A low dietary sodium intake reduces neuronal noradrenaline release and the blood pressure in spontaneously hypertensive rats

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Abstract
Summary Young male spontaneously hypertensive rats were placed on a low (0.5 mmol/100 g), normal (13 mmol/100 g) or high (120 mmol/100 g) sodium diet for 6 weeks. Subsequent to the assessment of the basal blood pressure and heart rate in freely moving animals, the rats were pithed. In some of the pithed rats dose-response curves were constructed to exogenously administered noradrenaline (NA) and sympathetic nerve stimulation (SNS) through the pithing rod before and after the administration of an inhibitor of the neuronal uptake mechanism (desipramine, DMI, 0.5 mg/kg). In other pithed rats the SNS-induced (2 Hz) increase in plasma NA levels was assessed before and after the administration of clonidine (30μg/kg), an agonist at the prejunctional α2-adrenoceptors. We found that following the dietary intervention period the basal blood pressure and heart rate were higher in the high sodium group and lower in the low sodium group compared to values obtained in the control group. The neurogenic pressor responses clearly differed between the various diet groups. These differences could not be explained by differences in postjunctional responsiveness to exogenous NA. Rather, they were probably due to differences in the amount of transmitter at the synapse since the low sodium group was associated with decreased levels of plasma NA during SNS. Pretreatment with DMI potentiated the pressor responses, but largely to the same degree in the various diet groups, suggesting that there were no major differences in the function of the neuronal uptake mechanism. Hence, it is concluded that a low sodium diet causes an attenuation of transmitter release/impulse from the sympathetic nerve terminal. This reduction of transmitter release was not due to a facilitated prejunctional α2-adrenoceptor mediated autoinhibition; in contrast the inhibitory influence of clonidine on SNS-induced plasma NA levels was significantly smaller in the low sodium group.