Movements of potassium have been observed in isolated right and left auricles of rabbits by means of radioactive tracer. The tissues have been immersed in a modified Krebs saline at 370, and in these conditions showed only small changes in ionic content over periods up to 6 hr.The extracellular volume, determined with inulin and with 2"Na, was large (44.1 ml./100 g. tissue), and accounted for about 3.8% of the tissue potassium. Left auricles exchanged without gross inhomogeneity: the rate of exchange was about 1.5% of the total tissue potassium/min., though it was probably higher in the first half-hour or hour after preparation. In right auricles the exchange was less homogeneous and included a faster component than that observed in left auricles. Digoxin and ouabain reduced the influx and had no appreciable effect on the efflux of potassium, so that the auricles lost potassium. The threshold concentrations which produced effects within 20 min. were of the order of 10-6 for ouabain and somewhat higher for digoxin. Irregularities or failure of contraction occurred when the tissue potassium was reduced by about 15%. Loss of potassium was accompanied by gain of sodium, but the tissue appeared unsuitable for making estimations of the rate of sodium movement.

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