Blood pressure and heart rate responses to mental stress in spontaneously hypertensive (SHR) and normotensive (WKY) rats on various sodium diets

Abstract

Normotensive (WKY) and hypertensive rats (SHR) were, from 5-12 wk of age, given low (LNa), control and high (HNa) Na diets (0.5, 5 and 50 mmol .cntdot. 100 g-1) food, respectively, during weekly recordings of body wt, conscious indirect systolic blood pressure (SBP) and heart rate (HR). During the last week, mean arterial pressure (MAP) and HR responses to standardized stress stimuli (air jet) were recorded before and after sequential cardiac nerve blockade. While resting, SBP was about equal in all WKY groups, but it was significantly reduced in SHR-LNa (152 mm Hg vs. 174 and 178 mm Hg in SHR controls and HNa; P < 0.05). In both LNa groups HR was elevated nearly 25% compared with controls, being in SHR 513 vs. 419 bpm [beats/min] (P < 0.01) and in WKY 489 vs. 393 bpm (P < 0.01). Cardiac nerve blockade indicated that this HR elevation was about equally due to elevations of sympathetic activity and intrinsic pacemaker activity. SHR-LNa also showed attenuated MAP elevations to acute mental stress. There were no significant differences between groups concerning hematocrit or plasma Na-K levels. SHR have a greater salt requirement than WKY, as Na restriction to 1/10 of normal led to a considerable MAP reduction in SHR despite compensatory sympathetic activation, and also to attenuated pressor responses to mental stress. Cardiovascular effects in SHR were much more extensive when on a low-Na diet than when Na intake was increased 10-fold above normal.