104 



MARTIN G. LARRABEE AND PAUL HOROWICY 



One possible interpretation of the smallness of the change in oxygen uptake 

 after withdrawal of glucose, would be that glucose is not oxidized to a signifi- 

 cant degree, even when available, but is all lost as lactate and other unoxidized 

 materials. In this case the approximate agreement which we have found between 

 the observed oxygen uptake and that calculated for glucose oxidation would 

 be fortuitous, and the substrate for oxidation would be all endogenous, even 

 when external glucose is supplied. 



At the present time, however, we prefer to continue with the assumption, 

 adopted elsewhere in this paper, that all the glucose which is not converted 

 to lactate is normally oxidized. If this be true, our oxygen measurements 

 show that the cells of a ganglion shift from glucose to the oxidation of endog- 

 enous substrates when the external sources of carbohydrate are withdrawn. 

 It does not seem likely that glycogen serves as the substitute for glucose: 

 if glycogen were oxidized at a rate sufficient to support the normal rate of 

 oxygen uptake there is no obvious reason why function should not also be 

 maintained. Possibly the endogenous substrates utilized during the glucose- 

 lack include proteins. In agreement with this suggestion are observations of 

 others on brain tissue (Kimura and Ito, 1952) and measurements on ganglia 

 in our own laboratory (Stekiel and Larrabee, 1955), which indicate that ammo- 

 nia production increases when glucose is withdrawn. 



Speculation on causes of failure of function, (a) Loss of energy production. 

 Since oxygen consumption does not drop significantly when glucose is with- 

 drawn, it is doubtful whether the decline in response should be ascribed to loss 

 of energy production. In this connection it is relevant that oxygen uptake can 

 be considerably depressed by other procedures without causing failure of 

 function (Table V). 



The disappearance of the response during withdrawal of glucose could seem- 

 ingly be explained on energetic grounds only by making the gratuitous (although 

 possibly correct) assumption that the energy derived from oxidation of endog- 

 enous substrates is less efficiently coupled to the mechanisms for energy utili- 

 zation than that derived from exogenous glucose. 



TABLE V 

 Effects on oxygen consumption and postsynaptic response in rat ganglia 



A. Control 



B. No Glucose 



C. Azide (0.5, 1.0 mM) 



D. Lactate + IAA (0.1 mM) 2 



Rate of 

 O2 Uptake 1 



100 

 91 



67,49 

 60, 68 



Postsynaptic Response 



Survived at least 12 hours. 



Failed rapidly between l 1 ^ & 2 ] 2 hrs. 



Survived at least 4 hours. 



Survived at least 6 hours. 



1 Two hours after changing bathing solution, expressed as percent of rate at start. 



2 In the absence of glucose. 



