The rate of exchange of ²⁴Na in cat nerves

Abstract

The present investigation was begun in an attempt to verify a result of Manery & Bale (1941) that Na in mammalian (rabbit) nerve exchanges only very slowly with plasma Na in vivo. As the investigation proceeded it became clear that roughly quantitative information concerning the flux of Na across the nerve surface might be obtained. However it did not prove possible to make sufficiently accurate quantitative studies of the Na flux in situ, so a number of experiments on the rate of loss of 24Na from desheathed cat nerves in vitro were carried out. From the results of the latter experiments estimations of the Na efflux were obtained. METHODSAll nine cats used in the experiments were anaesthetized with intraperitoneal pentobarbitone sodium (Nembutal), small doses being subsequently given intravenously to maintain a light level of anaesthesia.The solution injected was a mammalian saline (0.9 % NaCl), made up from irradiated NaCl and obtained from the Atomic Energy Research Establishment, Harwell. At the time of injection, its activity varied between 0 40 and 0 50 mc/ml. The amount of solution injected was 5-7 ml., equivalent to a total of 2-15-3-50 mc of 24Na.Experimental procedure 24Na exchange in situ. The solution was injected into a femoral or a jugular vein and washed in by a few ml. of inactive saline; the time required for this operation was about 1 min. At intervals after the injection, 4-5 cm long portions of the posterior tibial, the lateral popliteal, or, sometimes, the medial popliteal nerves were removed from one or the other hind limb; a sample of carotid arterial blood was obtained simultaneously. The nerves were rinsed very quickly in inactive saline to remove superficial contamination with blood, and, after lightly blotting with filter-paper, were immediately weighed in platinum crucibles of known weights. The crucibles were placed in an airoven at about 1000 C for at least 5-6 hr, and, after cooling in a desiccator, weighed again. They were then left overnight in a muffle furnace at a temperature not exceeding 5500C. In the morning, the ash was dissolved in 5 ml. nitric acid (0 75 N). A portion of this solution (about 3 ml.) was diluted about x 50, and the Na and K content then estimated by means of an Evans Electroselenium flame photometer, with an accuracy rather better than 5% for Na and 10% for K. Details of the method and its accuracy are given in Krnjevi6 (1955). The rest of the nitric acid solution (about 2 ml.) was used to determine the 24Na content of the nerve; this was always done within 24 hr of the end of the experiment. The heparinized blood samples were centrifuged soon * Beit Memorial Research Fellow.

The distribution of Na and K in cat nerves

“…This suggests that as the interstitial Na concentration decreases, ions leave the axoplasm thus lowering the internal ionic concentration. The K concentration of desheathed nerves soaked for 3 hr by Dainty & Krnjevic (1955) was only 43 m-mole/kg water, which corresponds to Ki = 105 m-mole/kg water, instead of the initial value of 181 m-mole/kg. As Nai had apparently increased to about 80 m-mole/kg, this was equivalent to the loss of about 40 m-mole/kg internal cation and must have been accompanied by an equivalent amount of anions.…”

“…The application of the correction (see Dainty & Krnjevic, 1955) leads to a value for the percentage of the nerve Na which is intracellular of 18 + 3. This agrees with the value (18-1 + 3-3 %) found by the second method, which involved similar experiments with desheathed nerves and 2ANa (Dainty & Krnjevic, 1955).…”

“…The application of the correction (see Dainty & Krnjevic, 1955) leads to a value for the percentage of the nerve Na which is intracellular of 18 + 3. This agrees with the value (18-1 + 3-3 %) found by the second method, which involved similar experiments with desheathed nerves and 2ANa (Dainty & Krnjevic, 1955). The intracellular Na may then be taken as 18% of the total: this suggests that the excess Na is to be found in the interstitial fluid, but an exact evaluation requires a knowledge of the relative amounts of intraand extracellular water.…”

Diffusion of acetylcholine in agar gels and in the isolated rat diaphragm

“…It may be objected that it is not justifiable to extrapolate in this way to zero time, because the rate of movement from the slow component should initially be much reduced by the high extracellular level of ACh (cf. Keynes, 1954, andDainty &Krnjevic, 1955). Inspection of the results suggests that a value of about 12 % (instead of 15 %) might be a better approximation to the true slow fraction of ACh, but the scatter of data hardly justifies a more detailed analysis.…”

The permeability of kidney cortex to chloride

“…Nevertheless, the difference in rates aerobically was about five-to tenfold which can be shown to be sufficient (cf. Dainty & Krnjevic, 1955) to justify extrapolation of the slower component to the ordinate to obtain a value of at/a. at zero time within 10-15 % of the true value.…”