Ouabain binding in tadpole ventral skin. I. Kinetics and effect on intracellular ions

Abstract

The short-circuit current across tadpole skin is inhibited by only 50% after 2 h of exposure to 10(-2) M ouabain [Am. J. Physiol. 237 (Regulatory Integrative Comp. Physiol. 6): R74-R79, 1979]. To determine the Na pump sensitivity to ouabain, the kinetics of ouabain binding in tadpole epidermis and the effect of ouabain on intracellular Na and K were tested. In tadpole epidermis a high-affinity dissociation constant (KD) for ouabain of 1.21 +/- 0.20 X 10(-7) M was found, which is similar to the KD in frog skin (1.27 +/- 0.16 X 10(-7) M). Incubation of tadpole epidermis in ouabain for 1 h increased intracellular Na concentration from 48.8 +/- 3.3 to 66.2 +/- 4.9 mM (P less than 0.005) and decreased K concentration from 115 +/- 7 to 104 +/- 9 mM (P less than 0.01). Two hours of ouabain exposure increased intracellular Na concentration from 41.4 +/- 2.4 to 73.3 +/- 4.3 mM (P less than 0.0005) and decreased K concentration from 113 +/- 10 to 85.9 +/- 5.3 mM (P less than 0.05). Regression analysis of the change in Na and K concentrations reveals a slope of 14.7 +/- 2.5 mM/h for the Na gain, which is similar in magnitude to the rate of K decrease (-13.8 +/- 4.7 mM/h). It is hypothesized that the residual short-circuit current seen in tadpole skin after ouabain exposure is a result of current being carried through the tissue by a slow gain of Na across the apical membrane and an equivalent loss of K across the basolateral membrane.