IN VITRO STUDIES OF BONE RESORPTIVE MECHANISMS 



571 



TIME 



DAYS 



Fig. 9. Radioactivity present in the collagen of metaphyseal bone from rats 

 at varying intervals following a 4-hour "pulse" labeling with proline-1-C^*. All 

 incubations were carried out in Krebs-Ringer bicarbonate-buffered media, pH 

 7.4, at 37.5 °C, with L-proline 0.5 mmole liter and glucose 2 mg. ml present 

 as substrates. Penicillin 5 u/ml and streptoniNcin 0.01 mg/ml were added to 

 prevent bacterial growth. 



stable collagen of ])oiie samples at times ranging from immediately 

 after transfer of the washed samples of bone into unlabeled media 

 to 48 hours. It can be seen that immediately after transfer, collagen 

 radioactivity increased, confirming Flanagan's observation of trans- 

 fer of proline label from the cells of the sample into collagen. This 

 was followed bv a small abrupt decline in counts whose significance 

 is not vet clear. From then on, however, collagen radioactivity re- 

 mained quite stable sa\'e for a very slow rise. The open circles are 

 for a longer experiment with less frequent sampling. At 4 days all 

 the labeled collagen was still present although the cells were still 

 metabolically active as evidenced by continued lactate production 



