Counterpoint: There is not capillary recruitment in active skeletal muscle during exercise

Abstract

COUNTERPOINT: THERE IS NOT CAPILLARY RECRUITMENT IN ACTIVE SKELETAL MUSCLE DURING EXERCISEThe notion that a substantial proportion of capillaries do not contain moving red blood cells (RBCs) in muscle at rest but are "recruited," i.e., begin flowing with RBCs during contractions, is one basis for our present understanding of blood-muscle exchange during exercise (20, 28). This concept emanates, in part, from August Krogh, who showed that many capillaries in resting muscle did not contain India ink after high pressure perfusion (19). Despite Krogh himself recognizing that India ink particles clumped together, more likely to prevent complete perfusion of the capillary bed at rest than during exercise, these experiments, and Krogh's O 2 diffusion model based on them, are still cited by researchers invoking capillary recruitment (e.g., Refs. 22). Today capillary recruitment during exercise is accepted by many to explain important physiological phenomena, including: 1) greater blood-muscle delivery and extraction of O 2 , free fatty acids, and glucose and 2) reduced capillaryto-mitochondrial diffusion distances. It makes great sense that, if there were a reserve of capillaries at rest, during exercise when the muscle demands for O 2 may increase up to 100-fold, all-or at least most-capillaries would contribute to meet that demand.Why, therefore, choose to oppose the concept of capillary recruitment during exercise? In Britain, the motto of The Royal Society is "Nullius in Verba" (Take nobody's word for it, see it for yourself). However, the majority of research papers invoking capillary recruitment have not visualized the capillary bed (e.g., Refs. 3, 7, 25). In his letter to the editor of The American Journal of Physiology, the eminent microcirculation expert, Professor Eugene Renkin (26) criticized Dr. Bentzer for entitling his paper "Capillary filtration coefficient is independent of number of perfused capillaries in skeletal muscle"(4) on the basis that ". . . its title is misleading. No direct measurements of [RBC-flowing] capillary number were made. . . ." The same criticism could be leveled at almost all papers that include the words capillary recruitment in their titles. Evidence that most capillaries already sustain flow in resting muscle would preclude the possibility that recruitment of previously nonperfused capillaries occurs to any great extent during contractions.Direct evidence for RBC flow in most capillaries in resting muscle. In resting muscle, intravital light microscopy shows that over 80% of capillaries support RBC flow, e.g., in rat spinotrapezius (14,17, 24), diaphragm (15), and extensor digitorum longus (1), hamster cremaster and sartorius (8), cat sartorius (6), rabbit tenuissimus (30). However, animals in these experiments were anesthetized to facilitate muscle exteriorization and viewing of the capillary beds. To address this, Bailey and colleagues (2) employed minimally invasive techniques to measure blood flow (radioactive microspheres) and microvascular oxygen partial pressure in muscle...

A new method to study changes in microvascular blood volume in muscle and adipose tissue: real-time imaging in humans and rat

“…This indicates that the MBV imaged immediately after exercise probably is a reliable value of MBV during actual exercise and demonstrates the value of imaging in real time. The existence of increased MBV during exercise was recently debated (8,25). The present data add to the evidence that changes in MBV do take place during dynamic exercise in humans (36) and during electrical stimulation in rat muscle (14).…”

Dynamics of muscle microcirculatory and blood–myocyte O2 flux during contractions

“…However, there is substantial heterogeneity of flow rates among capillaries (Erickson and Myrhage, 1972; Duling and Damon, 1987; Kindig et al 1999; Kindig and Poole, 1998) such that many may be of less relative importance for O 2 and substrate delivery. Thus, muscle contractions (and reactive hyperemia, Burton and Johnson, 1972) are accompanied by an increased flow principally within already flowing capillaries (Poole et al 2008b; right-hand side of Table 1). …”

Impaired microvascular perfusion: a consequence of vascular dysfunction and a potential cause of insulin resistance in muscle

“…Even so, there is some evidence that, when muscle is exposed and examined at rest by intravital microscopy, all capillaries are perfused (e.g., see Ref. 123). Clearly, if all capillaries are fully perfused with red blood cells at rest, then the observed increases in microvascular perfusion mediated by insulin or exercise create a dilemma.…”