OXYGEN-DEPENDENT AND PRESSURE-DEPENDENT FUNCTIONAL CAPILLARY DENSITY IN RABBIT TENUISSIMUS MUSCLE

Perfusion of capillaries was investigated in the tenuissimus muscle of young anesthetized New Zealand White rabbits during control condition s, complete aorta occlusion, and reactive hyperemia at various local o xygen tensions. Capillaries were visualized with bright-field microsco py. The number of capillaries perfused under experimental conditions w as compared with that during control conditions. Capillary diameter wa s measured to assess whether the interventions caused changes in lumin al diameter. During control conditions at a local pO(2) of about 20 mm Hg, capillary perfusion fluctuates; instantaneous capillary density i s smaller than anatomical capillary density. When the aorta is (partia lly) occluded, capillary perfusion becomes continuous and instantaneou s capillary density equals anatomical capillary density. The latter is also observed during the early phase of reactive hyperemia, prior to the reappearance of flowmotion. Capillary diameter is not invariant du ring these interventions, but decreases by 8% during occlusion and inc reases by 12% during reactive hyperemia. The concomitant change in per imeter and cross-sectional area should be factored in with functional capillary density, when tissue exchange surface area or volume flow ar e considered. When during control conditions, the muscle becomes local ly (under the microscope lens) exposed to a higher oxygen tension, cap illary diameter does not change. However, the relative number of capil laries perfused at complete aorta occlusion is unity at low local oxyg en, and diminishes with increasing local oxygen to become 0 at an oxyg en tension of about 70 mm Hg. In preparations in which capillary diame ter is not invariant under the experimental conditions, functional cap illary density can only be used to compare the number of perfused capi llaries with the number of capillaries anatomically present. Capillary diameter has to be factored in when tissue perfusion or exchange surf ace area are considered.