Plasma potassium may protect sodium pumps of toad hearts from an endogenous inhibitor.
Resibufogenin (3-hydroxy-14,15-epoxy-20,22-dienolide glycoside) is a potent sodium pump inhibitor present in toad toxin. It is present in the skin of the cane toad (Bufo marinus) at a concentration equivalent to ouabain of approximately 1 mM. Because toads, like other amphibians, have permeable skin, resibufogenin is also found in high concentrations in the blood. In the cane toad the blood concentration is estimated to be 1 microM (D. Lichtstein, S. Kachalsky, and J. Deutsch. Life Sci. 38: 1261-1270, 1986; D. Lichtstein, S. Samuelov, J. Deutsch, H. Xu, R. A. Lutz, S. S. Chernick and S. S. Chernick. Klin. Wochenschr. 65, Suppl. 8: 40-48, 1987), a concentration thousands of times that required to produce toxicity in humans (J. S. Flier, E. Matatos-Flier, J. A. Pallotta, and D. McIsaac. Nature Lond. 279: 341-343, 1979). In examining how the cane toad avoids inhibiting its own sodium pumps, work on the heart showed that 1) cane toads possess a similar number of cardiac sodium pumps as other vertebrates, and 2) normal plasma K+ levels completely prevent ouabain, and presumably resibufogenin, from binding to cardiac sodium pumps of the cane toad. Other species, i.e., rat (Rattus norvegicus) and salamander (Ambystoma mexicanum), did not show K+ protection of their cardiac ouabain binding sites up to normal plasma K+ levels. These species do not possess the high level of endogenous ouabain-like substance found in the toad. K+ demonstrated a capacity to protect the enzymatic activity of the toad heart sodium pumps from the inhibitory effects of ouabain.
Else
P L
PL
School of Biological and Chemical Sciences, Deakin University, Geelong, Victoria, Australia.
D000818:Animals; D001665:Binding Sites; D002024:Bufo marinus / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism*; D010042:Ouabain / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000494:pharmacology; D011188:Potassium / Q000097:blood*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism; D014562:Caudata
D010042:Ouabain; D011189:Potassium Chloride; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium