Inhibition of Human Aquaporin-1 Water Channel Activity by Carbonic Anhydrase Inhibitors

Abstract

Purpose:: Aquaporins are a family of water channel proteins that facilitates water transport across the plasma membrane. Studies with AQP1 and AQP4 knockout mice indicate that selective inhibition of these proteins might be useful in regulation of aqueous humor secretion and intraocular pressure (IOP). Earlier reports have claimed that acetazolamide, a carbonic anhydrase (CA) inhibitor, inhibits AQP1 activity. The purpose of this study was to determine if the effect of acetazolamide was a compound-specific or other CA inhibitors have the same effect on AQP1 activity.

Methods:: AQP1 cRNA was injected in Xenopus oocytes for measurement of osmotic water permeability. The oocytes were incubated for 2-3 days in ND96 buffer (96 mM NaCl₂/2 mM KCl/1.8 mM CaCl₂/1 mM MgCl₂/5 mM Hepes, pH 7.4) at 15°C. The effects of acetazolamide and two other CA inhibitors at 250 µM concentrations were studied on the water transport function of AQP1 by monitoring the osmotic swelling of AQP1 cRNA injected oocytes.

Results:: Osmotic water permeability coefficients (Pf values in µM/s) for oocytes microinjected with AQP1 cRNA were 247 +/- 64, whereas the Pf values for oocytes microinjected with H20 were 13 +/- 2 (p = 0.0016). Pf values of AQP1 cRNA injected oocytes were significantly lower in the presence of CA inhibitors. The Pf values for AQP1 cRNA injected oocytes in the presence of acetazolamide and two other CA inhibitors, AL-5733A and AL-6950 were 144 +/- 37 (p = 0.0143), 111 +/- 3 (p = 0.0015) and 83 +/- 12 (p = 0.0006), respectively.

Conclusions:: Water permeability of AQP1 cRNA injected oocytes was inhibited by three different carbonic anhydrase inhibitors. These three compounds share some features important for both CA and AQP1 inhibition suggesting that these inhibitors may play a role in IOP regulation partly by modulating water flow via AQP1 channel.

Keywords: inflow/ciliary body • intraocular pressure • aqueous