Skeletal muscle capillary geometry: adaptation to chronic hypoxia

The potential for gas and metabolite exchange across the capillary bed is determined largely by the capillary length and surface area available for blood-tissue transfer. It has been suggested that chronic exposure to hypoxia increases capillary tortuosity; however, the degree of orientation of capillaries in muscles of sea level animals chronically exposed to hypoxia has never been quantified rigorously. An augmented capillary tortuosity would increase capillary length per volume of muscle fiber, Jv(c,f), irrespective of whether new capillaries are formed. To resolve this issue, female rats (278 +/- 5 g) were maintained for 5 months in a temperate environment at 3800 m (PIO2 = 91 Torr). Capillary tortuosity and Jv(c,f) were estimated from transverse and longitudinal sections in perfusion-fixed M. Soleus and M. Gastrocnemius. Values were compared with weight-matched controls (274 +/- 7 g). Neither capillary density (normalized to sarcomere length 2.1 microns, hypoxic = 1292 +/- 79, control = 1282 +/- 43 mm-2) nor capillary-to-fiber ratio (hypoxic = 2.50 +/- 0.15, control = 2.57 +/- 0.05) were changed after altitude exposure. Capillary tortuosity was a function of sarcomere length in all animals and this relationship was not changed by hypoxia. Capillary length per volume of muscle fiber was unchanged (hypoxic = 1541 +/- 72, control = 1531 +/- 44 mm-2) as was mean capillary diameter. We conclude that chronic exposure to 3800 m does not change capillary tortuosity or surface area in rat M. Soleus or M. Gastrocnemius.