Transport of nitrite from large arteries modulates regional blood flow during stress and exercise.

Background: Acute cardiovascular stress increases systemic wall shear stress (WSS)-a frictional force exerted by the flow of blood on vessel walls-which raises plasma nitrite concentration due to enhanced endothelial nitric oxide synthase (eNOS) activity. Upstream eNOS inhibition modulates distal perfusion, and autonomic stress increases both the consumption and vasodilatory effects of endogenous nitrite. Plasma nitrite maintains vascular homeostasis during exercise and disruption of nitrite bioavailability can lead to intermittent claudication. Hypothesis: During acute cardiovascular stress or strenuous exercise, we hypothesize enhanced production of nitric oxide (NO) by vascular endothelial cells raises nitrite concentrations in near-wall layers of flowing blood, resulting in cumulative NO concentrations in downstream arterioles sufficient for vasodilation. Confirmation and implications: Utilizing a multiscale model of nitrite transport in bifurcating arteries, we tested the hypothesis for femoral artery flow under resting and exercised states of cardiovascular stress. Results indicate intravascular transport of nitrite from upstream endothelium could result in vasodilator-active levels of nitrite in downstream resistance vessels. The hypothesis could be confirmed utilizing artery-on-a-chip technology to measure NO production rates directly and help validate numerical model predictions. Further characterization of this mechanism may improve our understanding of symptomatic peripheral artery occlusive disease and exercise physiology.

Method for estimating pulsatile wall shear stress from one-dimensional velocity waveforms.

Wall shear stress (WSS)-a key regulator of endothelial function-is commonly estimated in vivo using simplified mathematical models based on Poiseuille's flow, assuming a quasi-steady parabolic velocity distribution, despite evidence that more rapidly time-varying, pulsatile blood flow during each cardiac cycle modulates flow-mediated dilation (FMD) in large arteries of healthy subjects. More exact and accurate models based on the well-established Womersley solution for rapidly changing blood flow have not been adopted clinically, potentially because the Womersley solution relies on the local pressure gradient, which is difficult to measure non-invasively. We have developed an open-source method for automatic reconstruction of unsteady, Womersley-derived velocity profiles, and WSS in conduit arteries. The proposed method (available online at https://doi.org/10.5281/zenodo.7576408) requires only the time-averaged diameter of the vessel and time-varying velocity data available from non-invasive imaging such as Doppler ultrasound. Validation of the method with subject-specific computational fluid dynamics and application to synthetic velocity waveforms in the common carotid, brachial, and femoral arteries reveals that the Poiseuille solution underestimates peak WSS 38.5%-55.1% during the acceleration and deceleration phases of systole and underestimates or neglects retrograde WSS. Following evidence that oscillatory shear significantly augments vasodilator production, it is plausible that mischaracterization of the shear stimulus by assuming parabolic flow leads to systematic underestimates of important biological effects of time-varying blood velocity in conduit arteries.

Hemodynamic modeling of the circle of Willis reveals unanticipated functions during cardiovascular stress.

The circle of Willis (CW) allows blood to be redistributed throughout the brain during local ischemia; however, it is unlikely that the anatomic persistence of the CW across mammalian species is driven by natural selection of individuals with resistance to cerebrovascular disease typically occurring in elderly humans. To determine the effects of communicating arteries (CoAs) in the CW on cerebral pulse wave propagation and blood flow velocity, we simulated young, active adult humans undergoing different states of cardiovascular stress (i.e., fear and aerobic exercise) using discrete transmission line segments with stress-adjusted cardiac output, peripheral resistance, and arterial compliance. Phase delays between vertebrobasilar and carotid pulses allowed bidirectional shunting through CoAs: both posteroanterior shunting before the peak of the pulse waveform and anteroposterior shunting after internal carotid pressure exceeded posterior cerebral pressure. Relative to an absent CW without intact CoAs, the complete CW blunted anterior pulse waveforms, although limited to 3% and 6% reductions in peak pressure and pulse pressure, respectively. Systolic rate of change in pressure (i.e., ∂P/∂t) was reduced 15%-24% in the anterior vasculature and increased 23%-41% in the posterior vasculature. Bidirectional shunting through posterior CoAs was amplified during cardiovascular stress and increased peak velocity by 25%, diastolic-to-systolic velocity range by 44%, and blood velocity acceleration by 134% in the vertebrobasilar arteries. This effect may facilitate stress-related increases in blood flow to the cerebellum (improving motor coordination) and reticular-activating system (enhancing attention and focus) via a nitric oxide-dependent mechanism, thereby improving survival in fight-or-flight situations.NEW & NOTEWORTHY Hemodynamic modeling reveals potential evolutionary benefits of the intact circle of Willis (CW) during fear and aerobic exercise. The CW equalizes pulse waveforms due to bidirectional shunting of blood flow through communicating arteries, which boosts vertebrobasilar blood flow velocity and acceleration. These phenomena may enhance perfusion of the brainstem and cerebellum via nitric oxide-mediated vasodilation, improving performance of the reticular-activating system and motor coordination in survival situations.

