Cosinor analysis of changes in circadian blood pressure rhythm with aging in spontaneously hypertensive rats

Abstract

Evolution of circadian rhythm of blood pressure with age in male spontaneously hypertensive rats (SHR) was compared with that in male Wistar Kyoto rats (WKY). Three different age groups (5-6 week old, 19-22 week old, 29-31 week old) were provided for each strain. Chronogram and cosinor method were used for time series analyses. In all age groups of WKY, a diurnal fall and a nocturnal rise in blood pressure (BP) as well as in heart rate (HR) were observed. There was no significant difference in acrophase between BP and HR in each age group of WKY. Regarding SHR, however, each age group demonstrated a different relation between circadian BP rhythm and HR rhythm. In 5-6 week old SHR, a difference in acrophase between BP and HR (about 6.9 hr, p less than 0.1) was observed, which became more conspicuous with the increase in age, eventually presenting an inverted relation between BP and HR in 19-22 week old SHR. The relation between circadian BP rhythm and circadian HR rhythm in 29-31 week old SHR was almost identical with that in 19-22 week old SHR. In other words, a phase lag of acrophase of BP from that of HR already observed in young SHR increased with aging. The results indicate that parasympathetic periodicity remained unchanged even in SHR since circadian HR rhythm was similar in all groups, while the periodicity of sympathetic neural tone relating to the regulation of circadian BP rhythm seems to be disturbed. The mechanism responsible for development and maintenance of high BP in SHR may be linked to a disturbance in the sympathetic mechanism which regulates BP periodicity.