Effects of oral salt loading on beta-adrenergic receptor responsiveness in normal and hypertensive subjects

The effect of oral salt loading (400 mmol per day of NaCl for 7 days) on cardiac and pancreatic beta-receptor responsiveness has been evaluated in 12 patients with established essential hypertension and in seven age-matched control subjects. Cardiac beta-receptor responsiveness was evaluated by assessing the dose of isoprenaline which increased a stable heart rate by 25% (chronotropic dose 25%, CD 25%). Pancreatic beta-receptor responsiveness was measured by the incremental areas of insulin secretion induced by iv infusion of increasing amounts of isoprenaline. Before salt load, CD 25% was significantly higher in hypertensives compared with controls (7.84 +/- 1.34 micrograms vs 3.9 +/- 0.48 micrograms, P less than 0.05) while there was no difference in the isoprenaline-induced insulin secretion between the two groups of subjects. After salt loading, CD 25% was significantly reduced in hypertensive patients but was not modified in normal subjects. Therefore, the difference in CD 25% was no longer detectable between the two groups (5.5 +/- 1.42 micrograms vs 3.2 +/- 0.48 micrograms in normal subjects and in hypertensives, respectively, NS). Furthermore, salt loading failed to induce any change in isoprenaline-induced insulin secretion in either groups. These results support the existence of a relationship between sodium intake and adrenergic beta-receptor responsiveness in human hypertension.