Importance of Intracellular Water Apparent Diffusion to the Measurement of Membrane Permeability

Abstract

The exchange of water across biological membranes is of fundamental significance to both animal and plant physiology. Diffusional membrane permeability (P(d)) for the Xenopus oocyte, an important model system for water channel investigation, is typically calculated from intracellular water pre-exchange lifetime, cell volume, and cell surface area. There is debate, however, whether intracellular water motion affects water lifetime, and thereby P(d). Mathematical modeling of water transport is problematic because the intracellular water diffusion rate constant (D) for cells is usually unknown. The measured permeability may be referred to as the apparent diffusional permeability, P, to acknowledge this potential error. Herein, we show that magnetic resonance (MR) spectroscopy can be used to measure oocyte water exchange with greater temporal resolution and higher signal-to-noise ratio than other methods. MR imaging can be used to assess both oocyte geometry and intracellular water diffusion for the same single cells. MR imaging is used to confirm the dependence of intracellular water lifetime on intracellular diffusion. A model is presented to relate intracellular lifetime to true membrane diffusional permeability. True water diffusional permeability (2.7 +/- 0.4 microm/s) is shown to be 39 +/- 6% greater than apparent diffusional permeability for 8 oocytes. This discrepancy increases with cell size and permeability (such as after water channel expression) and decreases with increasing intracellular water D.

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

“…A P f /P d ratio > 1 indicates facilitated diffusion involving the activity of water channels due to their high P f [36,38]. For example, the well-studied erythrocyte plasma membrane, having a high density of water channels, has a P f /P d ratio of about 7 [39].…”

Indirect measurement of regional axon diameter in excised mouse spinal cord with q-space imaging: Simulation and experimental studies

“…We are currently investigating bi-modal fitting algorithms to separate the ECS and ICS displacement profiles. Finally, while membrane and myelin permeability will lead to a broader FWHM, there is evidence to suggest that exchange between the ECS and ICS compartments is virtually non-existent at the diffusion time of 10 ms (Meier et al, 2003;Sehy et al, 2002). Furthermore, our previous simulations on histologic images (Ong et al, 2006) indicated that the displacement profile FWHM remained stable over the range of permeability values reported for lipid bilayers (Disalvo, 1995).…”

Amt/MEP/Rh proteins conduct ammonia

“…For equilibration experiments it can be important to have some estimate of the time needed for equilibration of a concentration gradient across the membrane. The time course of diffusional equilibration of a permeable solute across the membrane can be described by a single exponential with time constant 1/s=V/(AAEP d ) where V and A are volume and area of the permeated container [36,41]. For small spherical containers where internal diffusion times can be neglected, the time needed for equilibration, t (95%) then becomes about r/P d .…”

“…Stopped-flow techniques are needed to follow such rapid equilibration, but it is well known that the observed kinetics can be limited by unstirred layer effects [7]. Equilibration times in the minute range are expected for the much larger frog oocytes (typically 0.5-1 mm diameter) but in this case intracellular diffusional mixing may no longer be negligible [36].…”