Distribution of the myocardial tissue PO2 in the rat and the inhomogeneity of the coronary bed

Reduced oxygen tension culturing conditionally alters toxicogenic response of differentiated H9c2 cardiomyoblasts to acrolein

Acrolein is a reactive electrophilic aldehyde known to cause mitochondrial dysfunction, oxidative stress, and dysregulation of signaling transduction in vitro. Most in vitro systems employ standard cell culture maintenance conditions of 95% air/5% CO₂, translating to a culture oxygen tension of approximately 20%, far above most physiological tissues. The purpose of this investigation was to examine whether low-serum, retinoic acid differentiated H9c2 cells were less sensitive to acrolein insult when cultured under reduced oxygen tension. H9c2 cells were maintained separately in 20% and 5% oxygen, differentiated for 5 d, and then exposed to acrolein for 30 min in media containing varying concentrations of tricarboxylic acid and glycolytic substrates, followed by fresh medium replacement. Cells were then assessed for MTT reduction at 2 h and 24 h after acrolein insult. We showed that pyruvate supplementation in combination with lowered oxygen culturing significantly attenuated acrolein-induced viability loss at 24 h. Poly(ADP-ribose) polymerase inhibition and EGTA preferentially provided partial rescue to low oxygen cultures, but not for standard cultures. Collectively, these results offer evidence supporting altered toxicogenic response of H9c2 during physiologically relevant oxygen tension culturing.

Perfusion improves tissue architecture of engineered cardiac muscle

Cardiac muscle with a certain threshold thickness, uniformity of tissue architecture, and functionality would expand the therapeutic options currently available to patients with congenital or acquired cardiac defects. Cardiac constructs cultured in well-mixed medium had an approximately 100-microm-thick peripheral tissue-like region around a relatively cell-free interior, a structure consistent with the presence of concentration gradients within the tissue. We hypothesized that direct perfusion of cultured constructs can reduce diffusional distances for mass transport, improve control of oxygen, pH, nutrients and metabolites in the cell microenvironment, and thereby increase the thickness and spatial uniformity of engineered cardiac muscle. To test this hypothesis, constructs (9.5-mm-diameter, 2-mm-thick discs) based on neonatal rat cardiac myocytes and fibrous polyglycolic acid scaffolds were cultured either directly perfused with medium or in control spinner flasks. Perfusion improved the spatial uniformity of cell distribution and enhanced the expression of cardiac-specific markers, presumably due to the improved control of local microenvironmental conditions within the forming tissue. Medium perfusion could thus be utilized to better mimic the transport conditions within native cardiac muscle and enable in vitro engineering of cardiac constructs with clinically useful thicknesses.

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.

Cardiac tissue engineering: cell seeding, cultivation parameters, and tissue construct characterization

Cardiac tissue engineering has been motivated by the need to create functional tissue equivalents for scientific studies and cardiac tissue repair. We previously demonstrated that contractile cardiac cell-polymer constructs can be cultivated using isolated cells, 3-dimensional scaffolds, and bioreactors. In the present work, we examined the effects of (1) cell source (neonatal rat or embryonic chick), (2) initial cell seeding density, (3) cell seeding vessel, and (4) tissue culture vessel on the structure and composition of engineered cardiac muscle. Constructs seeded under well-mixed conditions with rat heart cells at a high initial density ((6-8) x 10(6) cells/polymer scaffold) maintained structural integrity and contained macroscopic contractile areas (approximately 20 mm(2)). Seeding in rotating vessels (laminar flow) rather than mixed flasks (turbulent flow) resulted in 23% higher seeding efficiency and 20% less cell damage as assessed by medium lactate dehydrogenase levels (p < 0.05). Advantages of culturing constructs under mixed rather than static conditions included the maintenance of metabolic parameters in physiological ranges, 2-4 times higher construct cellularity (p &le 0.0001), more aerobic cell metabolism, and a more physiological, elongated cell shape. Cultivations in rotating bioreactors, in which flow patterns are laminar and dynamic, yielded constructs with a more active, aerobic metabolism as compared to constructs cultured in mixed or static flasks. After 1-2 weeks of cultivation, tissue constructs expressed cardiac specific proteins and ultrastructural features and had approximately 2-6 times lower cellularity (p < 0.05) but similar metabolic activity per unit cell when compared to native cardiac tissue.

