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Cells were isolated by sequential collagenase digestion from the parietal segments of one day old mice (Swiss albino BNL strain) and characterized for osteoblast parameters by alkaline phosphatase histochemistry and bovine parathyroid hormone (bPTH- (1–34)) induced cAMP activity (protein binding assay). Phenytoin (DPH) reduced PTH stimulated cAMP activity nearly 3-fold in the presence and nearly 1.5-fold in the absence of added calcium. In the absence of PTH, DPH exerted no significant effect. Bay-K-8644, a calcium channel activator, appeared to approximate the PTH stimulation of cAMP activity, even in the presence of DPH. This study demonstrates that DPH has a direct effect on PTH stimulated cAMP activity in cultured murine osteoblasts.

Cathepsin B and L activities were examined with chicken osteoclasts isolated by sequential filtration and inhibitors were added to disaggregated rat osteoclasts on cortical bovine bone. Z-Phe-Phe-CHN2, a selective inhibitor of cathepsin L, at 1, 5, and 10 microM, inhibited bone resorption by rat osteoclasts 50, 85, and 100 per cent and, in chicken osteoclasts, cathepsin L activity was comparably inhibited. Cathepsin L in avian osteoclasts was also 25-fold higher than cathepsin B. Chicken osteoclasts treated with Z-Phe-Ala-CHN2, a generalized cysteine proteinase inhibitor, had both cathepsins inhibited to the same extent. Cathepsin L may play a key role in resorption.