Histamine treatment induces rearrangements of orthogonal arrays of particles (OAPs) in human AQP4-expressing gastric cells

To test the involvement of the water channel aquaporin (AQP)-4 in gastric a cid physiology, the human gastric cell line (HGT)-1 was stably transfected with rat AQP4. AQP4 was immunolocalized to the basolateral membrane of tran sfected HGT-1 cells, like in native parietal cells. Expression of AQP4 in t ransfected cells increased the osmotic water permeability coefficient (P-f) from 2.02 +/- 0.3 x 10(-4) to 16.37 +/- 0.5 x 10(-4) cm/s at 20 degreesC. Freeze-fracture EM showed distinct orthogonal arrays of particles (OAPs), t he morphological signature of AQP4, on the plasma membrane of AQP4-expressi ng cells. Quantitative morphometry showed that the density of OAPs was 2.5 +/- 0.3% under basal condition and decreased by 50% to 1.2 +/- 0.3% after 2 0 min of histamine stimulation, mainly due to a significant decrease of the OAPs number. Concomitantly, P-f decreased by similar to 35% in 20-min hist amine-stimulated cells. Both P-f and OAPs density were not modified after 1 0 min of histamine exposure, time at which the maximal hormonal response is observed. Cell surface biotinylation experiments confirmed that AQP4 is in ternalized after 20 min of histamine exposure, which may account for the do wnregulation of water transport. This is the first evidence for short term rearrangement of OAPs in an established AQP4-expressing cell line.