Evidence for impaired skeletal muscle contraction-induced rapid vasodilation in aging humans

We tested the hypothesis that aging is associated with an impaired contraction-induced rapid vasodilation in healthy adults. We reasoned that employing single contractions of a small muscle mass would allow us to isolate the local rapid vasodilatory responses independent of systemic hemodynamic and sympathetic neural influences on forearm hemodynamics. We measured forearm blood flow (Doppler ultrasound) and arterial blood pressure (Finapres) on a beat-by-beat basis and calculated the changes in forearm vascular conductance (DeltaFVC) in response to forearm contractions in 18 young (24 +/- 1 yr) and 13 older (62 +/- 2 yr) healthy subjects. Single, 1-s dynamic forearm contractions were performed with the experimental arm slightly above heart level at 5, 10, 20, and 40% of the subjects' maximal voluntary contraction (MVC) in random order. In general, muscle contractions evoked a rapid increase in FVC that reached a peak within approximately four to five cardiac cycles postcontraction in both age groups. At 5% MVC, there were no significant age-related differences in contraction-induced forearm vasodilation. However, the peak vasodilatory responses were impaired approximately 40-45% in older adults at 10, 20, and 40% MVC, as were the total vasodilatory responses (area under curve approximately 40-50%; all P < 0.05). Additionally, the immediate vasodilation (first cardiac cycle postcontraction) for the 20% and 40% MVC trials was also impaired approximately 50% with age (P < 0.05). There were no significant age-group differences in MVC or forearm fat-free mass, and these variables were not correlated with local vasodilation within a given exercise intensity. Under the experimental conditions employed, the blunted responses with age reflect impaired local contraction-induced rapid vasodilation.     

Attenuated carotid body hypoxic sensitivity after prolonged hypoxic exposure

Prolonged exposure to hypoxia is accompanied by decreased hypoxic ventilatory response (HVR), but the relative importance of peripheral and central mechanisms of this hypoxic desensitization remain unclear. To determine whether the hypoxic sensitivity of peripheral chemoreceptors decreases during chronic hypoxia, we measured ventilatory and carotid sinus nerve (CSN) responses to isocapnic hypoxia in five cats exposed to simulated altitude of 5,500 m (barometric pressure 375 Torr) for 3-4 wk. Exposure to 3-4 wk of hypobaric hypoxia produced a decrease in HVR, measured as the shape parameter A in cats both awake (from 53.9 +/- 10.1 to 14.8 +/- 1.8; P less than 0.05) and anesthetized (from 50.2 +/- 8.2 to 8.5 +/- 1.8; P less than 0.05). Sustained hypoxic exposure decreased end-tidal CO2 tension (PETCO2, 33.3 +/- 1.2 to 28.1 +/- 1.3 Torr) during room-air breathing in awake cats. To determine whether hypocapnia contributed to the observed depression in HVR, we also measured eucapnic HVR (PETCO2 33.3 +/- 0.9 Torr) and found that HVR after hypoxic exposure remained lower than preexposed value (A = 17.4 +/- 4.2 vs. 53.9 +/- 10.1 in awake cats; P less than 0.05). A control group (n = 5) was selected for hypoxic ventilatory response matched to the baseline measurements of the experimental group. The decreased HVR after hypoxic exposure was associated with a parallel decrease in the carotid body response to hypoxia (A = 20.6 +/- 4.8) compared with that of control cats (A = 46.9 +/- 6.3; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for hypoxic depression of CO2-ventilation response in carotid body-resected humans

Steady-state CO2-ventilation response curves with hyperoxia (end-tidal PO2 greater than 200 Torr) and mild hypoxia (end-tidal PO2 approximately equal to 60 Torr) were compared in five carotid body-resected (BR) patients and five control patients. The data were analyzed by fitting a linear equation, V = S(PETCO2-B), where V is minute ventilation S is the response curve slope. PETCO2 is end-tidal PCO2, and B is the response curve threshold. S slightly increased from hyperoxia to hypoxia in both BR and control groups. On the other hand, B moderately increased with hypoxia in BR patients, whereas it slightly decreased in controls. These changes were all not significant. However, in accordance with the change in B, the response curve to hypoxia at V of 10 1/min was significantly shifted in opposite directions in the two groups, i.e., rightward and leftward shift in BR and control groups, respectively. Thus the average magnitude of V calculated at PETCO2 of 40 Torr in hypoxia was significantly lower in BR patients than in controls (P less than 0.01). We conclude that this hypoxic depression of the CO2-ventilation response found in BR patients may have resulted, at least in part, from modulation of the brain stem neural mechanisms that were elicited by loss of afferent discharges from the carotid body.     

