Haversian bone-remodelling rates in the beagle after cessation of exposure to chronic low doses of lead.

This study reports the changes observed in bone-formation dynamics at the cell, tissue and organ level in cortical bone in four standardization research colony-raised male Beagles that were exposed to 0.7 mg of lead acetate per day for 6 months, followed by a 6-month period when no lead acetate was given. Four similar age-sex matched Beagles of the same origin served as controls. Tissue and blood lead levels were measured. Biochemical and hematological analysis were carried out on a monthly basis. Plasma levels of immunoreactive parathyroid hormone were measured. In the absence of any alteration of the dogs' internal milieu, following 6-months exposure to lead there was a decrease in bone formation at the cell (70 percent), tissue (66 percent) and organ (66 percent) levels. Six months after the lead exposure was interrupted, bone formation returned to normal (93 percent at tissue and 100 percent at organ levels), but there was still some depression of activity at the cell level (70 percent). It is not known whether that cell-level depression of activity is due to persistent enzymatic inhibition following the 6-months exposure or whether it is due to the fact that the cell continued to operate in a micro-environment in which tissue levels of lead are elevated, or both.