Sodium-calcium interactions and salt-sensitive hypertension. In humans with essential hypertension, salt-induced increases in blood pressure have been reported to correlate directly with salt-induced increases in intracellular free calcium [( Ca2+]i) in circulating mononuclear cells. These findings are consistent with the hypothesis that salt-induced increases in [Ca2+]i mediate the phenomenon of salt sensitivity. Circumstantial evidence suggests that salt-induced increases in intracellular sodium or in plasma levels of 1,25-dihydroxy vitamin D might mediate salt-induced increases in [Ca2+]i and blood pressure. However, in humans with salt-sensitive hypertension, it remains to be determined: (1) whether salt-induced increases in white blood cell [Ca2+]i reflect corresponding increases in vascular smooth muscle [Ca2+]i; (2) whether salt-induced increases in [Ca2+]i are a cause or consequence of salt-induced increases in blood pressure; and (3) whether salt-induced increases in 1,25-dihydroxy vitamin D or intracellular sodium precede salt-induced increases in [Ca2+]i. 