The location of chloride in single striated muscle fibers of the giant barnacle

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.