Leonardo F. Ferreira

Human femoral artery and estimated muscle capillary blood flow kinetics following the onset of exercise.

The purpose of this study was to compare the kinetics of estimated capillary blood flow to those of femoral artery blood flow and estimated muscle oxygen uptake . Nine healthy subjects performed a series of transitions from rest to moderate (below estimated lactate threshold, 6 min bouts) knee extension exercise. Pulmonary oxygen uptake was measured breath by breath, was measured continuously using Doppler ultrasound, and deoxyhaemoglobin ([HHb]) was estimated by near‐infrared spectroscopy over the rectus femoris throughout the tests. The time course of was estimated by rearranging the Fick equation (i.e. ), (arterio – venous O2 difference) using the primary component of to represent and [HHb] as a surrogate for (a−v)O2. The overall kinetics of (mean response time, MRT, 13.7 ± 7.0 s), (τ, 27.8 ± 9.0 s) and (MRT, 41.4 ± 19.0 s) were significantly (P < 0.05) different from each other. We conclude that for moderate intensity knee extension exercise, conduit artery blood flow kinetics may not be a reasonable approximation of blood flow kinetics in the microcirculation , the site of gas exchange. This temporal dissociation suggests that blood flow may be controlled differently at the conduit artery level than in the microcirculation.

Muscle blood flow–O2 uptake interaction and their relation to on-exercise dynamics of O2 exchange

A computer model was developed to provide a theoretical framework for interpreting the dynamics of muscle capillary O(2) exchange in health and disease. We examined the effects of different muscle oxygen uptake (V O(2m)) and CvO(2) profiles on muscle blood flow (Q (m)) kinetics (Q (m)=V O(2m)/[CaO(2)-CvO(2)]). Further, we simulated V O(2m) and Q (m) responses to predict the CvO(2) profile and the underlying dynamics of capillary O(2) exchange (CvO(2)=CaO(2)-V O(2m)/Q (m)). Exponential equations describing V O(2m), CvO(2) and Q (m) responses in vivo were used in the simulations. The results indicated that Q (m) kinetics were relatively insensitive to CvO(2) parameters, but directly associated with V O(2m) kinetics. The biphasic Q (m) response produced a substantial fall in CvO(2) within the first 15-20s of the exercise transition (phase 1 of Q (m)). These results revealed that the main determinant of CvO(2) (or O(2) extraction) kinetics was the dynamic interaction of Q (m) and V O(2m) kinetics during phase 1 of Q (m).

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 altered nitric oxide availability on rat muscle microvascular oxygenation during contractions.

Aim:  To explore the role of nitric oxide (NO) in controlling microvascular O2 pressure (Po2mv) at rest and during contractions (1 Hz). We hypothesized that at the onset of contractions sodium nitroprusside (SNP) would raise Po2mv and slow the kinetics of Po2mv change whereas l‐nitro arginine methyl ester (l‐NAME) would decrease Po2mv and speed its kinetics.

Muscle microvascular oxygenation in chronic heart failure: role of nitric oxide availability

Aim:  To test the hypothesis that diminished vascular nitric oxide availability might explain the inability of individuals with chronic heart failure (CHF) to maintain the microvascular PO2’s (PO2mv ∝ O2 delivery‐to‐uptake ratio) seen in healthy animals.

Dynamics of skeletal muscle oxygenation during sequential bouts of moderate exercise

