Free-running Rhythms in Blood Pressure and Heart Rate in Normotensive and Transgenic Hypertensive Rats

Circadian and ultradian rhythms in blood pressure, heart rate, and motor activity were studied in normotensive Sprague-Dawley rats and in transgenic hypertensive rats [TGR(mRen-2)27] during alternating lightdark cycles (LD 12: 12), constant darkness (DD), and constant light (LL). In LD 12: 12 conditions all parameters in both strains exhibited significant and dominant circadian rhythms, which were maintained under free-running conditions in DD. In transgenic hypertensive rats during DD, however, only motor activity and heart rate rhythms exhibited daily phase shifts, indicating period lengths of >24 h, whereas those in blood pressure remained in constant phase positions. During LL, circadian rhythmicity was almost abolished in all parameters and in both strains, whereas ultradian components became more dominant. These findings in transgenic hypertensive rats indicate a disturbance in the circadian regulation of blood pressure that, however, remains responsive to photic input.