1. We have used n.m.r. spectroscopy to measure rubidium concentrations in the skeletal muscle of live intact rats. Using a 1.9 T superconducting magnet and an ear-phone coil tuned to both protons ( 1 H) and rubidium ( 87 Rb), it was possible to make measurements of both tissue rubidium content and water content, and from these measurements to obtain the rubidium concentration. 2. The n.m.r. estimate of rubidium concentration in muscle in vivo was found to be a constant 31% ( sem 4%) of that estimated by flame atomic absorption spectroscopy in an extract of excised muscle. This is close to the predicted theoretical n.m.r. visibility of 33%. The visibility was constant for muscle rubidium concentrations ranging between 10 and 34 mmol/l. 3. Rubidium concentration measurement by this method is unaffected by variations in sample geometry, sample volume, tissue conductivity, coil tuning and amplifier gain. 4. By using this method to measure changes in tissue rubidium concentration with time in the same animal, it should now be possible to assess the activity of ion transport systems, such as sodium- and potassium-activated adenosine triphosphatase in vivo , by measuring the rates of change of tissue rubidium concentrations during the administration of rubidium salts. 5. This method could also be used to measure the absolute concentration of any n.m.r.-visible nucleus and could be applied to man.

1. We have measured cation transport in vivo in seven healthy volunteers under control conditions and after they had taken lithium carbonate for 21 days in doses which maintained the serum lithium concentration in the range 0.6–0.8 mmol/l. 2. We have measured cation transport in vivo after the administration of an oral load of rubidium chloride, and have found that, although intra-erythrocytic concentrations of rubidium were significantly lower 1 h after the administration of rubidium when the subjects were taking lithium, there was a significant increase in the rate of uptake of rubidium into the erythrocytes over the subsequent period of the test, suggesting a direct stimulation of sodium, potassium-activated adenosine triphosphatase by lithium. 3. Lithium administration did not affect the plasma concentration versus time profile of rubidium after the rubidium load, implying that the lithium-stimulated uptake of rubidium which occurs in erythrocytes does not necessarily occur in other cell types. 4. These results suggest that previous studies of cation transport using peripheral cells and assay systems in vitro do not necessarily reflect changes in cation transport in vivo in excitable tissues.

1. We have measured specific [ 3 H]ouabain binding and ouabain sensitive 86 rubidium influx in intact human lymphocytes incubated for up to 7 days in media containing different concentrations of fetal calf serum and human serum. 2. Incubation for periods of up to 7 days with fetal calf serum and human serum produced increases in both specific [ 3 H]ouabain binding and ouabain sensitive 86 rubidium influx that were dependent on concentration and time. 3. Neither specific [ 3 H]ouabain binding nor ouabain sensitive 86 rubidium influx was altered when dialysed serum was used, suggesting that both fetal calf serum and human serum contain a dialysable factor or factors which stimulate specific [ 3 H]ouabain binding and ouabain sensitive 86 rubidium influx in intact human lymphocytes. 4. To further elucidate the mechanisms underlying these changes we also measured the activity of two other enzymes of the lymphocyte plasma membrane, 5′-nucleotidase and γ-glutamyltransferase, the uptake of [ 3 H]thymidine by the intact cells, and the effects of cycloheximide, puromycin, and anisomycin, inhibitors of protein synthesis. 5. The activity of 5′-nucleotidase was increased after incubation of the lymphocytes in fetal calf serum for 72 h, but the activity of γ-glutamyltransferase was not changed, suggesting some selectivity of the stimulatory effect. 6. Measurements of [ 3 H]thymidine uptake by the lymphocytes showed that the major part of the observed changes in specific [ 3 H]ouabain binding and ouabain sensitive 86 rubidium influx was not attributable to transformation of the lymphocytes to lymphoblasts. 7. All three inhibitors of protein synthesis prevented the increase in specific [ 3 H]ouabain binding due to fetal calf serum.

1. We have measured intracellular sodium concentrations and specific 3 H-labelled glycoside binding characteristics in the erythrocytes and leucocytes of patients with untreated essential hypertension, and have compared the results with those in well-matched normotensive control subjects. 2. Intracellular sodium concentrations were increased in the leucocytes, but not in the erythrocytes, of patients with untreated essential hypertension. 3. There were no differences in the 3 H-labelled glycoside binding characteristics of either the erythrocytes or the leucocytes of hypertensive and normotensive subjects. 4. There was no difference in the ability of plasma samples from hypertensive and normotensive subjects to inhibit the binding of [ 3 H]-ouabain to intact leucocytes from normotensive subjects. 5. These findings are not consistent with the presence of increased concentrations of a substance which behaves like a cardiac glycoside in the circulation of patients with untreated essential hypertension.

