Reversed circadian rhythm of blood pressure in adult spontaneously hypertensive rats

The diurnal rhythms of blood pressure and heart rate were examined in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) when they were 5-6 weeks old (young), 17-20 weeks old (adult) and 30-32 weeks old (old). Blood pressure was recorded continuously for 24 h in conscious, unrestrained rats. Hourly averages of blood pressure and heart rate were calculated by an on-line computer. The circadian variation of blood pressure and heart rate remained unchanged regardless of age in WKY, but the difference in blood pressure between the daytime average and the night-time average decreased as they grew older. A well-defined circadian variation of blood pressure and heart rate was observed in young rats of both strains, i.e. a nocturnal rise in blood pressure and heart rate. An inverted circadian variation of blood pressure, a nocturnal fall in blood pressure, was observed in adult and old SHR, whereas the ordinary circadian variation of the heart rate remained unchanged. A significant positive correlation was observed between the hourly averages of blood pressure and heart rate in WKY in all the age groups (r = 0.9, P < 0.001 in each age group) and a weak positive correlation was observed in young SHR (r = 0.6, P < 0.05, 5-6 weeks old). This positive correlation disappeared in adult and old SHR and, instead, a negative correlation was observed (r = -0.5, P<0.02 for 17-20- week-old rats: r = -0.8, P < 0.001 for 30-32-week-old rats). These results suggest an impairment in the cardiovascular regulatory mechanism for the development and maintenance of circadian rhythm of blood pressure in SHR. The impairment may be linked with the development of hypertension in this strain.