Purpose:
To evaluate a perfusion system to measure outflow facility in micropore filters and in anesthetized BALB/c mice.
Methods:
A water column (inner diameter of 0.54 mm) was connected to a pressure transducer and to micropore filters as a means of producing resistance. The height of the fluid in the column was adjusted with a syringe pump. The transducer was connected to a computer running PowerLab software to record pressure. The fluid exiting the filters is proportional to the decline in height of the column over time. To determine sensitivity, simulated outflow facilities were measured using 5, 10, 15, and 20 filters arranged in series. The outflow facilities were measure by: i) calculating the slope of a pressure decline at 15, 25, and 35 mmHg (linear fit method) or ii) determining the exponential decay of pressure from 35 mmHg (exponential decay method). In the mouse experiments, the intraocular pressure (IOP) was measured by rebound tonometry (TonoLab) with and without ketamine/xylazine anesthesia. The linear fit method was used to calculate the outflow facility of mice. A microneedle was placed in the anterior chamber to measure in 15 minute intervals the pressure decline in the eye set at 15, 25, or 35 mmHg. The outflow facilities were measured over a 45 minute period starting 20 minutes after anesthesia in one eye and 80 minutes after anesthesia in the contralateral eye.
Results:
The simulated outflow facilities (Table) measured by the linear fit and exponential decay methods had a correlation coefficient of 0.997. The average IOP of BALB/c mice was 9.7 ± 0.3 mmHg (mean ± SEM) without anesthesia, and was significantly (p < 0.01, n=9) reduced with anesthesia. The outflow facility was not significantly (p > 0.8, n=8) different comparing the first eye (0.0055 ± 0.0004 uL/min/mmHg) and the second eye (0.0051 ± 0.0003 uL/min/mmHg).
Conclusions:
The water column perfusion system provides an accurate means of measuring outflow facility in mice in vivo. The anesthesia cocktail used in this study reduced IOP but did not change outflow facility during the 2 hour interval in which it was assessed.
Keywords: outflow: trabecular meshwork • aqueous • intraocular pressure