Calcium restriction accelerates salt-induced hypertension in young spontaneously hypertensive rats

To assess the effect of dietary calcium (Ca) and salt on blood pressure and vascular reactivity, 5-week-old male spontaneously hypertensive rats were placed on control (1% Ca diet+tap water), high Na (1% NaCl administered in drinking fluid), low Ca (0.2% Ca diet), and low Ca+high Na intake for 3 weeks (n = 8-10). During the observation period, systolic blood pressure increased in all groups, but the increase was markedly enhanced in the low Ca+high Na group. There were no significant differences in body weight and serum electrolytes. On high Na intakes, urine volume and urinary Na excretion increased and plasma renin activity and aldosterone concentration were suppressed, whereas Ca restriction did not affect these values. The dose-response curves of mesenteric arterial beds for in vitro contraction to norepinephrine (NE) were not affected by dietary manipulations. However, the time from maximum contraction to return to baseline levels of contractility by 10(-4) M NE was significantly prolonged in the low Ca+high Na group. These results indicate that Ca restriction accelerates salt-induced hypertension in young spontaneously hypertensive rats, and the cellular impairment in vascular smooth muscle might, at least in part, participate in the enhanced blood pressure elevation under this condition.