EFFECTS OF 2 ENDOGENOUS NA-ATPASE INHIBITORS, MARINOBUFAGENIN AND OUABAIN, ON ISOLATED RAT AORTA(,K+)

Previously, we reported that the venom of Bufo marinus toad contains a Na+,K+-ATPase inhibitor with potent vasoconstrictor activity. In the present study, using thin-layer chromatography in Silicagel 60 F-254+3 66, we separated a vasoactive substance from a mixture of steroids fro m Bufo marinus venom. Based on chromatographic mobility of this substa nce and typical color reaction after its vizualization with SbCl3, we identified it as a previously described steroid, marinobufagenin. Vaso constrictor and Na+,K+ pump inhibitory properties of marinobufagenin w ere studied in isolated rat aortic rings and compared with those of ou abain. Ouabain (10-100 mu mol.l(-1)) produced weak vasoconstriction, w hich was blocked by 2 mu mol.l(-1) phentolamine. 10 mu mol.l(-1) ouaba in stimulated, and at higher concentrations inhibited, the Na+,K+ pump . 2 mu mol.l(-1) phentolamine abolished the activating effect of 10 mu mol.l(-1) ouabain on the Na+,K+ pump, but did not alter the inhibitor y action of higher concentrations of ouabain. By contrast, marunibufag enin elicited rapid and strong vasoconstriction and inhibited ouabain- sensitive Rb-86 uptake. Antidigoxin antibody antagonized the vasoconst rictor responses to marinobufagenin, but not to ouabain. 2 mu mol.l(-1 ) phentolamine did not alter the constrictor effect of marinobufagenin . In solid-phase digoxin immunoassay, marinobufagenin demonstrated hig her digoxin-like immunoreactivity than ouabain.