A method for the study of cation transport in vivo: effects of digoxin administration and of chronic renal failure on the disposition of an oral load of rubidium chloride

Increase in Na+/K+ pump numbers in vivo in healthy volunteers taking oral lithium carbonate and further upregulation in response to lithium in vitro

1. We have measured [3H]-ouabain binding to lymphocyte membranes in eight healthy volunteers before and after they had taken lithium carbonate for 14 days in doses which maintained the serum lithium concentration in the range 0.5-1.0 mmol 1-1. 2. There was a statistically significant increase in the [3H]-ouabain binding capacity of the lymphocyte membranes (reflecting the number of Na+, K+-ATPase molecules) after 14 days of lithium administration in vivo. This suggests that a failure to increase pump numbers after similar exposure to lithium in vivo in patients with manic-depressive psychosis is a primary abnormality associated with the disease. 3. In vivo lithium administration did not alter the normal adaptive (upregulatory) response of lymphocyte Na+, K+-ATPase to standard pharmacological challenges, involving in vitro incubation for 3 days with lithium chloride (8 mmol 1-1) or sodium ethacrynate (1 mumol 1-1). 4. We have previously found that there is an impaired response of the Na+, K+-ATPase to these in vitro stimuli in patients with manic-depressive psychosis, and our present data suggest that this abnormality is attributable to the disease itself and not to in vivo lithium therapy. 5. The data also suggest that the increase in vivo Na+/K+ pump activity which we have previously described in healthy volunteers after 21 days of lithium administration is at least partly due to an increase in Na+/K+ pump numbers.

In vivo and in vitro sodium pump activity in subjects with thyrotoxic periodic paralysis

OBJECTIVE

To examine whether sodium pump activity plays a part in the pathogenesis of thyrotoxic periodic paralysis.

DESIGN

Measurement of platelet sodium-potassium ATPase and in vivo sodium pump activities in healthy subjects and thyrotoxic subjects with and without paralysis.

SETTING

University hospital in Hong Kong.

SUBJECTS

21 healthy subjects, 23 untreated thyrotoxic subjects, 13 untreated men with periodic paralysis, seven treated thyrotoxic subjects, and six treated men with periodic paralysis.

MAIN OUTCOME MEASURES

Platelet Na+, K(+)-ATPase activity and plasma rubidium concentration after oral loading.

RESULTS

Median (range) platelet Na+, K(+)-ATPase activity in thyrotoxic subjects was 253 (169-821) mumol inorganic phosphate/h/g protein--significantly higher than that in healthy subjects (134 (81-180) mumol/h/g protein; p less than 0.001). Na+, K(+)-ATPase activity in those with periodic paralysis was 374 (195-1196) mumol/h/g protein, again significantly higher than that in healthy subjects (p less than 0.001) and that in other thyrotoxic subjects (p less than 0.01) despite similar degrees of hyperthyroidism. Activities in treated thyrotoxic subjects with and without periodic paralysis were 148 (110-234) and 131 (86-173) mumol/h/g protein respectively. Mean (95% confidence interval) plasma rubidium concentration five hours after oral administration in thyrotoxic subjects (7.0 (6.6 to 7.5) mumol/l) was significantly lower than in healthy subjects (10.2 (9.5 to 10.9) mumol/l; p less than 0.001) and higher than in those with periodic paralysis (6.0 (5.7 to 6.3) mumol/l; p less than 0.01).

CONCLUSIONS

Sodium pump activity in untreated subjects with periodic paralysis is higher than in other thyrotoxic subjects, and this may be responsible for the hypokalaemia.

