Effects of hypercalcemia and parathyroid hormone on blood pressure in normal and renal-failure rats.

Hypertension is common in primary hyperparathyroidism, but the mechanisms are not clear. Significant hypercalcemia induces elevation in blood pressure (BP), whereas excessive parathyroid hormone (PTH) lowers BP. However, in chronic renal failure (CRF) and secondary hyperparathyroidism, the hypercalcemia-induced hypertension is more severe. We examined the interaction between PTH and calcium on BP in normal rats and in those with CRF. Calcium caused a dose-related rise in serum calcium and a rise in mean arterial pressure (MAP). For a comparable rise in serum calcium, the increment in MAP in parathyroidectomized (PTX) rats (7 +/- 3 mmHg) was significantly lower (P less than 0.05) than in sham PTX rats (19 +/- 7.3 mmHg). In PTX rats receiving PTH, the MAP response to calcium infusion (17 +/- 2.4 mmHg) was similar to that in the sham PTX rats. The infusion of similar amounts of calcium in CRF rats caused a greater rise in serum calcium. In CRF-PTX rats, the changes in MAP during calcium infusion were significantly lower (P less than 0.05) than in CRF-sham PTX animals, despite similar rise in serum calcium. For a comparable rise in serum calcium, the rise in MAP in CRF rats was greater than in normal rats. These data suggest that the presence of PTH plays an important permissive role for the hypertensive action of the hypercalcemia.