Effects of dietary sodium restriction on heart rate control in rats

The effects of dietary sodium restriction on heart rate control were evaluated in male Wistar rats maintained for 4 wk on a low- (22 meq/kg diet) or standard (141 meq/kg diet) sodium diet. In conscious low-sodium rats, atropine produced heart rate increases greater than in standard sodium animals, indicating that basal parasympathetic tone was elevated. Sodium restriction did not affect basal sympathetic tone, since similar reductions in heart rate were produced by propranolol in both dietary groups. However, sympathetically mediated tachycardic responses to sodium nitroprusside were augmented in low-sodium rats. This enhancement of sympathetic function was also observed in low-sodium animals after pithing. The increases in heart rate produced by spinal sympathetic stimulation were significantly accentuated; whereas, responses to norepinephrine were not affected by the diets. This finding is in agreement with the observation that the chronotropic responses elicited by isoproterenol in isolated atria were not altered. When neuronal influences on heart rate were blocked by combined administration of atropine and propranolol or chlorisondamine in conscious rats, the underlying intrinsic cardiac rate was found to be significantly higher in low-sodium rats. In summary, the present study identifies three effects of sodium restriction on heart rate control in the rat: 1) basal parasympathetic tone is increased, 2) heart rate responses to sympathetic activation are enhanced, and 3) the intrinsic cardiac rate is elevated.