Secreted protein acidic and rich in cysteine (SPARC) knockout mice have greater outflow facility

PURPOSE: Secreted protein acidic and rich in cysteine (SPARC) is a matricellular protein that regulates intraocular pressure (IOP) by altering extracellular matrix (ECM) homeostasis within the trabecular meshwork (TM). We hypothesized that the lower IOP previously observed in SPARC -/- mice is due to a greater outflow facility. METHODS: Mouse outflow facility (C(live)) was determined by multiple flow rate infusion, and episcleral venous pressure (P(e)) was estimated by manometry. The animals were then euthanized, eliminating aqueous formation rate (F(in)) and P(e). The C value was determined again (C(dead)) while F(in) was reduced to zero. Additional mice were euthanized for immunohistochemistry to analyze ECM components of the TM. RESULTS: The C(live) and C(dead) of SPARC -/- mice were 0.014 ± 0.002 μL/min/mmHg and 0.015 ± 0.002 μL/min/mmHg, respectively (p = 0.376, N/S). Compared to the C(live) = 0.010 ± 0.002 μL/min/mmHg and C(dead) = 0.011 ± 0.002 μL/min/mmHg in the WT mice (p = 0.548, N/S), the C(live) and C(dead) values for the SPARC -/- mice were higher. P(e) values were estimated to be 8.0 ± 0.2 mmHg and 8.3 ± 0.7 mmHg in SPARC -/- and WT mice, respectively (p = 0.304, N/S). Uveoscleral outflow (F(u)) was 0.019 ± 0.007 μL/min and 0.022 ± 0.006 μL/min for SPARC -/- and WT mice, respectively (p = 0.561, N/S). F(in) was 0.114 ± 0.002 μL/min and 0.120 ± 0.016 μL/min for SPARC -/- and WT mice (p = 0.591, N/S). Immunohistochemistry demonstrated decreases of collagen types IV and VI, fibronectin, laminin, PAI-1, and tenascin-C within the TM of SPARC -/- mice (p < 0.05). CONCLUSIONS: The lower IOP of SPARC -/- mice is due to greater aqueous humor outflow facility through the conventional pathway. Corresponding changes in several matricellular proteins and ECM structural components were noted in the TM of SPARC -/- mice.