Robert L. Hester

Differential Inhibition of Functional Dilation of Small Arterioles by Indomethacin and Glibenclamide

Indomethacin or glibenclamide treatments attenuate functional dilation of larger-diameter “feed” arterioles paired with venules in hamster cremaster muscle. We tested the hypothesis that release of cyclooxygenase products from venules is important for functional dilation of third- and fourth-order arterioles. We also tested whether ATP-sensitive potassium channels are important during functional dilation of smaller arterioles. The microcirculation of hamster cremaster muscle was visualized with in vivo video microscopy. We measured diameter responses of third- and fourth-order arterioles paired and unpaired with venules in response to 2 minutes of muscle field stimulation (40 &mgr;s, 10 V, 1 Hz). Control diameters of vessels were 31±2 (n=19), 13±1 (n=12), 12±2 (n=12), and 10±1 (n=12) for paired and unpaired third-order and paired and unpaired fourth-order arterioles, respectively. In all groups, field stimulation resulted in increases in mean control diameter of >80%. Indomethacin (28 &mgr;mol/L) superfused on the preparation was used to inhibit cyclooxygenase metabolism, or glibenclamide (10 &mgr;mol/L) was used to block ATP-sensitive potassium channels. Indomethacin attenuated arteriolar vasodilations to electrical stimulation in paired third-order vessels only, whereas glibenclamide attenuated this vasodilation in all 4 groups. These results support a role for ATP-sensitive potassium channels in functional dilation of arterioles of all sizes regardless of whether or not they are paired with venules. Conversely, a role for cyclooxygenase products is limited to larger “feed arterioles” paired with venules. This study provides further evidence that venules may be the source of prostaglandin release during functional hyperemia.

HumMod: A Modeling Environment for the Simulation of Integrative Human Physiology

Mathematical models and simulations are important tools in discovering key causal relationships governing physiological processes. Simulations guide and improve outcomes of medical interventions involving complex physiology. We developed HumMod, a Windows-based model of integrative human physiology. HumMod consists of 5000 variables describing cardiovascular, respiratory, renal, neural, endocrine, skeletal muscle, and metabolic physiology. The model is constructed from empirical data obtained from peer-reviewed physiological literature. All model details, including variables, parameters, and quantitative relationships, are described in Extensible Markup Language (XML) files. The executable (HumMod.exe) parses the XML and displays the results of the physiological simulations. The XML description of physiology in HumMods modeling environment allows investigators to add detailed descriptions of human physiology to test new concepts. Additional or revised XML content is parsed and incorporated into the model. The model accurately predicts both qualitative and quantitative changes in clinical and experimental responses. The model is useful in understanding proposed physiological mechanisms and physiological interactions that are not evident, allowing one to observe higher level emergent properties of the complex physiological systems. HumMod has many uses, for instance, analysis of renal control of blood pressure, central role of the liver in creating and maintaining insulin resistance, and mechanisms causing orthostatic hypotension in astronauts. Users simulate different physiological and pathophysiological situations by interactively altering numerical parameters and viewing time-dependent responses. HumMod provides a modeling environment to understand the complex interactions of integrative physiology. HumMod can be downloaded at http://hummod.org

Insulin resistance and impaired functional vasodilation in obese Zucker rats.

