Short-term haemodynamic 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 flow were performed during 1 h periods in conscious rats, before and after acute pharmacological blockade of the autonomic, renin-angiotensin and vasopressin systems. In the latter condition (areflexic state), arterial pressure was maintained with a continuous infusion of noradrenaline. 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 vasodilatation then developed while cardiac output returned to normal. Regional vasodilatations were also delayed from the onset of the falls in arterial pressure and were usually large enough to maintain blood flow. 3. Both time and frequency domain analyses confirmed that changes in systemic and regional vascular conductances lagged by about 1 s behind arterial pressure changes. 4. These results indicate that, in the absence of neurohumoral influences, autoregulatory-like mechanisms become dominant in the control of systemic and regional circulations and contribute to exaggeration of the spontaneous short-term variability of arterial pressure.