“…Serum of uremic patients interfered with the radiorubidium assays for digitalis (10); this observation and the reduced inotropic dose-response relationship to digoxin in uremic animals (11) led several authors to the conclusion that in uremic animals endogenous digoxin-like substances were present, presumably in supranormal concentrations. The presence of endogenous digitalis in normal mammals had been postulated half a century ago (12).…”