Determination of transmembrane water movement in crustacean axons by interference microscopy

Crustacean axons swell when subjected to K⁺-rich solutions. Interference microscopy was employed to quantitate volume and refractive index changes of axoplasm induced by K⁺-rich solutions. Above a critical value of 3–5 × (40–65 mM/L) normal K⁺ the diameter of the axon was directly related to the external K⁺ concentration. The relationship was approximately linear. The refractive index change was inversely related to external K⁺ concentration, also in linear fashion. K⁺-induced swelling occurred in a matter of seconds, while recovery after removal of K⁺ took several minutes. In high K⁺, 100 mM/m or above, recovery was not always complete. Axons pretreated with 10 × normal calcium (230 mM/L) or procaine (1 per cent) showed delayed onset of K⁺-induced volume increase by several minutes and the rate of swelling was slowed. Ouabain enhanced the swelling rate. Veratrine (1 pp. 1000) alone caused a slow swelling of the axons. Ferrocyanide, Cl-substituted, solution caused remarkably quick and conspicuous swelling and fringe reversal. Comparison of the swelling data to already existing electrophysiological data on crustacean axons suggests a correlation between water movement and membrane bioelectric activity.