Oxygen exchange in the isolated, arrested guinea pig heart: theoretical and experimental observations

A model of oxygen transport in perfused myocardial tissue is presented. Steady-state conditions are assumed in order to mimic the metabolic rate of the arrested heart. The model incorporates Michaelis-Menten dependence of mitochondrial oxygen consumption, oxymyoglobin saturation and oxyhemoglobin saturation on oxygen partial pressure (PO2). The transport equations model both the advective supply of oxygen via the coronary circulation and the diffusive exchange of oxygen between tissues and environment across the epicardial and endocardial surfaces. The left ventricle is approximated by an axisymmetric prolate spheroid and the transport equations solved numerically using finite element techniques. Solution yields the PO2 profile across the heart wall. Integration of this profile yields the simulated rate of metabolic oxygen uptake determined according to the Fick principle. Correction for the diffusive flux of oxygen across the surfaces yields the simulated true metabolic rate of oxygen consumption. Simulated values of oxygen uptake are compared with those measured experimentally according to the Fick principle, using saline-perfused, Langendorff-circulated, K(+)-arrested, guinea pig hearts. Four perfusion variables were manipulated: arterial PO2, environmental PO2, coronary flow and perfusion pressure. In each case agreement between simulated and experimentally determined rates of oxygen consumption gives confidence that the model adequately describes the advective and diffusive transport of oxygen in the isolated, arrested, saline-perfused heart.

Microvascular flow vectors in normal and hypertrophic myocardium as determined by the method of colored microspheres

Sequential in vivo infusion of two differently colored microsphere suspensions into the left atrium of normal and hypertrophic rat myocardium revealed that certain coronary capillaries contained microsphere aggregates of both colors. A capillary flow vector was established based on the sequence of colors embolized within each aggregate. Critical examination of flow vectors among neighboring capillaries enabled the characterization of capillary flow direction. Results indicated a predominance in concurrent flow direction, which decreased significantly (P less than 0.001) with capillaries further removed from an individual reference capillary. The percentage of concurrent flow was also found to be significantly lower in subendocardium (P less than 0.001) than in midmyocardium. Cardiac hypertrophy was not a contributing factor to the above findings. This study provides previously unattainable data regarding transmural capillary flow direction and suggests regional adaptations in coronary microvascular flow.

Improved distribution of regional oxygenation in denervated ischemic dog myocardium

The role of the adrenergic nervous system in the response to coronary artery occlusion has been examined using surgical and chemical denervation techniques. Experiments were conducted on four groups of dogs (n = 18): 1) untreated controls; 2) intrapericardial denervation immediately prior to coronary ligation; 3) surgical denervation 2 weeks prior to the experiment; and 4) chemical sympathectomy 5 days prior to the experiment with 6-hydroxydopamine (50 mg/kg). Small artery and vein O2 saturations were obtained microspectrophotometrically and combined with radioactive microsphere blood flow determinations to calculate regional myocardial O2 consumption in open chest dogs. Denervation significantly reduced the preocclusion heart rate from 165 +/- 16 beats/min in the control to 114 +/- 13 in the chronic surgically denervated and to 137 +/- 15 in the chemically sympathectomized groups. After 2 hours of occlusion, the O2 consumption and flow were similar in the nonischemic area except for lower values in the surgically denervated group. Total coronary blood flow and O2 consumption in the occluded regions were not significantly affected by chronic denervation. However, significant elimination of areas with low venous O2 saturation (less than 20%) were found in the ischemic myocardium of the chronically denervated groups as compared with the control or with the acutely denervated dogs. The mean venous O2 saturation was found to be significantly higher in all regions of these two groups as compared with the control. The O2 extraction was also lowered. Thus, chronic denervation reduced microregional heterogeneity of oxygenation in the ischemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of intracellular PO2 and conditions for blood-tissue O2 transport in heart and working red skeletal muscle

1. Neither anoxic nor hypoxic cells were found in epicardium of anaesthetized dogs, cats, rabbits and rats despite heterogeneity of flow (Wieringa et al., 1982) and haematocrit (Honig et al., in press) in the coronary capillary network. 2. Median PO2 in unstressed dog heart and cat heart are 4.8 and 5.2 torr, respectively. These values are close to the P50 of the oxymyoglobin dissociation curve, and well above PcritO2. 3. A dense, interconnected capillary network and high capillary haematocrit appear essential to achieve high O2 extraction at flows characteristic of maximally working myocardium. 4. Mb promotes O2 transport in myocardium by: a) maximizing the driving force for transcapillary diffusion, b) minimizing spatial variability in PmbO2, c) facilitating O2 diffusion in myocytes and, d) permitting close capillary packing without a diffusion shunt for O2. 5. The O2 conductance of the red cell-capillary system is a major determinant of O2 mass transfer in red muscle.

