Pulse pressure correlates in humans with a proscillaridin A immunoreactive compound

Endogenous digitalis-like factors in humans are presumably cardenolides and bufadienolides. To lest whether bufadienolide-like substances may circulate in human blood, we used antibodies from rabbits against the bufadienolide proscillaridin A to measure the concentration of cross-reacting material in human plasma with an indirect enzyme-linked immunosorbent assay. IgG had an apparent affinity of 2x10(-9) mol/L for proscillaridin A. It was specific for bufadienolides and did not cross-react with cardenolides or several steroid hormones. Extraction of human plasma with ethanol and fractionation of this extract over a high-performance liquid chromatographic reverse-phase C18 column with a propanol/isopropanol gradient resulted in the separation of three peaks of increasing hydrophobicity (ED(1), ED(2), ED(3)) that inhibited the sodium pump of human red blood cells and cross-reacted with proscillaridin A antibodies. The concentration of the proscillaridin A immunoreactivity ED(1) in normotensive subjects had a geometric mean of 0.1 nmol/L, with a dispersion factor of 8.77. ED(1) correlated positively in a group of 60 normotensive subjects, 22 patients with hypertension, and 19 patients with chronic renal failure with mean arterial blood pressure (log ED(1) [nmol/L]=0.013xmm Hg-2.17, r=.25; P<.05), systolic pressure (log ED(1) [nmol/L]=0.010xmm Hg-2.23, r=.32, P<.01): and pulse pressure (log ED(1) [nmol/L]=0.019xmm Hg-1.80, r=.38, P<.0001). There was no correlation with other parameters of the donors. We conclude that several substances cross-reacting with proscillaridin A antibodies and inhibiting the sodium pump of human red blood cells circulate in human blood. The level of one of these substances (ED(1)) correlates with mean arterial and pulse pressures.