Paul McDonough

Control of oxygen uptake during exercise.

Other than during sleep and contrived laboratory testing protocols, humans rarely exist in prolonged metabolic steady states; rather, they transition among different metabolic rates (V O2). The dynamic transition of V O2 (V O2 kinetics), initiated, for example, at exercise onset, provides a unique window into understanding metabolic control. This brief review presents the state-of-the art regarding control of V O2 kinetics within the context of a simple model that helps explain the work rate dependence of V O2 kinetics as well as the effects of environmental perturbations and disease. Insights emerging from application of novel approaches and technologies are integrated into established concepts to assess in what circumstances O2 supply might exert a commanding role over V O2 kinetics, and where it probably does not. The common presumption that capillary blood flow dynamics can be extrapolated accurately from upstream arterial measurements is challenged. From this challenge, new complexities emerge with respect to the relationships between O2 supply and flux across the capillary-myocyte interface and the marked dependence of these processes on muscle fiber type. Indeed, because of interfiber type differences in O2 supply relative to V O2, the presence of much lower O2 levels in the microcirculation supplying fast-twitch muscle fibers, and the demonstrated metabolic sensitivity of muscle to O2, it is possible that fiber type recruitment profiles (and changes thereof) might help explain the slowing of V O2 kinetics at higher work rates and in chronic diseases such as heart failure and diabetes.

Control of microvascular oxygen pressures in rat muscles comprised of different fibre types

In response to an elevated metabolic rate , increased microvascular blood–muscle O2 flux is the product of both augmented O2 delivery and fractional O2 extraction. Whole body and exercising limb measurements demonstrate that and fractional O2 extraction increase as linear and hyperbolic functions, respectively, of . Given the presence of disparate vascular control mechanisms among different muscle fibre types, we tested the hypothesis that, in response to muscle contractions, would be lower and fractional O2 extraction (as assessed via microvascular O2 pressure, P  mvO 2) higher in fast‐ versus slow‐twitch muscles. Radiolabelled microsphere and phosphorescence quenching techniques were used to measure and P  mvO 2, respectively at rest and across the transition to 1 Hz twitch contractions at low (Lo, 2.5 V) and high intensities (Hi, 4.5 V) in rat (n= 20) soleus (Sol, slow‐twitch, type I), mixed gastrocnemius (MG, fast‐twitch, type IIa) and white gastrocnemius (WG, fast‐twitch, type IIb) muscle. At rest and for Lo and Hi (steady‐state values) transitions, P  mvO 2 was lower (all P < 0.05) in MG (mmHg: rest, 22.5 ± 1.0; Lo, 15.3 ± 1.3; Hi, 10.2 ± 1.6) and WG (mmHg: rest, 19.0 ± 1.3; Lo, 12.2 ± 1.1; Hi, 9.9 ± 1.1) than in Sol (rest, 33.1 ± 3.2 mmHg; Lo, 19.0 ± 2.3 mmHg; Hi, 18.7 ± 1.8 mmHg), despite lower and in MG and WG under each set of conditions. These data suggest that during submaximal metabolic rates, the relationship between and O2 extraction is dependent on fibre type (at least in the muscles studied herein), such that muscles comprised of fast‐twitch fibres display a greater fractional O2 extraction (i.e. lower P  mvO 2) than their slow‐twitch counterparts. These results also indicate that the greater sustained P  mvO 2 in Sol may be important for ensuring high blood–myocyte O2 flux and therefore a greater oxidative contribution to energetic requirements.

Oxygen exchange profile in rat muscles of contrasting fibre types.

To determine whether fibre type affects the O2 exchange characteristics of skeletal muscle at the microcirculatory level we tested the hypothesis that, following the onset of contractions, muscle comprising predominately type I fibres (soleus, Sol, 86 % type I) would, based on demonstrated blood flow responses, exhibit a blunted microvascular PO2 (PO2,m, which is determined by the O2 delivery (Q̇O2) to O2 uptake (V̇O2) ratio) profile (assessed via phosphorescence quenching) compared to muscle of primarily type II fibres (peroneal, Per, 84 % type II). PO2,m was measured at rest, and following the rest‐contractions (twitch, 1 Hz, 2–4 V for 120 s) transition in Sol (n= 6) and Per (n= 6) muscles of Sprague‐Dawley rats. Both muscles exhibited a delay followed by a mono‐exponential decrease in PO2,m to the steady state. However, compared with Sol, Per demonstrated (1) a larger change in baseline minus steady state contracting PO2,m (ΔPO2,m) (Per, 13.4 ± 1.7 mmHg; Sol, 8.6 ± 0.9 mmHg, P < 0.05); (2) a faster mean response time (i.e. time delay (TD) plus time constant (τ); Per, 23.8 ± 1.5 s; Sol, 39.6 ± 4.3 s, P < 0.05); and therefore (3) a greater rate of PO2,m decline (ΔPO2,m/τ; Per, 0.92 ± 0.08 mmHg s−1; Sol, 0.42 ± 0.05 mmHg s−1, P < 0.05). These data demonstrate an increased microvascular pressure head of O2 at any given point after the initial time delay for Sol versus Per following the onset of contractions that is probably due to faster Q̇O2 dynamics relative to those of V̇O2.

