9.C.12 The Water Permeable Channels in Frog Auditory Papillar Hair Cells (APHC-C) Family

When amphibian papillar hair cell (APHCs) of the leopard frog, Rana pipiens, are osmotically challenged, they exhibit a characteristically asymmetric (rectifying) response: small decreases (5%, or less) in the extracellular solution's osmolarity do not significantly affect the cells' volume; larger decreases produce a relatively slow volume increase in APHCs, while exposure to a hyperosmotic medium leads to rapid shrinking of these cells. Furthermore, the rate of volume change appears to be a function of the rate of extracellular osmotic change. These characteristics make the application of methods devised for the estimation of the osmotic permeability coefficient (P(f)) for semipermeable membranes - i.e., those with significant permeability only to water - to APHC membrane rather futile. Farahbakhsh et al. (2010) have, therefore, devised a method that takes both the permeability to solutes as well as the kinetics of the osmolarity change into consideration, in order to obtain estimates of P(f) that are to a large degree independent of these factors. Using the new method, they estimated the P(f) of APHCs' plasma membrane to be in the 10^-2-cm/s range, and thus significantly larger than those reported for lipid bilayers. APHC's membrane P(f) appeared to be cell-size independent and insensitive to extracellular mercury. These results suggest that APHCs express water-permeable channels in their plasma membrane. Furthermore, asymmetric and rate dependent shape changes produced by osmolarity changes in APHCs imply the presence of significant permeability to solutes. The significance of transmembrane solute transport and water channel expression in amphibian auditory hair cells is discussed (Farahbakhsh et al., 2010).

The probable transport reaction is:

water (in) ⇌ water (out)