Individuals with metabolic syndrome exhibit insulin resistance and an attenuated functional vasodilatory response to exercise. We have shown that impaired functional vasodilation in obese Zucker rats (OZRs) is associated with enhanced thromboxane receptor (TP)-mediated vasoconstriction. We hypothesized that insulin resistance, hyperglycemia/hyperlipidemia, and the resultant ROS are responsible for the increased TP-mediated vasoconstriction in OZRs, resulting in impaired functional vasodilation. Eleven-week-old male lean Zucker rats (LZRs) and OZRs were fed normal rat chow or chow containing rosiglitazone (5 mg.kg(-1).day(-1)) for 2 wk. In another set of experiment, LZRs and OZRs were treated with 2 mM tempol (drinking water) for 7-10 days. After the treatments, spinotrapezius muscles were prepared, and arcade arteriolar diameters were measured following muscle stimulation and arachidonic acid (AA) application (10 muM) in the absence and presence of the TP antagonist SQ-29548 (1 muM). OZRs exhibited higher insulin, glucose, triglyceride, and superoxide levels and increased NADPH oxidase activity compared with LZRs. Functional and AA-induced vasodilations were impaired in OZRs. Rosiglitazone treatment improved insulin, glucose, triglyceride, and superoxide levels as well as NADHP oxidase activity in OZRs. Both rosiglitazone and tempol treatment improved vasodilatory responses in OZRs with no effect in LZRs. SQ-29548 treatment improved vasodilatory responses in nontreated OZRs with no effect in LZRs or treated OZRs. These results suggest that insulin resistance and the resultant increased ROS impair functional dilation in OZRs by increasing TP-mediated vasoconstriction.

Systemic Hemodynamics and Regional Blood Flow During Chronic Nitric Oxide Synthesis Inhibition in Pregnant Rats

Pregnancy-induced hypertension in women is associated with severe vasoconstriction and reductions in organ blood flow and cardiac output. Recent studies have indicated that nitric oxide (NO) synthesis inhibition during mid to late gestation in pregnant rats results in severe hypertension and proteinuria. The purpose of this study was to determine the systemic hemodynamic and regional blood flow alterations associated with chronic NO synthesis inhibition in the pregnant rat. The study was conducted in four groups of rats: virgin rats (n=6), pregnant rats (n=10), virgin rats treated with L-NAME (n=6), and pregnant rats treated with L-NAME (n=11). Rats were treated with L-NAME in drinking water at a dose of 1 mg/d for a week starting from day 13 of gestation in pregnant rats or an equivalent time for virgins. Mean arterial pressure (MAP), cardiac output, total peripheral resistance (TPR), and regional flows were measured by tracing radiolabeled microspheres in conscious rats. Pregnant rats that were given L-NAME showed significantly higher MAP (137+/-6 versus 96+/-2 mm Hg), higher TPR (5.08+/-0.58 versus 2.90+/-0.44 mm Hg/mL/min/100 g), and lower cardiac output (87.4+/-8.4 versus 113.3+/-11.1 mL/min) than pregnant controls. Chronic NO synthesis inhibition decreased the renal blood flow in pregnant rats at a significantly greater magnitude than in virgin rats. Significant reductions in regional blood flow to the heart, lungs, liver, diaphragm, and skeletal muscles were also observed in pregnant rats treated with L-NAME. The results of this study indicate that NO may play a role in mediating the alterations in systemic hemodynamics and regional blood flow in late pregnant rats.

Hemodynamic Alterations in Hypertensive Obese Rabbits

There is little information on changes in overall and regional hemodynamics in obesity-associated hypertension. Therefore, the purpose of this study was to determine alterations in overall and regional blood flows and resistances in adipose and nonadipose tissues in a new model of obesity-associated hypertension in rabbits. Sixteen female New Zealand White rabbits were fed either a maintenance or high-fat diet; after 8 to 12 weeks cardiac output and regional blood flows were measured with the use of radioactive microspheres. Obese rabbits (5.22 +/- 0.14 versus 3.66 +/- 0.04 kg) had higher blood pressure (113 +/- 3 versus 95 +/- 1 mm Hg), cardiac output (812 +/- 59 versus 593 +/- 47 mL/min), and heart rate (269 +/- 12 versus 219 +/- 9 beats per minute) and lower overall peripheral resistance (0.14 +/- 0.01 versus 0.17 +/- 0.01 mm Hg/[mL/min]) than lean rabbits. Compared with lean controls, obese rabbits had higher weights of the ventricles, kidneys, liver, ovaries, adrenals, diaphragm, and spleen. Absolute blood flows were greater in the ventricles, kidneys, lungs, and ovaries, but differences were minimized when flows were normalized for organ weight. Adipose tissue flow per gram weight was significantly lower and resistance higher in obese rabbits. However, calculated total adipose tissue flow was higher in obese rabbits (86 versus 45 mL/min). Absolute resistances were lower in the left ventricle, kidneys, and large intestine, but when resistances were indexed for organ weight, kidney resistance tended to be higher in obese rabbits. These results indicate that even short periods of obesity-associated hypertension result in marked overall and regional hemodynamic changes.