In rat muscle, faster dynamics of microvascular PO2 (approximately blood flow to O2 uptake ratio) after prior contractions that did not alter blood [lactate] have been considered to be a consequence of faster kinetics. However, in humans, prior exercise below the lactate threshold does not affect the pulmonary kinetics. To clarify this apparent discrepancy, we examined the effects of prior moderate exercise on the kinetics of muscle oxygenation (deoxyhaemoglobin, [HHb]α ) and pulmonary in humans. Eight subjects performed two bouts (6 min each) of moderate‐intensity cycling separated by 6 min of baseline pedalling. Muscle (vastus lateralis) oxygenation was evaluated by near‐infrared spectroscopy and was measured breath‐by‐breath. The time constant (τ) of the primary component of was not significantly affected by prior exercise (21.5 ± 9.2 versus 25.6 ± 9.7 s; Bout 1 versus 2, P= 0.49). The time delay (TD) of [HHb] decreased (11.6 ± 2.6 versus 7.7 ± 1.5 s; Bout 1 versus 2, P < 0.05) and τ[HHb] increased (7.0 ± 3.5 versus 10.2 ± 4.6 s; Bout 1 versus 2, P < 0.05), while the mean response time (TD +τ) did not change (18.6 ± 2.7 versus 17.9 ± 3.9 s) after prior moderate exercise. Thus, prior moderate exercise resulted in shorter onset and slower rate of increase in [HHb] during subsequent exercise. These data suggest that prior exercise altered the dynamic interaction between and following the onset of exercise.

Temporal profile of rat skeletal muscle capillary haemodynamics during recovery from contractions

In skeletal muscle capillaries, red blood cell (RBC) flux (FRBC), velocity (VRBC) and haematocrit (HctCAP) are key determinants of microvascular O2 exchange. However, the mechanisms leading to the changes in FRBC, VRBC and HctCAP during muscle contractions and recovery thereafter are not fully understood. To address this issue we used intravital microscopy to investigate the temporal profile of the rat spinotrapezius muscle (n= 5) capillary haemodynamics during recovery from 3 min of twitch muscle contractions (1 Hz, 4–6 V). Specifically, we hypothesized that (1) during early recovery FRBC and VRBC would decrease rapidly and FRBC would display a biphasic response (consistent with a muscle pump effect on capillary haemodynamics), and (2) there would be a dynamic relationship between changes (Δ) in VRBC and HctCAP. The values at rest (R) and end‐recovery (ER) were significantly lower (P < 0.05) than at end‐contraction (EC) for FRBC (in cells s−1, R = 30.1 ± 7.8, EC = 46.2 ± 7.3 and ER = 26.0 ± 6.1), VRBC (in μm s−1, R = 368 ± 83, EC = 497 ± 62 and ER = 334 ± 59) and HctCAP(R = 0.193 ± 0.016, EC = 0.214 ± 0.023 and ER = 0.185 ± 0.019). The first data point where a significant decrease in FRBC, HctCAP and VRBC occurred was at 5, 5 and 20 s post‐contraction, respectively. The decrease in FRBC approximated a monoexponential response (half‐time of ∼26 s). The relationship between ΔVRBC and ΔHctCAP was not significant (P > 0.05). Based on the early decrease in FRBC(within 5 s), overall dynamic profile of FRBC and the ∼20 s ‘delay’ to the decrease in VRBC we conclude that the muscle pump does not appear to contribute substantially to the steady‐state capillary haemodynamics in the contracting rat spinotrapezius muscle. Moreover, our findings suggest that alterations in VRBC do not obligate proportional changes in HctCAP within individual capillaries following muscle contractions.

Oxygen exchange in muscle of young and old rats: muscle–vascular–pulmonary coupling

Sustained performance of muscular exercise is contingent upon increasing muscle O2 delivery ( ; the product of blood flow and arterial O2 content, i.e. ) and utilization ( ) rapidly at exercise onset and sustaining necessary conductive and diffusive O2 fluxes throughout exercise. A tight co‐ordination of pulmonary, cardiovascular and muscle system responses is therefore required to prevent muscle microvascular O2 pressures (PmvO2) from falling to levels that impair blood–muscle O2 exchange and/or impact metabolic control and reduce exercise tolerance. Microvascular O2 pressures are determined by the balance between and , and emerging evidence indicates that this balance is regulated differently across muscle fibre types and also in aged muscle. Moreover, disease states such as diabetes (type I and II) and chronic heart failure (CHF) also impact PmvO2. This brief review primarily examines evidence obtained in animals that ageing: (1) redistributes exercising away from highly oxidative muscles and muscle fibres; (2) alters muscle capillary haemodynamics; and (3) reduces the O2 pressure head within the microcirculation (PmvO2) that serves to facilitate blood–muscle O2 transfer. In many respects, these alterations found in healthy ageing animals bear a striking resemblance to those present in some chronic diseases (e.g. diabetes, CHF) and may help explain the compromised exercise tolerance present in aged individuals. Putative mechanistic insights are explored within the context of current knowledge and future investigative approaches.

