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Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.

1. An in vitro aortic arch-aortic nerve preparation was used to characterize the pressure-discharge properties of aortic 'autoactive' baroreceptors (aBRs), a functionally unique group of baroreceptors that discharge continuously below pressure threshold (Pth). These units contrast to more familiar 'quiescent' BRs (qBRs) that are silent below Pth. This study also examined whether aBRs rapidly reset to sustained changes in mean arterial pressure (MAP), and whether they respond to local vasoconstriction, as found in qBRs. 2. Pressure-discharge curves were constructed using slow pressure ramps (2 mmHg s-1) in a total of fifty-four aBRs and sixty-four qBRs from fifty-three Wistar-Kyoto rats. Rapid resetting was tested by comparing the curves before and 15 min after adjusting MAP to selected levels between 40 and 180 mmHg. Response curves were also compared before and after constricting the arch with 10(-8) M angiotensin II. 3. Compared to qBRs, aBRs had significantly lower Pth values (83 +/- 2 versus 91 +/- 2 mmHg, mean +/- S.E.M., P < 0.05) but similar threshold frequencies (13 +/- 1 versus 16 +/- 2 Hz), higher saturation pressures (138 +/- 4 versus 123 +/- 2 mmHg) but similar saturation frequencies (55 +/- 3 versus 55 +/- 3 Hz), and lower suprathreshold sensitivities (slope of linear region: 1.1 +/- 0.1 versus 1.4 +/- 0.1 Hz mmHg-1) but wider operating ranges (57 +/- 4 versus 35 +/- 3 mmHg). 4. The aBRs rapidly reset to changes in MAP (n = 16), and the extent of resetting (delta Pth/delta MAP = 0.26) was similar to that in qBRs (0.23). 5. Vasoconstriction had no effect on aBR subthreshold discharge (n = 5), but inhibited suprathreshold responses to pressure much like that in qBRs. 6. These results suggest that aortic aBRs may extend the range of the baroreflex, but probably do not improve its sensitivity to transient fluctuations in pressure or its ability to correct changes in mean pressure over extended periods. 7. Subthreshold discharge in aBRs appears to be an intrinsic property of these units, which was not affected by resetting or changes in vascular tone. At suprathreshold pressures, contraction of local smooth muscle modulates the aBRs and qBRs in a similar fashion by mechanically unloading the sensory endings.

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