Influence of membrane polarization and hormonal stimulation on the action of lanthanum on frog skin sodium permeability

Microelectrode studies of the effect of lanthanum on the electrical potential and resistance of outer and inner cell membranes of isolated frog skin

Microelectrodes were used to investigate the effect of 0.5 mM mucosal lanthanum (La3+) on the intracellular potential and the resistance of outer and inner isolated frog skin (Rana esculenta) cell membranes. Under short-circuit conditions, the transapical membrane potential Vsco (mean value = -65.4 +/- 3.2 mV, inside negative) hyperpolarized to -108.7 +/- 2.3 mV in control skins, after addition of the sodium blocker amiloride. Current-voltage curves for the outer and inner membranes were constructed from the amiloride-inhibitable current versus the outer membrane potential Vo or the inner membrane potential Vi. The outer, and to a lesser degree the inner, membrane showed a characteristic nonlinearity with two slope resistances. Addition of La3+ to the outer medium increased the short-circuit current to 190% of the control value. Vsco concomitantly changed to -28 +/- 3.5 mV and outer and inner membrane resistances fell, considerably attenuating the nonlinearity seen in control skins. La3+ is suggested to raise the conductance by its effect on the surface potential. A secondary long-term inhibitory effect of La3+ on short-circuit current has been observed. It is ascribed to the penetration of La3+ into the sodium channels.

Surface potentials and sodium entry in frog skin epithelium

1. The effects which alterations in the surface potential of the apical membrane of isolated Rana catesbeiana skin have on Na entry were examined. 2. Changes in the external ionic strength have little effect upon the rate of Na transport across the frog skin epithelium. 3. Uranyl ion (UO2(2+), 2.5 mM) induces a +145 mV change in the surface potential of phosphatidylserine monolayers, and a +60mV change in the surface potential of monolayers made from phosphatidylcholine. 4. UO2(2+) inhibits the short-circuit current (Isc) by a maximum of 20% in R. catesbeiana skin, while stimulating Isc by 40% in R. temporaria skin. Neither Isc stimulation nor inhibition by UO2(2+) can be seen in the presence of 10(-4) M-amiloride. 5. From points 1 and 2 above, we conclude that the surface charge density in the neighbourhood of the Na-selective entry site located in the apical membrane is small (greater than 1e-/600 A2). The results obtained using UO2(2+) suggest that Na entry is not affected by changes in the membrane surface potential.

Effects of mucosal lanthanum on electrical parameters of isolated frog skin. Mechanism of action

The effect of mucosal La3+ on electrical parameters of isolated frog skins was studied on isolated frog skins with normally polarized or depolarized apical membrane. La3+ increases R8, the paracellular or shunt resistance and diminishes RNa, the resistance of the active sodium path, in both polarized and depolarized skins. The stimulatory effect of La3+ on short-circuit current (Is.c.) is correlated with this decrease in RNa. The characteristics of the stimulatory effect are: very rapid onset, ionic strength dependency, the possibility of being elicited by many other ions besides La3+. These features allow us to postulate that La3+ might affect the external interfacial potential which in turn affects the resistance of the sodium path.