80 GENERAL METABOLISM in vitrO 



must have arisen from sources other than inorganic phosphate 

 itself. In this respect the production of inorganic phosphate 

 resembles that occurring /// vivo after decapitation. Entry of 

 phosphate to the tissue slice is partly dependent upon the meta- 

 bolism of glucose (Fries et al., 1942) for the addition of this sub- 

 strate to slices metabolizing radioactive phosphorus markedly 

 increased the amount accumulating in the slices. Heald (19566) 

 studied the rate of incorporation of radioactive phosphate into the 

 labile phosphates and total inorganic phosphate of guinea pig 

 cerebral slices metabolizing glucose. Since the contribution to the 

 radioactivity of the tissue made by phosphate in the extracellular 

 space could not be adequately assessed (for discussion see also 

 Manery, 1952; Allen, 1955) no value was obtainable for the radio- 

 activity of the intracellular phosphate. However, rough calcula- 

 tions based on the radioactivity of the total inorganic phosphate of 

 the tissue slice indicated that between 10-15 /xmoles inorganic 

 phosphate/g wet wt. tissue hr~^ was transported between slices 

 and medium. Thus even on the basis of this low rate, transport 

 into the tissue appears to be more rapid than from the blood to the 

 brain. 



Levels of guanosine phosphates are not readily increased by the 

 inclusion of guanine in the medium, the quantities actually falling 

 from 0-38 /xmoles/g wet wt. in slices fixed immediately after cutting 

 to about 0-20 /xmoles/g in slices incubated for 100 min (Thomas, 

 1957). However, inclusion of both adenine (which increased levels 

 of adenosine triphosphate) and guanine in the medium partially 

 restored the levels of guanine nucleotides. This suggests that 

 increased quantities of adenosine triphosphate are necessary to the 

 synthesis of guanosine nucleotides, and is of interest in view of 

 recent evidence (Heald and Stancer, 1960) that adenosine triphos- 

 phate is a probable precursor of guanosine triphosphate in the 

 brain. The identity of phosphocreatine and the nucleotides has 

 been well established both by specific enzymic methods (Kratzing 

 and Narayanaswami, 1953) and by isolation and chromatographic 

 techniques (Gore ^^. al., 1950; Heald, 1956, 19576). Comparison 

 of the quantities resynthesized by cerebral slices from different 

 species is somewhat limited but in those so far examined there 

 appears to be no major differences. 



In addition to the phosphates listed, triphosphopyridine nucleo- 

 tide is present in guinea pig cerebral slices at levels of 0-04 /xmoles/g 



