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1. The effect of 5 consecutive days of hill walking on electrolyte balance, fluid homeostasis, plasma renin activity and plasma aldosterone concentration was studied in five male subjects. 2. The 5-day exercise period was preceded by a 4-day control period and followed by a 4-day recovery period. Throughout the 13-day study subjects ate a fixed diet. 3. After 5 days of exercise subjects had retained a mean of 264 mmol (SD 85) of sodium. Plasma sodium concentration remained constant at 142.0 mmol/l (SD 5.4). This indicates an expansion of the extracellular space by 1.84 litres. 4. By the end of the exercise period there was a positive water balance of about 0.9 litre. Thus there was a net movement of 0.94 litre of fluid from the intracellular to the extracellular space. 5. Packed cell volume decreased from a mean of 43.5% to 37.9% after 5 days of exercise, indicating that about 0.9 litre of the extracellular fluid entered the vascular compartment. The remaining fluid may be responsible for the significant increase in lower leg volume. 6. During the exercise period plasma aldosterone concentration and plasma renin activity rose, and there was a highly significant correlation between these values and the sodium retention. There was also a significant correlation between sodium retention and the increase in leg volume, which suggests that oedema may be the result of prolonged exercise of this type.

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Exposure of rats to a 50% NzO/oxygen mixture led to a rapid loss of methionine synthase activity in both liver and brain. This enzyme has vitamin B12 as a cofactor. There was impaired conversion of deoxyuridine to deoxythymidine by bone marrow cells and this defect followed loss of methionine synthase activity. There was no homocystinuria. Withdrawal of N2O was followed by a relatively slow recovery of methionine synthase activity over four days. The inactivation of vitamin B12 by N2O promises to be a valuable tool in the study of vitamin B12 metabolism.The anaesthetic gas, nitrous oxide (N20), is activated in vitro in the presence of vitamin B12 and is cleaved into nitrogen and oxygen [1,2]. This is accompanied by oxidation of vitamin B12 from the cob(1)-alamin to the inative cob(I1I)alamin form.Administration of N 2 0 to man over several hours is followed by neutropenia and if given for 10 h [ 3 ] erythropoiesis is altered to the megaloblastic form present in vitamin B12 deficiency in man. Administration for up to five days in the treatment of tetanus [4] was accompanied by marrow failure with a megaloblastic marrow. Amess et al.[ 3 ] also showed that the marrow cells from patients exposed to N20 failed to incorporate deoxyuridine normally into DNA, a step requiring both vitamin B12 and folate, and this defect was partially corrected by adding vitamin B12 to the marrow suspension. In addition, intermittent inhalation of N2O as a form of addiction in man produces a neuropathy not dissimilar to that resulting from vitamin B12 deficiency and reversed by cessation of N20 inhalation [5 -71.Mice and rats exposed to N2O have impaired folate polyglutamate synthesis and abnormal deoxyuridine utilization by marrow cells despite the fact that erythropoiesis remains normoblastic [8,9]. Although methionine synthase is impaired in rats exposed to N20, methylmalonyl-CoA mutase is unaffected by the action of N2O even after propionate stimulation [lo]. Both these enzymes require BIZ as a cofactor.Experimentally it has proved very difficult to produce animals with severe B12 deficiency, and B12 neuropathy in monkeys occurs only after several years Enzyme. Methionine synthase or 5-methyltetrahydro-folate : homocysteine methyltransferase (EC 2.1.1.13).on a B12-deficient diet [ll]. For this reason it has been difficult to study the effect of B12 deficiency and its relationship to folate other than in patients with untreated pernicious anaemia. If a similar effect were produced by inactivation of B12 by N20 it would be of great value to the biochemist and an understanding of how to circumvent the toxic effects of N20 in man would make it possible to use this valuable anaesthetic and analgesic agent on a much wider scale. The purpose of this study was to investigate the effect of N2O on methionine synthase and deoxyuridine utilization in greater detail. MATERIALS AND METHODS N20Exposure. Sprague-Dawley rats were kept in specially constructed chambers in an atmosphere of 50 % N 2 0 and 50 % oxygen, humidity and C02 concentration bein...

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