The Activity Coefficient of Sodium in Striated Muscle Fibers

Dear Sir: The statement by Brinley (1968) that for striated muscle fibers from the giant barnacle " . . . one can calculate an activity coefficient for sarcoplasmic sodium of ca. 0.5-0.6 which is not greatly different from what one might expect from a mixed salt solution having the composition of the sarcoplasm (0.65-0.75)" requires some comment. This calculation was made by dividing the average internal sodium activity (10-14 mmoles/kg H~O) of fibers analyzed by McLaughlin and Hinke (1966) by the average sodium concentration (21 mmoles/kg H,O) of fibers analyzed by Brinley (1968) and others. McLaughlin and Hinke (1966) measured the activity of sodium in the sarcoplasm of a barnacle muscle fiber by means of a longitudinally inserted cation-sensitive microelectrode, then analyzed the fiber for its total sodium content. The average concentration of sodium (51-81 mmoles/kg H~O) in the fibers they used was substantially higher than the concentration of sodium in the fibers used by Brinley, and they deduced an average value for the activity coefficient of about 0.2. Brinley (1968) was correct in noting that the rinse period in isosmotic sucrose (15 sec) used by McLaughlin and Hinke (1966) was insufficient to remove all the extracellular sodium from the clefts and reticulum of the fiber. He was, however, incorrect in implying that this is the major reason for the difference in the concentration measurements reported by himself and by McLaughlin and Hinke (1966) and hence the major reason for the low activity coefficient deduced by the latter investigators. It was apparently overlooked that McLaughlin and Hinke (1966), in a second series of experiments, made both concentration and activity measurements on fibers bathed for 45 rain in sodium-free sucrose Ringer. This is sufficient time for virtually all the sodium contained in compartments communicating with the bathing solution to have left the fiber (Hinke, 1967; McLaughlin and Hinke, 1968). The ratio of the sodium activity to concentration in these fibers was also found to be about 0.2. The difference between the sodium content of the fibers utilized in these two laboratories is perhaps best illustrated by a comparison of Fig. 1 of Brinley (1968) and Fig. 2 of McLaughlin and Hinke (1968). These are both "timed rinse" graphs in which the sodium concentration of single fibers is plotted as a function of time in sodium-free sucrose Ringer. After 25 rain, the slopes of both curves decreased to low values, but the sodium concentration of the fibers analyzed by Brinley (1968) was