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SUMMARY1. Intracellular activities of sodium, potassium and chloride ions, ai as, and at1were measured with ion-selective single-, double-and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, Rfac and r,.s and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, Aiac/Atcs were measured as criteria of satisfactory membrane penetration by the micro-electrodes.2. Under control conditions, aia was 12-3 + 0-8 mm, ai was 703 + 22 mm and at1was 20-3 + 1-6 mm with VRac averaging -38-0 + 3-2 mV. When 10-4 M-amiloride was added to the apical bathing fluid aia fell within 10 min to 118+001 mm and at1 to 52 + 09 mm, while a' increased to 86-2 + 3-8 mm as measured from the basolateral border of isolated epithelia.3. The sodium transport pool of the skin was measured from the fall in aia in the presence of amiloride and could be expressed as 33 x 10-9 mol cm-2 ofepithelium. The mean rate of fall of aia under these conditions corresponded to an efflux rate at the basolateral border of 30-1 x 10-9 mol cm-2 min-(48 ,sA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. 4. Reduction ofsodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 01 mm caused aka to fall in stages to 2 mm. Because Vfac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

In a previous paper (Carey, Conway & Kernan, 1959) it was shown that, when frog sartorii were immersed during the night in K-free RingerConway fluid containing 120 mM-Na, the average content of Na in the muscle was 59-0 + 0-7 m-equiv/kg (s.E. of mean for 166 expts). Three muscles were used for each experiment.On reimmersion for 2 hr at room temperature in similar fluid containing 104 mM-Na and 10 mM-K (referred to here as the 120,0/104,10 procedure) there was considerable active secretion of Na, which is in agreement with the results of Desmedt (1953). The secretion averaged 17-6 + 0-8 m-equiv Na/kg in a large number of experiments, allowing 2 m-equiv Na/kg for passive diffusion from the interspaces in the total Na loss. (In the oxygen-uptake experiments described here the conditions were somewhat different, and the average secretion was found to be 22 m-equiv Na/kg.)If the reimmersion fluid contained 120 mM-Na (the 120,0/120,10 procedure) there was, on the average, little or no excretion of sodium though a very occasional experiment might show it markedly. Also, if the K-free soaking fluid contained 104 mM-Na, and the reimmersion fluid contained also 104 mm-Na, there was likewise no significant Na secretion (Carey, et al. 1959).A partial explanation for this difference was given by the consideration that a secretion of Na in the second immersion, with 10 m r-K present, could be regarded as a restoration of the conditions following the effects of the K-free immersion. One would expect then that the limit of the secretion would be set by the Na levels in the muscles if 10 mM-K were present in both the soaking and the reimmersion fluids. A number of experiments carried out with 10 mM-K present throughout suggested that the maximum average loss of Na to be expected in the 120,0/104,10; 120,0/120,10 and the 104,0/104,10 procedures was 20-0 + 1-0; 3-6 + 3-5 and 7-8 + 1-1 m-equiv/kg respectively. The average experimental losses observed were 19-6 + 0-8, -5-2 + 9-6 and 1-0 + 0-9. The elucidation of the

SUMMARY1. Active transport of Na+ and K+ by Na-rich extensor digitorum and soleus muscles of rat was found to be increased considerably when muscles were innervated during enrichment with Na+ in K-free modified Krebs solution containing 160 mM-Na at 20 C and recovery in a similar fluid with 10 mM-K and 137 mM-Na at 370 C, bubbled with oxygen.2. Addition of acetylcholine (2 0 Fug/ml.) to recovery fluid containing denervated extensors increased active transport, whereas addition of eserine (50 ,tg/ml.), decamethonium (0.1 ,ug/ml.) and to a lesser extent tubocurarine (0-26 ,ug/ml.) inhibited active transport. Blocking of nerve conduction in innervated extensor inhibited K+ uptake more than Na+ excretion.3. The membrane potential of Na-rich extensor muscles measured soon after re-immersion in recovery fluid was higher in denervated than in innervated muscles. In the latter it was close to the K-equilibrium potential (EK). It is suggested that denervation here makes the Na-pump electrogenic by decreasing K+ uptake either by decreased permeability or by inactivating a K-pump. Evidence is presented that the latter is more likely.4. Addition of isoprenaline to Na-rich soleus muscles in recovery fluid increased active transport and reduced the membrane potential measured soon after re-immersion in recovery fluid. The Na-pump still remained electrogenic in the presence of isoprenaline. It was suggested that isoprenaline might also stimulate the Na-pump, perhaps through activation of lactic dehydrogenase.