Radioimmunoassays, ouabain-like material, and ouabain.

Radioimmunoassays, Ouabain-Like Material, and Ouabain To the Editor: We read with interest the article by Bauer et al1 on the changes in the plasma concentration of a ouabain-like compound associated with vigorous exercise in both humans and dogs. The authors comment on the result that is most striking to us, namely the remarkably high concentrations of the immunoassayable substance achieved without apparent ill effect. Bauer et al1 suggest that this may be a result of either the transient nature of the high concentrations measured, the “slow on-rate in forming the ouabain-Na/K ATPase complex,” or by the ouabain-like substance being bound by proteins. The most remarkable values of ouabain-like compound achieved were those in beagles with a mean “ouabain” concentration of 6882 1436 nmol/L. The raw data are not given but it is reasonable to assume that the concentration in at least one animal reached approximately 8000 nmol/L (4.68 mg/L). At such a ouabain concentration, close to 100% inhibition of any accessible sodium pumps would occur almost instantaneously. Whereas the numbers in the nontrained humans are less dramatic (176 68 nmol/L), it seems likely that the highest concentration seen in this population was in the region of 300 nmol/L (175 g/L). This would inhibit approximately 50% of the sodium pumps in proximity to the plasma. If the immunoassayable “ouabain” was uniformly distributed throughout the plasma this would correspond to a ouabain “dose” of approximately 0.8 mg in humans and 4.5 mg in beagles. The rapid administration of such an intravenous dose of ouabain would not be expected to be uneventful for the human recipient and even less so in the case of the beagle because the LD50 is around 0.1 mg/kg2 and the animals weighed 20 kg. If we were to assume a distribution volume equal to the extracellular fluid and make no allowance for binding by sodium pumps, we can calculate the total amount of ouabain secreted by the beagle in 13 minutes from the formula: ECF volume/plasma volume total dose in plasma; this is close to 13 mg. Given that significant amounts of endogenously secreted ouabain would have bound to the sodium pumps this is likely to be an underestimate. Whatever the explanation for these results, they do provide an excellent opportunity to resolve a division that exists in relation to endogenous ouabain. Since the mass-spectrometric identification of ouabain (or closely related substance) in an extract of human plasma,2 there have been those who have questioned its endogenous origins, whereas others, relying principally on radioimmunoassay, have equated the immunoassayable substance with authentic ouabain and use the terms “ouabain-like” and “ouabain” interchangeably. The quantities of the immunoassayable substance seen in these humans and, to a far greater extent in dogs, fall comfortably within the range required for physical characterization by mass spectrometry and, in view of the milligram amounts reported in beagles, proton NMR. If the substance responsible is indeed authentic ouabain the skeptics probably have to revise their position. If not, then the significance of plasma immunoassayable “ouabain” would be called into question. P.J. Hilton W. McKinnon Renal Laboratory St Thomas’ Hospital London, UK