Rapid Communication Diurnal Blood Pressure Variation and Dietary Salt in Spontaneously Hypertensive Rats

We have previously reported that high dietary salt exposure significantly increases daytime mean arterial pressure in spontaneously hypertensive rats (SHR) but not in normotensive Wistar-Kyoto (WKY) controls. In the present study, we used a telemetry monitoring system to evaluate the effects of high dietary salt exposure on diurnal variation of mean arterial pressure and heart rate in SHR and WKY rats. After implantation of a radio frequency transducer, SHR and WKY rats were maintained on either high (8%) or basal (1%) salt diets. Hemodynamic values were then analyzed for diurnal variation with the use of a nonlinear data-fitting program. After 2 weeks of dietary exposure, high salt-fed SHR had significantly greater 24-hour mean arterial pressure (156±3 mm Hg) than SHR receiving basal (135±2 mm Hg) and WKY rats receiving high (100±2 mm Hg) or basal (100±l mm Hg) Human and animal studies demonstrate characteristic blood pressure and heart rate (HR) rhythm related to the day/night cycle. This circadian rhythm is probably important in terms of cardiovascular morbidity and mortality because sudden death, stroke, and myocardial infarction in humans tend to occur most frequently in the morning and because increased circadian variability in blood pressure is accompanied by increased end-organ damage. Therefore, factors that alter diurnal cardiovascular variation would be expected to affect cardiovascular morbidity and mortality. Certain species and certain individuals within a species are known to be sensitive to dietary NaCl exposure; ie, with changes in dietary NaCl ingestion, significant changes in blood pressure occur.This laboratory has demonstrated that spontaneously hypertensive rats (SHR) obtained from Taconic Farms, Germantown, NY, are NaCl sensitive.Daytime blood pressure measurements obtained from indwelling femoral artery catheters have shown that SHR manifest an increase in mean arterial pressure (MAP) of 20 to 30 mm Hg when exposed to a high (8%) NaCl diet. Normotensive Wistar-Kyoto (WKY) rats, in contrast, manifest no significant change in blood pressure regardless of dietary NaCl exposure. Received December 16, 1993; accepted in revised form April 6, 1994. From the Vascular Biology and Hypertension Program, Division of Cardiovascular Disease, Departments of Medicine and Cell Biology, University of Alabama at Birmingham. Correspondence to David A. Calhoun, MD, Vascular Biology and Hypertension Program, 520 ZRB, University of Alabama at Birmingham, Birmingham, AL 35294. © 1994 American Heart Association, Inc. salt diets. Rhythm analysis indicated significant increases in both daytime and nighttime mean arterial pressure during high salt exposure in SHR. In WKY rats, high salt exposure increased nighttime but not daytime mean arterial pressure, with no net effect on 24-hour mean arterial pressure. High dietary salt exposure significantly decreased heart rate in both SHR and WKY rats, and it did not significantly alter the pattern of diurnal blood pressure or heart rate variation. These results indicate that WKY rats manifest an acute sensitivity to salt ingestion but have compensatory mechanisms sufficient to prevent sustained increases in mean arterial pressure; such mechanisms are lacking in SHR. (Hypertension. 1994;24:l-7.)