SPARC-null mice exhibit lower intraocular pressures

PURPOSE. SPARC is a matricellular protein that is highly expressed in remodeling tissues, including the trabecular meshwork and ciliary body. The hypothesis for the study was that SPARC contributes to the regulation of intraocular pressure (IOP). The IOPs of SPARC-null mice, their corresponding wildtype (WT), and heterozygous animals were compared. METHODS. Diurnal and nocturnal IOPs of C57Bl/6x129SvJ WT, SPARC-null, and heterozygous mice were measured. Fluorophotometric measurements were made to assess aqueous turnover. Central corneal thickness (CCT) was measured using histology, ultrasound biomicroscopy, and optical coherence tomography. Iridocorneal angles were examined using light microscopy (LM). RESULTS. During the day, the mean IOP of SPARC-null mice (n = 142, 16.9 ± 2.4 mm Hg) was lower than that of both WT mice (n = 104, 19.9 ± 2.9 mm Hg; P < 10^-12), and heterozygotes (n = 38, 19.3 ± 2.5 mm Hg; P < 10^-4). At night, SPARC-null mice also exhibited a blunted increase in IOP in comparison to WT and heterozygous mice. CCTs were not significantly different between WT and SPARC-null mice. Heterozygous mice tended to have thicker corneas (3.4%). Fluorophotometric measurements suggest that aqueous turnover rates in SPARC-null mice are equal to if not greater than rates in WT mice. LM of the SPARC-null iridocorneal angle revealed morphology that is indistinguishable from WT. CONCLUSIONS. SPARC-null mice have lower IOPs than do their WT counterparts with equal CCTs. The rate of aqueous turnover suggests that the mechanism is enhanced outflow resistance.