Altered in vitro adaptive responses of lymphocyte Na+,K(+)-ATPase in patients with manic depressive psychosis

When lymphocytes from healthy subjects are incubated in lithium (8 mM) or ethacrynate (1 microM) they show a time-dependent adaptive response, which consists of a significant increase in the number of Na+,K(+)-ATPase molecules in the lymphocyte membrane. We have studied the lymphocytes from nine euthymic drug-free patients with a history of manic depressive psychosis, and have found that this normal adaptive response was absent. It was also absent from the lymphocytes of euthymic patients taking lithium. We conclude that this altered in vitro adaptive response of lymphocyte Na+,K(+)-ATPase represents an enduring trait marker in manic depressive psychosis.

The effect of oral salbutamol on cation transport measured in vivo in healthy volunteers

1. We have measured the effect of oral salbutamol on cation transport in vivo by studying the disposition of an oral load of rubidium chloride in healthy treated volunteers and in untreated matched controls. 2. During the administration of salbutamol there was a significantly lower plasma rubidium concentration 5 h after the administration of the oral load of rubidium chloride, reflecting an increase in the net clearance of rubidium from the plasma into at least some tissues in vivo. 3. There was no difference in either intraerythrocytic rubidium concentrations or the pseudo-rate constant for erythrocyte rubidium uptake in vivo after salbutamol. 4. Ex vivo incubation of whole blood preloaded in vivo with rubidium showed that the clearance of rubidium from the plasma was inhibited by 95% in the presence of the Na+,K(+)-ATPase inhibitor digoxin. 5. These data suggest that salbutamol stimulates cation transport via Na+,K(+)-ATPase in vivo into some tissues but not into the erythrocyte. 6. This pattern of change in rubidium disposition after salbutamol is completely different from the patterns of change we have seen in patients with essential hypertension or acute manic illness. We therefore suggest that the changes in erythrocyte rubidium uptake which we have previously described in vivo in patients with essential hypertension or acute manic illness do not result from beta 2-adrenoceptor-mediated catecholamine stimulation of Na+,K(+)-ATPase.

The effect of lithium on cation transport measured in vivo in patients suffering from bipolar affective illness

We have investigated cation transport in vivo in patients being treated with lithium for bipolar affective illness by studying the disposition of rubidium after an oral load of rubidium chloride. The rate of erythrocyte cation transport was increased in the patients when compared with matched healthy volunteers. However, the rate of in-vivo erythrocyte rubidium accumulation in the euthymic treated patients was significantly lower than in a matched group of unmedicated manic patients. The regulation of specific pathways for cation transport may be altered in individuals predisposed to affective illness.

The measurement of transmembrane cation transport in vivo in acute manic illness

We have used a novel technique to assess the transport of cations across the erythrocyte membrane in vivo in unmedicated patients suffering an acute manic illness. The results show that erythrocyte cation transport via the sodium-pump enzyme Na+,K+-ATPase is increased in manic patients compared with healthy controls.

A study of the transport of lithium across the erythrocyte membrane in vivo and of the effects of the ion transport inhibitors digoxin and dipyridamole

1. We have given an oral load of lithium carbonate to healthy volunteers in order to investigate the transport of lithium across the erythrocyte membrane in vivo and the effects of known inhibitors of that transport. 2. Using this technique we have shown that pretreatment with either digoxin, an inhibitor of the sodium/potassium pump, or dipyridamole, an inhibitor of the anion transporter, does not alter the plasma or erythrocyte lithium concentration profiles, nor any of the pharmacokinetic variables derived from these data, and we conclude that these two transport pathways do not contribute significantly to the in vivo handling of lithium by erythrocytes. 3. We have also shown that erythrocyte lithium concentrations measured directly differ significantly from the predicted concentrations calculated using the two-compartment pharmacokinetic model which has been used in some earlier comparisons of in vitro and in vivo lithium handling. 4. We suggest that the in vivo administration of lithium carbonate may permit a specific measure of the in vivo activity of the sodium/sodium countertransport pathway.

