The Active Ion Transport through the Isolated Frog Skin in the Light of Tracer Studies

Abstract

Summary. Experiments with radio‐sodium (Na24) show that when an isolated frog skin separates two solutions containing NaCl (that on the inside being Ringer) the Na+‐flus from outside to inside is normally much higher than the outflux; this is true even when the outside NaCl concentration is only one millimole/l. The Na+‐influx (and thus the active sodium transport) shows a pronounced dependency on the pH of the inside solution. High pH values give high values for the Na+‐influx and vice versa. The pH of the outside solution has little effect on the Na+‐influx until pH is below‐ ca. 5. Then the 53+‐influs drops to a very low value, whereas the Cl–influx goes up. The Cl–influx is as a rule lower and often much lower than the simultaneous Na+‐influx. The variations in the Cl‐ influx generally go parallel to the variations in Na+‐influx. There is also a pronounced parallelism between the potential difference across the skin and the influx of Na+‐ and Cl‐ so that the more positive becomes the inside relative to the outside. the higher is the influx of Na + and Cl. When added on the inside of the skin adrenaline in the concentration 1/106 brings about an enormous increase, in the outflux of Na+ and a considerable increase in the influx of this ion. Immediately after the addition of adrenaline a violent drop in the P.D. is observed. This drop and the further course of the P.D. curve depends on the outside salt concentration and the magnitude of the P.D. before the adrenaline addition. It is concluded that the initial drop in P.D. is due to an increased permeability to Na+. The secondary rise in P.D. shortly after the drop must be due to an increased Na+‐transport. Cyanide poisoning reduces the Na+‐influx to 5–25% of the original value. The outflux of ions is not reduced. The theoretical basis for the use of tracers to determine the active transport of an ion species through a membrane is considered. In the frog skin the Na+‐influx is as a rule a fairly good measure of the Na+ actually transferred across the skin. But the total transport work performed by the transporting cells is greater than indicated by the flus because, due to back diffusion, some Na‐ions may be subjected to transport more than once before leaving the skin. Based on the experiments a hypothesis is put forward which will account for the interdependency between the in‐ and outflux of Na+ and Cl— and the potential difference across the skin.The radioactive isotopes used were kindly provided from the Institute of Theoretical Physics in Copenhagen. I wish to extent my gratitude to the head of this institute, Professor N. Bohr and to his staff, notably Dr. O. Lassen who prepared the radioactive samples.