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Chloride-sensitive Ag–AgCl microelectrodes were inserted into single striated muscle fibers of the giant barnacle, Balanus nubilus, to measure the activity of Cl− in the myoplasm, (aCl)i. Chemical analysis was also carried out to determine the total concentration of Cl− in the fiber, [Cl]i. In two sets of experiments, (aCl)i was 28.8 and 22.4 mM while [Cl]i was 75.1 and 66.8 mmoles/kg fiber water respectively. The transmembrane Cl− potential, calculated from the aCl measurements in the myoplasm and the bath, was slightly less than the membrane potential. To locate the large fraction of fiber Cl− that is not free in the myoplasm, Cl− washout studies were done in constant [K]o[Cl]o product Ringer solutions in which [Cl]o was reduced to 50% and 25% of the normal concentration. Fibers which were soaked in these solutions for 30 min showed no change in (aCl)i but a large drop in [Cl]i. From the extent of this drop, it was calculated that these muscle fibers have an extracellular space of about 5% of fiber volume. Electron microscopic studies indicate that this space is comprised of large clefts and smaller tubules which penetrate deeply into the fiber.

The activities of K+ and Cl− in the myoplasm of single fibers from the giant barnacle were measured at differing steady states by means of ion-selective microelectrodes. In one experiment, the fiber was equilibrated in a series of solutions in which [K]o and [Cl]o varied but [K]o × [Cl]o and [K]o + [Na]o were constant. In a second experiment, the fiber was equilibrated in solutions in which [K]o varied but [Cl]o and [K]o + [Na]o were constant. In a final experiment, the fiber was equilibrated in solutions in which [K]o, [Cl]o, and osmolarity were increased. The results support the view that the free K+ and Cl− activities in the myoplasm of the muscle fiber in the steady state can be defined by the equation (aK)o/(aK)i = (aCl)i/(aCl)o = exp (EF/RT), where a refers to ionic activity and E to the membrane potential. (Subscripts o and i refer to 'outside' and 'inside' the fiber, respectively.) The results are consistent with the results presented in the following paper which indicate that a significant fraction of the intracellular Cl− is not free in the myoplasm and that free myoplasmic Cl− is excluded from about 45% of the intracellular water. Free myoplasmic K+ appears to be excluded from only 15% of the intracellular water. The results also demonstrate that, because of the ion and water heterogeneity in the fiber, fiber concentrations of K+ and Cl− cannot be used in place of activities in the above equation.

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BHef letters to the Editor that make specific ~ientific reference to papers pubfished previously in T~m JOURNAL O1~ GENI~RAL PI~-~OLOOY are invited. Receipt of such letters will bc acknowledged, and those containing pertinent scientific comments and scientific criticisms will be published.]