CIRCADIAN VARIATIONS OF PLASMA ATRIAL-NATRIURETIC-PEPTIDE AND CORTICOSTERONE IN RATS WITH CONTINUOUS OR RESTRICTED ACCESS TO FOOD

In the present study plasma Atrial Natriuretic Peptide (ANP) and Corticosterone (B) levels were determined in 2 groups of Wistar rats, a Control group (C) with free access to food, and a Food Shift group (FS) with access to food only from 0900 to 1100 h for 2 weeks. Blood samples were collected by decapitation at 4 hour intervals. ANOVA indicated that B varied over time in both groups (F (6,46) = 10.14, p < 0.0001 for C and F (6,45) = 5.10, p = 0.0005 for FS). There was also time variation in plasma ANP levels in both groups (F (6,54) = 3.78, p = 0.003 for C and F (6,48) = 2.73, p = 0.02 for FS group). Plasma B presented circadian variations from 78 +/- 17 nmol/l (mean +/- SEM) at 0800 h to 339 +/- 79 nmol/l at 2000 h. The day peak plasma ANP level (pmol/l) was also attained at 2000 h (68 +/- 19). This value was higher than all others measured throughout the day. The restricted feeding regimen resulted in a 12 hour shift of peak B values (395 +/-39 vs 125 +/- 24). The FS rats presented the highest ANP level at 0800 h. The present study indicates similar circadian variations of plasma ANP and B in rats on an ad lib or restricted feeding regimen. ANP and B secretion may occur in anticipation of the feeding period or day activity.