Abnormal response of tumor vasculature to vasoactive drugs.

The effects of the vasoconstrictor phenylephrine and the vasodilator hydralazine on blood flow to tumor were studied and compared to those on blood flow to normal tissues in vivo. Regional blood flow and cardiac output were measured with the use of radioactive microspheres in 150- to 250-g inbred Harlan F344 rats bearing subcutaneous nodules of two types of transplantable carcinoma ("hard" and "soft") with microscopically different vascular patterns. Three groups of rats were treated with hydralazine, saline, or phenylephrine, and regional blood flow was determined at the time of maximum blood pressure response. Results were correlated with quantitative morphometric analysis of arteriolar and capillary wall thickness in tumor and normal tissue. Phenylephrine decreased and hydralazine increased normal tissue perfusion as indicated by cardiac output. Tumor blood flow remained low and was not significantly influenced by drug treatment, except for the phenylephrine effect on hard tumors. Histologic study of tumor vessel walls revealed an absence of smooth muscle capable of responding to the vasoactive drugs by constriction or dilation. Evidently, by their selective action on normal vessels, vasoactive drugs can change the ratio of tumor:normal tissue perfusion. In particular, the increase of normal tissue: tumor blood flow by vasodilator drugs may enhance the selectivity of local heat therapy.

Irradiation-hyperthermia in canine hemangiopericytomas: large-animal model for therapeutic response.

Results of irradiation-hyperthermia treatment in 11 dogs with naturally occurring hemangiopericytoma were reported. Similarities of canine and human hemangiopericytomas were described. Orthovoltage X-irradiation followed by microwave-induced hyperthermia resulted in a 91% objective response rate. A statistical procedure was given to evaluate quantitatively the clinical behavior of locally invasive, nonmetastatic tumors in dogs that were undergoing therapy for control of local disease. The procedure used a small sample size and demonstrated distribution of the data on a scaled response as well as transformation of the data through classical parametric and nonparametric statistical methods. These statistical methods set confidence limits on the population mean and placed tolerance limits on a population percentage. Application of the statistical methods to human and animal clinical trials was apparent.

CPR techniques that combine chest and abdominal compression and decompression: hemodynamic insights from a spreadsheet model.

BACKGROUND: This study was done to elucidate mechanisms by which newer cardiopulmonary resuscitation (CPR) techniques, including interposed abdominal compression (IAC), active compression-decompression (ACD), and Lifestick CPR, augment systemic perfusion pressure and forward flow and to compare the 3 techniques in the same test system. METHODS AND RESULTS: Mathematical models describing hemodynamics of the adult human circulation during cardiac arrest and CPR were created and exercised by use of spreadsheet software. Assumptions of the models are limited to normal human anatomy and physiology, the definition of compliance (volume change/pressure change), and Ohm's law (flow=pressure/resistance). Standard CPR generates 1.3 L/min forward and 25 mm Hg systemic perfusion pressure. In otherwise identical models, IAC-CPR generates 2.4 L/min and 45 mm Hg; ACD-CPR, 1.6 L/min and 30 mm Hg; and Lifestick CPR, which combines IAC and ACD, 3.1 L/min and 58 mm Hg. Augmented CPR techniques work by enhanced priming of either chest or abdominal pump mechanisms. CONCLUSIONS: Adjunctive maneuvers, combined with conventional chest compression, can produce substantial hemodynamic benefit in CPR by credible physiological mechanisms.