Attenuated thermoregulatory sweating and cutaneous vasodilation after 14-day bed rest in humans.

We investigated the effect of head-down bed rest (HDBR) for 14 days on thermoregulatory sweating and cutaneous vasodilation in humans. Fluid intake was ad libitum during HDBR. We induced whole body heating by increasing skin temperature for 1 h with a water-perfused blanket through which hot water (42 degrees C) was circulated. The experimental room was air-conditioned (27 degrees C, 30-40% relative humidity). We measured skin blood flow (chest and forearm), skin temperatures (chest, upper arm, forearm, thigh, and calf), and tympanic temperature. We also measured sweat rate by the ventilated capsule method in which the skin area for measurement was drained by dry air conditioned at 27 degrees C under similar skin temperatures in both trials. We calculated cutaneous vascular conductance (CVC) from the ratio of skin blood flow to mean blood pressure. From tympanic temperature-sweat rate and -CVC relationships, we assessed the threshold temperature and sensitivity as the slope response of variables to a given change in tympanic temperature. HDBR increased the threshold temperature for sweating by 0.31 degrees C at the chest and 0.32 degrees C at the forearm, whereas it reduced sensitivity by 40% at the chest and 31% at the forearm. HDBR increased the threshold temperature for cutaneous vasodilation, whereas it decreased sensitivity. HDBR reduced plasma volume by 11%, whereas it did not change plasma osmolarity. The increase in the threshold temperature for sweating correlated with that for cutaneous vasodilation. In conclusion, HDBR attenuated thermoregulatory sweating and cutaneous vasodilation by increasing the threshold temperature and decreasing sensitivity. HDBR increased the threshold temperature for sweating and cutaneous vasodilation by similar magnitudes, whereas it decreased their sensitivity by different magnitudes.     

Short-term hypoxic exposure at rest and during exercise reduces lung water in healthy humans.

Hypoxia and hypoxic exercise increase pulmonary arterial pressure, cause pulmonary capillary recruitment, and may influence the ability of the lungs to regulate fluid. To examine the influence of hypoxia, alone and combined with exercise, on lung fluid balance, we studied 25 healthy subjects after 17-h exposure to 12.5% inspired oxygen (barometric pressure = 732 mmHg) and sequentially after exercise to exhaustion on a cycle ergometer with 12.5% inspired oxygen. We also studied subjects after a rapid saline infusion (30 ml/kg over 15 min) to demonstrate the sensitivity of our techniques to detect changes in lung water. Pulmonary capillary blood volume (Vc) and alveolar-capillary conductance (D(M)) were determined by measuring the diffusing capacity of the lungs for carbon monoxide and nitric oxide. Lung tissue volume and density were assessed using computed tomography. Lung water was estimated by subtracting measures of Vc from computed tomography lung tissue volume. Pulmonary function [forced vital capacity (FVC), forced expiratory volume after 1 s (FEV(1)), and forced expiratory flow at 50% of vital capacity (FEF(50))] was also assessed. Saline infusion caused an increase in Vc (42%), tissue volume (9%), and lung water (11%), and a decrease in D(M) (11%) and pulmonary function (FVC = -12 +/- 9%, FEV(1) = -17 +/- 10%, FEF(50) = -20 +/- 13%). Hypoxia and hypoxic exercise resulted in increases in Vc (43 +/- 19 and 51 +/- 16%), D(M) (7 +/- 4 and 19 +/- 6%), and pulmonary function (FVC = 9 +/- 6 and 4 +/- 3%, FEV(1) = 5 +/- 2 and 4 +/- 3%, FEF(50) = 4 +/- 2 and 12 +/- 5%) and decreases in lung density and lung water (-84 +/- 24 and -103 +/- 20 ml vs. baseline). These data suggest that 17 h of hypoxic exposure at rest or with exercise resulted in a decrease in lung water in healthy humans.     

DNA adducts in humans after exposure to methylating agents

Human exposure to methylating agents appears to be widespread, as indicated by the frequent occurrence of methylated DNA adducts in human DNA. The high incidence of methylated DNA adducts even in humans thought not to have suffered extensive exposure to environmental methylating agents implies that chemicals of endogenous origin, probably N-nitroso compounds such as the strongly carcinogenic N-nitrosodimethylamine (NDMA), may be primarily responsible for their formation and raises the question of the carcinogenic risks associated with such exposure. In addition to accumulation of DNA damage, other factors (such as induced cell proliferation) appear to be important in determining the probability of induction of mutation or cancer by NDMA, implying that high to low dose risk extrapolations should not be based on the assumption of dose- or even adduct-linearity. Comparative studies of the accumulation and repair of methylated adducts in humans and animals treated with methylating cytostatic drugs do not reveal significant species differences. Based on this and the dosimetry of adduct accumulation in rats chronically exposed to very low doses of NDMA, it is suggested that the exposure needed to account for the levels of adducts found in human DNA may be of the order of hundreds of micrograms NDMA (or equivalent) per day, a level of exposure which may well represent a significant carcinogenic hazard for man.     

