Aspects of Calcium Metabolism in Obstructive Nephropathy

Abstract

In 28 patients with bilaterally recurring calcium – containing renal staghorn calculi, 14 with normal GFR and 14 with mild renal insufficiency, a metabolic investigation of calcium metabolism during intravenous phosphate loading and a morphologic (optical and electron microscopic) study by means of percutaneous bone biopsy were performed. The morphologic investigation showed evidence of osteomalacia in 28.5% of the patients with normal GFR and in 50% of those with renal insufficiency. Moreover, histological evidence of hyperparathyroidism was found in an additional 29% of the patients with renal insufficiency. The metabolic investigation demonstrated low values of Tm PO₄/GFR only in patients with morphologic evidence of hyperparathyroidism. Moreover, the fall in plasma calcium concentration during phosphate loading appeared to be more evident and significant in patients with osteodystrophy than in those with normal bone. It is possible that the disordered vitamin D metabolism in patients with renal insufficiency and the renal parenchymal lesions observed in all of the patients may have played some role in the pathogenesis of osteomalacia, while the repeated insults to the kidney secondary to ureteropelvic obstructions may have led to early secondary hyperparathyroidism. From the metabolic point of view, our results support the validity of phosphate infusions in assessing the Tm PO₄/GFR as a functional index of hyperparathyroidism, and the exaggerated fall in plasma calcium concentration as an index of metabolic bone disease.