Twenty-four hour survival in a canine model of cardiac arrest comparing three methods of manual cardiopulmonary resuscitation.

Two new modifications of manual cardiopulmonary resuscitation, high impulse compression at a rate of 120/min and interposed abdominal compression at a rate of 60/min, have been reported to produce better hemodynamic responses than standard cardiopulmonary resuscitation at 60/min. However, the effect of these two new methods on initial resuscitation success and 24 hour survival is unknown. In this study, 30 mongrel dogs were divided into three equal groups, each treated with one of three types of manual cardiopulmonary resuscitation. Ventricular fibrillation was induced electrically in morphinized, endotracheally intubated dogs emerging from halothane anesthesia. After 3 minutes of circulatory arrest without intervention, one of the three techniques of manual cardiopulmonary resuscitation was begun, and continued for 17 minutes. Defibrillation was performed at 20 minutes. Successful resuscitation was defined as a mean arterial blood pressure of at least 60 mm Hg, without chest compressions, 10 minutes after the initial defibrillation attempt. Intensive care was provided for 2 hours, including hemodynamic and respiratory monitoring, and drug intervention when required. Twenty-four hour survival and neurologic deficit were used as critical measures of outcome. Ten of 30 animals survived 24 hours with a mean neurologic deficit score of 5% (normal = 0, brain dead = 100). There was no difference in initial resuscitation success, 24 hour survival or neurologic deficit of the survivors among the three manual cardiopulmonary resuscitation methods. Aortic diastolic and calculated coronary perfusion pressures were similar for all three methods. Well performed standard manual cardiopulmonary resuscitation is as effective as these modified versions (high impulse compression and interposed abdominal compression) when compared in the same animal model.

Changes in expired end-tidal carbon dioxide during cardiopulmonary resuscitation in dogs: a prognostic guide for resuscitation efforts.

Expired end-tidal carbon dioxide (PCO2) measurements made during cardiopulmonary resuscitation have correlated with cardiac output and coronary perfusion pressure when wide ranges of blood flow are included. The utility of such measurements for predicting resuscitation outcome during the low flow state associated with closed chest cardiopulmonary resuscitation remains uncertain. Expired end-tidal PCO2 and coronary perfusion pressures were measured in 15 mongrel dogs undergoing 15 min of closed chest cardiopulmonary resuscitation after a 3 min period of untreated ventricular fibrillation. In six successfully resuscitated dogs, the mean expired end-tidal PCO2 was significantly higher than that in nine nonresuscitated dogs only after 14 min of cardiopulmonary resuscitation (6.2 +/- 1.2 versus 3.4 +/- 0.8 mm Hg; p less than 0.05). No differences in expired end-tidal PCO2 values were found at 2, 7 or 12 min of cardiopulmonary resuscitation. A significant decline in end-tidal PCO2 levels during the resuscitation effort was seen in the nonresuscitated group (from 6.3 +/- 0.8 to 3.4 +/- 0.8 mm Hg; p less than 0.05); the successfully resuscitated group had constant PCO2 levels throughout the 15 min of cardiac arrest (from 6.8 +/- 1.1 to 6.2 +/- 1.2 mm Hg). Changes in expired PCO2 levels during cardiopulmonary resuscitation may be a useful noninvasive predictor of successful resuscitation and survival from cardiac arrest.

Protection from reperfusion injury in the isolated rat heart by postischaemic deferoxamine and oxypurinol administration.

