O 2 ‐consumption, blood flow and PO 2 in bone

Osteocytes are bone cells encapsulated in a mineralized matrix. Since they are connected to nutrient blood vessels via narrow canaliculii which provide narrow, tortuous and often long diffusion pathways, the question arises as to how osteocytes are sufficiently supplied with O 2 and metabolites. Furthermore, different oxygen partial pressures (PO 2 ) ‐ resulting from O 2 supply and local oxygen consumption ‐ may influence cellular proliferation and differentiation. In this context, O 2 consumption rates of bone cells were measured and results were related to published blood flow values. This should allow to estimate mean venous PO 2 and PO 2 distribution in bone. O 2 consumption of bone cells inside spongious calvarial fragments of neonatal rats and adult guinea pigs were measured polarographically in a thermostabilized recording chamber containing Hepes‐buffered saline. PO 2 declined linearly as long as the PO 2 ranged above 20 mmHg. At 27°C and 37°C, the O 2 consumption rate of calvarial fragments from adult animals amounted to 0.06 and 0.1 ml/100 g✶min, respectively. Calvaria from newborn rats showed 5‐fold higher values. At 45 °C, oxygen consumption was irreversibly abolished. The blood flow to bones amounts to 5–6 ml/100 g✶min being equivalent to an oxygen delivery of about 1 ml/100 g✶min. Based on the hemoglobin‐oxygen binding curve and on an O 2 consumption of 0.1 ml/100 g ✶ min, venous PO 2 calculates to ca. 60 mmHg. This appears to be a luxurious oxygen supply in bone. With respect to the long diffusion pathways, however, high PO 2 values appear necessary to ensure sufficiently steep PO 2 gradients for the the supply of cells remote from nutrient vessels. The resulting local oxygen gradients may orchestrate proliferation and differentiation of bone cells via oxygen‐dependent gene expression. Based on these considerations a model is proposed which comprises known factors influencing blood flow and oxygen tension in bone. O 2 ‐Verbrauch, Blutfluss und PO 2 im Knochen Osteocyten sind Knochenzellen, die in einer mineralisierten Matrix eingekapselt und mit den versorgenden Gefäßen zumeist nur durch enge, verschlungene und oft lange Diffusionsstrecken verbunden sind. Es fragt sich, wie eine adäquate Energie‐ und O 2 ‐Versorgung dieser Zellen sichergestellt ist. Weiterhin ist zu fragen, ob die zu erwartenden unterschiedlichen O 2 ‐Partialdrucke (PO 2 ) Effekte auf Proliferation und Differenzierung von Knochenzellen haben. Um diese Fragen zu prüfen, wurde