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

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.