A Langendorff isolated rat heart preparation was used to determine the effect of oxypurinol, a xanthine oxidase inhibitor, and deferoxamine, an iron binding agent, on the extent of myocardial reperfusion injury after 60 minutes of ischaemia. Thirty rats were divided into three groups of 10, and an isolated heart preparation made from each rat. The isolated hearts were perfused for 15 minutes with a modified Krebs-Henseleit perfusate solution to permit stabilisation of the preparation. Each heart was then subjected to 60 minutes of total ischaemia at 37 degrees C followed by 60 minutes of reperfusion with either saline treated perfusate, oxypurinol treated perfusate (1.3 mmol.litre-1), or deferoxamine treated perfusate (0.61 mmol.litre-1). Reperfusion injury was assessed by the total amount of creatine phosphokinase released into the perfusate, by changes in myocardial vascular resistance, and by morphological examination. The saline treated group released significantly more creatine phosphokinase into the perfusate than either the oxypurinol treated group (p less than 0.05) or the deferoxamine treated group (p less than 0.05). The mean vascular resistance increased for all groups during the 60 minutes of reperfusion compared with that just before ischaemia but was significantly greater in the saline treated group than in the drug treated groups (p less than 0.01). Ultrastructural examination of a randomly selected heart from each group after 60 minutes of reperfusion showed pronounced attenuation of mitochondrial and endoplasmic reticulum swelling, increased maintenance of membrane integrity, and diminished separation of myofilaments in the oxypurinol treated and deferoxamine treated hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Histochemical demonstration of endothelial superoxide and hydrogen peroxide generation in ischaemic and reoxygenated rat tissues.

OBJECTIVE: The aims were to test and evaluate two novel and independent histochemical methods for detecting the initial postischaemic burst of superoxide and hydrogen peroxide in buffer perfused rat tissues during reflow after 60 min warm ischaemia. METHODS: The first is a high manganese/diaminobenzidine technique, in which superoxide oxidises Mn2+ to Mn3+, which in turn oxidises diaminobenzidine to form amber coloured polymers, observable by light microscopy. The second is a high iron/diaminobenzidine technique, in which hydrogen peroxide oxidises diethylenetriaminepenta-acetate chelated Fe2+ to form intermediate species, which in turn oxidise diaminobenzidine similarly to Mn3+. Various isolated organs of the rat were rendered ischaemic for 60 min, and reperfused with oxygen or air equilibrated buffers containing diaminobenzidine and either Mn2+ or Fe2+. Tissues were fixed by perfusion with Trump's solution and processed for light microscopy. RESULTS: Both manganese and iron methods consistently showed the appearance of reaction product on the luminal surfaces of arterial, capillary, and venular endothelial cells in lung, heart, and intestine of the rat during the first 2 to 3 min of reoxygenation after ischaemia. The histochemical reactions were nearly absent in non-manganese-treated and non-iron-treated controls. Superoxide dismutase strongly inhibited Mn2+/diaminobenzidine reaction product formation and catalase strongly inhibited Fe2+/diaminobenzidine reaction product formation, when tested in specially perfused lung preparations in which these specific antioxidant enzymes were concentrated. CONCLUSIONS: These histochemical techniques provide direct, visual evidence that a burst of reactive oxygen species is generated in postischaemic rat tissues. The Mn2+/diaminobenzidine and Fe2+/diaminobenzidine techniques permit investigation of the endothelium derived reactive oxygen by simple laboratory procedures available to almost any investigator at low marginal cost. The endothelial oxidants so revealed may be of pathophysiological significance in a variety of cardiovascular disorders.

Free radicals and the etiology of colon cancer.

This hypothesis paper reviews diverse evidence suggesting that intracolonic production of oxygen radicals may play a role in carcinogenesis. The hypothesis began to evolve when the author made the chance discovery that 1/10,000 dilutions of feces generated detectable quantities of highly reactive hydroxyl radicals (HO.). The rate of HO. formation, detected using DMSO as a molecular probe, was quite remarkable, corresponding to that which would be produced by over 10,000 rads of gamma irradiation per day, absorbed in the periphery of the fecal mass adjacent to the mucosa. The relatively high concentrations of iron in feces, together with the ability of bile pigments to act as iron chelators that support Fenton chemistry, may very well permit efficient HO. generation from superoxide and hydrogen peroxide produced by bacterial metabolism. Such free radical generation in feces could provide a missing link in our understanding of the etiology of colon cancer: the oxidation of procarcinogens either by fecal HO., or by secondary peroxyl radicals (ROO.) to form active carcinogens or mitogenic tumor promotors. Intracolonic free radical formation may explain the high incidence of cancer in the colon and rectum, compared to other regions of the GI tract, as well as the observed correlations of a higher incidence of colon cancer with red meat in the diet, which increases stool iron, and with excessive fat in the diet, which may increase the fecal content of procarcinogens and bile pigments.

