[Abnormal regulation of parathyroid hormone secretion by serum calcium level in myotonic dystrophy]

Although the presence of bone abnormalities has been well known in myotonic dystrophy (MyD), details about calcium metabolism in this condition remain unclear. We already reported that the intestinal calcium absorption is increased in MyD patients due to the elevation of plasma 1,25(OH)2D level. We also reported that serum immunoreactive parathyroid hormone (iPTH) levels and Nephrogenous cyclic AMP (NcAMP) levels are increased in MyD patients. This study was designed to elucidate the mechanism of hyperparathyroid state in MyD patients. Intravenous calcium tolerance test was performed in 7 patients with MyD and 7 control subjects, 3 patients with other neuromuscular disorder and 4 healthy subjects. Calcium was infused stepwise at the concentration of 0, 104.7, 194.9 and 235.5mg every hour. The basal plasma calcium levels were significantly higher in MyD (9.79 +/- 0.27 mg/dl, Mean +/- SD) as compared with control subjects (9.50 +/- 0.19; p less than 0.05). Basal serum iPTH levels were also significantly higher in MyD (514.0 +/- 188.9 pg/ml) than those in control subjects (333.7 +/- 113.5; p less than 0.05). After calcium infusion, serum calcium levels were increased in both groups, but the levels of serum calcium remained significantly higher in MyD than those in control subjects. On the other hand, urinary calcium excretion levels were not different between the two groups. At the same time, %TRP and calcium clearance were significantly lower in MyD than control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)