Flexion-relaxation response to gravity

The objective of this report was to study the influence of the orientation of gravitational loading on the behavior of anterior and posterior trunk muscles during anterior trunk flexion-extension. Participants (N=13) performed five (5) cycles of trunk flexion-extension while standing with gravity parallel to the body axis and five (5) cycles while in the supine condition (e.g. sit-ups) with gravity perpendicular to the body axis. Surface electromyographic (EMG) patterns from lumbar paraspinal, rectus abdominis, external oblique, rectus femoris, semimembranosis, and biceps femoris muscles were analyzed during each condition. EMG signals were synchronized with lumbar flexion and trunk inclination angles. Flexion-extension from the standing position resulted in a myoelectric silent period of the lumbar posterior muscles (e.g. flexion-relaxation phenomena (FRP)) as well as the hamstring muscles through deep angles during which activity was observed in abdominal muscles. Flexion-extension during sit-ups, however, resulted in a myoelectric silent period of the abdominal muscles and the quadriceps through deep angles during which the lumbar posterior muscles were active. In this condition, the FRP was not observed in posterior muscles. The new findings demonstrate the profound impact of the orientation of the gravity vector on the FRP, the abdominal muscles reaction to gravitational loads during sit-ups and its relationships with lumbar antagonists and thigh musculature. The new findings suggest that gravitational moments requirements dominate the FRP through the prevailing kinematics, load sharing and reflex activation-inhibition of muscles in various conditions. Lumbar kinematics or fixed sensory motor programs by themselves, however, are not the major contributor to the FRP. The new findings improve our insights into spinal biomechanics as well as understanding and evaluating low back disorders.     

Dihydroceramide-based response to hypoxia

To understand the mechanisms of ceramide-based responses to hypoxia, we performed a mass spectrometry-based survey of ceramide species elicited by a wide range of hypoxic conditions (0.2-5% oxygen). We describe a rapid, time-dependent, marked up-regulation of dihydroceramides (DHCs) in mammalian cells and in the lungs of hypoxic rats. The increase affected all DHC species and was proportional with the depth and duration of hypoxia, ranging from 2- (1 h) to 10-fold (24 h), with complete return to normal after 1 h of reoxygenation at the expense of increased ceramides. We demonstrate that a DHC-based response to hypoxia occurs in a hypoxia-inducible factor-independent fashion and is catalyzed by the DHC desaturase (DEGS) in the de novo ceramide pathway. Both the impact of hypoxia on DHC molecular species and its inhibitory effect on cell proliferation were reproduced by knockdown of DEGS1 or DEGS2 by siRNA during normoxia. Conversely, overexpression of DEGS1 or DEGS2 attenuated the DHC accumulation and increased cell proliferation during hypoxia. Based on the amplitude and kinetics of DHC accumulation, the enzymatic desaturation of DHCs fulfills the criteria of an oxygen sensor across physiological hypoxic conditions, regulating the balance between biologically active components of ceramide metabolism.     

Biochemical response to cadmium

The effect of daily oral administration of ethanol (2.5, 5, or 10% in drinking water for 8 wk), lead (10 mg/kg, po, once daily for 8 wk), or their combination on tissue trace-metal concentration and hematopoietic and hepatic biochemical indices was investigated in male rats. Ethanol (10%) ingestion enhanced the hepatic lipid peroxidation and decreased the calcium and magnesium content of blood and liver. Coexposure to lead and ethanol (5 and 10%) produced a more pronounced elevation of blood zinc protoporphyrin (ZPP) and hepatic lipid peroxidation. Combined lead-ethanol exposure also lowered the concentration of blood and hepatic magnesium and calcium and increased the amount of lead in the blood, liver, and brain compared to a group treated with lead alone. The results suggest that chronic alcohol ingestion results in calcium and magnesium loss. However, coexposure to lead and ethanol could result in more serious depletion of calcium and magnesium, and this could be the cause of suspected synergism between alcohol consumption and lead poisoning.     

