Aquaporins are highly selective water channels that are expressed in tissues which function involves rapid water movement across the cell membrane. The mechanisms regulating the presence of aquaporins are not completely understood. This study was undertaken to examine the role of osmoregulatory mechanism in aquaporin 5 (AQP5) expression in a rat mandibular acinar cell line.

Cells were first grown in a 500mOsm culture medium for 24 hrs and then in the standard 310 mOsm isosmotic culture medium with or without the mitogen-activated ERK-activating kinase (MEK) inhibitor U0126 (inhibits the MAP-kinase signalling pathway). Cells were harvested at 0, 4, 8, 12 and 24 hrs in the hyperosmotic medium, and at 3, 6 and 24 hrs after the isosmotic medium was reintroduced. RT-PCR and Western blot methodology was then used to analyse AQP5 mRNA and protein expression, respectively.

When cells were grown in hyperosmotic solutions, AQP5 mRNA and protein levels were upregulated. But when the cells were returned to the isosmotic medium, the AQP5 levels decreased towards normal. In the presence of the MEK inhibitor, there was a reduction in the amount of AQP5 mRNA and protein expressed.

Thus, hyperosmotic growth conditions will induce AQP5 expression. This induction can be regulated by the MAP-kinase signalling cascade.