Effect of lithium and of other drugs used in the treatment of manic illness on the cation-transporting properties of Na+,K+-ATPase in mouse brain synaptosomes

We have developed and used a novel technique to investigate the effects of lithium and other psychotropic drugs on the cation-transporting properties of the sodium- and potassium-activated ATPase enzyme (Na+,K+-ATPase) in intact synaptosomes. Rubidium-86 uptake into intact synaptosomes is an active process and is inhibited by approximately 75% in the presence of the Na+,K+-ATPase inhibitor acetylstrophanthidin. In vitro addition of lithium to synaptosomes prepared from untreated mice causes a progressive inhibition of acetylstrophanthidin-sensitive 86Rb uptake, but only at concentrations higher than the clinical therapeutic range. However, pretreatment of mice for 14 days in vivo with lithium, carbamazepine, and haloperidol, but not phenytoin, causes a significant stimulation of 86Rb uptake into synaptosomes via Na+,K+-ATPase.

A review of the biochemical and neuropharmacological actions of lithium

The pharmacological actions central to the therapeutic effects of lithium have not yet been established, despite almost 40 years of clinical use and scientific investigation. We review the biochemical and neuropharmacological data relating to this problem and attempt to identify profitable areas for further research.

Renal tolerance of rubidium chloride: short-term clinical evaluation

The rubidium and lithium ions are known to have opposite effects on a wide range of biochemical and behavioral parameters in experimental animals. Based on the proven effectiveness of lithium as an antimanic agent, several trials have been conducted with rubidium in the acute treatment of the depressive phase of bipolar illness. The results to date are promising. However, the 30- to 60-day biologic half-life of rubidium has mandated careful studies of potential toxicity before engaging in long-term administration of this ion to depressive subjects. One area of potential concern is the possibility of renal toxicity, which could be expressed as unexpectedly increased retention of rubidium. The data in this paper show that after 15 days of rubidium administration, there are no changes beyond the normal range in a variety of kidney function tests, including in four enzymes which are specific markers of tubule cell function.

Adrenaline causes potassium influx in skeletal muscle and potassium efflux in cardiac muscle in rats: the role of Na/K ATPase

Previous in vitro evidence suggests that adrenaline causes K influx in skeletal muscle by stimulating a ouabain sensitive Na/K ATPase membrane pump. However in rabbits, adrenaline induced hypokalaemia was not significantly altered by pretreatment with digoxin (50 micrograms/kg). Rats were infused with adrenaline or saline after being given a tracer dose of 42KCl. Adrenaline caused a highly significant uptake of 42K in skeletal muscle and a decrease in 42K uptake in ventricle. Rats were also studied after receiving a high dose of digoxin (1.4 mg/kg) which by itself produced a significant increase in plasma K, a decrease in plasma Na and a decreased uptake of 42K in ventricle and lung. These results suggest that adequate widespread Na/K ATPase inhibition had been achieved by this dose of digoxin but despite this, adrenaline still caused hypokalaemia and also still caused significant 42K tissue uptake by skeletal muscle. These results suggest that adrenaline causes K influx by skeletal muscle and K efflux by cardiac tissue. Furthermore, the former mechanism was not inhibited by pretreatment with digoxin.

In vivo cation transport during short-term and long-term digoxin therapy

We have studied the effects of digoxin on cation transport in vivo by measuring the changes in plasma and red cell rubidium concentrations following an oral load of rubidium chloride. In eight patients who had been taking digoxin for 7 to 10 days (mean plasma digoxin concentration 1.3 ng ml-1) the rise in plasma rubidium concentrations was enhanced and the rise in red cell rubidium concentrations was attenuated following the oral load of rubidium chloride, by comparison with the changes in well-matched controls. In contrast, the disposition of rubidium was not altered in 12 patients who had been taking digoxin for more than 3 months (mean plasma digoxin concentration 1.1 ng ml-1). These results suggest that the inhibitory effects of digoxin on cation transport are detectable in vivo during short-term therapy, but not during long-term therapy, and confirm our previous in vitro findings.