Expression of muscle capillary alkaline phosphatase is affected by hypoxia

Hypoxia stimulates angiogenesis in some microvascular beds, but no clear angiogenic effect of hypoxia has yet been demonstrated in adult skeletal muscle. In this study the distribution of alkaline phosphatase (APase) was compared with a novel microvascular marker, Griffonia simplicifolia I (GSI), to determine whether the respective markers were expressed by muscle capillaries during hypoxic conditions and to probe for the presence or absence of angiogenesis in response to short-term hypoxia. Mice were exposed to normobaric 8% oxygen for 7 or 21 days. Capillary density in the red and white areas of the gastrocnemius muscle was determined with the use of a double-labeling procedure for both APase and fluorescently tagged GSI. Little change in capillary density was found. Focal reductions in APase activity were observed within 1 wk of hypoxia, but no changes were observed in GSI binding. In controls, 74 and 92% of red and white muscle capillaries, respectively, were APase positive. This percentage declined to 60% in red and 43% in white muscle after 21 days of hypoxia. The results indicate that APase expression is labile under certain conditions and warrant a cautious approach to using the enzyme as a marker. Binding of the GSI lectin to muscle capillaries appeared to be unchanged by the exposure to hypoxia, indicating stability of this marker system. No significant change in the number of capillaries around individual muscle fibers was evident at 21 days when GSI was used to detect capillaries. These results confirm the absence of hypoxia-induced angiogenesis in muscle capillaries during the time period studied.