Abstract
Ethane-1-hydroxy-1,1-diphosphonate (EHDP) added ot an equilibrated suspension of hydroxyapatite crystals caused a loss of Ca 2+ from the ultrafiltrate of the suspension but an increase in non-sedimentable, non-ultrafiltrable, non-exchangeable Ca 2+ in the supernatant of the suspension. The evidence suggests that the loss of Ca 2+ from the ultrafiltrate was due to an entry of Ca 2+ into the hydration layer around the crystals. The increase in non-sedimentable, non-ultrafiltrable, non-exchangeable Ca 2+ in the supernatant was due to a dispersing or peptising effect of EHDP on the microcrystals of hydroxyapatite. Further experiments with methylene diphosphate, EHDP, dichloromethylene diphosphate and pyrophosphate (PP i) showed that all except PP i had a peptising effect on hydroxyapatite crystal suspensions. The order of peptising ability was dichloromethylene diphosphate > EHDP > methylene diphosphate > PP i = 0. In addition the diphosphanates and pyrophosphate increased the amount of Ca 2+ and decreased the amount of phosphate in the hydration layer around the crystals, the order of potency being PP i > methylene diphosphonate > EHDP > dichloromethylene diphosphonate. These findings may explain some of the in vivo effects of certain of the diphosphonates on the level of plasma Ca 2+ in thyroparathyroidectomised rats.
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