Effects of calcium loading on blood pressure in spontaneously hypertensive rats: attenuation of the vascular reactivity.

The effects of oral calcium loading on the blood pressure of spontaneously hypertensive rats (SHR) (n = 20) and age-matched normotensive Wistar-Kyoto rats (WKY) (n = 16) were investigated. Calcium loading was performed by adding 1.5% CaCl2 (Calcium chloride) to the drinking water. Calcium loading attenuated the development of hypertension in the SHR but not in the WKY, and at the end of a 3 week experiment, systolic blood pressure was 171+2 vs 197+3 mmHg (P less than 0.01). In spite of this reduction of blood pressure, there were no significant changes in sodium-water balance, plasma levels of norepinephrine and epinephrine, plasma renin activity, plasma aldosterone concentration and serum electrolytes due to the calcium treatment. On this basis, the depressor mechanism of calcium loading in the SHR was studied by observing the pressor response to norepinephrine and the vascular reactivity to norepinephrine in hind limb perfusion. It was found that both the pressor response and the vascular reactivity were significantly attenuated by the calcium treatment in the SHR but not in the WKY. These results suggest that the antihypertensive effects of calcium treatment in SHR may depend mainly on attenuation of the vascular reactivity.