AJP - Regulatory, Integrative and Comparative Physiology, Vol 267, Issue 3 754-R761, Copyright © 1994 by American Physiological Society
Plasma potassium may protect sodium pumps of toad hearts from an endogenous inhibitor
P. L. Else
School of Biological and Chemical Sciences, Deakin University, Geelong, Victoria, Australia.
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.
