFR (BFR-trial), systemic hypoxia (HYP-trial) or normoxia (control; CON-trial). Near-infrared spectroscopy was used to assess the muscle level of hypoxia. A muscle biopsy was collected at rest and 3 hours after exercise to quantify genes involved in cholesterol synthesis (PGC-1α2), glucose disposal (GLUT4) and capillary growth (HIF-1α; VEGFA), as well as mitochondrial respiration (PGC-1α2/3), uncoupling (UCP3) and expansion (p53; COXIV-1/2; CS; AMPKα1/2).

RESULTS

The muscle level of hypoxia was matched between the BFR-trial and HYP-trial (~90%; P > .05), which was greater than the CON-trial (~70%; P < .05). PGC-1α2 increased most in the BFR-trial (16-fold vs CON-trial; 11-fold vs HYP-trial; P < .05). GLUT4 and VEGFA selectively increased by 2.0 and 3.4-fold, respectively in BFR-trial (P < .05), which was greater than CON-trial (1.2 and 1.3 fold) and HYP-trial (1.2 and 1.8 fold; P < .05). UCP3 increased more in BFR-trial than the HYP-trial (4.3 vs 1.6 fold), but was not different between BFR-trial and CON-trial (2.1 fold) or between CON-trial and HYP-trial (P > .05). No trial differences were evident for other genes (P > .05).

CONCLUSION

Independent of the muscle level of hypoxia, BFR-exercise potentiates early markers of metabolic health associated with the regulation of cholesterol production and glucose homeostasis in man.

A Mesoscale Computational Model for Microvascular Oxygen Transfer

We address a mathematical model for oxygen transfer in the microcirculation. The model includes blood flow and hematocrit transport coupled with the interstitial flow, oxygen transport in the blood and the tissue, including capillary-tissue exchange effects. Moreover, the model is suited to handle arbitrarily complex vascular geometries. The purpose of this study is the validation of the model with respect to classical solutions and the further demonstration of its adequacy to describe the heterogeneity of oxygenation in the tissue microenvironment. Finally, we discuss the importance of these effects in the treatment of cancer using radiotherapy.

Methods to label, image, and analyze the complex structural architectures of microvascular networks

Microvascular networks play key roles in oxygen transport and nutrient delivery to meet the varied and dynamic metabolic needs of different tissues throughout the body, and their spatial architectures of interconnected blood vessel segments are highly complex. Moreover, functional adaptations of the microcirculation enabled by structural adaptations in microvascular network architecture are required for development, wound healing, and often invoked in disease conditions, including the top eight causes of death in the Unites States. Effective characterization of microvascular network architectures is not only limited by the available techniques to visualize microvessels but also reliant on the available quantitative metrics that accurately delineate between spatial patterns in altered networks. In this review, we survey models used for studying the microvasculature, methods to label and image microvessels, and the metrics and software packages used to quantify microvascular networks. These programs have provided researchers with invaluable tools, yet we estimate that they have collectively attained low adoption rates, possibly due to limitations with basic validation, segmentation performance, and nonstandard sets of quantification metrics. To address these existing constraints, we discuss opportunities to improve effectiveness, rigor, and reproducibility of microvascular network quantification to better serve the current and future needs of microvascular research.

Cellular cross talk, inflammatory signals, and enhanced microvascular permeability

Microvascular leakage remains a significant clinical problem for which there is no specific therapeutic available. This Special Topics issue of the journal Microcirculation features reviews pertaining to the cellular and molecular mechanisms that control the integrity of the microvascular barrier. (Meegan et al., Microcirculation, 2017) summarize recent research findings about how neutrophil extracellular traps and how these can cause injury to the microvascular barrier.( Zhang et al. Microcirculation, 2017) highlight the microvascular response to advanced glycation end-products that are formed as part of the pathophysiology of diabetes mellitus. These ongoing investigations are helping to clarify the mechanisms by which endothelial cells receive information and integrate it to generate cellular responses that fine-tune barrier function. Better understanding of these mechanisms is needed for the rational development of therapeutic strategies to reduce excessive microvascular leakage.

Obesity, insulin resistance, and microvascular adaptation

Two of the primary risk factors for the progressive evolution of cardiovascular disease are obesity and impaired glycemic control-including both insulin resistance and overt type 2 diabetes mellitus, leading to increased research emphasis on these conditions, their mechanistic bases, and their health outcomes. This Special Topics Issue of the journal Microcirculation summarizes a symposium at the recent Joint Meeting of the American Physiological Society and the Physiological Society, held in Dublin, Ireland, on July 30, 2016. This symposium, "Adaptive outcomes of microvascular networks to obesity and type 2 diabetes mellitus/insulin resistance," presented four lectures, each of which addressed the larger issue from a different perspective.