ELECTRIC FIELDS IN REGENERATING NEWT LIMB STUMPS (VIBRATING PROBE, NOTOPHTHALMUS VIRIDESCENS, WOUND EPITHELIUM, RESISTANCE)

Electrical measurements have been made in regenerating limb stumps of the newt Notophthalmus viridescens to further our understanding of the temporal control of stump currents and the magnitude of the internal electric fields associated with these currents. Following amputation of a limb, strong electric currents (50 (mu)A/cm('2)) leave the stump surface for several days before declining to low levels. Direct measurements of the resistance of the developing wound epithelium indicate that this resistance is responsible for the decline in the density of currents leaving the stump. The transepithelial potential of the trunk skin, which is the electromotive force that drives the currents, remains constant during this same time. Direct measurements were also made of the magnitude of the internal electric field in the limb stump. This field is distally negative and is well described in the proximal regions by equations describing the potential gradients in a cut cable. In the distal 0.5 mm of the stump, however, the field strength is greater then would be expected. The mechanism acting in this distal region is not known but several possibilities are considered. The effect is most likely caused by the presence of cellular injury currents. It was found that the field strengths measured in the limb stumps are of the same order of magnitude as the threshold field strength of 7 mV/mm known to affect cell migration and orientation in vitro. The proximal internal stump fields are initially 6.6 mV/mm and decline to 0.6 mV/mm within 24 hours, whereas the distal fields are over 50 mV/mm initially and decline to 5 mV/mm by 24 hours. These results indicate that the field strengths naturally present in amputated newt limbs are of a magnitude and polarity consistent with the hypothesis that the fields may play a role in initiating regeneration. The control of the currents and fields by the changing resistance of the developing wound epithelium indicates that potentially important modifications may be occurring in the physiology of this epithelium. Electrophysiological measurements of the developing wound epithelium may, therefore, be able to contribute to an understanding of the special role played by the wound epithelium in urodele limb regeneration.