Aging but not sodium loading significantly attenuated diurnal change in blood pressure in stroke-prone spontaneously hypertensive rats

In the present study, we evaluated the effect of aging on diurnal change in blood pressure in stroke-prone spontaneously hypertensive rats (SHRSP/Izm) using a telemetry system. Diurnal changes in blood pressure, heart rate, and locomotor activity were determined in unrestrained, freely moving condition in 24-week-old male SHRSP/Izm (n = 6) and 40-week-old male SHRSP/Izm (n = 6). Diurnal change in blood pressure was also investigated in 40-week-old male spontaneously hypertensive rats (SHR/Izm, n = 6) and Wistar-Kyoto rats (WKY/Izm, n = 5) as age-matched controls of the strain. All rats were kept in a 12-h light/dark cycle (light period from 06:00 to 18:00, and dark period from 18:00 to 06:00). Rats were active in dark phase and inactive in light phase. Mean blood pressures (MBP) were significantly higher in light phase as well as dark phase in old SHRSP/Izm compared with the other three groups. Light/dark phase ratio of MBP was significantly higher in old SHRSP/Izm compared with the other three groups. We observed a significant positive relationship between light/dark phase ratio of MBP and left ventricular mass index in a studied population of rats (r = 0.547, P< .01). Because SHRSP is a salt-sensitive model, the effect of high salt loading on the circadian pattern of blood pressure was also investigated. Male SHRSP/Izm, at the age of 22 weeks, were maintained on high salt (8%) for 2 weeks. High salt exposure significantly increased dark phase as well as light phase MBP in SHRSP/Izm. However, light/dark phase ratio of MBP was not significantly different from normal salt-fed (0.6%) SHRSP/Izm. These results indicate that aging and end-organ damage were associated with the alteration of diurnal change in blood pressure in SHRSP/Izm.