Oxygen radicals in ulcerative colitis.

This article reviews the pathophysiologic concept that superoxide and hydrogen peroxide, generated by activated leukocytes, together with low-molecular-weight chelate iron derived from fecal sources and from denatured hemoglobin, amplify the inflammatory response and subsequent mucosal damage in patients with active episodes of ulcerative colitis. The putative pathogenic mechanisms reviewed are as follows: (1) Dietary iron is concentrated in fecal material owing to normally limited iron absorption. (2) Mucosal bleeding, characteristic of ulcerative colitis, as well as supplemental oral iron therapy for chronic anemia, further conspire to maintain or elevate mucosal iron concentration in colitis. (3) Fenton chemistry, driven especially by leukocyte-generated superoxide and hydrogen peroxide, leads to formation of hydroxyl radicals. (4) The resultant oxidative stress leads to the extension and propagation of crypt abscesses, either through direct membrane disruption by lipid peroxidation or through generation of secondary toxic oxidants such as chloramines. (5) Chemotactic products of lipid peroxidation, including 4-hydroxynonenal, provide positive feedback to accelerate this inflammatory/oxidative process, leading to acute exacerbations of the disease. (6) Other oxidized products, such as oxidized tryptophan metabolites, created by free radical mechanisms in or near the mucosa, may act as carcinogens or tumor promotors that contribute to the exceedingly high incidence of colon carcinoma in patients suffering from chronic ulcerative colitis. In this way, self-sustaining cycles of oxidant formation may amplify flare-ups of inflammation and mucosal injury in ulcerative colitis. This concept, if proved correct by subsequent research, would provide a rationale for several novel clinical approaches to the management of ulcerative colitis, including use of SOD mimetics, iron chelators, and chain-breaking antioxidants.

Simulation of free radical reactions in biology and medicine: a new two-compartment kinetic model of intracellular lipid peroxidation.

To explore mechanisms of free radical reactions leading to intracellular lipid peroxidation in living systems, we developed a computational model of up to 109 simultaneous enzymatic and free radical reactions thought to be involved in the initiation, propagation, and termination of membrane lipid peroxidation. Rate constants for the various reactions were obtained from the published literature. The simulation model included a lipid membrane compartment and an aqueous cytosolic compartment, between which various chemical species were partitioned. Lipid peroxidation was initiated by the iron-catalyzed, superoxide-driven Fenton reaction. A "C" language computer program implemented numerical solution of the steady-state rate equations for concentrations of nine relevant free radicals. The rate equations were integrated by a modified Euler technique to describe the evolution with time of simulated concentrations of hydrogen peroxide, ferric and ferrous iron, unsaturated lipid, lipid hydroperoxides, superoxide anion, and biological antioxidants, including SOD and catalase. Initial results led to significant insights regarding mechanisms of membrane lipid peroxidation: 1. segregation and concentration of lipids within membrane compartments promotes chain propagation; 2. in the absence of antioxidants computed concentrations of lipid hydroperoxides increase linearly about 40 microM/min during oxidative stress; 3. lipid peroxidation is critically dependent upon oxygen concentration and the modeled dependence is similar to the experimental function; 4. lipid peroxidation is rapidly quenched by the presence of Vitamin E-like antioxidants, SOD, and catalase; 5. only small (1 to 50 microM) amounts of "free" iron are required for initiation of lipid peroxidation; 6. substantial lipid peroxidation occurs only when cellular defense mechanisms have been weakened or overcome by prolonged oxidative stress, hence understanding of the balance between free radical generation and antioxidant defense systems is critical to the understanding and control of free radical reactions in biology and medicine.

Hydroxyl radical generation by postischemic rat kidney slices in vitro.

