Calcitonin secretion in idiopathic renal stone formers

Several studies demonstrated a reduction in bone mineral content (BMC) in idiopathic renal stone formers (RSF). We found this reduction in association with a chronic low-calcium diet. Low calcium intake could theoretically result in calcitonin deficiency, responsible for increased bone resorption. This hypothesis was tested in 22 male RSF eating a low-calcium diet (350 +/- 72 SD mg/day) for 2 years or more, who showed a significant reduction in their BMC. When compared to 15 normal male subjects eating a free diet, RSF showed increases in serum alkaline phosphatase activity and fasting urinary excretion of hydroxyproline and calcium, suggesting increased bone turnover. Plasma calcitonin levels were measured by radioimmunoassay following an extraction-concentration technique (exCT). Basal plasma exCT levels were higher (P less than 0.005) in RSF (4.1 +/- 0.8 SEM pg/ml) than in normal subjects (2.8 +/- 0.4). Following a 5 minute infusion of 2 mg elemental calcium per kg, levels of plasma exCT tended to increase more, although not significantly, in RSF (51.3 +/- 9.4 pg/ml) than in normal subjects (36.6 +/- 9.7). The CT secretory response, taking into account changes in serum calcium concentration (delta exCT/delta Ca), was higher (P less than 0.05) in RSF (50.0 +/- 10.0) than in normal subjects (25.6 +/- 6.6). Our study thus demonstrates that RSF chronically fed a low-calcium diet have increased basal plasma CT levels and increased CT cells responsiveness. CT deficiency cannot therefore be considered a cause for the low BMC associated with a chronic low-calcium diet in RSF.