D. Walter Wray

Human skeletal muscle intracellular oxygenation: the impact of ambient oxygen availability

Intracellular oxygen (O2) availability and the impact of ambient hypoxia have far reaching ramifications in terms of cell signalling and homeostasis; however, in vivo cellular oxygenation has been an elusive variable to assess. Within skeletal muscle the extent to which myoglobin desaturates (deoxy‐Mb) and the extent of this desaturation in relation to O2 availability provide an endogenous probe for intracellular O2 partial pressure (P u2009iOu20092 ). By combining proton nuclear magnetic resonance spectroscopy (1H NMRS) at a high field strength (4 T), assessing a large muscle volume in a highly efficient coil, and extended signal averaging (30 min) we assessed the level of skeletal muscle deoxy‐Mb in 10 healthy men (30 ± 4 years) at rest in both normoxia and hypoxia (10% O2). In normoxia there was an average deoxy‐Mb signal of 9 ± 1%, which, when converted to P u2009iOu20092 using an O2/Mb half‐saturation (P50) of 3.2 mmHg, revealed an P u2009iOu20092 of 34 ± 6 mmHg. In ambient hypoxia the deoxy‐Mb signal rose to 13 ± 3% (P u2009iOu20092 = 23 ± 6 mmHg). However, intersubject variation in the defence of arterial oxygenation (S u2009aOu20092 ) in hypoxia (S u2009aOu20092 range: 86–67%) revealed a significant relationship between the changes in S u2009aOu20092 and P u2009iOu20092(r2= 0.5). These data are the first to document resting intracellular oxygenation in human skeletal muscle, highlighting the relatively high P u2009iOu20092 values that contrast markedly with those previously recorded during exercise (∼2–5 mmHg). Additionally, the impact of ambient hypoxia on P u2009iOu20092 and the relationship between changes in S u2009aOu20092 and P u2009iOu20092 stress the importance of the O2 cascade from air to cell that ultimately effects O2 availability and O2 sensing at the cellular level.

Onset exercise hyperaemia in humans: partitioning the contributors.

Using a step‐wise, reductionist approach we characterized the time course and degree to which mechanical, vasodilatory and cardiac mechanisms contribute to the increase in leg blood flow (LBF) at the onset of dynamic knee‐extensor exercise. Heart rate (HR) and LBF (ultrasound Doppler) were evaluated during (1) voluntary and (2) passive exercise in the seated position, (3) passive exercise in the supine position with the leg above the heart, and (4) passive exercise with measurements made in the non‐moving leg. In trials 2 and 3, the degree of change and time course of peak ΔHR (8.7 ± 2 bpm, seated; 10 ± 1 bpm, supine) and peak ΔLBF (518 ± 135 ml min−1, seated; 448 ± 179 ml min−1, supine) were similar, supporting the concept that the skeletal muscle pump was minimized. Even with the reduction of skeletal muscle pump and metabolic influences (trials 2, 3 and 4) a significant cardio‐acceleration and hyperaemia was seen. In the first 5 s of seated passive exercise, the retrograde component of the blood velocity profile was significantly greater than rest or the 5–20 s interval, which may suggest an arterial inflow that initially exceeded leg vasodilatation. Steady‐state LBF (minutes 2 and 3) remained elevated during voluntary exercise, but returned to near baseline during passive movement. Taken together, these data suggest that cardio‐acceleration (i.e. tachycardia) and mechanical forces other than the skeletal muscle pump play a role in reducing vascular resistance and ultimately increasing LBF at the onset of exercise, followed by steady‐state LBF which matches muscle metabolic demand.

Effect of acetazolamide on pulmonary and muscle gas exchange during normoxic and hypoxic exercise

Acetazolamide (ACZ) is used to prevent acute mountain sickness at altitude. Because it could affect O2 transport in several different and potentially conflicting ways, we examined its effects on pulmonary and muscle gas exchange and acid–base status during cycle exercise at ∼30, 50 and 90% in normoxia (F u2009IOu20092 = 0.2093) and acute hypoxia (F u2009IOu20092 = 0.125). In a double‐blind, order‐balanced, crossover design, six healthy, trained men (normoxic = 59 ml kg−1 min−1) exercised at both F u2009IOu20092 values after ACZ (3 doses of 250 mg, 8 h apart) and placebo. One week later this protocol was repeated using the other drug (placebo or ACZ). We measured cardiac output , leg blood flow (LBF), and muscle and pulmonary gas exchange, the latter using the multiple inert gas elimination technique. ACZ did not significantly affect , , LBF or muscle gas exchange. As expected, ACZ led to lower arterial and venous blood [HCO3−], pH and lactate levels (P < 0.05), and increased ventilation (P < 0.05). In both normoxia and hypoxia, ACZ resulted in higher arterial PO2 and saturation and a lower alveolar–arterial PO2 difference (AaDO2) due to both less mismatch and less diffusion limitation (P < 0.05). In summary, ACZ improved arterial oxygenation during exercise, due to both greater ventilation and more efficient pulmonary gas exchange. However, muscle gas exchange was unaffected.

Angiotensin II in the Elderly: Impact of Angiotensin II Type 1 Receptor Sensitivity on Peripheral Hemodynamics

Exercise hyperemia is attenuated in the elderly, which may be attributed to local vasoregulatory pathways within the skeletal muscle vasculature. Therefore, we sought to determine whether healthy aging is associated with changes in angiotensin II (Ang II) receptor sensitivity through measurements of leg blood flow in resting and exercising skeletal muscle. In 12 (n=6 young, 24±1 years; n=6 older, 68±3 years) healthy volunteers, we determined changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of Ang II (0.8 ng/mL of leg blood flow per minute). Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20% and 40% of the maximal work rate (WRmax). At rest, Ang II infusion decreased leg blood flow to a greater extent in older (−61±8%) subjects compared with younger subjects (−31±5%). Compared with rest, Ang II–mediated vasoconstriction (leg blood flow) during exercise was diminished in both older and younger subjects at 20% (older: −7±5%; younger: −21±2%) and 40% WRmax (older: −5±4%; younger: −9±3%). These data identify a clear age-related hypersensitivity to Ang II in the resting leg, which may contribute to the recognized decrement in leg blood flow in this cohort. However, the diminished vasoconstriction to Ang II during exercise suggests that the elevation in Ang II type 1 receptor sensitivity documented at rest does not contribute significantly to the blunted exercise hyperemia experienced with advancing age.

Diminished baroreflex-induced vasoconstriction following alpha-2 adrenergic receptor blockade in humans.

The relative contribution of alpha adrenergic receptor subtypes in the transduction of sympathetic nerve activity (SNA) during carotid baroreflex (CBR) engagement is not well understood. Therefore, we compared the hemodynamic consequence of CBR-mediated sympatho-excitation via neck pressure (NP) before and after alpha-2 adrenergic blockade with intra-arterial yohimbine. Leg blood flow was measured using 2D and Doppler ultrasound, and arterial blood pressure was determined directly. NP caused the expected vasoconstriction, and this response was significantly reduced (by 50-60%) when NP was repeated after yohimbine. These data indicate that alpha-2 adrenergic receptors contribute significantly to CBR-induced vasoconstriction in the human leg under resting conditions.