Evidence for the existence of an endogenous inhibitor of Na, K-ATPase in plasma from cats and rats.

Plasma samples from cats and rats were filtered through molecular filters to obtain a concentrate of the plasma molecules with a mass between 500 and 10,000. This concentrate was placed on a gel filtration column and eluted with a Tris buffer. Fractions corresponding to a molecular mass of 1500-2000 exhibited a marked Na,K-ATPase inhibitory 'activity', which was not influenced by heating to 95 degrees C for 10 min or by enzymatic degradation by pronase or trypsin.