Effects of ambient bicarbonate, phosphate and carbonic anhydrase inhibitors on fluid transport across rabbit corneal endothelium

Bicarbonate has been long held to be indispensable for fluid pumping by the endothelium; however, such need has been disputed recently. We investigated this issue and found that: (1) the corneal endothelium pumps fluid equally well (at 6-8 microliters hr-1 cm-2) whether the bathing solution contains 43 mM bicarbonate or 10 mM phosphate, (2) if bicarbonate and most of the phosphate are absent, fluid pumping is noticeably lowered (2-4 microliters hr-1 cm-2), (3) carbonic anhydrase inhibitors (5 mM acetazolamide; 0.1, 0.2 and 0.3 mM ethoxzolamide) block this lowered fluid pumping, and (4) in the absence of external bicarbonate, 20 mM HEPES is insufficient to preserve adequate fluid pumping. These results are consistent with existing models for endothelial transport in which exogenous and endogenous CO2 are converted to HCO3- by carbonic anhydrase, with HCO3- fueling the transport mechanism and therefore the fluid pump.