CIRCADIAN AND ULTRADIAN CONTROL OF CARDIAC-OUTPUT IN SPONTANEOUS HYPERTENSION IN RATS

The aim of the study was to test whether circadian and ultradian varia tions of cardiac output (CO) in spontaneously hypertensive rats (SHR) differ from those in normotensive Wistar-Kyoto rats (WKY). Twenty-four -hour beat-to-beat recordings of CO (by electromagnetic flow probe) an d mean arterial pressure (MAP) were performed in the absence and prese nce of cardiac autonomic blockade with metoprolol and atropine methyln itrate. Ultradian variability was analyzed by spectral analysis on bea t-to-beat data series (high-frequency range) and on averaged minute-to -minute data series (low-frequency range). in general, circadian and u ltradian rhythms of CO were similar in SHR (n = 10) and WKY (n = 9). V alues of CO were high during the dark and low during the light period, whereas total peripheral resistance was highest during the light peri od. During cardiac autonomic blockade, relative differences between av eraged values of CO over the dark and light periods were reduced. High -frequency spectral power of CO was mainly confined to fluctuations re lated to respiration and was not influenced by cardiac autonomic block ade. At low-frequency ranges, power spectra of CO lacked a dominant os cillator but showed 1/f characteristics, During cardiac autonomic bloc kade, low-frequency spectral power of CO fell without changing the 1/f characteristics. These findings suggest that dynamic control of CO is not altered in SHR and that autonomic effects on CO are frequency dep endent. In most frequency ranges, the relative variation of CO was hig her than that of MAP. Thus, over 24 h in both adult SHR and WKY, MAP i s controlled within a more narrow range than CO.