The Determination of Hemodialysis Blood Recirculation Using Blood Urea Nitrogen Measurements

The determination of blood recirculation using blood urea nitrogen (BUN) measurements in hemodialysis patients is a standard technique. The accuracy and reproducibility of these calculations have never been determined. Two pairs of recirculation studies (study A and study B) were performed in 13 patients during a single dialysis treatment. Blood samples were analyzed for BUN and recirculation was calculated. The first recirculation study (study A) was performed within 1 hour of the initiation of dialysis, with a duplicate test of recirculation performed within 15 minutes. In study B, the dialyzer blood lines were reversed in an attempt to enhance blood recirculation. After 15 minutes, duplicate tests of recirculation were again performed. Calculated recirculations before the line reversal (study A) ranged from -3.3% to 11.9% in the first test and -2.9% to 12.2% in the second test. In study A, there was no correlation (P > 0.05, r = 0.09) between the first and second calculated recirculations. In study B, an increase in recirculation was observed. Calculated recirculations ranged from 16.3% to 53.5% for the first test and 5.4% to 58.1% for the second test. A significant relationship was observed in the calculated recirculation in study B (P < 0.05, r = 0.81). The results from the present study show that the use of BUN measurements may not provide a consistent indicator of access recirculation in a patient with a low recirculation. This lack of consistency should be considered when determining further clinical treatment.

Role of Nitric Oxide, Adenosine, and ATP-Sensitive Potassium Channels in Insulin-Induced Vasodilation

The resistance of various tissues to the vasodilator and metabolic effects of insulin may be an important risk factor in the genesis of hypertension observed in several pathological states. Because of this, it is important to understand the mechanisms by which insulin causes vasodilation. Because insulin is known to raise metabolism, one mechanism by which insulin causes vasodilation could be through metabolic vasodilation. Recently, however, it has been suggested that the insulin-induced vasodilation is mediated by the release of endothelium-derived nitric oxide. Using a model of muscle microcirculation (hamster cremaster), we examined the interactions between insulin, nitric oxide, and tissue metabolism to understand the potential mechanisms by which insulin causes vasodilation. Topical application of insulin (200 microU/mL) to the cremaster resulted in significant increases in arteriolar diameter. Second-order arteriolar diameter increased from 69.6 +/- 6 to 79.8 +/- 5 microns and fourth-order arteriolar diameter from 11.3 +/- 1 to 15.1 +/- 2 microns (n = 8). During nitric oxide synthase inhibition, topical application of insulin caused significant vasodilation in both second- and fourth-order arterioles. In contrast, both adenosine receptor antagonism and blockade of ATP-sensitive potassium channels prevented insulin-induced increases in arteriolar diameter. Our findings suggest a role for increased tissue metabolism, particularly the metabolite adenosine, in mediating insulin-induced vasodilation.

