Increased dietary salt intake enhances the exercise pressor reflex

Abstract

Increased dietary salt in rats has been shown to sensitize central sympathetic circuits and enhance sympathetic responses to several stressors, including hyperinsulinemia, intracerebroventricular injection of angiotensin, and electrical stimulation of sciatic nerve afferents. These findings prompted us to test the hypothesis that increased dietary salt enhanced the exercise pressor reflex. Male Sprague-Dawley rats were fed 0.1% (low) or 4.0% (high) NaCl chow for 2 to 3 wk. On the day of the experiment, the rats were decerebrated, and the hind limb muscles were statically contracted for 30 s by electrically stimulating the cut peripheral ends of the L4 and L5 ventral roots. We found that contraction produced a significantly greater increase in mean arterial pressure of rats fed 4.0% (n = 26) vs. 0.1% (n = 22) NaCl (24 ± 2 vs. 15 ± 2 mmHg, respectively; P < 0.05). Baseline mean arterial pressure was not different between groups (0.1%, 77 ± 4 vs. 4.0% NaCl, 80 ± 3 mmHg). Likewise, the tension time indexes were not different between the two groups (P = 0.42). Section of the L4 and L5 dorsal roots greatly attenuated both the pressor and cardioaccelerator responses to contraction in both groups of rats, an effect showing that the responses were reflex in origin. Finally, electrical stimulation of the lumbar sympathetic chain produced similar increases in mean arterial pressure and decreases in femoral arterial blood flow and conductance between rats fed 0.1% vs. 4.0% NaCl diets. We conclude that increased dietary salt enhances the exercise pressor reflex.

Keywords: autonomic nervous system; muscle afferents; static contraction.

Fig. 1.

The pressor (A), cardioaccelerator (B), and tension-time-index (TTI; C) responses to static contraction of rats fed low NaCl (0.1%; black bars; n = 22) or high NaCl (4.0%; white bars; n = 26). #Significant differences (P < 0.05) between 0.1% vs. 4.0% NaCl rats. Values inside bars represent baseline values (means ± standard errors). MAP, mean arterial pressure; HR, heart rate; bpm, beats/min.

Fig. 2.

The pressor (A) and cardioaccelerator (B) responses to static contraction before (white bars) and after (black bars) cutting L4 and L5 dorsal roots in rats fed low (0.1%, n = 5) or high (4.0%, n = 7) NaCl diet. #Significant difference (P < 0.05) between before versus after dorsal root transection.

Fig. 3.

The pressor (A), femoral blood flow (B), and femoral artery vascular conductance (C) responses to lumber nerve stimulation (1-ms pulses of 50 μA at randomized frequencies of 1, 2.5, or 5 Hz for 1 min) in rats fed low (0.1%; ●; n = 6) or high (4.0%; □; n = 6) NaCl diet. There were no statistical differences in any variable at any frequency between rats fed low versus high NaCl diets.