State and distribution of potassium and sodium ions in frog skeletal muscle

Summary

A modified method for constructing cation-selective glass membrane microelectrodes is described. The method permits routine fabrication of electrodes with tip diameters less than 1 μ and exposed tip lengths of 2 to 5 μ. These electrodes had tip resistances in the range 10⁷ to 10⁹ ohms and gave stable and reproducible potentials in standard NaCl and KCl solutions. Potentials were unaffected by pH in the range 6 to 8 and satisfactory calibration curves were obtained over the temperature range 15 to 25°C. Pairs of electrodes with different K−Na selectivity coefficients were used to measure K⁺ and Na⁺ activities in frog sartorius fibers. In fibers containing normal amounts of these ions, Na⁺ activitywas much less than would be predicted from chemical analysis of the muscles, assuming that all the apparent muscle Na⁺ is present in an osmotically active form in the myoplasm. Possible origins of this discrepancy are discussed. Following a 48-hr soak at 5°C in a K-free medium, the apparent Na⁺ concentration and activity of the fibers both increased and the K⁺ concentration strongly suggests the existence in normal fibers of at least two intrafiber K⁺ compartments, both of which exchange with external Na⁺ but only one of which is detectible by a microelectrode located in the myoplasm. Ca⁺⁺ appears to exert a strong regulatory effect on Na⁺−K⁺ exchange between these compartments and the external medium and on the distribution of intrafiber Na⁺ and K⁺ between these compartments.