A comparison of aortic baroreceptor discharge in normotensive and spontaneously hypertensive rats.

Electrophysiological characteristics of individual aortic baroreceptors from normotensive rats (NTR) and spontaneously hypertensive rats (SHR) were compared. Impulse activity of afferent fibers was examined following the application of pressure steps to an in vitro aortic arch-aortic nerve preparation. Thirty-one fibers including seven unmyelinated fibers were studied completely over the range of 0-260 mm Hg, using steps 1-30 seconds in duration. The pressure threshold for peak transient discharge of baroreceptors of SHR's was 88.4 +/- 10.1 (mean +/- SE) mm Hg, whereas for baroreceptors of NTR's it was 77.5 +/- 9.3 mm Hg. The pressure threshold for steady state discharge was 137.3 +/- 5.2 mm Hg for SHR baroreceptors and 103.5 +/- 7.1 for NTR baroreceptors. These values compare favorably to measurements in vivo in the rat. The relationship between peak transient impulse frequency and pressure was linear, whereas the relationship between steady state impulse frequency and pressure was hyperbolic. The curvature of the steady state frequency-pressure curves was significantly reduced for baroreceptors of SHR's. The steady state pressure-volume curves of the aortas of SHR's and NTR's were similar, so that a reduction in distensibility could not account for the larger, significant differences in threshhold and sensitivity. Therefore resetting cannot be attributed simply to reduced vascular compliance.