The significance of duration of salt loading on cardiovascular response and urinary excretion of catecholamine in rats.

To analyze the conflicting data on the relationship between sodium intake and catecholamine release, the effect of the duration of high sodium loading on cardiovascular response and catecholamine release was examined in conscious rats. Urinary excretions of norepinephrine (NE), and dopamine (DA) were measured frequently over a 4 week period. Male Wistar rats at 4 weeks of age were given a diet containing either basal (0.3%) or high (3.1%) sodium content. Systolic blood pressure was measured weekly by the tail cuff method. Twenty-four hour urine collections were made for analysis of catecholamines in metabolic cages every other day during the initial 2 weeks and once a week in the following 2 weeks of salt loading. High sodium intake resulted in a rise in blood pressure and a reduction in heart rate. Bradycardia was significant during the initial 2 weeks and not significant during the following 2 weeks after the initiation of salt loading. Urinary excretion of NE did not change during the initial 2 weeks of salt loading but increased significantly following the 2 week period after salt loading. Urinary excretion of DA increased diphasically, showing the first peak at 1 week after salt loading and the second peak at 4 weeks after the initiation of salt loading. These results suggest that the heart rate and urinary excretion of catecholamine are influenced by the duration of salt loading. When we estimate the effect of salt loading on cardiovascular response and urinary excretion of catecholamine, we should draw attention to the importance of the duration of salt loading, because this duration of time further elicites delayed response in the sympathetic nervous system.