Two patterns of daily hypoxic exposure and their effects on measures of chemosensitivity in humans.

The purpose of this study was to compare chemoresponses following two different intermittent hypoxia (IH) protocols in humans. Ten men underwent two 7-day courses of poikilocapnic IH. The long-duration IH (LDIH) protocol consisted of daily 60-min exposures to normobaric 12% O(2). The short-duration IH (SDIH) protocol comprised twelve 5-min bouts of 12% O(2), separated by 5-min bouts of room air, daily. Isocapnic hypoxic ventilatory response (HVR) was measured daily during the protocol and 1 and 7 days following. Hypercapnic ventilatory response (HCVR) and CO(2) threshold and sensitivity (by the modified Read rebreathing technique) were measured on days 1, 8, and 14. Following 7 days of IH, the mean HVR was significantly increased from 0.47 +/- 0.07 and 0.47 +/- 0.08 to 0.70 +/- 0.06 and 0.79 +/- 0.06 l.min(-1).%Sa(O(2))(-1) (LDIH and SDIH, respectively), where %Sa(O(2)) is percent arterial oxygen saturation. The increase in HVR reached a plateau after the third day. One week post-IH, HVR values were unchanged from baseline. HCVR increased from 3.0 +/- 0.4 to 4.0 +/- 0.5 l.min(-1).mmHg(-1). In both the hyperoxic and hypoxic modified Read rebreathing tests, the slope of the CO(2)/ventilation plot was unchanged by either intervention, but the CO(2)/ventilation curve shifted to the left following IH. There were no correlations between the changes in response to hypoxia and hypercapnia. There were no significant differences between the two IH protocols for any measures, indicating that comparable changes in chemoreflex control occur with either protocol. These results also suggest that the two methods of measuring CO(2) response are not completely concordant and that the changes in CO(2) control do not correlate with the increase in the HVR.     

Methylene blue inhibits hypoxic cerebral vasodilation in awake sheep.

Cerebral vasodilation in hypoxia may involve endothelium-derived relaxing factor-nitric oxide. Methylene blue (MB), an in vitro inhibitor of soluble guanylate cyclase, was injected intravenously into six adult ewes instrumented chronically with left ventricular, aortic, and sagittal sinus catheters. In normoxia, MB (0.5 mg/kg) did not alter cerebral blood flow (CBF, measured with 15-microns radiolabeled microspheres), cerebral O2 uptake, mean arterial pressure (MAP), heart rate, cerebral lactate release, or cerebral O2 extraction fraction (OEF). After 1 h of normobaric poikilocapnic hypoxia (arterial PO2 40 Torr, arterial O2 saturation 50%), CBF increased from 51 +/- 5.8 to 142 +/- 18.8 ml.min-1 x 100 g-1, cerebral O2 uptake from 3.5 +/- 0.25 to 4.7 +/- 0.41 ml.min-1 x 100 g-1, cerebral lactate release from 2 +/- 10 to 100 +/- 50 mumol.min- x 100 g-1, and heart rate from 107 +/- 5 to 155 +/- 9 beats/min (P < 0.01). MAP and OEF were unchanged from 91 +/- 3 mmHg and 48 +/- 4%, respectively. In hypoxia, 30 min after MB (0.5 mg/kg), CBF declined to 79.3 +/- 11.7 ml.min-1 x 100 g-1 (P < 0.01), brain O2 uptake (4.3 +/- 0.9 ml.min-1 x 100 g-1) and heart rate (133 +/- 9 beats/min) remained elevated, cerebral lactate release became negative (-155 +/- 60 mumol.min-1 x 100 g-1, P < 0.01), OEF increased to 57 +/- 3% (P < 0.01), and MAP (93 +/- 5 mmHg) was unchanged. The sheep became behaviorally depressed, probably because of global cerebral ischemia. These results may be related to interference with a guanylate cyclase-dependent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostanoids contribute to cutaneous active vasodilation in humans

