AUTONOMIC CONTROL OF ULTRADIAN AND CIRCADIAN-RHYTHMS OF BLOOD-PRESSURE, HEART-RATE, AND BAROREFLEX SENSITIVITY IN SPONTANEOUSLY HYPERTENSIVE RATS

Objective To examine the influence of the autonomic nervous system on ultradian and circadian rhythms of blood pressure, heart rate and baro reflex sensitivity of heart rate (BRS) in spontaneously hypertensive r ats (SHR), Methods Spontaneous fluctuations in blood pressure, heart r ate and BRS in SHR were recorded continuously for 24 h using a compute rized system and compared with those in Wistar-Kyoto (WKY) rats. Furth ermore, 24 h recordings were performed in SHR during cardiac autonomic blockade by metoprolol and methyl-atropine, vascular autonomic blocka de by prazosin, ganglionic blockade by hexamethonium and vagal stimula tion by a low dose of scopolamine. The magnitudes of the ultradian flu ctuations in blood pressure, heart rate and BRS were assessed by wide- band spectral analysis techniques, Results The BRS was lower in SHR th an it was in WKY rats throughout the 24 h cycle. In both strains high values were found during the light, resting period, whereas low values were found during the first hours of the dark, active period, The cir cadian rhythmicity of the blood pressure in SHR was abolished complete ly during the infusions of prazosin and hexamethonium. In contrast, th e circadian rhythmicities of the blood pressure and heart rate were no t altered by infusions of metoprolol, methyl-atropine and the low dose of scopolamine. Power spectra of the blood pressure and heart rate la cked predominant peaks at ultradian frequencies and showed 1/f charact eristics, In the absence of autonomic tone, the ultradian fluctuations in heart rate, but not in blood pressure, were decreased, The ultradi an BRS spectra had no 1/f shape, but showed a major peak at approximat e to 20 min for 71% of the WKY rats and 42% of the SHR. Conclusions Th e influence of the autonomic nervous system on the blood pressure and heart rate in SHR is frequency-dependent The circadian, but not ultrad ian, blood pressure rhythmicity is controlled by vascular autonomic ac tivity, Conversely, the circadian, but not ultradian, heart rate rhyth micity is independent of autonomic tone, In rats, just as in humans, t he trough in baroreflex sensitivity occurred after the sleeping period , when locomotor activity is resumed.