How might Best2 affect IOP? IOP is determined from the sum of the EVP plus the ratio of aqueous humor formation and its drainage through the outflow facility. Since EVP was similar in all genotypes studied, Best2 must play a role in either inflow or outflow. We favor scenarios in which Best2 affects aqueous formation because of its localization on the basal surface of the NPE, its ability to generate or regulate Cl
− conductances, and the differential effects across
Best2 genotypes of drugs altering aqueous inflow
(Figs. 6B 6C) . It has long been recognized that water transport across the ciliary epithelium is coupled to the transepithelial transport of Cl
− by the NPE.
28 29 30 NPE cells secrete Cl
− via channels in their basal plasma membrane. Our data indicate that Best2 is present at the NPE basal plasma membrane, and so the simplest explanation is that Best2 is one of at least three species of Cl
− channels that have been described in the NPE.
28 The absence of one species of Cl
− channel may result in a diminished IOP similar our observations. Although there is no evidence that NPE Cl
− transport is Ca
2+ sensitive, Best2 Cl
− transport is Ca
2+ regulated.
12 However, the apparent affinity for Ca
2+ is high so that a considerable fraction of Best2 channels are active at “resting” intracellular Ca levels.
12 Furthermore, Best2 Cl
− channel activity is also volume sensitive.
27 Some Cl
− channels in the basal NPE membrane are volume sensitive,
31 and some models for fluid secretion by ciliary epithelia invoke volume-regulated anion channels. Cl
− transport in the NPE is coupled to HCO
3 − transport, which is affected by carbonic anhydrase inhibitors. The altered sensitivity of the
Best2 +/− and
Best2 −/− mice to brinzolamide
(Fig. 6B)is consistent with this idea. The β-blocker timolol brought IOP to the same level in all mice tested
(Fig. 6C) . Timolol affects adrenergic signaling via β2 adrenergic receptors in NPE cells.
32 33 This pathway is thought to govern inflow by regulating the number of Cl
− channels present in the basal membrane of the NPE,
28 and so it is possible that Best2 represents a population of channels that can be mobilized in response to adrenergic signaling.