The influence of genetic background on conventional outflow facility in mice

Abstract

Purpose: Intraocular pressure (IOP) varies between genetically distinct strains of mice. The purpose was to test the hypothesis that strain-dependent differences in IOP are attributable to differences in conventional outflow facility (C).

Methods: The IOP was measured by rebound tonometry in conscious or anesthetized BALB/cJ, C57BL/6J, and CBA/J mice (N = 6-10 per strain). Conventional outflow facility was measured by ex vivo perfusion of enucleated eyes (N = 9-10 per strain).

Results: Conscious IOP varied between strains, being highest in CBA/J (14.5 ± 0.9 mm Hg, mean ± SD), intermediate in C57BL/6J (12.3 ± 1.0 mm Hg), and lowest in BALB/cJ (10.6 ± 1.8 mm Hg) mice. Anesthesia reduced IOP and eliminated any detectable differences between strains. Conventional outflow facility also varied between strains, but, in contrast to IOP, C was lowest in CBA/J (0.0113 ± 0.0031 μL/min/mm Hg) and highest in BALB/cJ (0.0164 ± 0.0059 μL/min/mm Hg). Like IOP, C was intermediate in C57BL/6J (0.0147 ± 0.0029 μL/min/mm Hg). There was a strong correlation between conscious IOP and outflow resistance (1/C) from individual eyes across all three strains, revealing that 70% of the variation in IOP was attributable to variation in outflow resistance.

Conclusions: Differences in IOP among three genetically distinct murine strains are attributable largely to differences in conventional outflow facility. These results motivate further studies using mice to identify the morphologic and genetic factors that underlie IOP regulation within the conventional outflow pathway.

Keywords: intraocular pressure; mouse model; outflow facility; trabecular meshwork.

Figure 1

Calibration data from the rebound tonometer using cadaveric eyes in situ (two eyes per strain). Each eye was cannulated and manometric IOP_m was controlled by a fluid reservoir, while tonometric IOP_t was measured using the “manually-calculated” method (see Methods). The calibration relationship was not different between strains, and, therefore, data from all strains were lumped together to obtain the linear regression shown in the Figure. All tonometric IOP data in this study were corrected according to the inverse of this linear regression (Equation 2). Each data point refers to a single manually-calculated IOP measurement from a single eye.

Figure 2

A typical perfusion tracing showing the flow rate (solid tracing, left axis) and pressure (dashed tracing, right axis) as a function of time for one enucleated eye of a C57BL/6J mouse. The pressure increases in steps of 4, 8, 15, and 25 mm Hg, while the flow rate increases in proportion to the increase in pressure. The spikes in flow rate are due to rapid adjustments of the syringe pump as it attempts to maintain the desired pressure. The average flow rate at each pressure step is calculated by averaging over at least 10 minutes of data, excluding the spike values.

Figure 3

IOP measured in conscious (white) and anesthetized (black) mice from BALB/cJ (N = 8 and 10), C57BL/6J (N = 6 and 10), and CBA/J (N = 7 and 10) strains. The conscious mice were a subset of the anesthetized mice, and were fewer because of difficulties obtaining IOP measurements in conscious animals (see text and Table 2 for further details). IOP was corrected as described in Equation 2. Error bars: SD.

Figure 4

The average flow rate at each pressure step for BALB/cJ (N = 10), C57BL/6J (N = 9), and CBA/J (N = 9). Trend lines are plotted as the slope (C) and intercept (F_u) averaged over all eyes that were perfused for each strain (data given in Table 2). Flow rate and perfusion pressure correspond to F and P_p as defined in equation 3. Error bars: SD.

Figure 5

(A) Conscious IOP versus conventional outflow resistance (1/C) measured within individual mice for BALB/cJ (N = 6), C57BL/6J (N = 5), and CBA/J (N = 3). These data include only those mice where conscious IOP and C were measured in the same individual. The trendline and equation represents the linear regression of IOP versus 1/C excluding the one outlier (indicated by the filled triangle, see Equation 5). The slope of the regression provides an estimate of the conventional volumetric outflow rate (F_c = 0.068 ± 0.013 μL/min, see Equation 5), while the intercept provides an estimate of EVP (7.1 ± 1.0 mm Hg), assuming constant values for all mice. (B) The mean conscious IOP ( ) versus mean outflow resistance ( = the mean value of 1/C) measured for the entire set of BALB/cJ (N = 8 for and N = 9 for , with one outlier excluded), C57BL/6J (N = 6 and 9), and CBA/J (N = 7 and 9). These data include all mice of each strain where conscious IOP and C were measured, not necessarily in the same individual. The trendline and equation represents the linear regression of versus using only three data points (one per strain, see Equation 6). The population-based estimates of F_c (0.108 ± 0.022 μL/min) and EVP (4.2 ± 1.7 mm Hg) are not statistically different from the estimates based on individual eyes. Error bars: SD. See Discussion for further details.