Increased dietary sodium inhibits baroreflex-induced bradycardia during acute sodium loading

The present study investigated the effects of increased dietary sodium on the modification of cardiac baroreflex responses induced by acute sodium loading. Changes in blood pressure and heart rate during intravenous phenylephrine and nitroprusside administration were compared using a four-parameter sigmoid logistic function before and after a 30-min infusion of 0.6 or 1.0 M NaCl in conscious male Sprague-Dawley rats consuming only tap water (Tap) or isotonic saline (Iso) for 2-3 wk. In Tap animals, infusion of 1.0 M NaCl increased the baroreflex-induced heart rate minimum, reduced heart rate range, and increased the operating blood pressure. In contrast, infusion of 0.6 M NaCl in Tap rats reduced both heart rate minimum and maximum. However, infusion of 0.6 M NaCl in Iso animals produced responses similar to that shown in Tap rats infused with 1.0 M NaCl. In addition, the decreased heart rate minimum in Tap rats after infusion of 0.6 M NaCl was prevented by intravenous administration of a vasopressin V1-receptor antagonist. Furthermore, cardiac parasympathetic responses were similar in Tap and Iso rats before and after 0.6 M NaCl infusion. However, in animals receiving intravenous atropine, 0.6 M NaCl decreased heart rate minimum and maximum in Tap but did not alter the response parameters in Iso rats. These results demonstrate that the facilitation of cardiac baroreflex responses normally observed during moderate sodium loading is mediated by vasopressin and that increased dietary sodium ingestion reverses this facilitation by reducing sympathetic nervous system withdrawal.