Altered control of hindlimb vascular resistance by vagal afferents in spontaneously hypertensive rats. Difference in the early and late stage of hypertension

The aim of this study was to examine whether control of vascular resistance by vagal afferents is altered in the early as well as late stage of hypertension. We examined the effects of vagotomy on hindlimb vascular resistance as well as on arterial baroreflex control of hindlimb vascular resistance in spontaneously hypertensive rats and Wistar-Kyoto rats, 12 and 35 weeks old. Vagotomy in rats with the intact arterial baroreceptors increased hindlimb vascular resistance in all four groups. Hindlimb vascular responses to graded sympathetic nerve stimulation were closely linear up to 6 Hz in all groups, and the slope of the response was approximately 1.4 times steeper in spontaneously hypertensive than in Wistar-Kyoto rats, in both the young and old groups. The increase in hindlimb vascular resistance after vagotomy normalized by the slope of the response to sympathetic nerve stimulation was greater in young spontaneous hypertensive rats than in young Wistar-Kyoto rats, but was less in old spontaneously hypertensive rats than in old Wistar-Kyoto rats. Vagotomy increased the gain of arterial baroreflex control of hindlimb vascular resistance in young spontaneously hypertensive rats, and in two groups of Wistar-Kyoto rats, but not in old spontaneously hypertensive rats. The percent increase in the gain of arterial baroreflex control of hindlimb vascular resistance after vagotomy tended to be greater in young spontaneously hypertensive than in young Wistar-Kyoto rats, but was less in old spontaneously hypertensive than in old Wistar-Kyoto rats. Central venous pressure and left ventricular end-diastolic pressure were higher in spontaneously hypertensive than in Wistar-Kyoto rats in both young and old groups. Left atrial distensibility assessed by obtaining the atrial pressure-volume relationship was comparable between young spontaneously hypertensive and young Wistar-Kyoto rats, but was less in old spontaneously hypertensive than in old Wistar-Kyoto rats. These results indicate that vagal afferents exert tonic inhibition on control of hindlimb vascular resistance in spontaneously hypertensive as well as in Wistar-Kyoto rats, and that tonic inhibitory influence of vagal afferents on control of hindlimb vascular resistance is altered in spontaneously hypertensive rats, augmented in young, but attenuated in old, spontaneously hypertensive rats, compared with that in age-matched Wistar-Kyoto rats. It is considered that altered control of vascular resistance by vagal afferents in spontaneously hypertensive rats may result from changes in vagal afferent activity.