Baroreceptor function and changes in strain sensitivity in normotensive and spontaneously hypertensive rats

Baroreceptor resetting during hypertension has been attributed to a reduction in the distensibility of the vessel wall in which the receptors are located. According to this hypothesis, a simple increase in pressure is all that is required to overcome the increase in vessel wall stiffness. However, previous work from our laboratory suggested a more complicated situation. Measurements of both vessel wall mechanical properties and baroreceptor discharge characteristics in spontaneously hypertensive rats (SHR) showed that distortion thresholds for the receptors also undergo changes. Here we have examined the time course of resetting, measuring both aortic distensibility and baroreceptor properties in SHR and normotensive Wistar-Kyoto rats (WKY) from 5 to 30 weeks in age. An in vitro aortic arch-aortic nerve preparation was used. We found that for a given pressure the aortic radii of WKY were increasing much more rapidly than the aortic radii for SHR and, beyond 5 weeks of age, were much more distensible. The lower distensibility in SHR was accompanied by increased wall thickness. The discharge characteristics of single baroreceptors were expressed in terms of both pressure and distortion of circumferential wall strain. The change in distensibility of WKY aortas from 5 to 30 weeks was suitably matched by an increase in the strain threshold for discharge of WKY baroreceptors resulting in a constant pressure threshold for discharge. The lower distensibility of SHR aortas was accompanied by lower threshold strains in SHR baroreceptors, but the changes were not suitably matched, and progressive resetting of SHR baroreceptors to higher threshold pressures occurred. The two sets of receptors appear to be different as early as 5 weeks of age when blood pressures are similar and, furthermore, these differences are accentuated by age and hypertension.