Abstract
1 Previous work has shown that parathyroidectomy (PTX) decreases cardiovascular reactivity to noradrenaline in Sprague-Dawley rats with mineralocorticoid-induced (D) hypertension. 2. In order to explain this diminution of cardiovascular reactivity, we studied in vivo the influence of serum calcium levels on the cardiac and vascular response to noradrenaline (500 ng/kg). We used rats with or without parathyroids but fed a standard or a high-calcium (+Ca) diet that re-established a serum calcium level of about 100 mg/l. Work was performed in vagotomized, anaesthetized rats after ganglionic blockage with pentolinium and atropine sulphate. 3 Cardiac output was unchanged in all the experimental groups after 11 weeks of mineralocorticoid treatment. The enhancing effect of noradrenaline was not modified. 4. Since a serum calcium level of about 100 mg/l was sufficient to re-establish vascular reactivity to exogenous noradrenaline in the PTX-D rats, parathyroid hormone was not necessary. 5 The total and lanthanum-resistant calcium fractions of the walls of the aorta were reduced in the PTX-D rats. When serum calcium levels were re-established at about 100 mg/l, there was no difference between PTX-D and D rats. 6 It is postulated that the decreased storage of calcium in vascular smooth muscle cells of PTX-D rats depresses the vascular effect of noradrenaline.