A quantitative receptor assay for “digitalis-like” compounds in serum. Demonstration of raised concentrations in essential hypertension and correlation with arterial blood pressure

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Abstract
The influence of serum from patients with essential hypertension on [3H]ouabain binding to isolated (Na++K+)-ATPase and on the reactivity with digoxin-specific antibodies was investigated. [3H]Ouabain binding to (Na++K+)-ATPase was significantly decreased (P3H]ouabain at isolated (Na++K+)-ATPase. Therefore, it was possible to quantify this “digitalis-like” factor with a receptor assay in ouabain equivalents. Three times higher mean serum levels were found in hypertensive patients (234.8±48.7 nM) than in normotensive controls (76.3±9.3 nM). Deproteinization of the sera by ultrafiltration through steroid-adsorbing membranes and by boiling of acidified sera for 10 min led to a significant reduction of the inhibitory activity and to the complete loss of a difference between the sera of normotensives and hypertensives. After deproteinization by boiling for 15 min, sera of normotensives showed levels of “digitalis-like” compounds of 16.53±2.15 nM and hypertensives of 41.65±8.41 nM (P<0.05). Though significantly elevated concentrations of “digitalis-like” factor were measured with the receptor assay, no significant increase of digoxin-like activity could be detected with digoxin-specific antibodies in untreated serum.