Transmembrane K+ and Cl− Activity Gradients for the Muscle Fiber of the Giant Barnacle

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Abstract
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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