The Effects of Continuous Clonidine Infusion on the Circadian Rhythms of Arterial Blood Pressure, Heart Rate and Spontaneous Locomotor Activity in Normotensive Wistar-Kyoto Rats

In rats, the existence of circadian rhythms in many physiological, biochemical and behavioural systems has been well established (Sheving et al, 1968; Rusak and Zucker, 1975; Rosenfeld and Rice, 1979). However, controversy exists as to whether or not such rhythms exist for arterial blood pressure. Although Friberg et al (1982) reported that the blood pressure of normotensive rats was higher during the dark phase than the light phase, Norman et al (1980) did not find evidence for a circadian rhythm in this parameter. Nevertheless, it appears that the cardiovascular effects of drugs could be complicated by the presence of circadian rhythms in vagal and sympathetic activity, hormone release and the responsiveness of the heart and blood vessels (Rusak and Zuker, 1979; Hossman et al., 1980). For example, Brown and Lever (1982) demonstrated that the pressor effects of a continuous infusion of angiotensin II in normotensive rats were more dramatic during the dark phases of a four day infusion period.