Results of many clinical and experimental studies indicate an inverse relationship between
dietary calcium and the prevalence of
hypertension. Our study was designed to evaluate the alterations in arterial blood pressure and the changes in alpha-
adrenoceptor-mediated vascular reactivity in normotensive Sprague-Dawley and spontaneously hypertensive rats (SHR) fed from weaning (3 weeks of life) three diets: normal
calcium (Ca 1%), low
calcium (Ca 0.1%), and high
calcium (Ca 2.5%). The systolic and the diastolic arterial blood pressures were measured weekly by the tail cuff method. The plasma
calcium levels in the animals were also measured regularly by colourimetric methods, and the alpha-
adrenoceptor-mediated vascular reactivity was evaluated by measuring the pressor responses to alpha-
adrenoceptor agonists in pithed rats. These determinations were carried out at the end of the feeding periods (9 weeks of life in Sprague-Dawley rats and 20 weeks of life in SHR) and also at the moments when maximal differences in arterial blood pressure were observed between the conscious animals fed the normal
calcium diet and those fed the other two diets.
Dietary calcium deficiency increased arterial blood pressure in both strains but
calcium supplements were effective to lower this only in hypertensive animals. The plasma
calcium levels were altered in both strains when
calcium administration was not normal. The low-
calcium diet did not modify the pressor responses to either the alpha(1)-adrenoceptor agonist,
methoxamine, or the alpha(2)-adrenoceptor agonist,
B-HT 920 (5-allyl-2-amino-5,6,7, 8-tetrahydro-4H-thiazolo-(4,5-D)-acepin-dihydrochloride, talixepole), in the normotensive and the hypertensive rats. On the contrary, the high-
calcium diet caused a definite decrease in alpha(1)- and alpha(2)-adrenoceptor-mediated vascular reactivity in both strains. The changes in the alpha-
adrenoceptor-mediated
vasoconstrictor responses were observed in pithed 9-week old Sprague-Dawley rats and in pithed 20-week old SHR, but none were observed in pithed 15-week old SHR, although at this age maximal differences in arterial blood pressure between the animals fed the high- and the normal
calcium diet were observed. The results of this study suggest that the mechanisms implicated in the effects of
dietary calcium supplements on arterial blood pressure are clearly different from the mechanisms, which bring about changes in arterial blood pressure when the diet is deficient in
calcium. The results of this study also show that
calcium administration causes variations in alpha-
adrenoceptor-mediated vascular reactivity, but this is probably not the only mechanism implicated in the
calcium effect on arterial blood pressure.