Vitamin D-enhanced osteoclastic bone resorption at vascular canals

The responses of osteocytic and os-teoclastic bone resorption to pharmacologic doses of vitamin D were compared in rats. Resorptive re-sponses were assessed in cortical bone from meas-urements of the volume of bone pores ranging from 0.07 to 122μ in diameter by means of mercury porosimetry. This range includes two main classes of pores, osteocytic lacunae and canaliculi (0.07–0.3 μ diameter) and vascular canals (4–122 μ diameter). Rats treated with pharmacologic doses of vitamin D as compared with control rats developed hypercalcemia, a 7 increase in osteocyte lacunarcanalicular volume and a 225 increase in vascular canal volume. Tetracycline labeling and acid phosphatase staining revealed that the increase in osteocyte lacunar-canalicular volume was largely due to osteocytic resorption, and that the increase in vascular canal volume was entirely due to osteoclastic resorption. Thus, the changes in these two volumes were used to compare osteocytic and osteoclastic resorption. The absolute increase in the volume of bone removed was 24 times greater for osteoclasts than for osteocytes and more than 88% of the increase in intracortical bone resoVption was due to osteoclastic resorption. The product of vascular canal number and length was increased 69, indicating that the increase in cortical porosity was due in part to elongation of existing canals or creation of new canals. Increased porosity occurred despite increased osteoblastic bone formation at vascular canals.