Hemodynamic analysis of arterial pressure lability in sympathectomized rat

Male Sprague-Dawley rats, treated with either saline (n = 14) or guanethidine (n = 12) from 1 to 13 wk of age, were instrumented for the measurement of mean arterial pressure (MAP) and indexes of regional blood flows (pulsed Doppler flow probes) in the subdiaphragmatic aorta, superior mesenteric artery, and distal aorta (hindquarters). Hemodynamic parameters were simultaneously recorded in the conscious rats on a beat-to-beat basis by a computer. Chronic sympathectomy did not change the MAP level but nearly doubled its spontaneous variability, mainly due to large decreases in MAP associated with regional vasodilations that were slightly delayed with respect to the onset of the decreases in pressure, especially in the aortic and mesenteric circulations. In the hindquarters vascular bed, the onset of vasodilations sometimes coincided with the onset of the decreases in MAP. During depressor episodes, blood flow varied little (aortic and mesenteric circulations) or even increased (hindquarters). This hemodynamic pattern was not affected by acute beta-adrenoceptor blockade. We conclude that chronic sympathectomy sensitizes the cardiovascular system to decreases in arterial pressure mainly by unmasking local autoregulatory responses of regional circulations. The sympathetic control of vascular tone overrides these responses and considerably limits the MAP variability.