Kirsten Møller

Human Endotoxemia as a Model of Systemic Inflammation

Systemic inflammation is a pathogenetic component in a vast number of acute and chronic diseases such as sepsis, trauma, type 2 diabetes, atherosclerosis, and Alzheimers disease, all of which are associated with a substantial morbidity and mortality. However, the molecular mechanisms and physiological significance of the systemic inflammatory response are still not fully understood. The human endotoxin model, an in vivo model of systemic inflammation in which lipopolysaccharide is injected or infused intravenously in healthy volunteers, may be helpful in unravelling these issues. The present review addresses the basic changes that occur in this model. The activation of inflammatory cascades as well as organ-specific haemodynamic and functional changes after lipopolysaccharide are described, and the limitations of human-experimental models for the study of clinical disease are discussed. Finally, we outline the ethical considerations that apply to the use of human endotoxin model.

Interleukin‐6 release from the human brain during prolonged exercise

Interleukin (IL)‐6 is a pleiotropic cytokine, which has a variety of physiological roles including functions within the central nervous system. Circulating IL‐6 increases markedly during exercise, partly due to the release of IL‐6 from the contracting skeletal muscles, and exercise‐induced IL‐6 may be linked with central fatigue, which is enhanced by hyperthermia. Exercise‐induced IL‐6 may also stimulate hepatic glycogenolysis, which is important during prolonged and repeated exercise. Thus, in a randomised order and separated by 60 min of rest, eight young male subjects completed two 60 min exercise bouts: one bout with a normal (38 °C) and the other with an elevated (39.5 °C) core temperature. The cerebral IL‐6 response was determined on the basis of internal jugular venous to arterial IL‐6 differences and global cerebral blood flow. There was no net release or uptake of IL‐6 in the brain at rest or after 15 min of exercise, but a small release of IL‐6 was observed after 60 min of exercise in the first bout (0.06 ± 0.03 ng min−1). This release of IL‐6 from the brain was five‐fold greater at the end of the second bout (0.30 ± 0.08 ng min−1; P < 0.05) with no separate influence of hyperthermia. In conclusion, IL‐6 is released from the brain during prolonged exercise in humans and it appears that the duration of the exercise rather than the increase in body temperature dictates the cerebral IL‐6 response.

Ageing Is Associated with a Prolonged Fever Response in Human Endotoxemia

ABSTRACT The purpose of this study was to investigate whether an age-associated impaired acute-phase response exists. Nine healthy elderly volunteers (median, 66 years; range, 61 to 69 years) and eight young controls (median, 24 years; range, 20 to 27 years) were given an intravenous bolus of endotoxin (2 ng/kg). The rectal temperature was monitored continuously, and blood samples for cytokine measurements were obtained before endotoxin administration as well as 0.5, 1, 1.5, 2, 3, 4, 8, 12, and 24 h after the injection. The elderly subjects showed a more prolonged fever response compared to the young controls. Levels of tumor necrosis factor alpha (TNF-α), soluble TNF receptors (sTNFR-I), interleukin-6 (IL-6), IL-8, IL-10, and IL-1 receptor antagonist (IL-1ra) in plasma increased markedly following endotoxin administration in both groups. The elderly group showed larger initial increases in TNF-α and sTNFR-I levels and prolonged increased levels of sTNFR-I. Monocyte concentrations decreased in both groups, with the elderly group showing a more rapid decrease and a slower subsequent increase than did the young group. Furthermore, the elderly group had a more rapid increase in C-reactive protein levels than did the young group. In conclusion, ageing is associated with an altered acute-phase response including initial hyperreactivity, prolonged inflammatory activity, and prolonged fever response.

Unchanged cerebral blood flow and oxidative metabolism after acclimatization to high altitude.

The authors investigated the effect of acclimatization to high altitude on cerebral blood flow and oxidative metabolism at rest and during exercise. Nine healthy, native sea-level residents were studied 3 weeks after arrival at Chacaltaya, Bolivia (5,260 m) and after reacclimatization to sea level. Global cerebral blood flow at rest and during exercise on a bicycle ergometer was measured by the Kety-Schmidt technique. Cerebral metabolic rates of oxygen, glucose, and lactate were calculated by the Fick principle. Cerebral function was assessed by a computer-based measurement of reaction time. At high altitude at rest, arterial carbon dioxide tension, oxygen saturation, and oxygen tension were significantly reduced, and arterial oxygen content was increased because of an increase in hemoglobin concentration. Global cerebral blood flow was similar in the four conditions. Cerebral oxygen delivery and cerebral metabolic rates of oxygen and glucose also remained unchanged, whereas cerebral metabolic rates of lactate increased slightly but nonsignificantly at high altitude during exercise compared with high altitude at rest. Reaction time was unchanged. The data indicate that cerebral blood flow and oxidative metabolism are unaltered after high-altitude acclimatization from sea level, despite marked changes in breathing and other organ functions.

