The disruption of the baroreflex selectively eliminates
the circadian rhythm of BP, and the circadian rhythms of BP and
HR are modulated by the autonomic nervous system in rats. The circadian
rhythms of BP and HR are regulated by different mechanisms involving
the autonomic nervous system.
Chemical sympathectomy with guanethidine may activate the influence of the parasympathetic
nervous system on the heart. The possible reasons for the decrease of MBP during the dark period seem to be the elimination of the sympathetic nervous system or the activation of the
parasympathetic nervous system, or both.
Because the SBP and PP in sympathectomized rats during the dark period were suppressed, the decrease of stroke volume, which is reflected in SBP or PP, may be responsible for the decrease of MBP during the dark period. It seems definitive,
therefore, that the autonomic nervous system, particularly the sympathetic nervous system, is
important for the manifestation of 24-hour rhythms of BP and HR in rats.
The daily variations in MBP in intact rats, in which an abrupt rise was seen when the light was
turned off and a sharp fall in MBP was seen when the light was turned on,
were not observed in sympathectomized rats. These results suggest that a change in autonomic nervous tone, the majority being the sympathetic nervous tone, is required for the abrupt changes in BP at the time when the lighting is altered. In humans, BP often rises abruptly around the time of awakening.1 If this early morning surge in BP were due to the same mechanisms as in rats, a change in autonomic nervous tone would be important for the formation of this morning surge in humans. In fact, -sympathetic vasoconstrictor activity is reportedly related to this early morning surge.Furthermore, the autonomic nervous system may play some role in the high correlation between the HR and locomotor activity, because the slope of the regression line was decreased in sympathectomized rats. The correlation between the MBP (mean blood pressure) and locomotor activity was disrupted by both SAD and chemical sympathectomy.
The abrupt increase and decrease in BP observed at the time when the lighting conditions are changed are eliminated by chemical sympathectomy.(Circulation. 1997;96:1667-1674.)
© 1997 American Heart Association, Inc.
Mitsutaka Makino, MD; Hiroshi Hayashi, MD;
Hiroto Takezawa, MD; Makoto Hirai, MD;
Hidehiko Saito, MD; ; Shizufumi Ebihara, PhD