Limitations of tracer oxygen uptake in the canine coronary circulation

Theoretical models of oxygen transport in the myocardium have failed to account for low average tissue pO2 relative to to coronary sinus pO2, measured with pO2 electrodes and myoglobin saturation, and for hypoxic contractile failure at relatively high coronary sinus pO2 levels. These findings could be explained by either arteriovenous diffusional shunting or a limiting rate of transfer of oxygen from blood to tissue, or both. To gain new insights, we performed multiple indicator dilution tracer experiments across the coronary circulation in the dog, with 18O2 as the oxygen tracer and 51Cr-labeled red cells as the reference tracer for oxygen. 125I-Albumin and 22Na+ were included to provide the relative plasma flow rate. The tracer oxygen outflow curve consisted of a large early peak related to its reference red cell curve. No tracer emerged before the labeled red cells. The downslope, which contains the returning component of the tracer curve, decreased less steeply when oxygen consumption was reduced by propranolol. Fitting the tracer oxygen outflow curve with a distributed model including irreversible sequestration behind a resistance gave a transfer rate constant which was relatively small, and a relatively large rate constant for sequestration. Relative oxygen consumption (estimated from the arteriovenous difference) correlated closely with the rate constant for sequestration. Estimated average tissue oxygen concentrations were of the order of one-third blood concentration. Dimensional analysis indicates that the low transfer rate constant derives from hemoglobin-oxygen binding; this decreases fractional tracer oxygen transfer in proportion to the ratio of plasma:red cell oxygen pools.

An examination of the contribution of red cell spacing to the uniformity of oxygen flux at the capillary wall

It is generally assumed that capillary blood is homogeneous for O2 supply and that red cells can provide a constant, uniform flux of O2 out of the capillary regardless of the spacing between cells. Using a simplified model of red cells moving through a capillary in skeletal muscle, an approximate analysis is developed to study the effect of red cell spacing on the ability of erythrocytes to provide a constant, uniform flux of O2 at the capillary wall. The results suggest the existence of a critical red cell separation distance above which the flux of O2 at the capillary wall between red cells cannot remain uniform and the capillary blood is no longer homogeneous for O2 supply. In resting muscle the predicted critical separation distance is greater than four cell lengths. During maximal O2 consumption, the critical separation distance predicted by the model is one cell length. These predictions agree closely with in vivo observations of red cell spacing. The total red cell flux through a capillary is determined not only by red cell spacing (hematocrit) but also by erythrocyte velocity; a simple example is given which suggests that changes in each of these variables are not equivalent in maintaining a constant and uniform flux of O2 at the capillary wall.

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

Recently, we collected basic morphometric data from normal and hypertrophic rat hearts: median and mean values of the cell diameter and of the intercapillary distance as well as their variabilities. In the present communication we used these data, first for analysis of the effect of the heterogeneity of capillary spacing on the myocardial tissue PO2. Comparison of tissue PO2 histograms calculated for a situation in which the capillaries are evenly distributed as in the Krogh model, with a situation based on the same capillary but variable intercapillary distances clearly demonstrates the importance of heterogeneity of the capillary spacing as a separate oxygen determinant. This is even more important in the hypertrophic hearts which are characterized by longer and more variable intercapillary distances. In the second part, we compared the classical Krogh model with a model of concentric diffusion in which the oxygen consumption was either uniform or divided into two zones of distinctive rates. Oxygen profiles calculated for the Krogh model with excentric diffusion were similar to those derived for the two models of concentric diffusion.

