SHORT-TERM HEMODYNAMIC VARIABILITY IN THE CONSCIOUS AREFLEXIC RAT

1. Simultaneous measurements of arterial pressure and cardiac output ( n = 8), mesenteric blood flow (n = 7) or hindquarters (n = 8) blood fl ow were performed during 1 h periods in conscious rats, before and aft er acute pharmacological blockade of the autonomic, renin-angiotensin and vasopressin systems. In the latter condition (areflexic state), ar terial pressure was maintained with a continuous infusion of noradrena line. 2. In the areflexic state, spontaneous fluctuations in arterial pressure were markedly exaggerated, especially depressor episodes. At the onset of these falls in arterial pressure, there was an abrupt and transient decrease in stroke volume and cardiac output. Systemic vaso dilatation then developed while cardiac output returned to normal. Reg ional vasodilatations were also delayed from the onset of the falls in arterial pressure and were usually large enough to maintain blood flo w. 3. Both time and frequency domain analyses confirmed that changes i n systemic and regional vascular conductances lagged by about 1 s behi nd arterial pressure changes. 4. These results indicate that, in the a bsence of neurohumoral influences, autoregulatory-like mechanisms beco me dominant in the control of systemic and regional circulations and c ontribute to exaggeration of the spontaneous short-term variability of arterial pressure.