380 



F. C. MCLEAN AND R. E. ROWLAND 



PARATHYROID HORMONE 

 CALCITONIN (?) 



Fig. 4. Diagram of control system regulating Ca++ concentration in the 

 blood. For explanation, see text. 



cytes, and osteoclasts — are potentialK' capal^le of performing this 

 function; available evidence suggests that this is done by a com- 

 bination of osteoclastic resorption with release of H+ from the gly- 

 colytic cvcle of the osteocytes; there is no specific suggestion that 

 the osteoblasts contribute to the release of calcium from bone. 



In the absence of the parathyroid glands there is a decrease in 

 the concentration of calcium in the blood plasma, with 7 mg per 

 100 cc as a representative figure; the transfer of calcium between 

 bone and blood continues under these conditions. The dual mecha- 

 nism of McLean and Urist attributes this to ion transfer, not under 

 the control of the parathyroids. Osteoclastic resorption continues, as 

 demonstrated bv Jowsey et al. ( 1958 ) ; new haversian systems are 

 formed; and the glycolytic cycle of the cells of bone doubtless con- 

 tinues to be active. There must remain in bone all of the mecha- 

 nisms concerned in ion transfer or ion exchange; what is lost is the 

 parathyroid mechanism, which detects and corrects any deviation 

 from the normal concentration of Ca++ in the blood plasma. 



One further proposal remains for consideration, i.e., the possibility 

 that there is a change in the chemical nature of the mineral of bone, 



