PURPOSETo test whether blocking the Na+/H+ antiport reduces intraocular pressure (IOP) in the mouse.METHODSThe electrophysiologic approach (the servo-null micropipette system, SNMS) that had been adapted for continuously monitoring IOP in the mouse was used in a study of the effects of a series of transport inhibitors.RESULTSTopical application of three direct blockers of Na+/H+ exchangers produced comparable reductions in mouse IOP: dimethylamiloride (DMA, -5.0 +/- 0.7 mm Hg), ethylisopropylamiloride (EIPA, -4.1 +/- 1.0), and BIIB723 (-4.9 +/- 1.7 mm Hg). These effects were mediated locally, not systemically, because adding DMA to one eye had no effect on IOP in the contralateral eye. In contrast to the actions of selective inhibitors of Na+/H+ exchange, neither the low-potency inhibitor amiloride nor the inhibitor of Na+-K+-2Cl- cotransport bumetanide by itself was effective. Dorzolamide, which slows delivery of H+ and HCO3- to Na+/H+ and Cl-/HCO3- antiports, also reduced IOP by 2.9 +/- 0.6 mm Hg. After first blocking Na+/H+ exchange with DMA, EIPA, BIIB723, or dorzolamide, application of bumetanide produced an additional reduction in IOP of 3.8 to 4.0 mm Hg.CONCLUSIONSThe first step in formation of aqueous humor is uptake of NaCl by the ciliary epithelial cells from the stroma, possibly by both paired Na+/H+ and Cl-/HCO3- antiports and a bumetanide-sensitive Na+-K+-2Cl- symport. The present data are consistent with electron probe x-ray microanalyses of rabbit ciliary epithelium indicating that the antiports are the dominant mechanism. That bumetanide can produce a previously unobserved lowering of IOP when the Na+/H+ antiport is also inhibited substantiates a dominant antiport mechanism.