Endogenous electrical current leaves the limb and prelimb region of the Xenopus embryo

The electrical current pattern around the developing Xenopus laevis embryo was mapped with a vibrating probe. Current (taken as the movement of positive charge) was found to leave the emerging hind limb bud and to enter the gill region of the stage 47 embryo. The magnitude of the current leaving the limb was about 7 microA/cm2 and the current entering the gill was about 60 microA/cm2. Other regions of smaller outward current were found between the limb and gill. At stage 43, prior to the appearance of the limb bud, a highly localized region of the outward current existed in the general area from which the bud would later emerge. The inward current was localized to the gill bud, as in the older embryo. The ionic basis of the currents could not be determined. In about one-third of the cases studied, the inward current was sodium sensitive since the removal of external sodium or the addition of amiloride reversably blocked the current. In the remaining cases, however, removal of sodium did not change (or else increased) the current. No other external ion (Ca2+, Mg2+, K+) could be identified as the current-carrying ion; the possibility of an outward movement of some anion such as Cl- or HCO-2 remains.