Cerebral ammonia uptake and accumulation during prolonged exercise in humans

We evaluated whether peripheral ammonia production during prolonged exercise enhances the uptake and subsequent accumulation of ammonia within the brain. Two studies determined the cerebral uptake of ammonia (arterial and jugular venous blood sampling combined with Kety–Schmidt‐determined cerebral blood flow; n= 5) and the ammonia concentration in the cerebrospinal fluid (CSF; n= 8) at rest and immediately following prolonged exercise either with or without glucose supplementation. There was a net balance of ammonia across the brain at rest and at 30 min of exercise, whereas 3 h of exercise elicited an uptake of 3.7 ± 1.3 μmol min−1 (mean ±s.e.m.) in the placebo trial and 2.5 ± 1.0 μmol min−1 in the glucose trial (P < 0.05 compared to rest, not different across trials). At rest, CSF ammonia was below the detection limit of 2 μm in all subjects, but it increased to 5.3 ± 1.1 μm following exercise with glucose, and further to 16.1 ± 3.3 μm after the placebo trial (P < 0.05). Correlations were established between both the cerebral uptake (r2= 0.87; P < 0.05) and the CSF concentration (r2= 0.72; P < 0.05) and the arterial ammonia level and, in addition, a weaker correlation (r2= 0.37; P < 0.05) was established between perceived exertion and CSF ammonia at the end of exercise. The results let us suggest that during prolonged exercise the cerebral uptake and accumulation of ammonia may provoke fatigue, e.g. by affecting neurotransmitter metabolism.

Effect of transcutaneous electrical muscle stimulation on muscle volume in patients with septic shock.

Objective:Intensive care unit admission is associated with muscle wasting and impaired physical function. We investigated the effect of early transcutaneous electrical muscle stimulation on quadriceps muscle volume in patients with septic shock. Design:Randomized interventional study using a single-legged exercise design with the contralateral leg serving as a paired control. Setting:A mixed 18-bed intensive care unit at a tertiary care university hospital. Patients:Eight adult male intensive care unit patients with septic shock included within 72 hrs of diagnosis. Interventions:After randomization of the quadriceps muscles, transcutaneous electrical muscle stimulation was applied on the intervention side for 7 consecutive days and for 60 mins per day. All patients underwent computed tomographic scans of both thighs immediately before and after the 7-day treatment period. The quadriceps muscle was manually delineated on the computed tomography slices, and muscle volumes were calculated after three-dimensional reconstruction. Measurements and Main Results:Median age and Acute Physiology and Chronic Health Evaluation II score were 67 years (interquartile range, 64–72 years) and 25 (interquartile range, 20–29), respectively. During the 7-day study period, the volume of the quadriceps muscle on the control thigh decreased by 16% (4–21%, p = .03) corresponding to a rate of 2.3% per day. The volume of the stimulated muscle decreased by 20% (3–25%, p = .04) corresponding to a rate of 2.9% per day (p = .12 for the difference in decrease). There was no difference in muscle volume between the stimulated and nonstimulated thigh at baseline (p = .10) or at day 7 (p = .12). The charge delivered to the muscle tissue per training session (0.82 [0.66–1.18] coulomb) correlated with the maximum sequential organ failure assessment score. Conclusions:We observed a marked decrease in quadriceps volume within the first week of intensive care for septic shock. This loss of muscle mass was unaffected by transcutaneous electrical muscle stimulation applied for 60 mins per day for 7 days.

Exercise induces the release of heat shock protein 72 from the human brain in vivo

Abstract The present study tested the hypothesis that in response to physical stress the human brain has the capacity to release heat shock protein 72 (Hsp72) in vivo. Therefore, 6 humans (males) cycled for 180 minutes at 60% of their maximal oxygen uptake, and the cerebral Hsp72 response was determined on the basis of the internal jugular venous to arterial difference and global cerebral blood flow. At rest, there was a net balance of Hsp72 across the brain, but after 180 minutes of exercise, we were able to detect the release of Hsp72 from the brain (335 ± 182 ng/min). However, large individual differences were observed as 3 of the 6 subjects had a marked increase in the release of Hsp72, whereas exercise had little effect on the cerebral Hsp72 balance in the remaining 3 subjects. Given that cerebral blood flow was unchanged during exercise compared with values obtained at rest, it is unlikely that the cerebral Hsp72 release relates to necrosis of specific cells within the brain. These data demonstrate that the human brain is able to release Hsp72 in vivo in response to a physical stressor such as exercise. Further study is required to determine the biological significance of these observations.