Dynamics of oxygen uptake following exercise onset in rat skeletal muscle

Technical limitations have precluded measurement of the V(O(2)) profile within contracting muscle (mV(O(2))) and hence it is not known to what extent V(O(2)) dynamics measured across limbs in humans or muscles in the dog are influenced by transit delays between the muscle microvasculature and venous effluent. Measurements of capillary red blood cell flux and microvascular P(O(2)) (P(O(2)m)) were combined to resolve the time course of mV(O(2)) across the rest-stimulation transient (1 Hz, twitch contractions). mV(O(2)) began to rise at the onset of contractions in a close to monoexponential fashion (time constant, J = 23.2 +/- 1.0 sec) and reached its steady-state value at 4.5-fold above baseline. Using computer simulation in healthy and disease conditions (diabetes and chronic heart failure), our findings suggest that: (1) mV(O(2)) increases essentially immediately (< 2 sec) following exercise onset; (2) within healthy muscle the J blood flow (thus O(2) delivery, J Q(O(2)m)) is faster than JmV(O(2)) such that oxygen delivery is not limiting, and 3) a faster P(O(2)m) fall to a P(O(2)m) value below steady-state values within muscle from diseased animals is consistent with a relatively sluggish Q(O(2)m) response compared to that of mV(O(2)).

Dynamics of microvascular oxygen partial pressure in contracting skeletal muscle of rats with chronic heart failure.

OBJECTIVE This investigation tested the hypothesis that the dynamics of muscle microvascular O(2) pressure (PO(2)m, which reflects the ratio of O(2) utilization [V*O(2)] to O(2) delivery [Q*O(2)]) following the onset of contractions would be altered in chronic heart failure (CHF). METHODS Female Sprague-Dawley rats were subjected to a myocardial infarction (MI) or a sham operation (Sham). Six to 10 weeks post Sham (n=6) or MI (n=17), phosphorescence quenching techniques were utilized to determine PO(2)m dynamics at the onset of spinotrapezius muscle contractions (1 Hz). RESULTS MI rats were separated into groups with Moderate (n=10) and Severe (n=7) CHF based upon the degree of left ventricular (LV) dysfunction as indicated by structural abnormalities (increased right ventricle weight and lung weight normalized to body weight). LV end-diastolic pressure was elevated significantly in both CHF groups compared with Sham (Sham, 3+/-1; Moderate CHF, 9+/-2; Severe CHF, 27+/-4 mmHg, P<0.05). The PO(2)m response was modeled using time delay and exponential components to fit the PO(2)m response to the steady-state. Compared with Shams, the time constant (tau) of the primary PO(2)m response was significantly speeded in Moderate CHF (tau, Sham, 19.0+/-1.5; Moderate CHF, 13.2+/-1.9 s, P<0.05) and slowed in Severe CHF (tau, 28.2+/-3.4 s, P<0.05). Within the Severe CHF group, tau increased linearly with the product of right ventricular and lung weight (r=0.83, P<0.05). CONCLUSIONS These results suggest that CHF alters the dynamic matching of muscle V*O(2)-to-Q*O(2) across the transition from rest to contractions and that the nature of that perturbation is dependent upon the severity of cardiac dysfunction.

Measurement of muscle microvascular oxygen pressures: compartmentalization of phosphorescent probe.

Objective: To determine whether the phosphorescent probe Oxyphor R2 (a palladium porphyrin dendrimer) becomes extravasated within normotensive skeletal muscle, R2 perfusion and washout studies were performed using a perfused rat hindlimb preparation.

Blood flow and O2 extraction as a function of O2 uptake in muscles composed of different fiber types.

We examined how the greater vasodilatory capacity of slow--(ST) versus fast-twitch (FT) muscles impacts the relationship between blood flow (Q ) and O2 uptake (VO2) and, consequently, the O2 extraction (a-vO2 diff.)-to-VO2 relationship. Q was measured with radiolabelled microspheres, while VO2 was calculated by the Fick principle using measurements of microvascular O2 pressure (phosphorescence quenching) at rest, low--(2.5 V) and high-intensity contractions (4.5 V) for soleus (Sol; ST, n=5), mixed-gastrocnemius (MG; FT, n=7) and white-gastrocnemius (WG; FT, n=7). The slope of the Q-to-VO2 relationship (delta Q/delta VO2] ) was not different among muscles (Sol = 5.5 +/- 0.2, MG = 6.0 +/- 0.11 and WG = 5.8 +/- 0.06; P > 0.05). In contrast, the intercept was greater (P < 0.05) for Sol (16.3 +/- 2.7 ml min(-1) 100 g(-1)) versus MG and WG (in ml min(-1) 100 g(-1): 1.39 +/- 0.26 and 1.45 +/- 0.23, respectively; MG and WG, P > 0.05). In addition, the a-vO2 diff.-to-VO2] relationship for Sol was shifted rightward compared to MG and WG. These data suggest that the increase in Q for a given change in VO2 is similar for slow- and fast-twitch muscles, at least for the range of metabolic rates and muscles studied herein and that a-vO2 diff. differences result from the lower resting Q in FT muscles.

