i8 3 6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



(by a factor of 10 to loo) than the incorporation of 

 P 32 into the partial hydrolysis products (glyceryl- 

 phosphoryl-choline, -serine and -ethanolamine) of 

 the other phospholipids. There was no change in the 



absolute quantity of 'inositol diphosphate' obtained 

 under conditions making for wide variations in its 

 specific radioactivity; its synthesis must therefore have 

 been accompanied by an equally rapid breakdown of 

 the intrinsic diphosphoinositide in the tissue. In all 

 these experiments, .mother phosphate ester, believed 

 to be derived from a phosphatidic acid, became 

 labeled even more rapidly than 'inositol diphos- 

 phate'. Witter £225) has pointed out that this may be 

 an occurrence peculiar to work in vitro, since this 

 compound has not been found in experiments with 

 intact animals. 



It appears that the much slower biosynthesis ol 

 the better-known nitrogenous phosphatides (lecithin, 

 phosphatidylethanolamine, phosphatidylserine and 

 sphingomyelin) follow much the same routes as in 

 other tissues, and in slices, lecithin incorporates P 32 

 more rapidly than phosphatidylethanolamine or 

 phosphatidylserine, while in homogenates phosphat- 

 idylserine is the most active of these three (180). The 

 mechanism of the synthesis of diphosphoinositide is 

 still largely unknown, but it appears that the inositol 

 diphosphate moiety (51) has a much more rapid 

 turnover than the rest of the molecule, since the rate 

 ol incorporation of C! 1 ''-labeled glycerol into lecithin 

 and all the kephalins is about the same (82). Autolyz- 

 ing brain slices have also been found to lose 'kephalin' 

 and 'sphingomyelin,' but not other lipids, at sig- 

 nificant rates (85) which confirms the work of Tyrrell 

 (211 1, Goebel & Seckfort (63) and Sperry (189). 

 It has been shown that the cations associated with 

 diphosphoinositide and other acidic lipids can be 

 readilv exchanged for others (-,.;, 188). 



Ml rABOLISM MODIFIED BY XI-I-1 II 1 > MilMs 



Electrical and Cognate /ii/lm-nces 



Detailed assessments ol the effects of applied clec- 

 trical impulses upon the in vitro metabolism of carbo- 

 hydrates and phosphates together with surveys of the 

 literature have been published (76, 134, 136). Gen- 

 erally, application of alternating electrical potentials 

 to sines Hi (hopped preparations ol cerebral tissues 

 which retain cellular structure [bul not toother tissues 

 98 ults in ini reases ol the order of 100 per cent 

 in the oxygen uptake and aerobit la< tic acid produc- 



tion. Levels of creatine phosphate fall while those of 

 inorganic phosphate rise. Anaerobic glycolysis is 

 decreased. Examination of the sequence of these 

 reactions has revealed that the most rapid events 

 are the breakdown of creatine phosphate and an 

 increase in the levels of inorganic phosphate. In- 

 crease in lactic acid accompanies the change in 

 phosphates but glycogen levels arc not affected (147). 

 The changes arc rapid and occur within _> sec of 

 applying electrical pulses. No response is obtained 

 with homogenates or preparations of cerebral mito- 

 chondria. Owing to the nature of the technique used 

 to measure rapid changes in levels of the 

 intermediates, no accurate measurement of the rate 

 of onset of oxygen uptake has been made, but in- 

 creased uptake probably commences within jo sec. 

 of switching on the pulses. On switching off, creatine 

 phosphate is rapidlv resynthesized at 150 jumoles per 

 gin. wet wt. per hr., and the increased levels of tissue 

 lactic acid return to normal (73, 147). The rates of 

 change of the phosphates and lactic acid are high. 

 Thus creatine phosphate is metabolized at 1200 to 

 1400 /imoles per gm wet wt. per hi., while inorganic 

 phosphate increases at about 800 /Limoles P per gm 

 wet wt. per hr. Lactate is formed at a rate of 420 

 pinoles per gm per hr. Consideration of these rates in 

 relation to the maximal oxygen uptake of cerebral 

 tissues under the influence of applied pulses (some 

 100 pmoles Oj per gin wet wt. per hr. for slices of 

 guinea-pig cerebral tissue) shows that the rate of 

 breakdown of creatine phosphate cannot be ac- 

 counted for on the assumption (3) that the decrease 

 is solely due to inhibition of oxidative phosphoryla- 

 tion by the pulses. Examination of the pathway ol 

 creatine phosphate breakdown bv radioactive tracer 

 techniques (74, 75) has shown that other cerebral 

 Constituents, including adenosine triphosphate, guano- 

 sine triphosphate and 'phosphoprotein,' are involved 

 in an exchange reaction which is likelv to proceed at 

 the rate of creatine phosphate breakdown. 



In view of the influence of inorganic phosphate 

 and of phosphate acceptors, such as ere. nine, upon 

 o\v gen uptake in tissue preparations described abov e, 

 it is understandable that increase in their levels 

 should lead to increased oxygen uptake ol intact 



cerebral slices. In this respect th<- effect of electrical 



pulses inav be regarded as removing a constraint 

 imposed upon metabolism bv the lowered levels of 

 inorganic phosphate ,\]M.\ acceptors which norm. ilk 

 exist within the slice, permitting the tissue to exhibit 

 its maximal metabolic potential. 