Muscle microvascular hemoglobin concentration and oxygenation within the contraction-relaxation cycle.

Inability to directly measure microvascular oxygen distribution and extraction in striated muscle during a contraction/relaxation cycle limits our understanding of oxygen transport to and utilization by contracting muscle. We examined muscle microvascular hemoglobin concentration (total [Hb/Mb]) and oxygenation within the contraction-relaxation cycle to determine if microvascular RBC volume would be preserved and if oxygen extraction continued during the actual contraction phase. Eight subjects performed dynamic knee extension exercise (40 contractions/min) at moderate ( approximately 30% of peak work rate) and heavy ( approximately 80% of peak) work rates. Total hemoglobin/myoglobin (total [Hb/Mb]) and deoxy-hemoglobin/myoglobin (deoxy-[Hb/Mb]) were measured in the rectus femoris using NIRS to determine if microvascular total [Hb/Mb] would be preserved during the contraction, and to estimate microvascular oxygen extraction, respectively. Mean values during the relaxation (RP) and contractile phases and the peak values during the contractile phase for both moderate and heavy exercise were calculated. Total [Hb/Mb] increased from rest to steady-state exercise (6.36+/-5.08 microM moderate; 5.72+/-4.46 microM heavy exercise, both P<0.05), but did not change significantly within the contraction/relaxation cycle. Muscle contractions were associated with a significant (1.29+/-0.98 microM moderate; 2.16+/-2.12 microM heavy exercise, P<0.05) increase in deoxy-[Hb/Mb] relative to RP. We conclude that (a) microvascular RBC volume is preserved during muscle contractions (i.e., RBCs are present in the capillaries), and (b) the cyclical pattern of deoxygenation/oxygenation during the respective contraction/relaxation phases of the contraction cycle suggests that oxygen extraction is not restricted to the relaxation phase but continues to occur during muscle contractions.

Matching of blood flow to metabolic rate during recovery from moderate exercise in humans

It is unclear whether measurement of limb or conduit artery blood flow during recovery from exercise provides an accurate representation of flow to the muscle capillaries where gas exchange occurs. To investigate this, we: (a) examined the kinetic responses of femoral artery blood flow ( ), estimated muscle capillary blood flow ( ) and estimated muscle oxygen uptake ( ) following cessation of exercise; and (b) compared these responses to verify the adequacy of O2 delivery during recovery. Pulmonary ( ) was measured breath by breath, was measured using Doppler ultrasonography, and deoxy‐haemoglobin/myoglobin (deoxy‐[Hb/Mb]) was estimated by near‐infrared spectroscopy over the rectus femoris in nine healthy subjects during a series of transitions from moderate knee‐extension exercise to rest. The time course of was estimated by rearranging the Fick equation [i.e. ], using the primary component of to represent and deoxy‐[Hb/Mb] as a surrogate for arteriovenous O2 difference. There were no significant differences among the overall kinetics of (τ, 31.4 ± 8.2 s), [mean response time (MRT), 34.5 ± 20.4 s] and (MRT, 31.7 ± 14.7 s). The kinetics were also significantly correlated (P < 0.05) with those of both and . Both and appear to be coupled with during recovery from moderate knee‐extension exercise, such that extraction falls (thus cellular energetic state is not further compromised) throughout recovery.