Human skeletal muscle sympathetic nerve activity, heart rate and limb haemodynamics with reduced blood oxygenation and exercise.

Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Denmark.
The Journal of Physiology (Impact Factor: 4.38). 09/2003; 551(Pt 2):635-47. DOI: 10.1113/jphysiol.2003.044024
Source: PubMed

ABSTRACT Acute systemic hypoxia causes significant increases in human skeletal muscle sympathetic nerve activity (MSNA), heart rate and ventilation. This phenomenon is thought to be primarily mediated by excitation of peripheral chemoreceptors sensing a fall in arterial free oxygen partial pressure (Pa,O2). We directly tested the role of Pa,O2 on MSNA (peroneal microneurography), heart rate, ventilation and leg haemodynamics (n = 7-8) at rest and during rhythmic handgrip exercise by using carbon monoxide (CO) to mimic the effect of systemic hypoxia on arterial oxyhaemoglobin (approximately 20 % lower O2Hba), while normalising or increasing Pa,O2 (range 40-620 mmHg). The four experimental conditions were: (1) normoxia (Pa,O2 approximately 110 mmHg; carboxyhaemoglobin (COHb) approximately 2 %); (2) hypoxia (Pa,O2 approximately 40 mmHg; COHb approximately 2 %); (3) CO + normoxia (Pa,O2 approximately 110 mmHg; COHb approximately 23 %); and (4) CO + hyperoxia (Pa,O2 approximately 620 mmHg; COHb ~24 %). Acute hypoxia augmented sympathetic burst frequency, integrated MSNA, heart rate and ventilation compared to normoxia over the entire protocol (7-13 bursts min-1, 100-118 %, 13-17 beats min-1, 2-4 l min-1, respectively, P < 0.05). The major new findings were: (1) CO + normoxia and CO + hyperoxia also elevated MSNA compared to normoxia (63-144 % increase in integrated MSNA; P < 0.05) but they did not increase heart rate (62-67 beats min-1) or ventilation (6.5-6.8 l min-1), and (2) despite the 4-fold elevation in MSNA with hypoxaemia and exercise, resting leg blood flow, vascular conductance and O2 uptake remained unchanged. In conclusion, the present results suggest that increases in MSNA with CO are not mediated by activation of the chemoreflex, whereas hypoxia-induced tachycardia and hyperventilation are mediated by activation of the chemoreflex in response to the decline in Pa,O2. Our findings also suggest that Pa,O2 is not an obligatory signal involved in the enhanced MSNA with reduced blood oxygenation.

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    PeerJ. 01/2013; 1:e37.
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    ABSTRACT: Hypobaric hypoxia, physical and psychosocial stress may influence key cardiovascular parameters including blood pressure (BP) and pulse pressure (PP). We investigated the effects of mild hypobaric hypoxia exposure on BP and PP reactivity to mental and physical stress and to passive elevation by cable car. 36 healthy volunteers participated in a defined test procedure consisting of a period of rest 1, mental stress task (KLT-R), period of rest 2, combined mental (KLT-R) and physical task (bicycle ergometry) and a last period of rest both at Graz, Austria (353 m asl) and at the top station Dachstein (2700 m asl). Beat-to-beat heart rate and BP were analysed both during the test procedures at Graz and at Dachstein and during passive 1000 m elevation by cable car (from 1702 m to 2700 m). A significant interaction of kind of stress (mental vs. combined mental and physical) and study location (Graz vs. Dachstein) was found in the systolic BP (p = .007) and PP (p = .002) changes indicating that during the combined mental and physical stress task sBP was significantly higher under hypoxic conditions whereas sBP and PP were similar during mental stress both under normobaric normoxia (Graz) and under hypobaric hypoxia (Dachstein). During the passive ascent in cable car less trivialization (psychological coping strategy) was associated with an increase in PP (p = .004). Our data show that combined mental and physical stress causes a significant higher raise in sBP and PP under hypoxic conditions whereas isolated mental stress did not affect sBP and PP under hypoxic conditions. PP-reaction to ascent in healthy subjects is not uniform. BP reactions to ascent that represents an accumulation of physical (mild hypobaric hypoxia) and psychological stressors depend on predetermined psychological traits (stress coping strategies). Thus divergent cardiovascular reactions can be explained by applying the multidimensional aspects of the biopsychosocial concept.
    PLoS ONE 01/2014; 9(5):e89005. · 3.53 Impact Factor


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