Relation of Membrane Properties of the Giant Muscle Fiber of a Barnacle to Internal Ionic Composition

Giant muscle fibers 1 to 2 mm in diameter and 4 to 5 cm in length are found in the barnacle Balanus nubilus. The preparation offers an excellent opportunity for studying membrane mechanisms of excitation by controlling the internal ionic composition of the fiber. The internal ionic composition was altered by intracellular injection through a glass pipette longitudinally inserted into the fiber. A test solution of a given ionic composition was injected over the whole length of the fiber until the diameter increased by a factor of 1.5 to 2.0. This should make the internal ionic composition of the treated fiber close to that of the injected solution. The final concentration of K or Na inside each treated fiber was determined by flame photometry after each experiment. The internal C1 concentration was also determined by the mercuric nitrate method. To obtain a given concentration of the internal Ca++ or Mg++ concentration the injection was made with a solution containing EDTA or EGTA and Ca or Mg salts in varying proportions. The resting potential of the fiber decreased approximately linearly with the logarithm of the external K+ concentration when an increasing amount of the external NaCl was replaced with KC1, but the slope was smaller than 58 my for a tenfold change of the concentration. The slope became 58 mv when the K + concentration was altered by keeping the product of [K+]out and [Cl-]ou, constant. This result indicates that the resting potential of the barnacle muscle fiber is determined by the concentration gradient of K+ and C1across the membrane. When the internal K concentration was decreased below the normal value (150 to 200 m) by replacing a given amount of K in the injecting solution with sucrose, the resting potential decreased approximately linearly with the logarithm of the internal K concentration. The increase of the internal C1 concentration above the normal value (40 to 60 mM) always decreased the resting potential. These results, therefore, agree with the conclusion obtained in experiments with change of the external ionic composition. However, the result deviated when the internal K concentration was raised