Intracellular potassium activity in guinea pig papillary muscle during prolonged hypoxia

During prolonged hypoxia, intracellular potassium concentration, [K](i) has been reported to fall by 70% with a concomitant decrease in the calculated potassium equilibrium potential, E(K). Nevertheless, resting membrane potential, V(m), declined only slightly. Because V(m) depolarized very little in relation to the calculated E(K), it was hypothesized that electrogenic Na-K pumping contributed up to 40 mV to V(m) during prolonged hypoxia. To further test this hypothesis we studied what changes prolonged hypoxia makes in the thermodynamically active fraction of cellular potassium, intracellular potassium activity, alpha(K) (i), and how change in alpha(K) (i) affects the relationship between V(m), E(K) and, by inference, the Na-K pump. Using double-barrel K-selective electrodes, V(m) and alpha(K) (i) were measured in quiescent guinea pig right ventricular papillary muscles superfused for 8 h with hypoxic Tyrode's solution. Over the 8-h period both V(m) and alpha(K) (i) decreased. However, the decline in V(m) was paralleled by a decrease in the E(K) calculated from alpha(K) (i). At no time was there hyperpolarization of V(m) beyond E(K). After 8 h the Na-K pump was inhibited by exposing the muscles to 0.1 mM ouabain. The onset of an increase in extracellular potassium activity, measured with a double-barrel electrode, was used to mark the amount of depolarization of V(m) due solely to pump inhibition. After hypoxia, V(m) depolarized 8.4+/-4.4 mV before extracellular potassium activity (alpha(K) (e)) increased. Thus, the decrease in alpha(K) (i) during hypoxia is much less than that reported for [K](i). The parallel decline in V(m) and E(K) and the small depolarization of V(m) with ouabain suggest that after prolonged hypoxia the Na-K pump continues to contribute to V(m), but the amount of this contribution is substantially less than previously reported.

Lognormal distribution of intercapillary distance in normal and hypertrophic rat heart as estimated by the method of concentric circles: its effect on tissue oxygenation

The inhomogeneity of the capillary net in the cardiac muscle was estimated using our morphometric measurements in normal and hypertrophic rats hearts. As entry data we used the distribution of tissue at different distances from the nearest capillary as measured by the method of concentric circles and the mean intercapillary distance independently calculated from the capillary density. The derived distribution of intercapillary distances was approximated by lognormal distribution in which the spread can be characterized by a single parameter, namely the log standard deviation. The effect of the log standard deviation on tissue oxygenation was evaluated in normal and hypertrophic hearts, at normoxia and at hypoxia. The mean tissue PO2 and the percentage of anoxic tissue at the venous end of the tissue cylinder were calculated using Krogh's model. Two boundary situations were considered: A) the end-capillary PO2 was assumed to be equal in all capillaries due to compensatory adjustment in blood flow; B) the same flow in all capillaries was assumed resulting in varying end-capillary PO2. The real situation is expected to be between situations A and B. Increased variability of intercapillary distance proved to impair considerably the tissue oxygenation, especially when the results were expressed as a percentage of anoxic tissue. The percentage of anoxic tissue turned out to be a better index of tissue oxygenation than the mean PO2 particularly at hypoxia. The results suggest the presence of at least a partial adjustment of blood flow with respect to the width of tissue cylinder. Without such adjustment, a large part of tissue would become anoxic already in normal hearts at normoxia and this would be further aggravated by hypertrophy and/or hypoxia.

Diffusional shunting in the canine myocardium

A test for the existence of a diffusional shunt in the myocardium was performed in closed-chest, chloralose-anesthetized dogs. Coronary blood flow was varied from 0.21 to 1.17 ml/min per g with a roller pump via a stainless steel coronary perfusion cannula. Following intracoronary artery bolus indicator injection, the coronary sinus venous appearance times of hydrogen, plasma protein labeled with indocyanine green dye, and red blood cells labeled with 99mTc were measured. Venous appearance time was calculated, either as the time from injection until the indicator reached 5% of its peak venous concentration or the time from injection until 0.1% of the injected material arrived at the coronary sinus. At low coronary flow rates (0.4 sec dye appearance time) hydrogen appearance preceded by about 1 second the appearance of the intravascular 99mTc-labeled red blood cells and the dyed plasma, indicating that hydrogen traversed an extravascular shunt path from small arteries to veins. As flow was increased, this hydrogen precession was reduced, and at high flows detectable levels of hydrogen arrived in the coronary sinus after the intravascular indicators. The difference in appearance time between hydrogen and the intravascular indicators was related linearly to the dye appearance time (proportional to l/flow), as would be expected with diffusional shunting. These data indicate that there is arterial-venous shunting of hydrogen in the myocardium. Furthermore, diffusional shunting may be important in the delivery of oxygen to, or removal of carbon dioxide from, the heart.