HIGH-CALCIUM DIET PREVENTS BAROREFLEX IMPAIRMENT IN SALT-LOADED SPONTANEOUSLY HYPERTENSIVE RATS

To investigate the role of the sympathetic control mechanism in the an tihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sym pathetic nerve activity, heart rate, and overall and central propertie s of the arterial baroreceptor reflex in salt-loaded young spontaneous ly 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 ar terial pressure with nitroprusside or phenylephrine, The overall prope rty of the arterial baroreceptor reflex was assessed by the median mea n arterial pressure (MAP(50)) and maximal gain (G(max)) of the relatio n between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central propert y of the arterial baroreceptor reflex was assessed by reflex inhibitio n of renal sympathetic nerve activity and heart rate elicited by elect rical stimulation of the aortic depressor nerve. Compared with the con trol group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and ren al sympathetic nerve activity but not heart rate. Moreover, the arteri al baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP(50) and decrease in G(max) of the two relations and an attenuation of rifler inhibition of renal sympathetic nerve act ivity by aortic depressor nerve stimulation. There were no significant differences in mean arterial pressure, renal sympathetic nerve activi ty, or the overall and central properties of the arterial baroreceptor reflex among the control, high salt/high calcium. and normal salt/hig h calcium groups. In conclusion, dietary calcium supplementation preve nted accelerated hypertension with sympathetic overactivity as well as impairment of the arterial baroreceptor reflex in salt-loaded young S HR. It is suggested that normalization of both tonic and reflex contro l of sympathetic discharges underlies the antihypertensive effect of a high calcium diet on salt-induced hypertension.