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POINT-COUNTERPOINT
It is also important to define precisely what we are talking about. Professor Clark and colleagues' (1) opening statement that "The key issue here is whether resting skeletal muscle is fully perfused" is clearly erroneous. "Perfusion," (from the Latin perfusus), infers nothing about the distribution of blood, which, along with the veracity of presumptions made to assess that distribution, is what is at issue here.
Technique 1: Red blood cell occupancy in cryosections. PRESUMPTIONS. An RBC in capillary cross-section indicates capillary was flowing (4).
CHALLENGES. 1) Cannot discriminate stopped versus moving RBCs. 2) Low capillary hematocrit means large inter-RBC (plasma) spaces.
CONCLUSION. Apparent increase in capillaries "recruited" might simply reflect elevated hematocrit during hyperemia (6, 7).
Technique 2: direct intravital microscopy observation. Reports of a majority of nonperfused capillaries in resting tenuissimus muscles (8) are opposed by others (10).
Technique 3: increased metabolism of 1-methyl xanthine by xanthine oxidase. PRESUMPTIONS. Xanthine oxidase (XO) found predominantly in capillary endothelial cells and increased 1-methyl xanthine (1-MX) metabolism occurs: 1) in direct proportion to the number of RBC-perfused capillaries and 2) is unaffected by capillary hemodynamics.
CHALLENGES. 1) XO is found in plasma (9) and smooth muscle (3), 2) increased 1-MX metabolism results from increased delivery to low-flow capillaries.
Technique 4: contrast-enhanced ultrasound. PRESUMPTIONS. Microbubbles are distributed the same as RBCs.
CHALLENGE. If true (doubtful), concentration in already-flowing capillaries would increase two- (or more) fold in hyperemia without obligatory capillary "recruitment" (5–7).
Technique 5: laser Doppler flowmetry shows "... an increase in intramuscular hyperemia...". PRESUMPTION. Hyperemia parallels capillary recruitment.
CHALLENGE. Presumption is baseless: hyperemia reflects primarily increased RBC flux in already flowing capillaries (2, 5, 6).
The famous Indian/Chinese/African legend "The Blind Men and the Elephant," popularized by John Godfrey Saxe in 1878, bears some similarity to my opponents' approach to this debate. When several blind men seek to discover what an elephant is like, each man only touches one part and argues as follows:
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Hence, although all were partly right, all were wrong. This 2,000 year-old missive argues for a better integration of current techniques and knowledge if we are to understand muscle capillary exchange.
Nullius in Verba!
REFERENCES
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