PARATHYROID HORMONE AND RAT CALVARIA METABOLISM 593 



constancy of the rate over an incubation period of 8 hours, all sub- 

 sequent studies were conducted for 3 hours or less. The average 

 Qo.2 of 10 different experiments, 18.4 ± 2.5 /u,moles/gm wet wt/hr., 

 is intermediate between earlier values reported for metaphysis and 

 for a truly active soft tissue such as liver: 



Qot Ainioles/mg Qot Mmoles/mg 



NCN DNA 



Metaphysis (Borle et al, 1960a, 19606) 1.1 



Metaphysis (Laskin and Engel, 1956) 57 



Calvarium 2.0 230 



Liver (Spector, 1956) 850 



No effect on the Qoo could be found upon addition of a variety 

 of substrates, including glucose, pyruvate, acetate, fumarate, malate, 

 oxaloacetate, citrate, and alpha ketoglutarate either singly or in 

 various combinations best calculated to stimulate oxygen uptake. 

 Similar findings were reported bv Laskin and Engel using meta- 

 physeal preparations (Laskin and Engel, 1956). 



Succinate was the only substrate found to have any effect (Fig. 

 IB) : a 50 per cent increase at a concentration of 10 mM or higher. 

 This stimulation was abolished by the subsequent addition of mal- 

 onate at an equal or higher concentration. Note that, whereas the 

 succinate effect is immediate, the malonate effect takes some 15 to 

 20 minutes to develop. Because of its ability to participate in sub- 

 strate-linked oxidative phosphorylation independent of the pyr- 

 idine nucleotides, however, succinate occupies a special place in 

 the Krebs ( TCA ) cycle, and stimulation of oxygen uptake in a tissue 

 upon its addition is not evidence of increased TCA cycle activity, 

 but only of the availability of an excess capacity in the succinic de- 

 hydrogenase-flavoprotein system. Malonate does more than abolish 

 the increase due to succinate. It produces, in turn, a further reduc- 

 tion in the Q02 which, relatively to the endogenous rate, is equal to 

 that produced by malonate additions alone ( not shown ) . In experi- 

 ments both with and without succinate, a concentration effect of 

 malonate was noted: 1 mM not affecting the Q0.2, 30 niM reducing 

 it by some 25 to 30 per cent, 10 niM being intermediate. The con- 



