Rebuttal from Drs. Poole, Brown, Hudlicka

Abstract

POINT-COUNTERPOINTRebuttal from Drs. Poole, Brown, HudlickaPublished Online:01 Mar 2008https://doi.org/10.1152/japplphysiol.00779.2007cMoreSectionsPDF (44 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmail My learned colleagues are to be congratulated for considering different techniques (1–5 below) to assess changes in the proportion of flowing capillaries in muscle. However, the resultant data must be judged in light of limitations inherent in those techniques.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: 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!REFERENCES1 Clark MG, Rattigan S, Barrett EJ, Vincent MA. Point: There is capillary recruitment in active skeletal muscle during exercise. J Appl Physiol; doi:10.1152/japplphysiol.00779.2007.Link | ISI | Google Scholar2 Hargreaves D, Egginton S, Hudlicka O. Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle. Microvasc Res 40: 14–28, 1990.Crossref | PubMed | ISI | Google Scholar3 Hellsten-Westing Y. Immunohistochemical localization of xanthine oxidase in human cardiac and skeletal muscle. Histochemistry 100: 215–222, 1993.Crossref | PubMed | Google Scholar4 Honig CR, Odoroff CL, Frierson JL. Capillary recruitment in exercise: rate, extent, uniformity, and relation to blood flow. Am J Physiol Heart Circ Physiol 238: H31–H42, 1980.Link | ISI | Google Scholar5 Hudlicka O, Zweifach BW, Tyler KR. Capillary recruitment and flow velocity in skeletal muscle after contractions. Microvasc Res 23: 201–213, 1982.Crossref | PubMed | ISI | Google Scholar6 Kindig CA, Richardson TE, Poole DC. Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer. J Appl Physiol 92: 2513–2520, 2002.Link | ISI | Google Scholar7 Klitzman B, Duling BR. Microvascular hematocrit and red cell flow in resting and contracting striated muscle. Am J Physiol Heart Circ Physiol 237: H481–H490, 1979.Link | ISI | Google Scholar8 Lindbom L. Microvascular blood flow distribution in skeletal muscle. Acta Physiol Scand Suppl 525: 1–40, 1983.PubMed | Google Scholar9 Newaz MA, Adeeb NNN. Detection of xanthine oxidase in human plasma. Med J Malaysia 53: 70–75, 1998.Google Scholar10 Vrielink HH, Slaaf DW, Tangelder GJ, Reneman RS. Does capillary recruitment exist in young rabbit skeletal muscle? Int J Microcirc Clin Exp 6: 321–332, 1987.Google Scholar Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 104Issue 3March 2008Pages 893-894 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00779.2007cHistory Published online 1 March 2008 Published in print 1 March 2008 PDF download Metrics Downloaded 70 times