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

The effect of mucosal La3+ on short-circuit current (s.c.c.) has been studied on isolated frog skins with normaly polarized or depolarized apical membrane. With skins in "polarized condition", La3+ stimulates transiently the s.c.c. and after a long-lasting presence on the mucosal side (1 h) attenuates significantly the stimulation promoted by oxytocin. With skins in "depolarized conditions", lanthanum stimulates permanently the s.c.c. and does not modifying the oxytocin effect, even over long period of continuous stimulation. In contrast, an inhibitory action of mucosal La3+ develops, when skins submitted to repetitive hormonal stimulation are forced to oscillate from the stimulated to the normal resting state. The significance of this particular mode of action is discussed in terms of a possible electrical potential variation of apical membrane during oxytocin stimulation.