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POINT-COUNTERPOINT
However, the data are only as good as the techniques used to acquire them, and the authors themselves (6) acknowledge some of the problems associated with exposing a muscle such as the spinotrapezius for viewing under a microscope. These problems, which could alter the extent of capillary recruitment, include the effect of manipulations required to expose the muscle; the question of whether the thin muscles that can be exposed are representative of the cylindrical load-bearing muscles; the effects of anesthesia; and the superfusion PO2. Since the weight of argument against exercise-mediated capillary recruitment is based on studies using intravital microscopy it is essential that the findings from this system can be extrapolated to in vivo. Not least among the problems is the superfusion PO2 where suffusate buffer for intravital microscopy of isolated muscles is invariably gassed with 5% CO2 in 95% N2 [e.g. tenuissimus (4); spinotrapezius (3); cremaster (2)]. Comparison with a suffusate PO2 dose curve (7) suggests this could have a marked impact to increase the number of capillaries perfused at rest.
In contrast, our endeavours in this field have focused on noninvasive estimations of microvascular perfusion in bulk load-bearing muscle in vivo that has not been manipulated nor perturbed by invasive conditions. Ultrasound imaging of a region of interest comprising 
15 g of human forearm muscle of conscious healthy humans has indicated capillary recruitment to occur in response to insulin infusion under euglycemic conditions, to a mixed meal, and to exercise (8), and that the insulin response is impaired in obese insulin-resistant patients (1).
Finally, despite the differences in our two approaches, an impaired blood-muscle exchange has been identified in animal models of type 2 diabetes (5, 9). The percentage of RBC-perfused capillaries is decreased in the Goto-Kakizaki rat (5) and we find that the obese Zucker rat has impaired insulin-mediated capillary recruitment (9). Future experiments may reveal that the vascular dysfunction that is responsible for the impaired insulin response in vivo (9) is also that which prevents capillary recruitment when the muscle is exposed, irrigated with N2 buffer, and viewed microscopically (5).
"The voyage of discovery is not in seeking new landscapes but in having new eyes."—Marcel Proust
REFERENCES
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