Effect of x-irradiation upon glucose catabolism in the intact rat Public Deposited

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  • Beginning with the discovery of x-rays by Wilhelm Konrad Roentgen in 1895, man has had the capacity to greatly increase radiation levels of his environment, especially with the advent of nuclear weapons in recent years. Radioisotopes are finding progressively wider use in medicine, research, power production, etc., with increased hazards to the workers and communities involved. The biological effects that follow exposure to ionizing radiation are a realistic problem. It would, therefore, be desirable to understand more about the mode of radiation action so that possible hazards associated with ionizing radiation can be fully controlled. There is at present a need to investigate further the effects of ionizing radiation on metabolic processes and factors involved in the metabolic mechanism of the cell in intact animals. The purpose in the preparation and work involved in this thesis was to investigate one aspect of metabolism in animals after x-irradiation, namely, carbohydrate metabolism. Radiorespirometric experiments were designed to study pathways of glucose catabolism in mature rats and changes in relative participation of glucose pathways after x-irradiation at the LD 50 dose. To successfully carry out these experiments, a respiratory CO₂ and ¹⁴CO₂ analyzer was developed featuring accurate and continuous analysis of respiratory CO₂ for four concurrent experiments. ¹⁴CO₂ from the metabolizing animal was measured by means of a vibrating reed electrometer and total CO₂ excretion by means of infra-red absorption spectrometry. Readout of data was accomplished in digital form in a manner which allowed a minute by minute tally of ¹⁴CO₂ yield from the administered ¹⁴C labeled substrate in each of the concurrent experiments. The precision of such measurement is estimated to be less than one percent. The radiorespirometric method involves the use of differences in rates and yields of ¹⁴CO₂ formation from ¹⁴C specifically labeled carbon atoms of a substrate. Such information reflects directly the nature of the catabolic sequence traversed by the substrate. An aqueous solution of ¹⁴C specifically labeled glucose containing 1.5 g of glucose was administered to the rat via stomach tubing. Production of respiratory ¹⁴CO₂ was followed until the end of the complete time course of utilization of the labeled substrate. Radiorespirometric data obtained in experiments with normal rats indicated that more than 50% of the administered glucose was utilized via the EMP-TCA sequence. Smaller, but significant, portions of the administered glucose was routed into the pentose phosphate pathway and the glucuronic acid pathway. With rats immediately after 800 R of x-irradiation, the EMP-pyruvate decarboxylation pathway was drastically inhibited, with greater portions of the administered glucose being routed into the pentose phosphate pathway and the glucuronic acid pathway. There was evidence that the fructose-6-phosphate derived from the pentose phosphate pathway was routed more extensively through the pentose cycle pathway. At 80 hours after x-irradiation at 800 R in rats, the operation of the pentose phosphate and the glucuronic acid pathways had increased approximately twofold in relative extent when compared to the normal rat. The EMP-pyruvate decarboxylation pathway was still inhibited, to some extent, 80 hours after x-irradiation. No change could be found in nicotinamide coenzyme levels or NADH oxidase activity in liver homogenates of x-irradiated rats that could explain the significant inhibition of the EMP-pyruvate decarboxylation pathway in glucose catabolism observed after x-irradiation. The observed change in the catabolic picture of rats after x-irradiation leads one to speculate that the enzyme F-6-P kinase, or some other enzyme in the imme diate subsequent steps of the EMP scheme, may have been preferentially inhibited by x-irradiation.
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