The gap between in vitro and in vivo assays has inspired biomimetic model development. Tissue engineered models that attempt to mimic the complexity of microvascular networks have emerged as tools for investigating cell-cell and cell-environment interactions that may be not easily viewed in vivo. A key challenge in model development, however, is determining how to recreate the multi-cell/system functional complexity of a real network environment that integrates endothelial cells, smooth muscle cells, vascular pericytes, lymphatics, nerves, fluid flow, extracellular matrix, and inflammatory cells. The objective of this mini-review is to overview the recent evolution of popular biomimetic modeling approaches for investigating microvascular dynamics. A specific focus will highlight the engineering design requirements needed to match physiological function and the potential for top-down tissue culture methods that maintain complexity. Overall, examples of physiological validation, basic science discoveries, and therapeutic evaluation studies will emphasize the value of tissue culture models and biomimetic model development approaches that fill the gap between in vitro and in vivo assays and guide how vascular biologists and physiologists might think about the microcirculation.

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Hemoglobin (Hb)-based oxygen (O2) carriers (HBOC) have been proposed as an alternative for blood transfusion. Phase III clinical trials of previous HBOC generations presented issues, such as vasoconstriction and myocardial infarction, that were not detected in pre-clinical trials. These problems have been attributed to extravasation of the low molecular weight (LMW) Hb species, nitric oxide (NO) scavenging and O2 oversupply to the vasculature. However, the mechanisms responsible for the cardiac toxicity of HBOCs remain unclear. To explore the effects of acellular (acell) Hb and HBOC in cardiac tissues, we exposed cardiomyocytes (CMCs) to highly purified stroma-free acell bovine Hb (bHb) or human Hb (hHb). CMCs were loaded with Ca2+ fluorescent indicator (fluo-3) and put on laminin-coated coverslips. Cells were superfused with standard Tyrode solution with and without acell Hb. CMCs were stimulated with current pulses given at 1 Hz, and free cytosolic Ca2+ concentration was measured. To eliminate the effects of NO scavenging on CMCs, L-Name was added to the superfused with bHb and hHb. To study the effect of acell Hb MW on CMCs, LMW polymerized hHb (PolyhHb) was used to challenge CMCs. To explore the effect of Hb oxidation on CMCs, LMW MetHb was used to challenge the CMCs and compared to LMW PolyhMetHb. To explore the effects of antioxidants and heme toxicity, the studies with hHb and MethHb were repeated in the presence of the antioxidant N-acetylcysteine (NAC) or purified human Haptoglobin (Hp). Mitochondrial reactive oxygen species (ROS) production was measured using the fluorescent indicator MitoSOX Red after removing the Hb by washing the cells. After ROS determination, the maximal ROS production by CMCs was measured by challenging the cells with a high concentration of H2O2. Acell Hb increased the Ca2+ transient decay time constant (tau) relative to control. Removing the effects of NO production with L-Name, increased the tau of the Ca2+ transient in control and acell Hb challenge cells. Challenging CMCs with PolyhHb reduced the effects observed with acell Hb in Ca2+ transients. MetHb appears to be worse than reduced Hb in terms of the changes induced to CMCs Ca2+ transients. The antioxidant NAC mitigated the effects of acell Hb and MetHb but did not eliminate the changes on increased Ca2+ transients. The Hb scavenger, Hp, mitigated the effects of acell hHb and MethHb on Ca2+ transients. The MitoSOX Red fluorescence was higher for CMCs exposed to acell Hb and MetHb compared to the control. PolyhHb and PolyMetHb challenge decreased the MitoSOX compared to acell hHb and MethHb. Cells with higher levels of ROS showed lower changes in fluorescence when exposed to H2O2. Here, we show that the toxicity of acell Hb is impart determined by facilitated diffusion, NO scavenging, heme release, and ROS, potentially affecting the energy supply and metabolism of CMCs. This study was supported by National Institutes of Health grants R01HL162120, R01HL159862 and the Department of Defense under grants W81XWH1810059. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Aging is the most significant risk factor for many chronic diseases. Studies of heterochronic blood exchange show that blood and plasma from old-age animals have senescent effects in young mice. However, the underlying mechanisms of how bloodborne factors promote aging remain largely unknown, and studies of blood transfusions' effects on donors of different ages are currently unavailable. This study aimed to compare the microcirculatory effects of blood transfusions from young or old donors in anemic young animals. Blood was collected from anesthetized young (8 weeks old) and older (52-60 weeks old) golden Syrian hamsters via cardiac puncture into CP2D (from an AS-3 blood preparation kit). The supernatant and buffy coat were removed. The AS-3 additive was added according to manufacture instructions, and packed red cells were mixed gently. For microhemodynamic studies, young golden Syrian hamsters instrumented with a dorsal window chamber were subjected to an isovolemic hemodilution of 30% of the animal's blood volume with 5% human serum albumin to induce a normovolemic anemic state. The next day, the anemic animals were randomly assigned to young or older blood and received a transfusion of a volume equivalent to a single unit of red cells. The microhemodynamics in arterioles lower and greater than 60 μm and venules between 20 and 80 μm and functional capillary density (FCD) were measured. Hemodilution acutely reduced hematocrit (Hct) to 60% of the baseline and increased microvascular blood flow in venules and arterioles while decreasing the FCD. Transfusion of young or old donors’ blood increased the Hct. There was no significant change post-transfusion in the diameter or the flow of the arterioles in the young donors' blood group compared to old donors’ blood. The animals transfused with blood from young donors restored venules flow and diameter similarly to baseline (pre-anemia). However, the old donors’ blood group did not show the same recovery. More importantly, the FCD significantly improved after younger donors' blood transfusions compared to the old ones. These results suggest that recovery from anemia is improved when using blood from younger donors, as this ensures higher FCD and supports proper tissue homeostasis. This study intentionally tried to recapitulate how blood is processed in the USA and transfuse only a conservative amount of blood into anemic animals. Studies have shown that heterochronic parabiosis (surgically joining younger and older animals in which blood, organs, and environments are shared) or a blood exchange rejuvenates old tissues. Here using a heterochronic blood transfusion, we show that the donor's age determines the recovery of FCD after transfusion. These deficits could be due to capillary collapse preventing oxygen from arriving uniformly through the tissue, as the FCD determines the microvascular oxygen diffusion field. Future studies will look at the implication of the recovery of FCD after transfusion. This study was supported by National Institutes of Health grants R01HL162120, R01HL159862 and the Department of Defense under grants W81XWH1810059. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.