A response to Purdy

The authors respond to Laura Purdy's article, "Surrogate mothering: exploitation or empowerment?," in the same issue of Bioethics. They contend that focusing on what is necessarily wrong with surrogate motherhood contracts allows Purdy to overlook the contingent features of classist, patriarchal society that make such contracts morally wrong and to marginalize feminist concerns. Theoretical fallacies within Purdy's consequentialist framework create too individualistic and narrow a discussion of the possible harms of surrogate contracts, ignoring influences upon women as a group and the psychological risks to the surrogate mother. If surrogacy contracts have the potential to empower women, then we must see some specific changes that would make this a reality, given that Purdy does not mean surrogacy as it is currently practised.     

Cockroach heart response to cardio-accelerators

Stimulation of the segmental nerve bundle of the American cockroach, Periplaneta americana caused a short increase in heartbeat rate of the isolated heart. The amount of rate increase was related sigmoidally to the stimulation frequency. Very low frequency stimulation of the segmental bundle caused initial slowing of the heartbeat rate.The cardio-accelerating property of cockroach blood and of lateral cardiac nerve cord extracts could not be compared with in vitro stimulation of the segmental nerve.The blood factor was heat-stable, dialyzable and did not withstand ashing. The factor was not a simple ion imbalance, and acted both on the cardiac neurons and the denervated myocardium by causing an increase in activity.The effect of the blood factor could be reproduced by assay of of low concentrations of a molecule with calcium-chelating properties. Since the cardio-acceleration had no specific site of action, and its effects could be reproduced by moleccules with calcium-chelating abilities, the so-called cardio-excitatory factors were considered artifacts until their actions are more clearly defined in vivo.

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Zusammenfassung Das isolierte Herz von Periplaneta americana reagiert auf Segmentalnervenreize mit Anstieg der Herzschlagfrequenz. Die Pulszunahme ist sigmoidal von der Stimulationsfrequenz abhängig.Blut-und Herzseitennerven-Extrakte, die eine Erhöhung des Pulsschlags des isolierten Herzens verursachen, wirkten nachweislich durch Verstärkung der Aktivität der Herzneurone und durch direkten Einfluß auf das denervierte Myocard. Zwischen Stimulation und den extrahierten herzanregenden Faktoren wurden keine Beziehungen gefunden.Der Blutfaktor war hitzestabil, dialysierbar und gegenüber den üblichen chromatographischen Verfahren beständig. Da die Herzbeschleunigung keinen spezifischen Wirkungsort hat und ihre Wirkungen durch Moleküle mit Calcium-chelierenden Eigenschaften hervorgerufen werden konnten, wären die sogenannten herzerregenden Faktoren als Artefakte zu betrachten, bis ihre Wirkungen klarer in vivo definiert werden können.

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Supported by USPHS Training Grant PH-GM-1076.

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Address until September 1969 is Dept. Zoology, The University, Glasgow, W.2, Scotland.     

Immune response to heavy exertion

Nieman, David C. Immune response to heavyexertion. J. Appl. Physiol. 82(5):1385-1394, 1997.Epidemiological data suggest that enduranceathletes are at increased risk for upper respiratory tract infectionduring periods of heavy training and the 1- to 2-wk period followingrace events. There is growing evidence that, for several hourssubsequent to heavy exertion, several components of both the innate(e.g., natural killer cell activity and neutrophil oxidative burstactivity) and adaptive (e.g., T and B cell function) immune systemexhibit suppressed function. At the same time, plasma pro- andanti-inflammatory cytokines are elevated, in particular interleukin-6-and interleukin-1-receptor antagonist. Various mechanisms explainingthe altered immunity have been explored, including hormone-inducedtrafficking of immune cells and the direct influence of stresshormones, prostaglandin-E₂, cytokines, and other factors. The immune response to heavy exertion istransient, and further research on the mechanisms underlying the immuneresponse to prolonged and intensive endurance exercise is necessarybefore meaningful clinical applications can be drawn. Some attemptshave been made through chemical or nutritional means (e.g.,indomethacin, glutamine, vitamin C, and carbohydrate supplementation) to attenuate immune changes following intensive exercise.