High calcium diet prevents baroreflex impairment in salt-loaded spontaneously hypertensive rats

Abstract

To investigate the role of the sympathetic control mechanism in the antihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sympathetic nerve activity, heart rate, and overall and central properties of the arterial baroreceptor reflex in salt-loaded young spontaneously hypertensive rats (SHR). Six-week-old SHR were fed either a normal (0.66%) or high (8.00%) salt diet with either a normal (1.17%) or high (4.07%) calcium content for 4 weeks. The arterial baroreceptor reflex was elicited with rats under halothane anesthesia by altering mean arterial pressure with nitroprusside or phenylephrine. The overall property of the arterial baroreceptor reflex was assessed by the median mean arterial pressure (MAP50) and maximal gain (G(max)) of the relation between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central property of the arterial baroreceptor reflex was assessed by reflex inhibition of renal sympathetic nerve activity and heart rate elicited by electrical stimulation of the aortic depressor nerve. Compared with the control group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and renal sympathetic nerve activity but not heart rate. Moreover, the arterial baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP50 and decrease in G(max) of the two relations and an attenuation of reflex inhibition of renal sympathetic nerve activity by aortic depressor nerve stimulation. There were no significant differences in mean arterial pressure, renal sympathetic nerve activity, or the overall and central properties of the arterial baroreceptor reflex among the control, high salt/high calcium, and normal salt/high calcium groups. In conclusion, dietary calcium supplementation prevented accelerated hypertension with sympathetic overactivity as well as impairment of the arterial baroreceptor reflex in salt-loaded young SHR. It is suggested that normalization of both tonic and reflex control of sympathetic discharges underlies the antihypertensive effect of a high calcium diet on salt-induced hypertension.