Changes in autonomic activity and baroreflex sensitivity with the hypertension process and age in rats.

1. Autonomic activity and baroreflex sensitivity (BRS) were compared in age-matched conscious groups of Wistar Kyoto (WKY) rats, spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHRSP). 2. Male WKY rats, SHR and SHRSP aged 4-30 weeks were used. Autonomic activity and BRS were estimated by power spectral and cross-spectral analysis of systolic blood pressure (SBP) and SBP-SBP (SS) interval fluctuations, respectively. 3. The time-course of heart rate (HR), SBP, the amplitude of the low-frequency component of SBP fluctuation (SBP-LF; prazosin-sensitive index) and the amplitude of the high-frequency component of the SS interval fluctuation (SS-HF; atropine-sensitive index) consisted of two periods. In the first period (up to 10 or 15 weeks of age), BP, SBP-LF and SS-HF increased with age. The order of SBP-LF was SHRSP > SHR > WKY rats throughout this period. During the second period, BP was sustained at certain levels in all strains, but changes in SBP-LF and SS-HF with age were different among strains. In particular, in SHRSP, SBP-LF markedly decreased with age after 10 weeks. Baroreflex sensitivity in WKY rats increased gradually with age, whereas the BRS in SHR and SHRSP decreased before 6 weeks of age and remained lower than that in WKY rats. 4. In conclusion, the present study shows that both prazosin-sensitive and atropine-sensitive indices are associated with the elevation of BP in all strains studied. However, hypertension after 15 weeks of age in SHRSP is sustained despite a paradoxical reduction in sympathetic activity with an abnormal control of BRS. Therefore, the contribution of autonomic activity to hypertension may be discussed separately as a developmental period and a sustained period.