Hyperresponsiveness of sympathoadrenal system in conscious DOCA-NaCl and SHR rats in response to acute hemorrhagic hypotension.

The sympathoadrenal basal tone and reactivity were evaluated by the measure of plasma norepinephrine (NE) and epinephrine (EPI) levels in chronically cannulated awake and unrestrained animals under basal conditions and following an acute withdrawal of blood volume necessary to decrease the mean arterial pressure (MAP) by 50 mmHg in normotensive, in deoxycorticosterone acetate (DOCA) and salt (NaCl) hypertensive and in spontaneously hypertensive (SHR) (10-11 weeks) rats. In DOCA-NaCl and SHR rats the basal sympathoadrenal tone was found to be increased as was reflected by the higher plasma levels of both catecholamines. Moreover, an enhanced reactivity of the sympathoadrenal system was also observed in DOCA-NaCl hypertensive rats as well as a slight sympathetic hyperreactivity in SHR in response to the hemorrhagic hypotension suggesting the possibility of alterations in the baroreflex functions or in the local modulatory mechanisms. In addition, the acute hemorrhagic hypotension triggered compensatory mechanisms which permitted a rapid return of the MAP to the baseline values in normotensive rats and in the SHR. However, in DOCA-NaCl hypertensive rats, the return of the MAP was delayed and remained below initial levels even 60 min after the hemorrhagic hypotension, suggesting a less efficient compensatory mechanism in these animals despite a markedly potentiated sympathoadrenal response. To explain such a discrepancy, it may be hypothesized that the potentiated sympathoadrenal reactivity could induce a decrease in vascular reactivity by an acute desensitization of the vascular alpha-adrenoceptors in DOCA-NaCl hypertensive rats.