Objectives: This study investigated effects of acute plasma volume expansion on plasma levels and urinary output of two endogenous Na,K-ATPase inhibitors, marinobufagenin-like and ouabain-like immunoreactive substances.
Methods: Plasma volume was expanded for 3 h via intravenous saline infusion in three groups of anesthetized dogs--nontreated (n = 5); pretreated with rabbit antidigoxin (n = 5); and pretreated with rabbit antimouse (control) antibody (n = 4).
Results: Plasma marinobufagenin-like immunoreactivity increased to 11.87 +/- 3.16 nmol.l-1 (vs. 0.30 +/- 0.16 nmol.l-1) within 10 min of volume expansion, in parallel with a 15% increase in LVdP/dt, then decreased to 2.21 +/- 0.59 nmol.l-1, and in 90 min increased to 11.8 +/- 2.8 nmol.l-1, in parallel with the maximal natriuretic response. Plasma concentrations of ouabain-like immunoreactive material were increased after 90 min of saline infusion (0.019 +/- 0.004 nmol.l-1 vs. 0.139 +/- 0.056 nmol.l-1). Pretreatment of the animals with antidigoxin antibody blocked the positive inotropic and reduced natriuretic response to volume expansion, and decreased the urinary release of marinobufagenin-like, but not ouabain-like, material.
Conclusions: These results show the presence of marinobufagenin-like immunoreactive substance in dog plasma and suggest that mammalian EDLF may have a bufodienolide nature. Endogenous marinobufagenin-like immunoreactive substance, which is likely to cross-react with antidigoxin antibody, is involved in the natriuretic and positive inotropic responses to plasma volume expansion.