Hemodynamic responses to electrical stimulation of the aortic depressor nerve in awake rats

Changes in mean arterial pressure (MAP), heart rate (HR), and vascular resi stance (hindquarter and mesenteric territories) in response to electrical s timulation (ES) of the aortic depressor nerve (ADN) were evaluated in consc ious freely moving rats. Platinum electrodes were implanted into the ADN of all rats studied, and some of these animals were also implanted with minia turized Doppler probes around the superior mesenteric artery and inferior a bdominal aorta (hindquarter). In both groups, the femoral artery and vein w ere catheterized one day before the experiments. In the first group of rats (n = 7), the control ES of the ADN in the range from 0.5 to 3.0 V (50 Hz, 10 ms) produced bradycardia and hypotension in an intensity-dependent manne r, and treatment with methylatropine (intravenously) blocked the bradycardi a but produced no significant changes in the hypotensive response. In a sec ond group (n = 6), ES of the ADN was performed with the intensity fixed at 3 V and the frequency of the stimuli varying from 10 to 50 Hz. In this grou p, the hypotensive response was frequency dependent, whereas the bradycardi c response was not. In a third group of rats (n = 6), ES of the ADN (2.5 V) produced hypotension (-35 +/- 4 mmHg), minor changes in the mesenteric (+5 +/- 14%), and vasodilation in hindquarter (-32 +/- 6%) vascular beds. The data show that 1) ES of the ADN produces a fall in pressure, bradycardia, v asodilation in the hindquarter, and no changes in the mesenteric vascular r esistance, 2) methylatropine blocked the bradycardia and produced no effect on the hypotensive response to ES of the ADN, and 3) the baroreceptor affe rent fibers involved in the hypotensive response to ES of ADN are sensitive to the variation of the frequency of the stimuli, whereas the fibers invol ved in the bradycardic response are not.