The specific mechanisms by which skin blood flow increases in response to a rise in core body temperature via cutaneous active vasodilation are poorly understood. The primary purpose of this study was to determine whether the cyclooxygenase (COX) pathway contributes to active vasodilation during whole body heat stress (protocol 1; n = 9). A secondary goal was to verify that the COX pathway does not contribute to the cutaneous hyperemic response during local heating (protocol 2; n = 4). For both protocols, four microdialysis fibers were placed in forearm skin. Sites were randomly assigned and perfused with 1) Ringer solution (control site); 2) ketorolac (KETO), a COX-1/COX-2 pathway inhibitor; 3) NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor; and 4) a combination of KETO and L-NAME. During the first protocol, active vasodilation was induced using whole body heating with water-perfused suits. The second protocol used local heaters to induce a local hyperemic response. Red blood cell flux (RBC flux) was indexed at all sites using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC; RBC flux/mean arterial pressure) was normalized to maximal vasodilation at each site. During whole body heating, CVC values at sites perfused with KETO (43 +/- 9% CVCmax), L-NAME (35 +/- 9% CVCmax), and combined KETO/L-NAME (22 +/- 8% CVCmax) were significantly decreased with respect to the control site (59 +/- 7% CVCmax) (P < 0.05). Additionally, CVC at the combined KETO/L-NAME site was significantly decreased compared with sites infused with KETO or L-NAME alone (P < 0.05). In the second protocol, the hyperemic response to local heating did not differ between the control site and KETO site or between the L-NAME and KETO/L-NAME site. These data suggest that prostanoids contribute to active vasodilation, but do not play a role during local thermal hyperemia.     

Effects of live high, train low hypoxic exposure on lactate metabolism in trained humans.

We determined the effect of 20 nights of live high, train low (LHTL) hypoxic exposure on lactate kinetics, monocarboxylate lactate transporter proteins (MCT1 and MCT4), and muscle in vitro buffering capacity (betam) in 29 well-trained cyclists and triathletes. Subjects were divided into one of three groups: 20 consecutive nights of hypoxic exposure (LHTLc), 20 nights of intermittent hypoxic exposure [four 5-night blocks of hypoxia, each interspersed with 2 nights of normoxia (LHTLi)], or control (Con). Rates of lactate appearance (Ra), disappearance (Rd), and oxidation (Rox) were determined from a primed, continuous infusion of l-[U-14C]lactic acid tracer during 90 min of steady-state exercise [60 min at 65% peak O2 uptake (VO(2 peak)) followed by 30 min at 85% VO(2 peak)]. A resting muscle biopsy was taken before and after 20 nights of LHTL for the determination of betam and MCT1 and MCT4 protein abundance. Ra during the first 60 min of exercise was not different between groups. During the last 25 min of exercise at 85% VO(2 peak), Ra was higher compared with exercise at 65% of VO(2 peak) and was decreased in LHTLc (P < 0.05) compared with the other groups. Rd followed a similar pattern to Ra. Although Rox was significantly increased during exercise at 85% compared with 65% of VO(2 peak), there were no differences between the three groups or across trials. There was no effect of hypoxic exposure on betam or MCT1 and MCT4 protein abundance. We conclude that 20 consecutive nights of hypoxia exposure decreased whole body Ra during intense exercise in well-trained athletes. However, muscle markers of lactate metabolism and pH regulation were unchanged by the LHTL intervention.     

Effects of supplemental oxygen on forearm vasodilation in humans

Crawford, Paul, Peter A. Good, Eric Gutierrez, Joshua H. Feinberg, John P. Boehmer, David H. Silber, and Lawrence I. Sinoway. Effects of supplemental oxygen on forearm vasodilation in humans.J. Appl. Physiol. 82(5):1601-1606, 1997.Supplemental O₂ reduces cardiac output andraises systemic vascular resistance in congestive heart failure. Inthis study, 100% O₂ was given tonormal subjects and peak forearm flow was measured. Inexperiment 1, 100%O₂ reduced blood flow andincreased resistance after 10 min of forearm ischemia (flow 56.7 ± 7.9 vs. 47.8 ± 6.7 ml · min¹ · 100 ml¹;P < 0.02; vascular resistance 1.7 ± 0.2 vs. 2.4 ± 0.4 mmHg · min · 100 ml · ml¹;P < 0.03). Inexperiment 2, lower body negativepressure (LBNP; 30 mmHg) and venous congestion (VC) simulatedthe high sympathetic tone and edema of congestive heart failure.Postischemic forearm flow and resistance were measured under fourconditions: room air breathing (RA); LBNP+RA; RA+LBNP+VC; and 100%O₂+LBNP+VC. LBNP and VC did notlower peak flow. However, O₂raised minimal resistance (2.3 ± 0.4 RA; 2.8 ± 0.5 O₂+LBNP+VC,P < 0.04). When O₂ alone(experiment 1) was compared withO₂+LBNP+VC(experiment 2), no effect of LBNP+VCon peak flow or minimum resistance was noted, although the return rateof flow and resistance toward baseline was increased.O₂ reduces peak forearm flow evenin the presence of LBNP and VC.

