Characterization of the baroreceptor heart rate reflex during development in spontaneously hypertensive rats.

1. We have examined the baroreceptor-heart rate (HR) reflex in weight-matched conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats during development. 2. Graded steady-state changes in mean arterial pressure (MAP) and the corresponding HR responses before and after vagal blockade with methylatropine were fitted to an S-shaped logistic function. 3. At 6 weeks of age, SHR had a 17% higher MAP than WKY and an increased baroreflex gain (slope) compared with WKY due to an increased curvature of the MAP-HR relationship. The HR range (the difference between the upper and lower HR plateaus) was similar in the two strains at this time. 4. From 9-14 weeks of age, the baroreflex gain progressively increased in WKY and decreased in SHR due to corresponding alterations in HR range. 5. By 20 weeks the baroreflex gain was 23% lower in SHR than WKY due to a 37% lower HR range. 6. There were no differences between the two strains in the sympathetic component of the baroreflex at any age, suggesting that the changes to baroreflex properties were confined to the cardiac vagus. 7. Pretreatment with enalapril from 4-9 weeks reduced the hypertension of SHR at 14 and 20 weeks by 38% and abolished all baroreceptor-HR reflex differences between the two strains. 8. These studies suggest that the major alteration to the baroreceptor-heart rate reflex in the SHR during development was a reduction in the maximum vagal capacity to respond to changes in blood pressure. This effect developed after the onset of hypertension and was prevented by antihypertensive treatment early in life. The lack of effect on the cardiac sympathetic component suggests that altered arterial baroreceptor afferents are not unlikely to be responsible.