Effects of Type II diabetes on muscle microvascular oxygen pressures

We tested the hypothesis that muscle microvascular O2 pressure (PmvO2; reflecting the O2 delivery (QO2) to O2 uptake (VO2) ratio) would be lowered in the spinotrapezius muscle of Goto-Kakizaki (GK) Type II diabetic rats (n=7) at rest and during twitch contractions when compared to control (CON; n=5) rats. At rest, PmvO2 was lower in GK versus CON rats (CON: 29+/-2; GK: 18+/-2Torr; P<0.05). At the onset of contractions, GK rats evidenced a faster change in PmvO2 than CON (i.e., time constant (tau); CON: 16+/-4; GK: 6+/-2s; P<0.05). In contrast to the monoexponential fall in PmvO2 to the steady-state level seen in CON, GK rats exhibited a biphasic PmvO2 response that included a blunted (or non-existent) PmvO2 decrease followed by recovery to a steady-state PmvO2 that was at, or slightly above, resting values. Compared with CON, this decreased PmvO2 across the transition to a higher metabolic rate in Type II diabetes would be expected to impair blood-muscle O2 exchange and contractile function.

Nitric oxide synthase inhibition speeds oxygen uptake kinetics in horses during moderate domain running

Within the moderate exercise intensity domain, the speed of oxygen uptake (V(O(2))) kinetics at the transition to a higher metabolic rate is thought to be limited by an inertia of the oxidative machinery. Nitric oxide (NO)-induced inhibition of O(2) consumption within the electron transport chain may contribute to this inertia. This investigation tested the hypothesis that a reduction or removal of any such NO effect via infusion of N(omega)-nitro-L-arginine methyl ester (L-NAME; a NOS inhibitor) would speed V(O(2)) kinetics at the onset of moderate exercise. Five Thoroughbred geldings underwent four transitions to running speeds of 7 m sec(-1) (two control, C, 2 L-NAME [20 mg kg(-1)]) on an equine treadmill during which pulmonary gas exchange was determined using a bias flow system. Consistent with exercise in the moderate intensity domain, in none of the transitions was a V(O(2)) slow component elicited. The L-NAME treatment significantly accelerated V(O(2)) kinetics via a reduction of the primary amplitude time constant (C, 17.3 +/- 1.7; L-NAME, 11.8 +/- 1.5 sec, P < 0.05) concomitant with faster overall dynamics (i.e. T(50) and T(75) both P < 0.05) and a trend toward a decreased O(2) deficit (C, 6.4 +/- 0.7; L-NAME, 4.7 +/- 1.2 L; P = 0.06). These data support the notion that NO contributes prominently to the oxidative enzyme inertia and thus the speed of V(O(2)) kinetics at the onset of moderate intensity exercise in the horse.

Rat Muscle Microvascular PO2 Kinetics During the Exercise Off-Transient

Dependent upon the relative speed of pulmonary oxygen consumption (V̇O2) and blood flow (Q˙) kinetics, the exercise off‐transient may represent a condition of sub‐ or supra‐optimal perfusion. To date, there are no direct measurements of the dynamics of the V̇O2/Q˙ relationship within the muscle at the onset of the work/recovery transition. To address this issue, microvascular PO2 (PO2,m) dynamics were studied in the spinotrapezius muscles of 11 female Sprague‐Dawley rats (weight ∼220 g) during and following electrical stimulation (1 Hz) to assess the adequacy of Q˙ relative to V̇O2 post exercise. The exercise blood flow response (radioactive microspheres: muscle Q˙ increased ∼240%), and post‐exercise arterial blood pH (7.40 ± 0.02) and blood lactate (1.3 ± 0.4 mM l−1) values were consistent with moderate‐intensity exercise. Recovery PO2,m (i.e. off‐transient) rose progressively until baseline values were achieved (Δend‐recovery exercise PO2,m, 14.0 ± 1.9 Torr) and at no time fell below exercising PO2,m. The off‐transient PO2,m was well fitted by a dual exponential model with both fast (τ= 25.4 ± 5.1 s) and slow (τ= 71.2 ± 34.2 s) components. Furthermore, there was a pronounced delay (54.9 ± 10.7 s) before the onset of the slow component. These data, obtained at the muscle microvascular level, support the notion that muscle V̇O2 falls with faster kinetics than muscle Q˙ during the off‐transient, such that PO2,m increases systematically, though biphasically, during recovery.