To quantitate the formation of hydroxyl radicals (HO.) in ischemia and reoxygenation, dimethyl sulfoxide (DMSO) was added to "trap" evolving HO. in normal, in ischemic, and in ischemic and reoxygenated rat kidney slices, incubated in short-term organ culture in vitro. Hydroxyl radical generation was measured as the accumulation of the specific product of DMSO oxidation by HO., methane sulfinic acid (MSA) in the kidney tissue and surrounding medium using a new colorimetric assay. A mean difference of 7 nmol cumulative HO./gram tissue was detected in rat kidney slices subjected to ischemia and reoxygenation. This amount of HO. generation was not significantly greater than that found in nonischemic or in ischemic but not reoxygenated control tissues, and does not appear to represent the highly toxic burst of HO. radicals implied in current theoretical discussions of reperfusion injury. However, the addition of EDTA chelated iron (1:1) to the incubation medium led to marked postischemic HO. generation. We conclude that clearly toxic numbers of HO. radicals are not formed during reoxygenation in rat kidney slices, either because there is insufficient iron, because only a small fraction of cells in the kidney tissue make oxygen radicals, or because cellular defenses against HO. formation are more powerful than currently appreciated.

Scatchard analysis of methane sulfinic acid production from dimethyl sulfoxide: a method to quantify hydroxyl radical formation in physiologic systems.

A major impediment to the confirmation of free radical mechanisms in pathogenesis is a lack of direct, chemical evidence that oxygen centered free radicals actually arise in living tissues in quantities sufficient to cause serious damage. This investigation was conducted to validate the use of dimethyl sulfoxide (DMSO) as a quantitative molecular probe for the generation of hydroxyl radicals (HO.) under physiologic conditions. Reaction of HO. with DMSO produces methane sulfinic acid (MSA) as a primary product, which can be detected by a simple colorimetric assay. To develop a method for estimating total HO. production, we studied two model systems: the superoxide driven Fenton reaction in vitro, using xanthine oxidase as the source of superoxide, and a computer model of Fenton chemistry. Measured MSA production both in vitro and in the computer model was a predictable function of the concentrations of DMSO and competing scavengers of HO., according to the principle of competition kinetics. Both experimental results and model calculations showed that Scatchard analysis may be used to infer total HO. generation, despite the presence of scavengers other than DMSO, such as mannitol. Thus, methane sulfinic acid production from DMSO holds promise as an easily measured marker for HO. formation in biologic systems pretreated with DMSO, and Scatchard analysis of repeated experiments with varying DMSO concentrations can yield an estimate of total HO. generation.

Effect of oxygen concentration on the formation of malondialdehyde-like material in a model of tissue ischemia and reoxygenation.

This study was conducted to explore the functional relationship between oxygen concentration during tissue reoxygenation after ischemia and the extent of postischemic lipid peroxidation, an indicator of reoxygenation injury. Excised rat liver or kidney tissue was rendered ischemic for 1 h at 37 degrees C, minced into 1 mm3 fragments, and then reoxygenated for 1 h in flasks of buffered salt solution containing various amounts of oxygen. Production of malondialdehyde-like material (MDA) was measured to indicate lipid peroxidation. MDA production was minimal at oxygen tensions less than 10 mmHg, increased sharply from 10 to 50 mmHg, and plateaued at approximately 100 mmHg. A similar functional relationship was produced by a simple mathematical model of free radical mediated lipid peroxidation in biological membranes, suggesting that MDA production is indeed caused by free radical oxidation of membrane phospholipids and that the oxygen effect is governed by simple competition between chain propagation and chain termination reactions within the membrane. These experimental and analytical results confirm that relatively low concentrations of oxygen are sufficient to produce oxidative damage in post-ischemic tissues.

Detection and quantitation of hydroxyl radical using dimethyl sulfoxide as molecular probe.

Generation of clearly harmful amounts of hydroxyl radicals in biological systems can be studied using DMSO as a molecular probe. DMSO is oxidized by HO. to form the stable, nonradical compound methanesulfinic acid, which is not normally found in living systems and which can be easily extracted from tissue and measured spectrophotometrically. The present method provides a simple, inexpensive assay for methanesulfinic acid in biological materials. As little as 10 nmol of sulfinate can be detected, and interference from diverse biological compounds is minimal. Additionally, there is no interference from a large excess of dimethyl sulfoxide, which is necessary if the assay is to be applied directly to tissues pretreated with DMSO. When straightforward cleanup procedures are utilized, there is minimal interference from glutathione or sulfate, and potentially troublesome interference from detergentlike substances can usually be minimized. Owing to its relative specificity for sulfinic acids at acid pH, the diazonium coupling reaction can thus be exploited to provide an efficient and inexpensive means of detecting methanesulfinic acid in DMSO-pretreated biological materials. The results provide a direct chemical means for measuring cumulative HO. generation.