Altered free radical metabolism in acute mountain sickness: implications for dynamic cerebral autoregulation and blood–brain barrier function

We tested the hypothesis that dynamic cerebral autoregulation (CA) and blood–brain barrier (BBB) function would be compromised in acute mountain sickness (AMS) subsequent to a hypoxia‐mediated alteration in systemic free radical metabolism. Eighteen male lowlanders were examined in normoxia (21% O2) and following 6 h passive exposure to hypoxia (12% O2). Blood flow velocity in the middle cerebral artery (MCAv) and mean arterial blood pressure (MAP) were measured for determination of CA following calculation of transfer function analysis and rate of regulation (RoR). Nine subjects developed clinical AMS (AMS+) and were more hypoxaemic relative to subjects without AMS (AMS–). A more marked increase in the venous concentration of the ascorbate radical (A•−), lipid hydroperoxides (LOOH) and increased susceptibility of low‐density lipoprotein (LDL) to oxidation was observed during hypoxia in AMS+ (P < 0.05 versus AMS–). Despite a general decline in total nitric oxide (NO) in hypoxia (P < 0.05 versus normoxia), the normoxic baseline plasma and red blood cell (RBC) NO metabolite pool was lower in AMS+ with normalization observed during hypoxia (P < 0.05 versus AMS–). CA was selectively impaired in AMS+ as indicated both by an increase in the low‐frequency (0.07–0.20Hz) transfer function gain and decrease in RoR (P < 0.05 versus AMS–). However, there was no evidence for cerebral hyper‐perfusion, BBB disruption or neuronal–parenchymal damage as indicated by a lack of change in MCAv, S100β and neuron‐specific enolase. In conclusion, these findings suggest that AMS is associated with altered redox homeostasis and disordered CA independent of barrier disruption.

Neuro-oxidative-nitrosative stress in sepsis

Neuro-oxidative-nitrosative stress may prove the molecular basis underlying brain dysfunction in sepsis. In the current review, we describe how sepsisinduced reactive oxygen and nitrogen species (ROS/RNS) trigger lipid peroxidation chain reactions throughout the cerebrovasculature and surrounding brain parenchyma, due to failure of the local antioxidant systems. ROS/RNS cause structural membrane damage, induce inflammation, and scavenge nitric oxide (NO) to yield peroxynitrite (ONOO−). This activates the inducible NO synthase, which further compounds ONOO− formation. ROS/RNS cause mitochondrial dysfunction by inhibiting the mitochondrial electron transport chain and uncoupling oxidative phosphorylation, which ultimately leads to neuronal bioenergetic failure. Furthermore, in certain ‘at risk’ areas of the brain, free radicals may induce neuronal apoptosis. In the present review, we define a role for ROS/RNS-mediated neuronal bioenergetic failure and apoptosis as a primary mechanism underlying sepsis-associated encephalopathy and, in sepsis survivors, permanent cognitive deficits.

Interleukin-6 Markedly Decreases Skeletal Muscle Protein Turnover and Increases Nonmuscle Amino Acid Utilization in Healthy Individuals

CONTEXT IL-6 is a key modulator of immune function and suggested to be involved in skeletal muscle wasting as seen in sepsis. OBJECTIVE Our objective was to determine the role of IL-6 in human in vivo systemic and skeletal muscle amino acid metabolism and protein turnover. SUBJECTS AND METHODS There were 12 healthy men infused for 3 h with saline (saline, n = 6) or recombinant human IL (rhIL)-6 (n = 6). Systemic and muscle protein turnover was determined with a combination of tracer dilution methodology, primed constant infusion of L-[ring-(2)H(5)]phenylalanine, and femoral arterial-venous blood differences and m. vastus lateralis biopsies after 2-h basal, 3-h infusion, and 3 h after infusion. RESULTS The IL-6 concentration after 30-min infusion was approximately 4 (saline) and 140 pg/ml (rhIL-6). Three-hour rhIL-6 infusion caused an approximate 50% decrease in muscle protein turnover, albeit synthesis was more suppressed than breakdown, causing a small increase in net muscle protein breakdown. Furthermore, rhIL-6 decreased arterial amino acid concentration with 20-40%, despite the increase net release from muscle. CONCLUSIONS We demonstrated that IL-6 profoundly alters amino acid turnover. A substantial decrease in plasma amino acids was observed with a concomitant 50% decrease in muscle protein turnover, however, modest increase in net muscle degradation. We hypothesize that the profound reduction in muscle protein turnover and modest increase in net degradation are primarily caused by the reduced plasma amino acid availability and not directly mediated by IL-6.