DIGITALIS-LIKE AND VASOCONSTRICTOR EFFECTS OF ENDOGENOUS DIGOXIN-LIKEFACTOR(S) FROM THE VENOM OF BUFO-MARINUS TOAD

Digitalis glycoside-like properties of the Bufo marinus toad crude ven om and one of its constituents, bufalin, were studied in various assay systems. In concentrations 0.3-30 mug/ml crude venom increased the co ntractility of isolated electrically driven rat atria, constricted rat aortic rings, inhibited ouabain-sensitive Na+,K+-ATPase in rat erythr ocytes and the Na+,K+-pump in rat aorta, and cross-reacted with antidi goxin antibody from the dissociation enhanced lanthanide fluoroimmunoa ssay (DELFIA). These effects were unaffected by adrenoceptor blockers and the 5-HT antagonist, deseril, but were blocked by antidigoxin anti body. Bufalin (10-30 muM) increased myocardial contractility and inhib ited Na+,K+-ATPase in rat erythrocytes similarly to crude Bufo marinus venom. In rat aorta bufalin showed weak and delayed vasoconstrictor a ctivity which was antagonized by 2 muM phentolamine, and had a biphasi c effect on the Na+,K+-pump; 0.5-1.0 muM bufalin stimulated the pump, while higher concentrations inhibited its activity. Although the effec ts of bufalin were blocked by antidigoxin antibody, bufalin showed ver y low digoxin-like immunoreactivity in the DELFIA. These observations suggest that, in addition to bufalin, Bufo marinus venom contains at l east one more digitalis-like steroid with significant intrinsic vasoco nstrictor activity which, unlike bufalin, constricts the blood vessels acting directly via inhibition of the sodium pump in the vascular smo oth muscle membrane.