Effects of endogenous and exogenous parathyroid hormone on tubular reabsorption of calcium in pseudohypoparathyroidism.

Resistance to the proximal tubular actions of PTH is a well defined feature in patients with pseudohypoparathyroidism type I (PsH). However, it is less clear whether there also is resistance to the distal tubular effect of PTH on calcium reabsorption in these patients. Thus, we examined the effects of endogenous and exogenous PTH on calcium reabsorption in seven patients with PsH before and during treatment with 1 alpha-hydroxyvitamin D3 (1 alpha OHD3). Eleven patients with idiopathic hypoparathyroidism served as controls. Before treatment, urinary calcium excretion was comparable in the two groups and did not decrease significantly after PTH infusion in either group. During treatment with 1 alpha OHD3, in contrast, steady state urinary calcium excretion in patients with PsH was much lower than that in patients with idiopathic hypoparathyroidism at comparable serum calcium concentrations and did not exceed the upper limit of the normal range when PsH patients were normocalcemic. Infusion of PTH into 1 alpha OHD3-treated PsH patients led to a significant reduction in urinary calcium excretion, even though PTH had no effect on their urinary cAMP or phosphate excretion. These findings suggest that the renal resistance to PTH in patients with PsH is confined to its proximal tubular actions and does not include its distal tubular effect on calcium reabsorption, at least during treatment with active vitamin D metabolites.