MECHANISM OF PARATHYROID HORMONE ACTION 581 



sible for dissolving the minerals is in the Embden-Meyerhof-Krebs 

 system, and the honnone effect also is exerted in this system. 



There are several apparent contradictions to our carbonic acid 

 theory that must be resolved. Vaes and Nichols ( 1962) used glucose- 

 U-C^^ as a substrate and could find no change in the CO2. However, 

 they used a substrate concentration of 11.1 mM, and we have found 

 ( Cohn and Forscher, 1962b ) that in bone the CO2 effect is sensitive 

 to the concentration of the substrate. We have demonstrated this for 

 glucose-U-C^^ as well as for glucose-6-C^^. With the uniform label, 

 the effect could not be detected when the concentration of the 

 substrate was 4 mM or more. Using manometric methods, Laskin 

 and Engel (1956) studied the effects of pretreatment with para- 

 thyroid extract on the metabolic activity of slices of bone and did 

 not detect the changes in the production of CO2 which we describe 

 as evidence of the basic action of the hormone. We also did not find 

 these changes when we used a Warburg respirometer; as Laskin 

 and Engel reported, we found no evidence of utilization of exogenous 

 glucose with such incubations. In fact, were it not for the greater 

 sensitivity of radioisotope tracer methods, one might conclude that 

 there was no utilization of exogenous glucose. From the appearance 

 of C" in the CO2, in lactate, and in other intermediates, we know 

 that the added substrate is metabolized. 



Our hypothesis is that excess parathyroid hormone causes an in- 

 creased production of CO2 from glucose or from metabolites derived 

 from glucose. Calculations based on the specific activity of the added 

 glucose and on the radioactivity of the CO2 produced show that the 

 difference between the production of CO2 in the control and in the 

 experimental flasks would be about 0.2 /A per hour, an increment 

 which is not detectable in the standard Warburg respirometer. The 

 apparent failure of in vitro slice preparations to use an exogenous 

 substrate is known (Robinson, 1949), and we make the assumption 

 that, in the body, exogenous glucose is a major substrate for cellular 

 metabolism. 



Two alternate explanations of our data must be settled. One possi- 

 bility is that the metabolic effect we observed was due to a general 

 or random influence of the hormone and was not necessarily related 



