No upregulation of digitalis glycoside receptor (Na,K-ATPase) concentration in human heart left ventricle samples obtained at necropsy after long term digitalisation

Study objective - The aim was to evaluate the hypothesis that digitalis glycosides increase the concentration of their specific receptor (Na,K-ATPase) in human myocardial tissue, thereby possibly reducing the inotropic effect of long term digitalis treatment.Design - Intact samples of left ventricle were obtained at necropsy from patients who had been on long term treatment with digoxin and from patients not previously given digoxin. Digitalis glycoside receptors were quantified using vanadate facilitated ³H-ouabain binding before and after washing samples in buffer containing excess digoxin antibody fragments for 16 h at 30°C. This washing procedure has previously been shown to reduce prior specific digoxin binding in human left ventricle by 95% and to allow subsequent vanadate facilitated complete quantification of ³H-ouabain binding sites. In this context it was performed to reduce occupancy of digitalis glycoside receptors by digoxin, caused by digitalisation before ³H-ouabain binding.Subjects - 11 patients who had been on long term treatment with digoxin and eight who had not previously been given digoxin were studied. Left ventricle samples were obtained at necropsy at around 15 h after death.Measurements and main results - Standard ³H-ouabain binding was 39% less in samples from digitalised than from undigitalised subjects (p<0.001). Washing samples in buffer containing excess digoxin antibody fragments induced an increase in ³H-ouabain binding from 174(SEM 10) to 265(20) pmol·g^-1 wet weight (n=11, p<0.001) in samples from digitalised patients. After washing, the digitalis glycoside receptor concentration in left ventricle samples showed a tendency to a lower value (14%, p>0.10) in patients exposed to digoxin compared to left ventricle samples from individuals unexposed to digitalis glycoside treatment. Calculating ³H-ouabain binding relative to dry ventricular muscle weight confirmed the results obtained using wet weight as reference.Conclusions - The results suggest that digoxin treatment in life is associated with a 34% occupancy of digitalis glycoside receptors with digoxin. In the human heart there was no evidence for upregulation of digitalis glycoside receptor concentration due to long term digitalisation. Thus at receptor level there was no evidence for development of tolerance to digoxin therapy. The lower digitalis glycoside receptor concentration in the left ventricle observed in the heart failure patients may support the report of a relationship between Na,K-ATPase concentration as evaluated by ³H-ouabain binding and left ventricular function.