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C:catioil-sensitive glass rlmicroelectrodes were ilnscrtcd into 4ngle striated muscle fibers of the giant barnacle, Bulanzls nubilus, to measure thc activity of sodiuin and pot:~ssiun~a in the myoplasm. These rne;~s~arernents, con~bincd with : i h~iowledge of the total cellular water L~~~d sodium and pot;issi~nm content (flanne photo~netry), permitted the rni~lim,il percentage of L)ound sodium1 and welter to be calculated. These values were 845% and 42% respectively. When m~mscle fibers were so,iked in sucrose Ringer solution, abo~mt 30% of the totdl sodi~m~n was removed proportionally from the bourld and the free fractioris. Potnssiutm replaced sodiutrl in the cellular bound fractio~r. In some experiments the chloride conteut of m u d e fibers was determined. -4ssa1ming alo binding and a passive distrib~mtion of this ion, the res~alts predict 65:' : binding crf the fiber w'lter. This value for water bindii~g W;I> used with the previous data to c-alculatc that !)I(;.', of the intracell~alar sodiu~ai :tnd 38:3 of the irrtracellular potassium were bound.T h i s work was supported by the Johra and Mary R. Markle l~c~iii~dation, the British Coliimlbia hledical Research Foundation, and the British Col~r~nbia Ileart b;ouild:~t~on."Vith the technical assistance of 1). C. Gaytoll.Cariadian Jouraial of Physiulogy ant1 Pharmacology. Volume 44 (l!#fjG)Can. J. Physiol. Pharmacol. Downloaded from www.nrcresearchpress.com by Depository Services Program on 11/18/14For personal use only.

The relative self-diffusion coefficients D/Do, of water in various solutions, in fresh barnacle muscle fibers, and in membrane-damaged fibers equilibrated with several media have been estimated from NMR relaxation rates in the presence of applied field gradients. A model has been developed to account for the contributions to the observed reduction in D/Do from small organic solutes, and from the hydration and obstruction effect of both soluble macromolecules and myofilament proteins. Intracellular ions do not affect D/Do, but all tested organic solutes do. Solute effects are additive. When artificially combined in the proportions found in barnacle muscle ultracentrifugate (measured D/Do = 0.77), organic acids, small nitrogenous solutes, and proteins give D/Do = 0.77. After correcting the D/Do measured in fibers for this value, we calculate the myofilament hydration, Hm, in fresh muscle to be 0.65 g H2O/g macromolecule. Only in membrane-damaged fibers, highly swollen by salt-rich media, was this significantly increased. Because our earlier NMR relaxation measurements indicate only 0.07 g H2O bound/g myofilament protein, we conclude that the "hydration" water measured by reduction of D/Do cannot be described by stationary layers of water molecules; instead, we propose that nonpolar groups on the proteins cause extensive, hydrophobically-induced interactions among a large fraction of solvent molecules, slowing their translational motion.