Correlations between plasma levels of an endogenous digitalis-like substance and haemodynamic parameters measured during cardiac catheterization

It has been postulated that one or more plasma digitalis-like compounds may play an important role in body fluid regulation and in essential hypertension, although very little is known about their possible role in general haemodynamics. We therefore measured plasma inhibition of human kidney Na+,K+-ATPase and plasma cross-reactivity with digoxin antibodies in 11 normotensive cardiopathic subjects admitted to our clinic for heart catheterization. Possible correlations with haemodynamic parameters were studied. Plasma digoxin-like activity correlated directly with left atrial pressure and with pulmonary circulation data. The ability of the plasma to inhibit Na+,K+-ATPase showed an inverse correlation with cardiac output and cardiac index. No correlations were found with any of the other parameters measured, notably systemic resistance, blood pressure and natriuresis. These findings suggest the presence of more than one substance sharing chemical properties with digitalis: (1) a substance cross-reacting with digoxin antibodies and dependent on pulmonary vascular congestion; and (2) a substance capable of inhibiting the Na+-K+ pump and present in large amounts in heart diseases with a reduced cardiac index.