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

Abstract

Baroreceptor resetting during hypertension was attributed to a reduction in distensibility of the vessel wall in which the receptors are located. According to this hypothesis, an increase in pressure is required to overcome the increase in vessel wall stiffness. Previous work 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 undergo changes. The time course of resetting, measuring aortic distensibility and baroreceptor properties was studied in SHR and normotensive Wistar-Kyoto rats (WKY) from 5-30 wk in age. An in vitro aortic arch-aortic nerve preparation was used. For a given pressure the aortic radii of WKY were increasing much more rapidly than the aortic radii for SHR and, beyond 5 wk 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 or circumferential wall strain. The change in distensibility of WKY aortas from 5 to 30 wk 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. The changes were not suitably matched, and progressive resetting of SHR baroreceptors to higher threshold pressures occurred. The 2 sets of receptors appear to be different as early as 5 wk of age when blood pressures are similar. These differences are accentuated by age and hypertension.