Objective: to evaluate the circadian rhythm of type-C Natriuretic Peptide (CNP)
concentration in rabbit aqueous humor. Methods: 41 male New Zealand
white rabbits were entrained to a lighting schedule of alternating 12 h period
of light and dark for at least 2 weeks prior to the beginning of the
experiment. Lights on was defined as 00:00 circadian time (CT) and lights off
at 12:00 CT. Animals were killed at 6 intervals (Light: 2h, 6h, 10h, Dark:
14h,18h, 22h), four animals for each interval except for interval 6h (11
animals) and 22h (12 animals). Aqueous was collected, anti-proteases (1 ml EDTA/2mM, 1 ml PMSF/2 mM and 5ml pepstatin 0,2 mM) were added and the
samples stored at-84ºC.CNP was measured by CNP-22 radioimmunoassay. Statistical
analyses: ANOVA, unpaired t-test and linear regression. Results: The CNP
concentrations in pg/200ml were not statistically different when
rabbits were divided only into light (mean: 2.62 ±0.91) and dark groups (mean: 3.02 ±1.16). Statistical differences were also not
found when the 6 intervals were compared (F= 1.72 P=0.157), where the highest
differences on concentration were found between 22h (3.41 ±1.24) and 10h (mean:2.20 ±0.72). There was no significant tendency of
change concentration over time (r2=7.4%, p=0.085). Conclusion:
The CNP concentration in rabbit aqueous humor does not have a significant
fluctuation over circadian time.