CALCIUM SUPPLEMENT ENHANCES BAROREFLEX SENSITIVITY IN SALT-LOADED SPRAGUE-DAWLEY RATS

Dietary calcium is known to prevent salt-induced hypertension, although the exact mechanism responsible for this remains unknown. One of the proposed mechanisms of the pathogenesis of salt-induced hypertension is the impairment of baroreflex sensitivity. Hence we investigated the effect of calcium supplement on baroreceptor in salt-loaded rats. The experiment was performed in male sprague-Dawley rats fed with measured salt and/or calcium diets and given tap water ad libitum for 6 weeks. Blood pressure and heart rate measurments were done in anaesthetised animals through direct invesive method using Grass Polygraph. Bilateral carotid occlusion test was used to determine the baroreflex sensitivity in the rats. There was increase in mean arterial pressure (MAP) of salt-loaded rats relative to control (132.6 ± 2.3 vs. 90.1 ± 1.5mmHg; n=8; P<0.05), while dietary calcium alone did not have any significant effect on the MAP (84.5 ± 1.7mmHg; n=8). During the feeding period, the salt consumed by salt-loaded rats was significantly higher than those of control but lower than that of salt-loaded-fed rats. Also, water intake was highest in salt-loaded rats compared with other experimental rats. However, the volume of urinary excretion was higher in salt-loaded-calcium-fed rats than salt-loaded rats but both were higher than control. These resulted in attenuated baroreflex sensitivity of salt-loaded rats relative to control (0.55 ± 0.2 vs. 1.25± 0.1 beats/mmHg, n=7; P<0.05). However, dietary calcium enhanced baroreflex sensitivity in salt-loaded-fed rats (2.21 ± 0.2 beats/min/mmHg, n=7; P<0.05) compared with control and salt-loaded rats. The study shows that salt-loading led to hypertension probably through alteration of haemodynamic function and impairment of baroreflex sensitivity. Calcium supplement prevention of salt-induced hypertension seems to reverse these, thus resulting in maintenance of water balance and baroreceptor integrity.

Keywords
Salt-induced hypertension, baroreflex sensitivity, calcium supplement, Sprague-Dawley rats