Noradrenaline: a circulating inhibitor of sodium transport

Intracellular electrolytes in cardiac failure

In congestive heart failure (CHF) there are several compensatory mechanisms operating which may influence electrolyte metabolism. The activation of the renin-angiotensin-aldosterone system causes retention of sodium (Na) and losses of potassium (K) and magnesium (Mg). The secondary hyperaldosteronism may give rise to high intracellular Na and low intracellular K through a direct permeability effect on the cell membrane. The Mg deficiency may lead to a further increase of intracellular Na and decrease of intracellular K since Mg is a necessary ion for the function of the Na-K pump. In 297 patients with diuretic treated CHF we found that 42% had hypokalemia, 37% hypomagnesemia and 12% hyponatremia. We also found that 57% had excess muscle Na, 52% had depletion of muscle K and 43% had low muscle Mg. We have also shown that the low muscle K cannot be corrected by K supplementation when there is a concomitant Mg deficiency and that Mg infusions may change the disturbed relation between extra- and intracellular electrolytes towards normal.

Reduced ouabain binding to erythrocytes in epilepsy--evidence for a membrane abnormality

The number of sodium pump sites on erythrocytes was measured using tritiated ouabain in 20 untreated fasted patients with recent-onset untreated primary generalised or partial seizures and in 22 age- and sex-matched controls. The ouabain binding was significantly lower at 1393 +/- 392 molecules ouabain/pl cells compared to 1677 +/- 366 molecules ouabain/pl cells in the group with epilepsy (P less than 0.02). The differences between ouabain binding in patient and control groups were greater for males than for females. There were no significant differences between patients with generalised and those with partial seizures. These results are consistent with a generalised membrane abnormality in epilepsy which, if present in the brain, might predispose to seizures.

Endogenous cardiac glycosidelike compounds

The possibility that endogenous inhibitors of the sodium pump exist and bind to the cardiac glycoside binding site on Na+,K+-adenosine triphosphatase (ATPase) has been a source of much controversy. Although numerous hormones and inorganic ions that modulate Na+,K+-ATPase activity have been described, most of these affect the sodium pump indirectly by varying the intracellular sodium concentration or by increasing the number of enzyme units. None of these endogenous compounds has been shown conclusively to modulate sodium pump activity by binding to the cardiac glycoside binding site on Na+,K+-ATPase. However, the near-universal presence of three high-affinity binding sites on the alpha-subunit of the enzyme has engendered much speculation that endogenous ligands for these receptors must exist. In addition, none of the hormones known to indirectly affect sodium pump activity in vivo has been shown to modulate Na+,K+-ATPase activity in response to extracellular volume expansion or to play a role in the pathogenesis of hypertension or chronic renal failure, conditions in which a circulating inhibitor of Na+,K+-ATPase has been implicated. This report presents a condensed history of the search for endogenous inhibitors of Na+,K+-ATPase and describes recent advances in the field. Despite progress in identifying and characterizing compounds that could affect Na+,K+-ATPase activity in vivo, definitive proof for the existence of endogenous ligands for the cardiac glycoside binding site remains elusive.

Assay of a circulating sodium pump inhibitor in patients with essential hypertension and normotensive subjects

The plasma levels of a sodium pump inhibitor (Na+ PI) were measured by a modified method of Hamlyn et al, using dog kidney Na+, K+-ATPase. When the level of Na+ PI was expressed as the % inhibition of the enzyme and compared with that of a control solution, it was found to be 9.0 +/- 0.7% in 43 untreated patients with essential hypertension. This was significantly higher than 5.0 +/- 0.4% for 56 normotensive subjects (p less than 0.01). Male patients with essential hypertension showed the highest mean value of 10.5 +/- 1.1%, disclosing an apparent sex difference in the patient group (p less than 0.01). Only in female patients was there a significant positive correlation between the inhibitor's level and the mean blood pressure (r = 0.649, p less than 0.01). These results provided additional evidence for increased Na+ PI in the plasma of patients with essential hypertension, which might bear an important role in the pathogenesis of the disease.

Disturbed hypothalamic control of Na,K-ATPase: a cause of somatic symptoms of depression

Current theories of the causes of somatic symptoms in depression neglect evidence for a general inhibition of Na,K-ATPase. Experimentally, sodium pump inhibition is associated with depolarization of electrically active tissue: the threshold of peripheral nerves is decreased and smooth muscle function altered. Symptoms compatible with these changes are found both in depression and intoxication with digoxin, which inhibits Na,K-ATPase. The hypothalamus contains inhibitors of Na,K-ATPase which are capable of producing depolarization. In depression, changes in hypothalamic activity may increase endogenous inhibition and contribute to the somatic symptoms.