Chronic salt loading and adrenergic mechanisms in the Sprague-Dawley rat

The effect of chronic salt loading on adrenergic mechanisms was evaluated in Sprague-Dawley rats (and NMRI mice) maintained on a high sodium (8%) or normal sodium (0.3%) regime for 4 weeks. The basal blood pressure (carotid artery) was not influenced by the high salt diet but the heart rate and blood pressure increases to mental stress (jet air) were larger in the salt loaded rats. There were indications of an increased sympathetic tone in rats on the high salt diet since in these rats sympathoinhibitory treatment with ganglionic blockade or clonidine induced larger falls in blood pressure and heart rate than in the controls. Central catecholamines (brain stem, striatum, hemispheres) were determined spectrofluorimetrically after cation exchange chromatography. The high salt diet influenced neither the endogenous levels of noradrenaline nor central noradrenaline turnover (disappearance of noradrenaline after synthesis inhibition by alpha-methyltyrosine and accumulation of dihydroxyphenylalanine after decarboxylase inhibition by 3-hydroxybenzylhydrazine). There were no changes in central alpha 2-adrenoceptor responsiveness when assessed as clonidine-induced deceleration of noradrenaline turnover in the brain and in central alpha 1-adrenoceptor responsiveness (clonidine-induced increase of flexor reflex in spinalized rats and clonidine-induced increase of motor activity in reserpinized mice). Peripheral sympathetic function was assessed in pithed rats. The pressor responses to intravenously administered noradrenaline (0.01-10 micrograms/kg) and electrical stimulation of the spinal sympathetic nerves (SNS, 0.25-2 Hz) were similar in the two groups, suggesting that salt did not influence vascular alpha-adrenoceptor responsiveness or transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)