1. In order to study cation transport in vivo we have measured the changes in plasma and intra-erythrocytic rubidium concentrations following an oral load of rubidium chloride. The changes in plasma rubidium concentration are related to the distribution of rubidium to all the body tissues and the changes in intra-erythrocytic rubidium concentrations provide an example of rubidium uptake by one particular tissue. 2. In eight healthy volunteers pretreatment with a loading dose of digoxin (20 μg/kg) enhanced the rise in plasma rubidium concentrations and attenuated the rise in intra-erythrocytic rubidium concentrations after the oral load of rubidium chloride. 3. Ten patients with chronic renal failure, compared with a well-matched control group, were found to have changes similar to, but more marked than, those caused by digoxin, i.e. a much greater rise in plasma rubidium concentrations and a much smaller rise in intra-erythrocytic rubidium concentrations, after the oral load of rubidium chloride. 4. These findings are consistent with widespread reduction in Na + ,K + -ATPase activity in subjects who have taken a loading dose of digoxin and patients with chronic renal failure. They are, therefore, consistent with the findings of previous studies in vitro and show that it is possible to demonstrate changes in cation transport in vivo .

1. The conversion of cytosine arabinoside into its active metabolite cytosine arabinoside triphosphate, and catabolism by deamination to uracil arabinoside, was measured in intact marrow myeloblasts from patients with acute myeloid leukaemia. The ratio of uracil arabinoside/cytosine arabinoside triphosphate ranged from 0.32 to 19.11. 2. The effect of tetrahydrouridine, an inhibitor of cytosine arabinoside deamination, on cytosine arabinoside triphosphate production was studied. The greatest increase of cytosine arabinoside triphosphate production caused by addition of tetrahydrouridine was 27%. 3. The increase in cytosine arabinoside triphosphate production was not related to the ratio of uracil arabinoside/cytosine arabinoside triphosphate or to the deaminase activity per 10 6 cells. It was proportional to the percentage change of cytosine arabinoside in the incubation medium. 4. The sensitivity of DNA synthesis to inhibition by cytosine arabinoside was measured in myeloblasts from 11 patients. Addition of tetrahydrouridine did not increase the sensitivity of the marrow to cytosine arabinoside. 5. Cytosine arabinoside deamination is unlikely to be an important mechanism of resistance in myeloblasts in vivo , although it may produce apparent resistance in vitro.

1. Monoamine oxidase activity in platelets prepared from the blood of patients with iron-deficiency anaemia was significantly lowered when compared with that in platelets from normal subjects. 2. The K m values of the platelet enzyme for the substrates dopamine, 5-hydroxytryptamine, phenylethylamine and kynuramine were similar for the platelet enzyme from iron-deficient and normal groups. 3. Heat-inactivation studies showed that the platelet monoamine oxidase from iron-deficient subjects was more labile to this treatment, when compared with the platelet enzyme from normal subjects. 4. The sensitivity of platelet monoamine oxidase to the inhibitors, clorgyline and deprenil, was increased in iron-deficiency anaemia. 5. Binding studies with the 14 C-binding irreversible monoamine oxidase inhibitor, deprenil, showed that the amount of enzyme capable of binding this inhibitor was lowered by 48% in platelets from iron-deficient patients when compared with platelets from normal subjects. 6. The results show that there is a lowered amount of active enzyme in platelets from iron-deficient subjects. It is suggested that iron is necessary either for the synthesis of monoamine oxidase apoenzyme or is a cofactor for an enzyme which attaches flavin—adenine dinucleotide covalently to the monoamine oxidase apoenzyme.

1. The activity of monoamine oxidase, when assayed with four substrates, was significantly lowered in platelets prepared from the blood of patients with iron-deficiency anaemia. 2. Treatment with oral iron preparations restored platelet monoamine oxidase activity to normal in those patients whose serum iron concentrations also returned to normal. 3. Platelet monoamine oxidase activity remained low if treatment failed to restore serum iron concentration to within normal limits.