3 Measurement of Diffusional Water Permeability of Cell Membranes Using N M R without Addition of Paramagnetic Ions*
Magnetic Resonance in Colloid and Interface Science Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/27/19. For personal use only.
D. G. STOUT† Department of Floriculture and Ornamental Horticulture, Cornell University, Ithaca, Ν. Y. 14853 R. M. C O T T S Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, Ν . Y. 14853 1
Conlon and Outhred have described a means for determining the mean residence time and the diffusional water permeability for water in a cell surrounded by a permeable membrane. They measure the value of an NMR (nuclear magnetic resonance) spin relaxation time of the mobile water as a function of concentra tion of paramagnetic impurities outside the cell. They used Mn ions. If the relaxation time of the extracellular water can be made sufficiently small the Conlon-Outhred experiment can be intrepreted with little difficulty, but, especially in bio logical systems, there is, as Conlon and Outhred point out, the concern that the presence of Mn concentrations in non-physio logical amount might perturb the cell membrane enough to alter its water permeability and the mean residence time for intra cellular water molecules. The experiment described below eliminates the need for addition of paramagnetic salts in systems having small cells. The extracellular water which must have relatively unrestricted motion can have its transverse relaxation rate enhanced by ap plication of a pair of pulsed magnetic field gradients as is done in NMR measurements of the self diffusion coefficient. The intracellular water has, in small cell systems restricted diffusion and responds with a relaxation rate that is less enhanced. The amplitude of this "slow" component of the signal decays with a rate partially determined by the mean residence time. Let p and p equal the fractions of mobile intracellular and extracellular water, respectively, with intrinsic trans verse relaxation rates (T ) and (T ). We assume that T >> T and T >> T where T = (R /D ), R is the measure of the cell size, and D is the self diffusion coefficient of p . We also assume that gradients in the applied D.C. magnetic field are negligible. Let T equal the mean residence time for water within the cell. If T ≥ T then T is determined by dif fusional water permeability, P , of the cell membrane and R/Pd with a proportionality constant the order of two and T 31 ++
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determined by c e l l shape . The two pulse spin echo experiment i s done with a f i e l d gradient G = (dl^/dZ) turned on for a time δ immediately after the π/2 r . f . pulse and similarly after the π r . f . pulse a time τ later. For the water pj,, the echo amplitude 4
