The dependence of nerve membrane potentials upon extracellular ionic strength.

Phospholipids and proteins are assumed to be components of cellular membranes. Both these substances have groups ionizable in aqueous media. We attempted to ascertain to what extent the behavior of cellular membranes is similar to that of nonbiological ion-exchange interfaces. One approach is the measurement of the dependence of the membrane potential in nerve upon the extracellular concentration of electrolytes. The results are presented here and seem to be consistent with the idea that the axon membrane behaves as an ionexchange system. Methods.-The giant axon of the stellar nerve of Loligo pealii or Loligo vulgaris was used. The axons were 180-1200 ,u in diameter and 3-6 cm in length. Room temperature was 19-210C. The details of methods of (a) cleaning and mounting the axon, (b) insertion of intracellular pipettes or wire electrodes, (c) intracellular injection, (d) stimulation of the axon, and (e) recording the membrane responses and resistance are described elsewhere (e.g., ref. 1). The chamber on which the axon was mounted had three compartments. The lateral compartments were 5-15 mm in width, and the middle 0.5-35 mm. Thin partitions were made of rubber (thickness: 0.05-0.2 mm). A hole was made in the rubber with a diameter slightly less than or equal to the diameter of the axon. The membrane potential was measured with calomel KCI electrodes (Beckman): an intracellular pipette electrode and an extracellular gross reference electrode (Fig. 1A). Details of their construction can be found in previous publications." 2 The extracellular medium was stirred constantly. In dilute electrolyte solutions the potential of the external electrode was affected by stirring; this difficulty was circumvented by dividing the middle pool (in Fig. 1A) in two and immersing the electrode in one and the axon in the other. Only the latter pool was stirred. Fluid contact between the pools was made through a connecting pore. The intracellular pipette was filled with either 0.5 M KC1 or 3 M KCl. Methods of perfusion: The entire axon, with the exception of end or cannulated portions, was perfused. A pipette (100-300 it) was inserted into the axon while suction was applied; when this pipette emerged from the opposite end, the contents were blown out. In some experiments, while the pipette was being withdrawn, air was blown through the axon. This appeared to facilitate maintenance of a through-channel. Air (or mineral oil) could be blown through for 1 hr or longer; the only apparent electrophysiological properties affected were those dependent upon the longitudinal internal resistance (e.g., conduction velocity). The pipette was reinserted and withdrawn while the perfusate was being introduced. In the method illustrated in Figure 1B, a drainage cannula of glass, Teflon, nylon, or Plexiglas was used. In the method illustrated in Figure 1C, part of the axon itself was used as a drainage cannula. After insertion of the electrode, the axon (cleaned in all such instances) was cut at the point of insertion and pulled over the electrode for a length of 1-1.5 cm. This portion of the axon was suspended in air. A platinum retractor in conjunction with the electrode held the cut end of the axon ellipsoidally flared. The portion of the axon in air was treated for a few minutes with 10-2 or 10-a M AuCl2. This resulted in stiffening of the axonal surface; subsequently, the treated portion of the axon was washed, dried, painted with mineral oil, and its contents