Role of baroreflex and central alpha2-adrenergic receptor systems in the diurnal variation of blood pressure and heart rate in normotensive and hypertensive rats.

1. To elucidate whether baroreflex could contribute to manifest the diurnal blood pressure variations (DBPV) in normotension and hypertension, DBPV were recorded continuously via a femoral artery in awake normotensive (NT) rats and spontaneously hypertensive rats (SHR) with and without sinoaortic denervation (SAD). To determine the role of central alpha2-adrenergic receptor system in DBPV in hypertension, guanabentz (0.5-1.0 mu g/kg per min) was infused in SHR. 2. There were no differences in mean arterial pressure (MAP) variability (SD) of MAP of 24 h (MAP -- SD/MAP) and SD of 24 h heart rate (HR -- SD/HR) between SHR and NT. SAD did not elevate MAP and HR in both SHR and NT. 3. However, in SAD rats, MAP -- SD/MAP was significantly greater than those in sham-operated rats in both SHR and NT, while MAP -- SD/MAP and HR -- SD/HR did not show any difference between the two groups even after SAD. On the other hand, SAD did not augment HR -- SD/HR in either strain. 4. During guanabentz infusion, MAP was significantly lowered in sham-operated and SAD -- SHR. Moreover, MAP -- SD/MAP was significantly reduced in sham-operated, but it was not in SAD -- SHR. 5. These findings suggest that baroreflex could suppress tonically the diurnal change of blood pressure in NT and SHR. The regulation of diurnal blood pressure by baroreflex via a central alpha-adrenergic receptor system may be altered in SHR.