Effects of H1 blocking agents on age-related osteopenia in the mouse.

Old female B6AF1 mice were given acidified tap water, distilled water, one of five H1 blockers or chlorpheniramine (an H1 blocker) and trifluoperazine (a phenothiazine with no H1 blocking activity) in their drinking water for 5 months, and the effects of these agents on bone mineral metabolism were assessed by determining ash weights of femur, ilium and sacrum at the end of the study. In one experiment 24 h whole-body retention (WBR) of Tc 99m methylene diphosphonate (Tc 99m MDP, an indicator of bone metabolism) was measured at the beginning of the study and 40 days later. It was found that: promethazine and dimenhydrinate were the most effective of the H1 blockers in preventing age-related loss of bone mass; distilled water, chlorpheniramine, and chlorpheniramine plus trifluoperazine had no effect on the loss of bone mass; mean bone mass in the groups given meclizine and pyrilamine were greater than but not significantly different from that in the control group given acidified tap water; and only promethazine induced a significant reduction in the WBR of Tc 99m (the other H1 blockers induced small but not significant reductions).