Enhanced Sympathoadrenal Reactivity to Haemorrhagic Stress in DOCA-salt Hypertensive Rats

The effect of haemorrhagic hypotension on plasma catecholamine levels was studied in anesthetized normotensive and DOCA-salt hypertensive rats. The basal levels of plasma norepinephrine (NE) were significantly higher in DOCA-salt hypertensive rats than in normotensive rats. Moreover, the elevations in plasma NE and epinephrine (E) levels induced by haemorrhagic hypotension were found to be markedly potentiated in DOCA-salt hypertensive rats. Pretreatment with the re-uptake blocker (desmethylimipramine) increased both basal and haemorrhage NE levels in DOCA-salt hypertensive as well as in normotensive rats. Consequently, basal and haemorrhage NE plasma levels remained significantly higher in the DOCA-salt hypertensive animals than in the normotensive rats even following neuronal re-uptake blockage. This suggests that the elevated NE concentrations found in the plasma of DOCA-salt hypertensive rats both under basal condition and during haemorrhagic hypotension do not reflect a defective re-uptake. Moreover, in contrast with what is observed in normotensive animals, bilateral adrenalectomy did not induce any increase in basal or haemorrhage NE levels in the DOCA-salt hypertensive rats. This constitutes yet more evidence supporting the existence of an impaired balance of the sympatho-adrenal axis in this hypertension model. The present study therefore suggests that the potentiated plasma catecholamine response to haemorrhage in DOCA-salt hypertensive rats is the consequence of an increased sympathoadrenal reactivity and not of an altered neuronal uptake. This hyperreactivity may result from an impaired regulation of the sympatho-adrenal axis in that hypertension model.