Circadian and ultradian control of cardiac output in spontaneous hypertension in rats.

The aim of the study was to test whether circadian and ultradian variations 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) and mean arterial pressure (MAP) were performed in the absence and presence of cardiac autonomic blockade with metoprolol and atropine methylnitrate. Ultradian variability was analyzed by spectral analysis on beat-to-beat data series (high-frequency range) and on averaged minute-to-minute data series (low-frequency range). In general, circadian and ultradian rhythms of CO were similar in SHR (n = 10) and WKY (n = 9). Values of CO were high during the dark and low during the light period, whereas total peripheral resistance was highest during the light period. During cardiac autonomic blockade, relative differences between averaged values of CO over the dark and light periods were reduced. High-frequency spectral power of CO was mainly confined to fluctuations related to respiration and was not influenced by cardiac autonomic blockade. At low-frequency ranges, power spectra of CO lacked a dominant oscillator but showed 1/f characteristics. During cardiac autonomic blockade, 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 dependent. In most frequency ranges, the relative variation of CO was higher than that of MAP. Thus, over 24 h in both adult SHR and WKY, MAP is controlled within a more narrow range than CO.