Diurnal rhythms of blood pressure, heart rate, and locomotor activity in adult and old male Wistar rats

The diurnal rhythms of systolic blood pressure (SBP), diastolic BP (DBP), heart rate (HR), and spontaneous locomotor activity (SLA) were determined in adult (6-month-old) and old (24-month-old) male Wistar rats by using radiotelemetry. The rhythm parameters (mesor, amplitude, acrophase, and percent rhythm) were analyzed by Fast-Fourier Transform and Cosinor methods. We found that the 12-h mean values of SBP, DBP, HR, and SLA were significantly (p<0.001) higher in the dark phase than in the light phase in both adult and old rats. The nocturnal 12-h mean value of HR was significantly (p<0.01) lower in old than in adult rats. In addition, the differences between diurnal and nocturnal 12-h mean values of SBP and HR were reduced in old rats. There was no significant difference in the 12-h mean values of SBP, DBP, and SLA between old and adult rats. Otherwise, the diurnal HR rhythm amplitude was significantly (p<0.01) reduced in old rats (32+/-2 vs. 46+/-2 bpm). A nearly 1-h delay in SBP and DBP acrophases (03h55 +/- 00h19 vs. 02h57 +/- 00h14 and 03h17 +/- 00h13 vs. 01h53 +/- 00h22, respectively) was found in old rats comparing to adult rats. No significant change in SLA diurnal rhythm was observed in old rats. In conclusion, these results show a decrease in the nocturnal 12-h mean value of HR and an alteration in cardiovascular diurnal rhythms by a 1-h delay of SBP and DBP acrophases and a reduction of HR rhythm amplitude in old Wistar rats.