Autonomic mediation of short-term cardiovascular oscillations after acute hemorrhage in conscious rats

The role of the autonomic and the renin-angiotensin (R-A) activities in short-term cardiovascular control during the bradycardic phase following severe hemorrhage was investigated in conscious rats. Spectral analysis of beat-to-beat fluctuations of the R-R interval (RRI), systolic (SBP) and diastolic (DBP) blood pressure in the 0.01-5-Hz range was carried out under control conditions and following a bleeding of 30% of total blood volume, with and without i.v. injection of atropine (2 mg/kg), prazosin (2 mg/kg), propranolol (5 mg/kg) or captopril (7 mg/kg). The bradycardic stage was characterized by: (i) an increase of the three oscillatory components exhibited by RRI variability which appears driven by vagal activity and buffered by beta-adrenergic activity, while the increase of the slower LF (0.01-0.2 Hz) oscillations seems driven by the slow alpha-adrenergic control; (ii) a decrease of SBP and DBP oscillations and absence of SBP-RRI correlation in the MF band (0.2-0.6 Hz) possibly related to a decrease in the sympathetic drive of SBP-MF and DBP-MF oscillations and in RRI baroreflex control; (iii) an increase in LF oscillations of SBP and even more of DBP that seems driven by the slow alpha-and beta-adrenergic control and buffered by the R-A control, responses possibly related to an increase of LF oscillations of peripheral resistance; (iv) the persistence of high SBP-RRI correlation in the LF band and in the respiratory band (1-3 Hz); the former seem to reflect the existence of a slow baroreflex control mediated by beta-adrenergic activity, the latter appears to be caused by feedforward mechanical effects of RRI changes on SBP.