title: Human platelet diffusional water permeability measured by nuclear magnetic resonance.
abstract: To examine whether water transport in human platelets is mediated by an aqueous pore or channel, the ratio of osmotic to diffusional water permeability coefficients (Pf/Pd) was measured. Pd was measured from protein spin-lattice relaxation times in a dense platelet suspension (approximately 20% intracellular exchangeable water) using 20 mM solution Mn as a paramagnetic quencher. The decay of magnetization was biexponential with time constants of 1.3 and 6.9 ms (10 MHz, 37 degrees C), corresponding to a Pd of (2.9 +/- 0.2) X 10(-3) cm/s (SE; n = 8). Pd did not depend on concentrations of Mn (6-20 mM) or of platelets (2-4 X 10(10) platelets/ml), but increased to 4.5 X 10(-3) cm/s with addition of gramicidin (6 micrograms/10(10) platelets). 54Mn uptake studies showed less than 1% of equilibrium uptake of Mn into platelets in 30 min at 37 degrees C. The activation energies (Ea) for Pd were 4.5 kcal/mol (less than 28 degrees C) and 16.3 kcal/mol (greater than 28 degrees C). Pf was measured by a stopped-flow light scattering technique as reported previously [M. M. Meyer and A. S. Verkman, Human platelet osmotic water and nonelectrolyte transport, Am. J. Physiol. 251 (Cell Physiol. 20): C549-C557, 1986], in which the time course of platelet volume was measured in response to a 100 mM inwardly directed sucrose gradient. At 37 degrees C, Pf was (2.7 +/- 0.2) X 10(-3) cm/s and independent of [Mn]. The measured platelet Pf/Pd of 0.93 +/- 0.1 suggests that unlike water transport in erythrocytes, platelet water transport is not associated with an aqueous channel.
mesh_terms: D001792:Blood Platelets / Q000166:cytology*; D002463:Cell Membrane Permeability; D004058:Diffusion; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D009995:Osmosis; D013696:Temperature; D014867:Water
chemical_list: D014867:Water