Reduced Cardiac Contractile Responsiveness to Isoproterenol in Obese Rabbits

Although obesity is characterized by increased sympathetic nervous system activity, there is often a paradoxical reduction in cardiovascular end-organ response to sympathetic stimulation. Mechanisms involved in reduced sympathetic responsiveness in obesity have not been well characterized. Therefore, we determined cardiac contractile responsiveness to beta-stimulation in the obese rabbit model using both isolated heart (IH) and isolated papillary muscle (IPM) preparations. Female New Zealand White rabbits were fed control (IH: n=9; IPM: n=6) or 10% fat diets (IH: n=9; IPM: n=7) for 12 weeks. Contractile responsiveness in the IH was determined using a modified Langendorff preparation to evaluate the dose-response relationship between isoproterenol and 1) peak developed pressure/g of left ventricular wet weight and 2) maximal rate of pressure development (+dP/dt/P). Contractile responsiveness in the IPM was determined using right ventricular papillary muscles to evaluate the dose-response relationship between isoproterenol and (1) peak developed tension (T)/mm2 cross-sectional area (CSA) and (2) maximal rate of tension development (dT/dt/CSA). In the IH, baseline and maximum developed pressure/g were reduced in obese rabbits by 37% and 31%, respectively (P< or =.05). In the IPM, baseline and maximum T/CSA responses were reduced in obese rabbits by 59% and 33%, respectively (P< or =.05). Potency of isoproterenol as reflected by the EC50 did not differ between lean and obese animals in either preparation. These results demonstrate that left ventricular contractility in obesity is reduced at baseline and in response to stimulation with isoproterenol and suggest that decreased responsiveness to beta-stimulation may be a factor in the obesity-related systolic dysfunction.

Validation of a computational platform for the analysis of the physiologic mechanisms of a human experimental model of hemorrhage

Computational models of integrative physiology may serve as a framework for understanding the complex adaptive responses essential for homeostasis in critical illness and resuscitation and may provide insights for design of diagnostics and therapeutics. In this study a computer model of human physiology was compared to results obtained from experiments using Lower Body Negative Pressure (LBNP) analog model of human hemorrhage. LBNP has been demonstrated to produce physiologic changes in humans consistent with hemorrhage. The computer model contains over 4000 parameters that describe the detailed integration of physiology based upon basic physical principles and established biologic interactions. The LBNP protocol consisted of a 5min rest period (0mmHg) followed by 5min of chamber decompression of the lower body to -15, -30, -45, and -60mmHg and additional increments of -10mmHg every 5min until the onset of hemodynamic decompensation (n=20). Physiologic parameters recorded include mean arterial pressure (MAP), cardiac output (CO), and venous oxygen saturation (SVO(2); from peripheral venous blood), during the last 30s at each LBNP level. The computer model analytic procedure recreates the investigational protocol for a virtual individual in an In Silico environment. After baseline normalization, the model predicted measurements for MAP, CO, and SVO(2) were compared to those observed through the entire range of LBNP. Differences were evaluated using standard statistical performance error measurements (median performance error (PE) <5%). The simulation results closely tracked the average changes observed during LBNP. The predicted MAP fell outside the standard error measurement for the experimental data at only LBNP -30mmHg while CO was more variable. The predicted SVO(2) fell outside the standard error measurement for the experimental data only during the post-LBNP recovery point. However, the statistical median PE measurement was found to be within the 5% objective error measure (1.3% for MAP, -3.5% for CO, and 3.95% for SVO(2)). The computer model was found to accurately predict the experimental results observed using LBNP. The model should be explored as a platform for studying concepts and physiologic mechanisms of hemorrhage including its diagnosis and treatment.

Non-invasive determination of recirculation in the patient on dialysis.

Recirculation of blood flow occurs when the fistula flow rate is inadequate to support the desired dialyzer blood flow. The percentage recirculation is normally calculated using the blood urea nitrogen of blood samples from the two dialyzer blood lines and a peripheral blood sample. However, this method is time consuming, costly, and may not always give accurate measurements. A technique was developed to measure recirculation using the injection of saline into the venous dialysis line. For this technique, an optical detector is placed across the arterial dialysis tubing, and the light intensity, which is proportional to the hematocrit, is continually measured using a computerized data collection system. After a baseline data collection period, 10 ml of saline is injected into the venous dialysis line using the sampling port. The saline that appears in the arterial dialysis line as a result of recirculation will cause a dilution of the blood and an increase in light intensity. In vitro testing showed an excellent correlation between the area under the dilution curve and percentage recirculation. This technique will provide a quick, inexpensive, and reliable measurement of recirculation.