     

Impaired glucose homeostasis after a transient intermittent hypoxic exposure in neonatal rats

This initial report presents a neonatal rat model with exposure to a transient intermittent hypoxia (IH), which results in a persisting diabetes-like condition in the young rats. Twenty-five male pups were treated at postnatal day 1 with IH exposure by alternating the level of oxygen between 10.3% and 20.8% for 5 h. The treated animals were then maintained in normal ambient oxygen condition for 3 week and compared to age-matched controls. The IH treated animals exhibited a significantly higher fasting glucose level than the control animals (237.00 ± 19.66 mg/dL vs. 167.25 ± 2.95 mg/dL; P = 0.003); and a significantly lower insulin level than the control (807.0 ± 72.5 pg/mL vs. 1839.8 ± 377.6 pg/mL; P = 0.023). There was no difference in the mass or the number of insulin producing beta cells as well as no indicative of inflammatory changes; however, glucose tolerance tests showed a significantly disturbed glucose homeostasis. In addition, the amount of C-peptide secreted from the islets harvested from the IH animals were decreased significantly (from 914 pM in control to 809 pM in IH; P = 0.0006) as well. These observations demonstrate that the neonatal exposure to the IH regimen initiates the development of deregulation in glucose homeostasis without infiltration of inflammatory cells.     

Antioxidant status in humans after exposure to hyperbaric oxygen.

Hyperbaric oxygen (HBO) treatment (i.e., exposure to 100% oxygen at a pressure of 2.5 atmosphere absolute (ATA) for a total of 3 x 20 min periods) of human subjects induced DNA damage in the alkaline comet assay with leukocytes and protected against the DNA damaging effects of subsequent in vivo HBO exposures. Furthermore, blood taken 24 h after the first HBO was well protected against the in vitro induction of genotoxic effects by hydrogen peroxide. To investigate the mechanisms which led to this apparent adaptive response, we determined the antioxidant status of blood from subjects before and after HBO. We did not find differences in the plasma concentrations of the antioxidant vitamins A, C and E after HBO treatment. HBO had also no effect on the 'antioxidant power' of the plasma as measured with the FRAP-assay or on the concentration of reduced glutathione determined in the plasma or in lymphocytes. Red cell concentrate activities of superoxide dismutase, catalase, glutathione peroxidase were not influenced by HBO. In contrast, synthesis of the heat shock protein HSP70 which has been implicated to play an important role in cellular protection against oxidative stress, was significantly induced in lymphocytes after a single HBO treatment. To investigate whether intake of antioxidants may protect against HBO-induced DNA damage, we supplemented subjects with vitamin E (800 mg for 7 days) or with N-acetylcysteine (400 mg, 1 h before the HBO treatment). However, these supplementations did not influence the induction of DNA damage by HBO.     

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.     

Adenosine contributes to hypoxia-induced forearm vasodilation in humans.

In humans, hypoxia leads to increased sympathetic neural outflow to skeletal muscle. However, blood flow increases in the forearm. The mechanism of hypoxia-induced vasodilation is unknown. To test whether hypoxia-induced vasodilation is cholinergically mediated or is due to local release of adenosine, normal subjects were studied before and during acute hypoxia (inspired O(2) 10.5%; approximately 20 min). In experiment I, aminophylline (50-200 microg. min(-1). 100 ml forearm tissue(-1)) was infused into the brachial artery to block adenosine receptors (n = 9). In experiment II, cholinergic vasodilation was blocked by atropine (0.4 mg over 4 min) infused into the brachial artery (n = 8). The responses of forearm blood flow (plethysmography) and forearm vascular resistance to hypoxia in the infused and opposite (control) forearms were compared. During hypoxia (arterial O(2) saturation 77 +/- 2%), minute ventilation and heart rate increased while arterial pressure remained unchanged; forearm blood flow rose by 35 +/- 6% in the control forearm but only by 5 +/- 8% in the aminophylline-treated forearm (P < 0.02). Accordingly, forearm vascular resistance decreased by 29 +/- 5% in the control forearm but only by 9 +/- 6% in the aminophylline-treated forearm (P < 0.02). Atropine did not attenuate forearm vasodilation during hypoxia. These data suggest that adenosine contributes to hypoxia-induced vasodilation, whereas cholinergic vasodilation does not play a role.