The 370 Da inhibitor of the sodium pump in the plasma of haemodialysis patients

Background: Previous studies have shown that a molecule of mass 370 Da that inhibits the sodium pump can be extracted from human placentas and from the concentrated plasma or ultrafiltrate of volume-expanded patients. Aim: To study the abundance of the 370 Da molecule and its changes across dialysis in a population of patients with renal failure treated by haemodialysis. Methods: 4 mL pre- and post-dialysis blood samples (2 mL plasma) were taken from patients receiving intermittent haemodialysis and analysed by high performance liquid chromatography (HPLC) coupled to high sensitivity mass spectrometry. Results: In over half the study population, the 370 Da molecule was present in an abundance that exceeded the limit of quantitation. Most patients experienced a marked fall in the abundance of the molecule over a haemodiafiltration (HDF) session, though exceptions were seen in two individuals both of whom showed clear evidence for the presence of two structural isomers of the 370 Da molecule. Conclusions: Advanced renal failure is frequently accompanied by an increased abundance of a 370 Da inhibitor of the sodium pump and that abundance is strongly impacted by haemodialysis. The technique described here could readily be applied to other clinical situations where sodium pump inhibition might be anticipated, such as hypertension, pregnancy and fetal medicine and thereby lead to a better understanding of the physiology and patho-physiology of these conditions.

Cardiac Glycosides in Human Physiology and Disease: Update for Entomologists

Cardiac glycosides, cardenolides and bufadienolides, are elaborated by several plant or animal species to prevent grazing or predation. Entomologists have characterized several insect species that have evolved the ability to sequester these glycosides in their tissues to reduce their palatability and, thus, reduce predation. Cardiac glycosides are known to interact with the sodium- and potassium-activated adenosine triphosphatase, or sodium pump, through a specific receptor-binding site. Over the last couple of decades, and since entomologic studies, it has become clear that mammals synthesize endogenous cardenolides that closely resemble or are identical to compounds of plant origin and those sequestered by insects. The most important of these are ouabain-like compounds. These compounds are essential for the regulation of normal ionic physiology in mammals. Importantly, at physiologic picomolar or nanomolar concentrations, endogenous ouabain, a cardenolide, stimulates the sodium pump, activates second messengers, and may even function as a growth factor. This is in contrast to the pharmacologic or toxic micromolar or milimolar concentrations achieved after consumption of exogenous cardenolides (by consuming medications, plants, or insects), which inhibit the pump and result in either a desired medical outcome, or the toxic consequence of sodium pump inhibition.

Calcium adaptation to sodium pump inhibition in a human megakaryocytic cell line

The unique characteristics of the platelet Na/Ca exchanger, i.e., its dependence on both transmembrane Na and K gradients, render it highly sensitive to Na pump inhibition. In this project, we observed that the human megakaryocytic cell line CHRF-288 expresses both the α1- and α3-isoforms of the Na-K-ATPase. Inhibition of the Na pump increased the RNA and protein expressions of sarco(endo)plasmic reticulum Ca-ATPase 2b, cytosolic Na and Ca, and the freely exchangeable Ca in the endoplasmic reticulum. These changes occurred in concert with diminished store-operated Ca entry and an increase in the maximal activity of the Na/Ca exchanger. Inhibition of the Na pump by ouabain was more effective in inducing these changes than diminishing medium K. Collectively, these observations point to an integrative effort to counteract the impact of Na pump inhibition by Ca sequestration into the endoplasmic reticulum, diminished Ca entry, and increased activity of the Na/Ca exchanger. The implications of these findings in platelet biology are discussed.