Loss of Diurnal Rhythms of Blood Pressure and Heart Rate Caused by High-Fat Feeding

Background Diet-induced obesity using ad libitum high-fat feeding in rabbits causes losses in diurnal rhythms of blood pressure (BP) and heart rate (HR). Because obesity is associated with hypertension, it is difficult to determine independent effects of ad libitum feeding and obesity in altering diurnal rhythms. We studied diurnal rhythms of BP and HR after controlling BP during obesity development using hydralazine. Methods New Zealand white rabbits were divided into lean control (LC), lean hydralazine-treated (LH), obese control (OC), and obese hydralazine-treated (OH) groups. Lean animals consumed a maintenance diet, whereas obese animals consumed an ad libitum high-fat diet. Over 12 weeks, BP and HR were monitored from 11:00 to 07:00 using telemetry. Hydralazine treatment consisted of 6 mg/kg/day and 10 to 14 mg/kg/day for LH and OH, respectively. Diurnal rhythms were evaluated using day–night values (day, 11:00 to 16:00 average; night, 02:00 to 07:00 average). Results Compared with control values, diurnal HR rhythm was abolished on day 1 of high-fat feeding (61.4 ± 3.6 v 3.1 ± 4.2 beats/min, respectively; P ≤ .05), whereas OC and OH did not differ significantly in day–night HR over 12 weeks. Compared with control values, diurnal BP rhythm was abolished by day 2 of high-fat feeding (4.6 ± 1.6 v −0.1 ± 0.9 mm Hg, respectively; P ≤ .05), whereas OC and OH did not differ in day–night BP despite lower BP in OH. Conclusion In this study ad libitum high-fat feeding in rabbits caused immediate losses of diurnal rhythms of BP and HR that were independent of weight gain and BP elevation.