Association of magnesium deficiency with the blood pressure-lowering effects of calcium.

The role of dietary calcium and magnesium in the development of hypertension was studied in nine groups, each consisting of nine spontaneously hypertensive rats aged 8-31 weeks. The animals were fed AIN 76A semi-purified diets varying in calcium (0.075, 0.5 and 2.5%) and magnesium (0.01, 0.05 and 0.75%) concentrations according to a 3 x 3 factorial design. Dietary calcium and systolic blood pressure were inversely related, significantly (P less than 0.05) after 12 weeks. Total and ultrafilterable serum calcium concentrations were also significantly negatively correlated with blood pressure (r = -0.46; P = 0.001 and r = -0.57; P = 0.001, respectively). Repeated measures analysis of variance indicated that dietary magnesium had no effect on systolic blood pressure, and no calcium x magnesium interaction on blood pressure was observed. Signs of magnesium deficiency, calcium deposits in the kidneys, and histological lesions were observed in groups on a high-calcium diet receiving normal and low levels of magnesium. Thus a lowering of blood pressure by calcium supplementation, without concomitant magnesium supplementation, was accompanied by biochemical and histological abnormalities in this animal model.

Methylene blue as an inhibitor of superoxide generation by xanthine oxidase. A potential new drug for the attenuation of ischemia/reperfusion injury.

Tissue oxidases, especially xanthine oxidase, have been proposed as primary sources of toxic oxygen radicals in many experimental models of disease states. Among these, ischemia-reperfusion injury may be of the greatest clinical interest. In this paper we propose the use of methylene blue as a means of suppressing the production of superoxide radicals O2- by acting as an alternative electron acceptor for xanthine oxidase. Previous work has indicated that methylene blue accepts electrons from xanthine oxidase at the iron-sulfur center. Initial experiments in our laboratory demonstrated that (1) pairs of electrons from each enzymatic oxidation are transferred to methylene blue, (2) the reduction of methylene blue can be achieved by model iron-sulfur centers, similar to the iron-sulfur center of xanthine oxidase, (3) reduced methylene blue auto-oxidizes to produce H2O2 directly, rather than O2-, and (4) methylene blue is effective at non-toxic levels (2-5 mg/kg) in preventing free radical damage to liver and kidney tissues in an in vitro model of ischemia and reoxygenation. Accordingly, we propose that methylene blue may represent a new class of antioxidant drugs that competitively inhibit reduction of molecular oxygen to superoxide by acting as alternative electron acceptors for tissue oxidases. We have termed these agents "parasitic" electron acceptors.

Improved preferential tumor hyperthermia with regional heating and systemic blood cooling: a balanced heat transfer method.

Absorption of power in large body volumes can occur with some approaches used for hyperthermia treatment of cancer. A systemic heat absorption rate exceeding the heat dissipation rate can lead to systemic temperature elevation that limits the magnitude and duration of application of power and hence the degree of preferential tumor temperature rise. We describe a hyperthermia approach consisting of regional electromagnetic power absorption and extracorporeal blood cooling with regulation of both systemic heat absorption and dissipation rates ("balanced heat transfer"). A test of this approach in five dogs with nonperfused tumor models demonstrated intratumoral temperatures greater than 42 degrees C, while systemic temperature remained at 33 degrees C and visceral temperatures within the heated region equilibrated between 33 and 42 degrees C. Solutions of the bioheat transfer equation were obtained for a simplified model with a tumor perfusion rate lower than surrounding normal tissue perfusion rate. In this model, the use of arterial blood temperatures less than 37 degrees C allowed higher power densities to be used, for given normal tissue temperatures, than when arterial temperature was greater than or equal to 37 degrees C. As a result, higher intratumoral temperatures were predicted. Control of arterial blood temperature using extracorporeal cooling may thus (1) limit systemic temperature rise produced by regional heating devices and (2) offer a means of improving intratumoral temperature elevations.