Renal and systemic magnesium metabolism during chronic continuous PTH infusion in normal subjects

Renal and systemic magnesium metabolism has not been adequately characterized in states of prolonged PTH excess in humans. Whereas acute experimental PTH administration uniformly results in enhanced renal magnesium reabsorption in many species, including humans, numerous clinical reports have documented renal magnesium wasting in human primary hyperparathyroidism. The possibility has been raised, therefore, that secondary consequences of sustained hyperparathyroidism (eg, hypercalcemia, nephrocalcinosis) might override the direct renal effects of PTH. Accordingly, the present studies assessed the effects of chronic (12 days) continuous intravenous (IV) b-(1-34)-PTH infusion in four normal human subjects on plasma, urinary, and intestinal magnesium and calcium homeostasis under metabolic balance conditions. Chronic PTH infusion resulted in a steady-state of hypercalcemia, hypercalciuria, and persistent negative calcium balance, which returned to baseline values in a recovery period. In contrast to plasma calcium concentration, plasma magnesium concentration was not altered by PTH infusion. Significant hypermagnesuria was observed during the period of PTH administration (control, 8.21 +/- 0.43 mEq/24 hours; PTH days 7-12, 10.75 +/- 0.74 mEq/24 hours, P less than 0.05) resulting in an initial, but transient, negative magnesium balance. During days 7-12 of PTH administration, net intestinal magnesium absorption increased sufficiently to result in a return to control magnesium balance. These findings suggest that hypermagnesuria associated with clinical primary hyperparathyroidism results from either direct or indirect effects of PTH excess, per se, and does not require the long-term consequences or complications of the clinical disorder (eg, nephrocalcinosis, renal insufficiency, acidosis).