The electrical transients produced on the isolated abdominal skin obtained from Bufo arenarurn Hensel, under the influence of inward current pulses of constant intensity have been studied. When both faces of the skin are bathed with Ringer's solution, short pulses of inward current give rise to transient variations of the potential difference between both faces of the skin with "all-or-nothing" characteristics (action potentials, A.P). When the outer face is bathed with a modified Ringer solution with low sodium content (2.4 mu), the transients are longer and they are only evident when the pulse is several hundred milliseconds long. With even longer pulses (several seconds) a repetitive activity can be elicited, with the electrical characteristics of a "pacemaker" activity. In all these "excitability" phenomena Na + may be replaced by Li + in the outer solution. The logarithm of the duration of AP's is inversely related to the logarithm of the increase in concentration of Na + or Li + in the solution bathing the external face of the skin. The duration of AP's is increased when the Ca ++ concentration in the outer solution is raised. This effect is more evident with low sodium concentration on the outside. The evolution of the slope conductance during repetitive activity has been determined. The site and mechanisms of the "excitable" behavior of the skin and the induced repetitive activity are discussed. Under the experimental conditions employed the behavior of the skin is compared with that of normally excitable plasma membranes. I N T R O D U C T I O NDuring the last decade a number of studies have been carried out to elucidate the mechanism of active sodium transport in isolated surviving frog skin. The information obtained gives insight on ion transporting properties of other epithelial membranes. Koefoed-Johnsen and Ussing (1) demonstrated that the electrical potential through the skin is dependent upon both the concentrations of sodium bathing the outer surface and of potassium bathing the inner surface. T h e y proposed a model based on their results to explain the electrical behavior of the skin. This model assumes the existence of two major diffusion barriers: (a) an outward facing barrier impermeable to all cations except Na+ and Li +, and (b) an inward facing barrier permeable to K+ and
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